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The resistance arises from the prevailing limitations on availability (local concentration) and mobility of the ions whose motion between the electrolyte and the electrode constitutes the faradaic current. The capacitance is that of the capacitor formed by the electrolyte and the electrode, separated by the Debye screening length and giving rise to the double-layer capacitance at the electrolyte-electrode interface. When the supply of ions does not meet the demand created by the potential the resistance increases, the effect being that of a constant current source or sink, and the cell is then said to be polarized at that electrode. The extent of polarization, and hence the faradaic impedance, can be controlled by varying the concentration of electrolyte ions and the temperature, by stirring the electrolyte, etc. The chemistry of the electrolyte-electrode interface is also a crucial factor.
Electrodes constructed as smooth planar sheets of metal have the least surface area. The area can be increased by using a woven mesh or porous or sintered metals. In this case faradaic impedance may be more appropriately modeled as a transmission line consisting of resistors in series coupled by capacitors in parallel. | 7 | Physical Chemistry |
TRIM24 has been shown to interact with Mineralocorticoid receptor, TRIM33, Estrogen receptor alpha and Retinoid X receptor alpha. | 1 | Biochemistry |
A variety of alloys exhibit the shape-memory effect. Alloying constituents can be adjusted to control the transformation temperatures of the SMA. Some common systems include the following (by no means an exhaustive list):
* Ag-Cd 44/49 at.% Cd
* Au-Cd 46.5/50 at.% Cd
* Co-Ni-Al
* Co-Ni-Ga
* Cu-Al-Be-X(X:Zr, B, Cr, Gd)
* Cu-Al-Ni 14/14.5 wt.% Al, 3/4.5 wt.% Ni
* Cu-Al-Ni-Hf
* Cu-Sn approx. 15 at.% Sn
* Cu-Zn 38.5/41.5 wt.% Zn
* Cu-Zn-X (X = Si, Al, Sn)
* Fe-Mn-Si
* Fe-Pt approx. 25 at.% Pt
* Mn-Cu 5/35 at.% Cu
* Ni-Fe-Ga
* Ni-Ti approx. 55–60 wt.% Ni
* Ni-Ti-Hf
* Ni-Ti-Pd
* Ni-Mn-Ga
* Ni-Mn-Ga-Cu
* Ni-Mn-Ga-Co
* Ti-Nb | 8 | Metallurgy |
An ideal cycle is simple to analyze and consists of:
# TOP (A) and BOTTOM (C) of the loop: a pair of parallel isobaric processes
# RIGHT (B) and LEFT (D) of the loop: a pair of parallel isochoric processes
If the working substance is a perfect gas, is only a function of for a closed system since its internal pressure vanishes. Therefore, the internal energy changes of a perfect gas undergoing various processes connecting initial state to final state are always given by the formula
Assuming that is constant, for any process undergone by a perfect gas.
Under this set of assumptions, for processes A and C we have and , whereas for processes B and D we have and .
The total work done per cycle is , which is just the area of the rectangle. If the total heat flow per cycle is required, this is easily obtained. Since , we have .
Thus, the total heat flow per cycle is calculated without knowing the heat capacities and temperature changes for each step (although this information would be needed to assess the thermodynamic efficiency of the cycle). | 7 | Physical Chemistry |
Mainly associated with abnormalities that result in neurodegenerative diseases. Tau proteins stabilize microtubules, and thus shift the reaction kinetics in favor of addition of new subunits, accelerating microtubule growth. Tau has the additional function of facilitating bundling of microtubules within the nerve cell. The function of tau has been linked to the neurological condition Alzheimers disease. In the nervous tissue of Alzheimers patients, tau forms abnormal aggregates. This aggregated tau is often severely modified, most commonly through hyperphosphorylation. As described above, phosphorylation of MAPs causes them to detach from microtubules. Thus, the hyperphosphorylation of tau leads to massive detachment, which in turn greatly reduces the stability of microtubules in nerve cells.[9] This increase in microtubule instability may be one of the main causes of the symptoms of Alzheimer's disease. | 1 | Biochemistry |
Because the urea conversion is incomplete, the urea must be separated from the unconverted reactants, including the ammonium carbamate. Various commercial urea processes are characterized by the conditions under which urea forms and the way that unconverted reactants are further processed. | 0 | Organic Chemistry |
Parasite Rex: Inside the Bizarre World of Natures Most Dangerous Creatures is a nonfiction book by Carl Zimmer that was published by Free Press in 2000. The book discusses the history of parasites on Earth and how the field and study of parasitology formed, along with a look at the most dangerous parasites ever found in nature. A special paperback edition was released in March 2011 for the tenth anniversary of the books publishing, including a new epilogue written by Zimmer. Signed bookplates were also given to fans that sent in a photo of themselves with a copy of the special edition.
The cover of Parasite Rex includes a scanning electron microscope image of a tick as the focus, along with illustrations in the centerfold of parasites and topics discussed in the book. | 1 | Biochemistry |
Many prior technologies exist for preparing wide-gap diamond for use in electronic devices or as a substrate for single-crystal diamond growth. The more stable forms of carbon have lower gaps and different crystal structures, and their presence must be carefully controlled. The Marchywka Effect has been characterised and compared to alternative means to create a desired surface for several applications.
Removal of non-diamond carbon with wet chemicals had been accomplished by boiling in mixtures of sulfuric and chromic acid. When applied to a diamond substrate with an ion implantation damage profile as may be used for basic science, crystal growth, or device fabrication, the electrochemical approach makes it easier to preserve the thin film of less damaged diamond lying above the implant range, and it has been used in annealing experiments to fix the diamond after implantation damage has occurred. In some cases, thermal cycling may be an issue and selectivity to various masks may be important, so the lower temperatures and more flexible chemistry may offer benefits over prior art.
The method does not require the use of non-volatile materials such as chrome, possibly reducing contamination problems in some applications. The ability to control the etching direction and speed with an applied voltage or electrode configuration, as with electrochemical machining, gives additional capabilities not available with isotropic chemical-only approaches. Dry processing methods such as hot oxygen or plasmas can also burn off the graphite faster than the diamond, as can a simple acetylene torch. These require higher temperatures and do not have the same high selectivity that can be achieved with the electrochemical approach.
Surface termination is often an issue with both solid state and vacuum devices, and the details of final surface band structure have been compared with alternatives in various device structures. | 7 | Physical Chemistry |
In and of themselves, pH indicators are usually weak acids or weak bases. The general reaction scheme of acidic pH indicators in aqueous solutions can be formulated as:
:HInd + +
where, "HInd" is the acidic form and "Ind" is the conjugate base of the indicator.
Vice versa for basic pH indicators in aqueous solutions:
:IndOH + + +
where "IndOH" stands for the basic form and "Ind" for the conjugate acid of the indicator.
The ratio of concentration of conjugate acid/base to concentration of the acidic/basic indicator determines the pH (or pOH) of the solution and connects the color to the pH (or pOH) value. For pH indicators that are weak electrolytes, the Henderson–Hasselbalch equation can be written as:
:pH = pK + log
:pOH = pK + log
The equations, derived from the acidity constant and basicity constant, states that when pH equals the pK or pK value of the indicator, both species are present in a 1:1 ratio. If pH is above the pK or pK value, the concentration of the conjugate base is greater than the concentration of the acid, and the color associated with the conjugate base dominates. If pH is below the pK or pK value, the converse is true.
Usually, the color change is not instantaneous at the pK or pK value, but a pH range exists where a mixture of colors is present. This pH range varies between indicators, but as a rule of thumb, it falls between the pK or pK value plus or minus one. This assumes that solutions retain their color as long as at least 10% of the other species persists. For example, if the concentration of the conjugate base is 10 times greater than the concentration of the acid, their ratio is 10:1, and consequently the pH is pK + 1 or pK + 1. Conversely, if a 10-fold excess of the acid occurs with respect to the base, the ratio is 1:10 and the pH is pK − 1 or pK − 1.
For optimal accuracy, the color difference between the two species should be as clear as possible, and the narrower the pH range of the color change the better. In some indicators, such as phenolphthalein, one of the species is colorless, whereas in other indicators, such as methyl red, both species confer a color. While pH indicators work efficiently at their designated pH range, they are usually destroyed at the extreme ends of the pH scale due to undesired side reactions. | 7 | Physical Chemistry |
Selenols are structurally similar to thiols, but the bond is about 8% longer at 196 pm. The angle approaches 90°. The bonding involves almost pure p-orbitals on Se, hence the near 90 angles. The bond energy is weaker than the bond, consequently selenols are easily oxidized and serve as H-atom donors. The Se-H bond is much weaker than the bond as reflected in their respective bond dissociation energy (BDE). For Benzeneselenol|, the BDE is 326 kJ/mol, while for Benzenethiol|, the BDE is 368 kJ/mol.
Selenols are about 1000 times stronger acids than thiols: the pK of Methaneselenol| is 5.2 vs 8.3 for Methanethiol|. Deprotonation affords the selenolate anion, , most examples of which are highly nucleophilic and rapidly oxidized by air.
The boiling points of selenols tend to be slightly greater than for thiols. This difference can be attributed to the increased importance of stronger van der Waals bonding for larger atoms. Volatile selenols have highly offensive odors. | 0 | Organic Chemistry |
Synthesis of nucleosides involves the coupling of a nucleophilic, heterocyclic base with an electrophilic sugar. The silyl-Hilbert-Johnson (or Vorbrüggen) reaction, which employs silylated heterocyclic bases and electrophilic sugar derivatives in the presence of a Lewis acid, is the most common method for forming nucleosides in this manner. | 0 | Organic Chemistry |
Catalytic triads perform covalent catalysis via an acyl-enzyme intermediate. If this intermediate is resolved by water, the result is hydrolysis of the substrate. However, if the intermediate is resolved by attack by a second substrate, then the enzyme acts as a transferase. For example, attack by an acyl group results in an acyltransferase reaction. Several families of transferase enzymes have evolved from hydrolases by adaptation to exclude water and favour attack of a second substrate. In different members of the α/β-hydrolase superfamily, the Ser-His-Asp triad is tuned by surrounding residues to perform at least 17 different reactions. Some of these reactions are also achieved with mechanisms that have altered formation, or resolution of the acyl-enzyme intermediate, or that don't proceed via an acyl-enzyme intermediate.
Additionally, an alternative transferase mechanism has been evolved by amidophosphoribosyltransferase, which has two active sites. In the first active site, a cysteine triad hydrolyses a glutamine substrate to release free ammonia. The ammonia then diffuses though an internal tunnel in the enzyme to the second active site, where it is transferred to a second substrate. | 1 | Biochemistry |
Osteomalacia is a disease in adults that results from vitamin D deficiency. Characteristics of this disease are softening of the bones, leading to bending of the spine, proximal muscle weakness, bone fragility, and increased risk for fractures. Osteomalacia reduces calcium absorption and increases calcium loss from bone, which increases the risk for bone fractures. Osteomalacia is usually present when 25-hydroxyvitamin D levels are less than about 10ng/mL. Although the effects of osteomalacia are thought to contribute to chronic musculoskeletal pain, there is no persuasive evidence of lower vitamin D levels in people with chronic pain or that supplementation alleviates chronic nonspecific musculoskeletal pain. Osteomalacia progress to osteoporosis, a condition of reduced bone mineral density with increased bone fragility and risk of bone fractures. Osteoporosis can be a long-term effect of calcium and/or vitamin D insufficiency, the latter contributing by reducing calcium absorption. | 1 | Biochemistry |
Mineral sequestration aims to trap carbon in the form of solid carbonate salts. This process occurs slowly in nature and is responsible for the deposition and accumulation of limestone over geologic time. Carbonic acid in groundwater slowly reacts with complex silicates to dissolve calcium, magnesium, alkalis and silica and leave a residue of clay minerals. The dissolved calcium and magnesium react with bicarbonate to precipitate calcium and magnesium carbonates, a process that organisms use to make shells. When the organisms die, their shells are deposited as sediment and eventually turn into limestone. Limestones have accumulated over billions of years of geologic time and contain much of Earth's carbon. Ongoing research aims to speed up similar reactions involving alkali carbonates. | 5 | Photochemistry |
There is an emerging understanding of skeletal muscle as a secretory organ, and of myokines as mediators of physical fitness through the practice of regular physical exercise (aerobic exercise and strength training), as well as new awareness of the anti-inflammatory and thus disease prevention aspects of exercise. Different muscle fiber types – slow twitch muscle fibers, oxidative muscle fibers, intermediate twitch muscle fibers, and fast twitch muscle fibers – release different clusters of myokines during contraction. This implies that variation of exercise types, particularly aerobic training/endurance training and muscle contraction against resistance (strength training) may offer differing myokine-induced benefits. | 1 | Biochemistry |
The fact that the binding motif appears as a "triplet" of the conserved TxT repeat, as well as the observation that blastp queries have returned no viable matches, has led some researchers to suggest that RiAFP represents a new type of AFP – one that differs from the heavily studied TmAFP (from T. molitor), DcAFP (from D. canadensis), and CfAFP (from C. fumiferana). On the basis of these observations, it has been predicted that the need for insect AFPs came about after insect evolutionary divergence, much like the evolution of fish AFPs; thus, different AFPs most likely evolved in parallel from adaptations to cold (environmental) stress. As a result, homology modelling with TmAFP, DcAFP, or CfAFP would prove to be fruitless.
Secondary structure modelling algorithms have determined that the internal repeats are spaced sufficiently to tend towards β-strand configuration; no helical regions include the conserved repeats; and all turn regions are located at the ends of β-strand regions. These data suggest that RiAFP is a well-folded β-helical protein, having six β-strand regions consisting of 13-amino acids (including one TxTxTxT binding motif) per strand.
Primary crystallographic studies, have been published on a RiAFP crystal (which diffracted to 1.3Å resolution) in the trigonal space group P321 (or P321), with unit-cell parameters a = b = 46.46, c = 193.21Å. | 1 | Biochemistry |
The clinical use of most antibiotics result in a mutation of the pathogen organism leading to their resistance against the drug. Therefore, development of new drugs is always needed. A potential first step in developing new drugs against currently threatening diseases (e.g. tuberculosis) is to find new drug targets in the causative agent of the disease, i.e. the pathogen microorganism, let it be either a bacterium, or a protozoan parasite. After finding the target protein in the bacterium (or protozoan parasite), one could design small molecular drug compounds that bind to the protein and inhibit it.
Public availability of biological network data makes the process of searching for new drug targets easier than it was before. By using the available metabolic networks, it is possible to find important nodes with link analysis algorithms, like PageRank. In a recently published paper, biochemical reactions are treated as nodes of the metabolic network. In this directed network, reaction A has a directed edge towards reaction B if the product of the former enters the latter reaction as a substrate or co-factor.
To select important nodes that could serve as drug targets, we might think of selecting high in-degree nodes (hubs; nodes with many incoming edges). It was shown however[2], that targeting hub proteins with many vital functions may unintentionally harm the living cell as well. A PageRank-based scoring method could detect important nodes that are not hubs and therefore might be better drug targets.
The PageRank of a node A is the stationary limit probability distribution that the random walker is at node A. In its original application, the personalization vector w captured the personal interest of a web-surfer: interesting websites to a surfer appeared with a higher probability in the distribution given in vector w. In this metabolic network, w is personalized to proteins; w is larger for those proteins that appear in higher concentrations in the proteomics analysis of certain diseases. This personalized PageRank may identify other related proteins to the disease.
However, by using only the personalized PageRank to identify important nodes, hubs still get a high score on average. To find non-hub important nodes instead, we should consider scoring the nodes by their "relativized personalized PageRank"; i.e. their personalized PageRank scores over the number of edges pointing towards them (over their in-degree):
The relativized personalized PageRank (rPPR(v)) for a node v is given by:
where PpageRank(v) is the personalized PageRank score of node v, and d_(v) is its in-degree. It was shown, that by using this method, numerous already validated drug targets can be found (e.g. in the Mycobacterium tuberculosis), therefore, new, currently unknown targets might be detected as well. | 1 | Biochemistry |
The transition dipole moment is useful for determining if transitions are allowed under the electric dipole interaction. For example, the transition from a bonding orbital to an antibonding orbital is allowed because the integral defining the transition dipole moment is nonzero. Such a transition occurs between an even and an odd orbital; the dipole operator, , is an odd function of , hence the integrand is an even function. The integral of an odd function over symmetric limits returns a value of zero, while for an even function this is not necessarily the case. This result is reflected in the parity selection rule for electric dipole transitions. The transition moment integral
of an electronic transition within similar atomic orbitals, such as s-s or p-p, is forbidden due to the triple integral returning an ungerade (odd) product. Such transitions only redistribute electrons within the same orbital and will return a zero product. If the triple integral returns a gerade (even) product, the transition is allowed. | 5 | Photochemistry |
Some caveats should be considered with the above.
1. Like all statistical mechanical results according to the MaxEnt school, this increase in thermodynamic entropy is only a prediction. It assumes in particular that the initial macroscopic description contains all of the information relevant to predicting the later macroscopic state. This may not be the case, for example if the initial description fails to reflect some aspect of the preparation of the system which later becomes relevant. In that case the "failure" of a MaxEnt prediction tells us that there is something more which is relevant that we may have overlooked in the physics of the system.
It is also sometimes suggested that quantum measurement, especially in the decoherence interpretation, may give an apparently unexpected reduction in entropy per this argument, as it appears to involve macroscopic information becoming available which was previously inaccessible. (However, the entropy accounting of quantum measurement is tricky, because to get full decoherence one may be assuming an infinite environment, with an infinite entropy).
2. The argument so far has glossed over the question of fluctuations. It has also implicitly assumed that the uncertainty predicted at time t for the variables at time t will be much smaller than the measurement error. But if the measurements do meaningfully update our knowledge of the system, our uncertainty as to its state is reduced, giving a new S which is less than S. (Note that if we allow ourselves the abilities of Laplaces demon, the consequences of this new information can also be mapped backwards, so our uncertainty about the dynamical state at time t is now also reduced from S to S').
We know that S > S; but we can now no longer be certain that it is greater than S = S. This then leaves open the possibility for fluctuations in S. The thermodynamic entropy may go "down" as well as up. A more sophisticated analysis is given by the entropy Fluctuation Theorem, which can be established as a consequence of the time-dependent MaxEnt picture.
3. As just indicated, the MaxEnt inference runs equally well in reverse. So given a particular final state, we can ask, what can we "retrodict" to improve our knowledge about earlier states? However the Second Law argument above also runs in reverse: given macroscopic information at time t, we should expect it too to become less useful. The two procedures are time-symmetric. But now the information will become less and less useful at earlier and earlier times. (Compare with Loschmidt's paradox.) The MaxEnt inference would predict that the most probable origin of a currently low-entropy state would be as a spontaneous fluctuation from an earlier high entropy state. But this conflicts with what we know to have happened, namely that entropy has been increasing steadily, even back in the past.
The MaxEnt proponents' response to this would be that such a systematic failing in the prediction of a MaxEnt inference is a "good" thing. It means that there is thus clear evidence that some important physical information has been missed in the specification the problem. If it is correct that the dynamics "are" time-symmetric, it appears that we need to put in by hand a prior probability that initial configurations with a low thermodynamic entropy are more likely than initial configurations with a high thermodynamic entropy. This cannot be explained by the immediate dynamics. Quite possibly, it arises as a reflection of the evident time-asymmetric evolution of the universe on a cosmological scale (see arrow of time). | 7 | Physical Chemistry |
Eutrophication may cause competitive release by making abundant a normally limiting nutrient. This process causes shifts in the species composition of ecosystems. For instance, an increase in nitrogen might allow new, competitive species to invade and out-compete original inhabitant species. This has been shown to occur in New England salt marshes. In Europe and Asia, the common carp frequently lives in naturally eutrophic or hypereutrophic areas, and is adapted to living in such conditions. The eutrophication of areas outside its natural range partially explain the fish's success in colonizing these areas after being introduced. | 2 | Environmental Chemistry |
In 1999, the Royal Society of Chemistry closed the journal Analytical Communications because it felt that the material submitted to that journal would be best included in a new communications section of Analyst. Predecessor journals of Analytical Communications were Proceedings of the Society for Analytical Chemistry, 1964–1974; Proceedings of the Analytical Division of the Chemical Society, 1975–1979; Analytical Proceedings, 1980–1993; Analytical Proceedings including Analytical Communications, 1994–1995. | 3 | Analytical Chemistry |
Induction is the redistribution of electron density through a traditional sigma bonded structure according to the electronegativity of the atoms involved. The inductive effect drops across every sigma bond involved limiting its effect to only a few bonds.
Conjugation is a redistribution of electron density similar to induction but transmitted through interconnected pi-bonds. Conjugation is not only affected by electronegativity of the connected atoms but also affected by the position of electron lone pairs with respect to the pi-system. Electronic effects can be transmitted throughout a pi-system allowing their influence to extend further than induction.
In the context of electronic redistribution, an electron-withdrawing group (EWG) draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. Similarly, an electron-releasing group (ERG) or electron-donating group (EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
In electrophilic aromatic substitution and nucleophilic aromatic substitution, substituents are divided into activating groups and deactivating groups. Resonance electron-releasing groups are classed as activating, while Resonance electron-withdrawing groups are classed as deactivating. | 7 | Physical Chemistry |
The development of the Isa Process tank house technology had its beginning in the zinc industry. During the mid-1970s, MIM Holdings Limited (“MIM”) was considering building a zinc refinery in Townsville to treat the zinc concentrate produced by its Mount Isa operations. As a result, MIM staff visited the zinc smelters using the best-practice technology and found that modern electrolytic zinc smelters had adopted permanent cathode plate and mechanised stripping technology.
MIM recognised that the performance of traditional copper refineries was constrained by the poor cathode geometry inherent in using copper starter sheets.
MIM then developed a research program aimed at developing similar permanent cathode technology for copper refining. CRL had been operating in Townsville since 1959, using conventional starter-sheet technology and treating blister copper produced in the Mount Isa Mines Limited copper smelter at Mount Isa in Queensland. CRL incorporated the permanent cathode technology in its 1978 refinery modernisation project. The material initially selected was 316L stainless steel, stitch-welded to a 304L stainless-steel hanger bar. The hanger-bar assembly was then electroplated with copper to a thickness of 1.3 millimeters (“mm”) (later increased to 2.5 mm and then 3.0 mm to improve the corrosion resistance of the hanger bar) to approximately 15 mm down onto the blade, which provided sufficient electrical conductivity and gave the assembly some corrosion resistance.
Electrodeposited copper adheres quite firmly to the stainless steel so that it does not detach during refining. The vertical edges of the stainless steel plates are covered with tight-fitting polymer edge strips to prevent copper depositing around the edge of the cathode plate and so make it easier to strip the cathode copper from them. The bottom of the cathode plates were masked with a thin film of wax, again to prevent the copper depositing around the bottom edge. Wax was used rather than an edge strip to avoid having a ledge that would collect falling anode slimes and contaminate the cathode copper.
Wax was also used on the vertical edges to prolong the life of the vertical edge strip.
The original cathode stripping machine was based on that used at the Hikoshima plant of the Mitsui Mining and Smelting Company of Japan. However, considerable development work was necessary to modify the design to handle the copper cathodes, which were heavier than those at Hikoshima, and to process the cathode plates without damaging them. The machines also had to be redesigned to allow for waxing the sides and bottoms of the cathode plates to allow the next copper cathode sheets to be removed easily.
The stripping machines included receiving and discharge conveyors, washing, separation, cathode stacking and discharging, cathode plate separation for refurbishing, and the wax applications for the sides and bottoms of the cathode plates.
The original CRL stripping machine had the capability of stripping 250 cathode plates per hour.
The lower cost of the cathode plates compared to starter sheets means that shorter cathode cycle times are possible. The cycle time can range from 5 to 14 days, but a seven-day cathode cycle is common. This shorter cycle time improves current efficiency as less short circuits occur and there is less nodulation of the cathode surface.
Initially, other refinery operators regarded the developments at CRL with scepticism. Stainless steel had been tried unsuccessfully as mother-plate material for copper starter sheets. They suffered from rapid deterioration of their strippability, resulting in “an almost daily increase in difficulty of stripping”. However, following the success of early installations in Townsville, Timmins, and many other places, the permanent stainless steel cathode technology has had widespread introduction. | 8 | Metallurgy |
Although some control exists at the transcriptional level, the regulation of cellular iron levels is ultimately controlled at the translational level by iron-responsive element-binding proteins IRP1 and especially IRP2. When iron levels are low, these proteins are able to bind to iron-responsive elements (IREs). IREs are stem loop structures in the untranslated regions (UTRs) of mRNA.
Both ferritin and ferroportin contain an IRE in their 5 UTRs, so that under iron deficiency their translation is repressed by IRP2, preventing the unnecessary synthesis of storage protein and the detrimental export of iron. In contrast, TFR1 and some DMT1 variants contain 3 UTR IREs, which bind IRP2 under iron deficiency, stabilizing the mRNA, which guarantees the synthesis of iron importers. | 1 | Biochemistry |
When the pre-mRNA has been transcribed from the DNA, it includes several introns and exons. (In nematodes, the mean is 4–5 exons and introns; in the fruit fly Drosophila there can be more than 100 introns and exons in one transcribed pre-mRNA.) The exons to be retained in the mRNA are determined during the splicing process. The regulation and selection of splice sites are done by trans-acting splicing activator and splicing repressor proteins as well as cis-acting elements within the pre-mRNA itself such as exonic splicing enhancers and exonic splicing silencers.
The typical eukaryotic nuclear intron has consensus sequences defining important regions. Each intron has the sequence GU at its 5 end. Near the 3 end there is a branch site. The nucleotide at the branchpoint is always an A; the consensus around this sequence varies somewhat. In humans the branch site consensus sequence is yUnAy. The branch site is followed by a series of pyrimidines – the polypyrimidine tract – then by AG at the 3' end.
Splicing of mRNA is performed by an RNA and protein complex known as the spliceosome, containing snRNPs designated U1, U2, U4, U5, and U6 (U3 is not involved in mRNA splicing). U1 binds to the 5' GU and U2, with the assistance of the U2AF protein factors, binds to the branchpoint A within the branch site. The complex at this stage is known as the spliceosome A complex. Formation of the A complex is usually the key step in determining the ends of the intron to be spliced out, and defining the ends of the exon to be retained. (The U nomenclature derives from their high uridine content).
The U4,U5,U6 complex binds, and U6 replaces the U1 position. U1 and U4 leave. The remaining complex then performs two transesterification reactions. In the first transesterification, 5 end of the intron is cleaved from the upstream exon and joined to the branch site A by a 2,5-phosphodiester linkage. In the second transesterification, the 3 end of the intron is cleaved from the downstream exon, and the two exons are joined by a phosphodiester bond. The intron is then released in lariat form and degraded. | 1 | Biochemistry |
Water quality guidelines for South Africa are grouped according to potential user types (e.g. domestic, industrial) in the 1996 Water Quality Guidelines. Drinking water quality is subject to the South African National Standard (SANS) 241 Drinking Water Specification. | 3 | Analytical Chemistry |
Oxidative addition into cyclopropylimines gives a metalloenamine intermediate similar to oxidative addition to cyclopropylketones giving alkylmetalloenolates. These intermediates can also reaction with alpha-beta unsaturated ketones to give disubstituted cyclopentane products following reductive elimination.
With rhodium, the intermediate metalloenamine reacts with tethered alkynes. and alkenes to give cyclized products such as pyrroles and cyclohexenones, respectively. | 0 | Organic Chemistry |
The Journal of Physical Organic Chemistry is a monthly peer-reviewed scientific journal, published since 1988 by John Wiley & Sons. It covers research in physical organic chemistry in its broadest sense and is available both online and in print. The current editor-in-chief is Rik Tykwinski (University of Alberta). | 7 | Physical Chemistry |
In Mexico, modafinil is not listed as a controlled substance, in the National Health Law, and can be purchased in pharmacies without prescription. | 4 | Stereochemistry |
This section discusses bremsstrahlung emission and the inverse absorption process (called inverse bremsstrahlung) in a macroscopic medium. We start with the equation of radiative transfer, which applies to general processes and not just bremsstrahlung:
is the radiation spectral intensity, or power per (area × × photon frequency) summed over both polarizations. is the emissivity, analogous to defined above, and is the absorptivity. and are properties of the matter, not the radiation, and account for all the particles in the medium - not just a pair of one electron and one ion as in the prior section. If is uniform in space and time, then the left-hand side of the transfer equation is zero, and we find
If the matter and radiation are also in thermal equilibrium at some temperature, then must be the blackbody spectrum:
Since and are independent of , this means that must be the blackbody spectrum whenever the matter is in equilibrium at some temperature – regardless of the state of the radiation. This allows us to immediately know both and once one is known – for matter in equilibrium. | 7 | Physical Chemistry |
Like a camera, most spectrometers allow the user to select the exposure time and quantity of samples to be collected. Setting the integration time and the number of scans is an important step. Too long of an integration time can cause saturation. (In a camera photo this could appear as a large white spot, where as in a spectrometer it can appear as a dip, or cut off peak) Too short an integration time can generate noisy results (In a camera photo this would be a dark or blurry area, where as in a spectrometer this may appear are spiky or unstable readings).
The exposure time is the time the light falls on the sensor during a measurement. Adjusting this parameter changes the overall sensitivity of the instrument, as changing the exposure time does for a camera. The minimum integration time varies by instrument with a minimum of .5 msec and a maximum of about 10 minutes per scan. A practical setting is in the range of 3 to 999 ms depending on the light intensity.
The integration time should be adjusted for a signal which does not exceed the maximum counts (16-bit CCD has 65,536, 14-bit CCD has 16,384). Saturation occurs when the integration time is set too high. Typically, a peak signal of about 85% of the maximum is a good target and yields a good S/N ratio. (ex: 60K counts or 16K counts respectively)
The number of scans indicates how many measurements will be averaged. Other things being equal, the Signal-to-Noise Ratio (SNR) of the collected spectra improves by the square root of the number N of scans averaged. For example, if 16 spectral scans are averaged, the SNR is improved by a factor of 4 over that of a single scan.
S/N ratio is measured at the input light level which reaches the full scale of the spectrometer. It is the ratio of signal counts Cs (usually at full scale) to RMS (root mean square) noise at this light level. This noise includes the dark noise Nd, the shot noise Ns related to the counts generated by the input light and read out noise. This is the best S/N ratio one can get from the spectrometer for light measurements. | 7 | Physical Chemistry |
The journal included reviews of research, books, and academic conferences, as well as primary results in the form of brief reports. It also reviewed what it considered to be notable aspects of patents and relevant scientific literature. Occasionally articles on the history, geological occurrences, and exploitation of platinum group metals were also published. | 8 | Metallurgy |
Crystal twinning occurs when two or more adjacent crystals of the same mineral are oriented so that they share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals that are tightly bonded to each other. The surface along which the lattice points are shared in twinned crystals is called a composition surface or twin plane.
Crystallographers classify twinned crystals by a number of twin laws, which are specific to the crystal structure. The type of twinning can be a diagnostic tool in mineral identification. There are three main types of twinning. The first is growth twinning which can occur both in very large and very small particles. The second is transformation twinning, where there is a change in the crystal structure. The third is deformation twinning, in which twinning develops in a crystal in response to a shear stress, and is an important mechanism for permanent shape changes in a crystal. | 3 | Analytical Chemistry |
Steric effects play a major role in the assignment of configurations in cyclohexanes. One can use steric hindrances to determine the propensity of a substituent to reside in the axial or equatorial plane. It is known that axial bonds are more hindered than the corresponding equatorial bonds. This is because substituents in the axial position are relatively close to two other axial substituents. This makes it very crowded when bulky substituents are oriented in the axial position. These types of steric interactions are commonly known as 1,3 diaxial interactions. These types of interactions are not present with substituents at the equatorial position.
There are generally considered three principle contributions to the conformational free energy:
# Baeyer strain, defined as the strain arising from deformation of bond angles.
# Pitzer strain, defined as the torsional strain arising from 1,2 interactions between groups attached to contiguous carbons,
# Van der Waals interactions, which are similar to 1,3 diaxial interactions. | 4 | Stereochemistry |
Positive charges of soil minerals can retain anions by the same principle as cation exchange. The surfaces of kaolinite, allophane and iron and aluminium oxides often carry positive charges. In most soils the cation-exchange capacity is much greater than the anion-exchange capacity, but the opposite can occur in highly weathered soils, such as ferralsols (oxisols). | 9 | Geochemistry |
Predictive biomarkers measure the effect of a drug and tell if the drug is having its expected activity, but do not offer any direct information on the disease. Predictive biomarkers are highly sensitive and specific; therefore they increase diagnostic validity of a drug or toxin's site-specific effect by eliminating recall bias and subjectivity from those exposed. For example, when an individual is exposed to a drug or toxin, the concentration of that drug or toxin within the body, or the biological effective dose, provides a more accurate prediction for the effect of the drug or toxin compared to an estimation or measurement of the toxin from the origin or external environment. | 1 | Biochemistry |
DAST converts alcohols to the corresponding alkyl fluorides as well as aldehydes and unhindered ketones to geminal difluorides. Carboxylic acids react no further than the acyl fluoride. Sulfur tetrafluoride, SF, effects the same transformation but will also convert the acyl fluoride to the trifluoromethyl derivative. For laboratory-scale operations, DAST is used in preference to SF, which is far less expensive but less easily handled. A slightly thermally more stable compound is morpho-DAST. Acid-labile substrates are less likely to undergo rearrangement and elimination since DAST is less prone to contamination with acids. Reaction temperatures are milder as well – alcohols typically react at −78 °C and ketones around 0 °C. | 0 | Organic Chemistry |
In solution, reducing monosaccharides exist in equilibrium between their acyclic and cyclic forms with less than 1% in the acyclic form. The open chain form can close to give the pyranose and furanose with both the α- and β-anomers present for each. The equilibrium population of conformers depends on their relative energies which can be determined to a rough approximation using steric and stereoelectronic arguments. It has been shown that cations in solution can shift the equilibrium. | 0 | Organic Chemistry |
ptxP is the pertussis toxins promoter gene. There is a well documented emergence and global spread of ptxP3 strains evolving from and replacing the native ptxP1 strains, associated with an increased production of the toxin, and thus an increased virulence. Such spread has been documented in multiple countries, and sometimes but not always linked to the resurgence of pertussis in the end of the 20th century. Countries with a documented spread of ptxP3' include Australia, Denmark, Finland, Iran, Italy, Japan, the Netherlands, and Sweden. | 1 | Biochemistry |
As shown in Scheme 3, diol 19 was protected with phosgene as a carbonate ester (20). The terminal alkene group of 20 was next converted to a methyl ester using ozonolysis followed by oxidation with potassium permanganate and esterification with diazomethane. Ring expansion to give the cyclohexane C ring 24 was achieved using a Dieckman condensation of lactone 23 with lithium diisopropylamide as a base at -78 °C. Decarboxylation of 24 required protection of the hydroxyl group as the 2-methoxy-2-propyl (MOP) ether (25). With the protecting group in place, decarboxylation was effected with potassium thiophenolate in dimethylformamide to give protected hydroxy ketone 26. In the next two steps the MOP protecting group was removed under acidic conditions, and alcohol 27 was reprotected as the more robust benzyloxymethyl ether 28. The ketone was converted to the trimethylsilyl enol ether 29, which was subsequently oxidized in a Rubottom oxidation using m-chloroperbezoic acid to give the trimethylsilyl protected acyloin 30. At this stage the final missing carbon atom in the Taxol ring framework was introduced in a Grignard reaction of ketone 30 using a 10-fold excess of methylmagnesium bromide to give tertiary alcohol 31. Treatment of this tertiary alcohol with the Burgess reagent (32) gave exocyclic alkene 33. | 0 | Organic Chemistry |
Pyruvate dehydrogenase is inhibited when one or more of the three following ratios are increased: ATP/ADP, NADH/NAD and acetyl-CoA/CoA.
In eukaryotes PDC is tightly regulated by its own specific pyruvate dehydrogenase kinase (PDK) and pyruvate dehydrogenase phosphatase (PDP), deactivating and activating it respectively.
* PDK phosphorylates three specific serine residues on E1 with different affinities. Phosphorylation of any one of them (using ATP) renders E1 (and in consequence the entire complex) inactive.
* Dephosphorylation of E1 by PDP reinstates complex activity.
Products of the reaction act as allosteric inhibitors of the PDC, because they activate PDK. Substrates in turn inhibit PDK, reactivating PDC.
During starvation, PDK increases in amount in most tissues, including skeletal muscle, via increased gene transcription. Under the same conditions, the amount of PDP decreases. The resulting inhibition of PDC prevents muscle and other tissues from catabolizing glucose and gluconeogenesis precursors. Metabolism shifts toward fat utilization, while muscle protein breakdown to supply gluconeogenesis precursors is minimized, and available glucose is spared for use by the brain.
Calcium ions have a role in regulation of PDC in muscle tissue, because it activates PDP, stimulating glycolysis on its release into the cytosol - during muscle contraction. Some products of these transcriptions release H2 into the muscles. This can cause calcium ions to decay over time. | 1 | Biochemistry |
The Mayo-Lewis method uses a form of the copolymer equation relating to :
For each different monomer composition, a line is generated using arbitrary values. The intersection of these lines is the , for the system. More frequently, the lines do not intersect in a single point and the area in which most lines intersect can be given as a range of , and values. | 7 | Physical Chemistry |
In the kinetic theory, heat is explained in terms of the microscopic motions and interactions of constituent particles, such as electrons, atoms, and molecules. The immediate meaning of the kinetic energy of the constituent particles is not as heat. It is as a component of internal energy.
In microscopic terms, heat is a transfer quantity, and is described by a transport theory, not as steadily localized kinetic energy of particles. Heat transfer arises from temperature gradients or differences, through the diffuse exchange of microscopic kinetic and potential particle energy, by particle collisions and other interactions. An early and vague expression of this was made by Francis Bacon. Precise and detailed versions of it were developed in the nineteenth century.
In statistical mechanics, for a closed system (no transfer of matter), heat is the energy transfer associated with a disordered, microscopic action on the system, associated with jumps in occupation numbers of the energy levels of the system, without change in the values of the energy levels themselves. It is possible for macroscopic thermodynamic work to alter the occupation numbers without change in the values of the system energy levels themselves, but what distinguishes transfer as heat is that the transfer is entirely due to disordered, microscopic action, including radiative transfer. A mathematical definition can be formulated for small increments of quasi-static adiabatic work in terms of the statistical distribution of an ensemble of microstates. | 7 | Physical Chemistry |
The Volta potential measured by SKP is directly proportional to the corrosion potential of a material, as such SKP has found widespread use in the study of the fields of corrosion and coatings. In the field of coatings for example, a scratched region of a self-healing shape memory polymer coating containing a heat generating agent on aluminium alloys was measured by SKP. Initially after the scratch was made the Volta potential was noticeably higher and wider over the scratch than over the rest of the sample, implying this region is more likely to corrode. The Volta potential decreased over subsequent measurements, and eventually the peak over the scratch completely disappeared implying the coating has healed. Because SKP can be used to investigate coatings in a non-destructive way it has also been used to determine coating failure. In a study of polyurethane coatings, it was seen that the work function increases with increasing exposure to high temperature and humidity. This increase in work function is related to decomposition of the coating likely from hydrolysis of bonds within the coating.
Using SKP the corrosion of industrially important alloys has been measured. In particular with SKP it is possible to investigate the effects of environmental stimulus on corrosion. For example, the microbially induced corrosion of stainless steel and titanium has been examined. SKP is useful to study this sort of corrosion because it usually occurs locally, therefore global techniques are poorly suited. Surface potential changes related to increased localized corrosion were shown by SKP measurements. Furthermore, it was possible to compare the resulting corrosion from different microbial species. In another example SKP was used to investigate biomedical alloy materials, which can be corroded within the human body. In studies on Ti-15Mo under inflammatory conditions, SKP measurements showed a lower corrosion resistance at the bottom of a corrosion pit than at the oxide protected surface of the alloy. SKP has also been used to investigate the effects of atmospheric corrosion, for example to investigate copper alloys in marine environment. In this study Kelvin potentials became more positive, indicating a more positive corrosion potential, with increased exposure time, due to an increase in thickness of corrosion products. As a final example SKP was used to investigate stainless steel under simulated conditions of gas pipeline. These measurements showed an increase in difference in corrosion potential of cathodic and anodic regions with increased corrosion time, indicating a higher likelihood of corrosion. Furthermore, these SKP measurements provided information about local corrosion, not possible with other techniques.
SKP has been used to investigate the surface potential of materials used in solar cells, with the advantage that it is a non-contact, and therefore a non-destructive technique. It can be used to determine the electron affinity of different materials in turn allowing the energy level overlap of conduction bands of differing materials to be determined. The energy level overlap of these bands is related to the surface photovoltage response of a system.
As a non-contact, non-destructive technique SKP has been used to investigate latent fingerprints on materials of interest for forensic studies. When fingerprints are left on a metallic surface they leave behind salts which can cause the localized corrosion of the material of interest. This leads to a change in Volta potential of the sample, which is detectable by SKP. SKP is particularly useful for these analyses because it can detect this change in Volta potential even after heating, or coating by, for example, oils.
SKP has been used to analyze the corrosion mechanisms of schreibersite-containing meteorites. The aim of these studies has been to investigate the role in such meteorites in releasing species utilized in prebiotic chemistry.
In the field of biology SKP has been used to investigate the electric fields associated with wounding, and acupuncture points.
In the field of electronics, KPFM is used to investigate the charge trapping in High-k gate oxides/interfaces of electronic devices. | 7 | Physical Chemistry |
Gene therapy is a medical technology that aims to produce a therapeutic effect through the manipulation of gene expression or through altering the biological properties of living cells.
The first attempt at modifying human DNA was performed in 1980, by Martin Cline, but the first successful nuclear gene transfer in humans, approved by the National Institutes of Health, was performed in May 1989. The first therapeutic use of gene transfer as well as the first direct insertion of human DNA into the nuclear genome was performed by French Anderson in a trial starting in September 1990. Between 1989 and December 2018, over 2,900 clinical trials were conducted, with more than half of them in phase I. In 2003, Gendicine became the first gene therapy to receive regulatory approval. Since that time, further gene therapy drugs were approved, such as Glybera (2012), Strimvelis (2016), Kymriah (2017), Luxturna (2017), Onpattro (2018), Zolgensma (2019), Abecma (2021), Adstiladrin, Roctavian and Hemgenix (all 2022). Most of these approaches utilize adeno-associated viruses (AAVs) and lentiviruses for performing gene insertions, in vivo and ex vivo, respectively. AAVs are characterized by stabilizing the viral capsid, lower immunogenicity, ability to transduce both dividing and nondividing cells, the potential to integrate site specifically and to achieve long-term expression in the in-vivo treatment. ASO / siRNA approaches such as those conducted by Alnylam and Ionis Pharmaceuticals require non-viral delivery systems, and utilize alternative mechanisms for trafficking to liver cells by way of GalNAc transporters.
Not all medical procedures that introduce alterations to a patient's genetic makeup can be considered gene therapy. Bone marrow transplantation and organ transplants in general have been found to introduce foreign DNA into patients. | 1 | Biochemistry |
Through the recent study of untranslated regions, general information has been gathered about the nature and function of these elements. However, there is still much that is unknown about these regions of mRNA. Since the regulation of gene expression is critical in the proper function of cells, this is an area of study that needs to be investigated further. It is important to consider that mutations in 3' untranslated regions have the potential to alter the expression of several genes that may appear unrelated. We are only beginning to understand the links between proper untranslated region function, and disease states of cells. | 1 | Biochemistry |
The RPA process employs three core enzymes – a recombinase, a single-stranded DNA-binding protein (SSB) and strand-displacing polymerase.
Recombinases are capable of pairing oligonucleotide primers with homologous sequence in duplex DNA.
SSB bind to displaced strands of DNA and prevent the primers from being displaced.
Finally, the strand displacing polymerase begins DNA synthesis where the primer has bound to the target DNA.
By using two opposing primers, much like PCR, if the target sequence is indeed present, an exponential DNA amplification reaction is initiated. No other sample manipulation such as thermal or chemical melting is required to initiate amplification. At optimal temperatures (37–42 °C), the reaction progresses rapidly and results in specific DNA amplification from just a few target copies to detectable levels, typically within 10 minutes, for rapid detection of viral genomic DNA or RNA, pathogenic bacterial genomic DNA, as well as short length aptamer DNA.
The three core RPA enzymes can be supplemented by further enzymes to provide extra functionality. Addition of exonuclease III allows the use of an exo probe for real-time, fluorescence detection akin to real-time PCR. Addition of endonuclease IV means that a nfo probe can be used for lateral flow strip detection of successful amplification. If a reverse transcriptase that works at 37–42 °C is added then RNA can be reverse transcribed and the cDNA produced amplified all in one step. Currently only the TwistAmp exo version of RPA is available with the reverse transcriptase included, although users can simply supplement other TwistAmp reactions with a reverse transcriptase to produce the same effect.
As with PCR, all forms of RPA reactions can be multiplexed by the addition of further primer/probe pairs, allowing the detection of multiple analytes or an internal control in the same tube. | 1 | Biochemistry |
Transnasal evaporative cooling is a method of inducing the hypothermia process and provides a means of continuous cooling of a person throughout the early stages of targeted temperature management and during movement throughout the hospital environment. This technique uses two cannulae, inserted into a person's nasal cavity, to deliver a spray of coolant mist that evaporates directly underneath the brain and base of the skull. As blood passes through the cooling area, it reduces the temperature throughout the rest of the body.
The method is compact enough to be used at the point of cardiac arrest, during ambulance transport, or within the hospital proper. It is intended to reduce rapidly the person's temperature to below while targeting the brain as the first area of cooling. Research into the device has shown cooling rates of per hour in the brain (measured through infrared tympanic measurement) and per hour for core body temperature reduction. | 1 | Biochemistry |
Modafinil's patent history involves several key developments. The original patent, , was granted to Laboratoire L. Lafon in 1990, covering the chemical compound of modafinil. This patent expired in 2010. In 1994, Cephalon filed a patent for modafinil in the form of particles of a defined size, represented by , which expired in 2015.
Following the nearing expiration of marketing rights in 2002, generic manufacturers, including Mylan and Teva, applied for FDA approval to market a generic form of modafinil, leading to legal challenges by Cephalon regarding the particle size patent. The patent RE 37,516 was declared invalid and unenforceable in 2011.
In addition, Cephalon entered agreements with several generic drug manufacturers to delay the sale of generic modafinil in the US. These agreements were subject to legal scrutiny and antitrust investigations, culminating in a ruling by the Court of Appeals in 2016, which found that the settlements did not violate antitrust laws. | 4 | Stereochemistry |
Dialysis is the process used to change the matrix of molecules in a sample by differentiating molecules by the classification of size. It relies on diffusion, which is the random, thermal movement of molecules in solution (Brownian motion) that leads to the net movement of molecules from an area of higher concentration to a lower concentration until equilibrium is reached. Due to the pore size of the membrane, large molecules in the sample cannot pass through the membrane, thereby restricting their diffusion from the sample chamber. By contrast, small molecules will freely diffuse across the membrane and obtain equilibrium across the entire solution volume, thereby changing the overall concentration of these molecules in the sample and dialysate (see dialysis figure at right).
Osmosis is another principle that makes dialysis work. During osmosis, fluid moves from areas of high water concentration to lower water concentration across a semi-permeable membrane until equilibrium. In dialysis, excess fluid moves from sample to the dialysate through a membrane until the fluid level is the same between sample and dialysate.
Finally, ultrafiltration is the convective flow of water and dissolved solute down a pressure gradient caused by hydrostatic forces or osmotic forces. In dialysis, ultrafiltration removes molecules of waste and excess fluids from sample.
For example, dialysis occurs when a sample contained in a cellulose bag and is immersed into a dialysate solution. During dialysis, equilibrium is achieved between the sample and dialysate since only small molecules can pass the cellulose membrane, leaving only larger particles behind.
Once equilibrium is reached, the final concentration of molecules is dependent on the volumes of the solutions involved, and if the equilibrated dialysate is replaced (or exchanged) with fresh dialysate (see procedure below), diffusion will further reduce the concentration of the small molecules in the sample.
Dialysis can be used to either introduce or remove small molecules from a sample, because small molecules move freely across the membrane in both directions. Dialysis can also be used to remove salts. This makes dialysis a useful technique for a variety of applications. See dialysis tubing for additional information on the history, properties, and manufacturing of semipermeable membranes used for dialysis. | 1 | Biochemistry |
The period of pulsation is an increasing function of the amplitude .
When ,
the period
When , the period
In the whole range
, the period and frequency can be approximated by
to at least 8 significant figures. The relative error of this approximation does not exceed . | 7 | Physical Chemistry |
Bioadhesives are natural polymeric materials that act as adhesives. The term is sometimes used more loosely to describe a glue formed synthetically from biological monomers such as sugars, or to mean a synthetic material designed to adhere to biological tissue.
Bioadhesives may consist of a variety of substances, but proteins and carbohydrates feature prominently. Proteins such as gelatin and carbohydrates such as starch have been used as general-purpose glues by man for many years, but typically their performance shortcomings have seen them replaced by synthetic alternatives. Highly effective adhesives found in the natural world are currently under investigation. For example, bioadhesives secreted by microbes and by marine molluscs and crustaceans are being researched with a view to biomimicry. Furthermore, thiolation of proteins and carbohydrates enables these polymers (thiomers) to covalently adhere especially to cysteine-rich subdomains of proteins such as keratins or mucus glycoproteins via disulfide bond formation. Thiolated chitosan and thiolated hyaluronic acid are used as bioadhesives in various medicinal products. | 1 | Biochemistry |
Mitochondria can repair oxidative DNA damage by mechanisms analogous to those occurring in the cell nucleus. The proteins employed in mtDNA repair are encoded by nuclear genes, and are translocated to the mitochondria. The DNA repair pathways in mammalian mitochondria include base excision repair, double-strand break repair, direct reversal and mismatch repair. Alternatively, DNA damage may be bypassed, rather than repaired, by translesion synthesis.
Of the several DNA repair process in mitochondria, the base excision repair pathway has been most comprehensively studied. Base excision repair is carried out by a sequence of enzyme-catalyzed steps that include recognition and excision of a damaged DNA base, removal of the resulting abasic site, end processing, gap filling and ligation. A common damage in mtDNA that is repaired by base excision repair is 8-oxoguanine produced by oxidation of guanine.
Double-strand breaks can be repaired by homologous recombinational repair in both mammalian mtDNA and plant mtDNA. Double-strand breaks in mtDNA can also be repaired by microhomology-mediated end joining. Although there is evidence for the repair processes of direct reversal and mismatch repair in mtDNA, these processes are not well characterized. | 1 | Biochemistry |
In the early 1990s, Monsanto faced several lawsuits over harm caused by PCBs from workers at companies such as Westinghouse that bought PCBs from Monsanto and used them to build electrical equipment. Monsanto and its customers, such as Westinghouse and GE, also faced litigation from third parties, such as workers at scrap yards that bought used electrical equipment and broke them down to reclaim valuable metals. Monsanto settled some of these cases and won the others, on the grounds that it had clearly told its customers that PCBs were dangerous chemicals and that protective procedures needed to be implemented.
In 2003, Monsanto and Solutia Inc., a Monsanto corporate spin-off, reached a $700 million settlement with the residents of West Anniston, Alabama, who had been affected by the manufacturing and dumping of PCBs. In a trial lasting six weeks, the jury found that "Monsanto had engaged in outrageous behavior, and held the corporations and its corporate successors liable on all six counts it considered – including negligence, nuisance, wantonness and suppression of the truth."
In 2014, the Los Angeles Superior Court found that Monsanto was not liable for cancers claimed to be from PCBs permeating the food supply of three plaintiffs who had developed non-Hodgkins lymphoma. After a four-week trial, the jury found that Monsantos production and sale of PCBs between 1935 and 1977 were not substantial causes of the cancer.
In 2015, the cities of Spokane, San Diego, and San Jose initiated lawsuits against Monsanto to recover cleanup costs for PCB contaminated sites, alleging that Monsanto continued to sell PCBs without adequate warnings after they knew of their toxicity. Monsanto issued a media statement concerning the San Diego case, claiming that improper use or disposal by third-parties, of a lawfully sold product, was not the company's responsibility.
In July 2015, a St Louis county court in Missouri found that Monsanto, Solutia, Pharmacia and Pfizer were not liable for a series of deaths and injuries caused by PCBs manufactured by Monsanto Chemical Company until 1977. The trial took nearly a month and the jury took a day of deliberations to return a verdict against the plaintiffs from throughout the USA. Similar cases are ongoing. "The evidence simply doesnt support the assertion that the historic use of PCB products was the cause of the plaintiffs harms. We are confident that the jury will conclude, as two other juries have found in similar cases, that the former Monsanto Company is not responsible for the alleged injuries," a Monsanto statement said.
In May 2016, a Missouri state jury ordered Monsanto to pay $46.5 million to three plaintiffs whose exposure to PCB caused non-Hodgkin lymphoma.
In December 2016, the state of Washington filed suit in King County. The state sought damages and clean up costs related to PCBs. In March 2018 Ohio Attorney General Mike DeWine also filed a lawsuit against Monsanto over health issues posed by PCBs.
On November 21, 2019, a federal judge denied a bid by Monsanto to dismiss a lawsuit filed by LA County calling the company to clean up cancer-causing PCBs from Los Angeles County waterways and storm sewer pipelines. The lawsuit calls for Monsanto to pay for cleanup of PCBs from dozens of waterways, including the LA River, San Gabriel River and the Dominguez Watershed.
In June 2020, Bayer agreed to pay $650 million to settle local lawsuits related to Monsanto's pollution of public waters in various areas of the United States with PCBs.
In 2023, over 90 Vermont school districts joined a lawsuit against Monsanto alleging that PCBs created by the company were used in the construction of their schools. The Vermont Attorney General's office also filed its own lawsuit against Monsanto related to the use of its PCBs. | 2 | Environmental Chemistry |
This - Calphad calculated value of numerical derivative - Q has some interesting applications in the field of metal solidification. In fact, Q reflects the phase diagram of the alloy system and its reciprocal has been found to have a relationship with grain size d on solidification, which empirically has been found in some cases to be linear:
where a and b are constants, as illustrated with some examples from the literature for Mg and Al alloys. Before Calphad use, Q values were calculated from the conventional relationship:
Q=m*c0(k−1)
where m is the slope of the liquidus, c0 is the solute concentration, and k is the equilibrium distribution coefficient.
More recently some other possible correlation of Q with grain size d have been found, for instance:
where B is a constant independent of alloy composition. | 8 | Metallurgy |
Pyruvate decarboxylation or pyruvate oxidation, also known as the link reaction (or oxidative decarboxylation of pyruvate), is the conversion of pyruvate into acetyl-CoA by the enzyme complex pyruvate dehydrogenase complex.
The reaction may be simplified as:
:Pyruvate + NAD + CoA → Acetyl-CoA + NADH + CO
Pyruvate oxidation is the step that connects glycolysis and the Krebs cycle. In glycolysis, a single glucose molecule (6 carbons) is split into 2 pyruvates (3 carbons each). Because of this, the link reaction occurs twice for each glucose molecule to produce a total of 2 acetyl-CoA molecules, which can then enter the Krebs cycle.
Energy-generating ions and molecules, such as amino acids and carbohydrates, enter the Krebs cycle as acetyl coenzyme A and oxidize in the cycle. The pyruvate dehydrogenase complex (PDC) catalyzes the decarboxylation of pyruvate, resulting in the synthesis of acetyl-CoA, CO, and NADH. In eukaryotes, this enzyme complex regulates pyruvate metabolism, and ensures homeostasis of glucose during absorptive and post-absorptive state metabolism. As the Krebs cycle occurs in the mitochondrial matrix, the pyruvate generated during glycolysis in the cytosol is transported across the inner mitochondrial membrane by a pyruvate carrier under aerobic conditions. | 1 | Biochemistry |
Karrer was born in Moscow, Russia to Paul Karrer and Julie Lerch, both Swiss nationals. In 1892 Karrer's family returned to Switzerland where he was educated at Wildegg and at the grammar school in Lenzburg, Aarau, where he matriculated in 1908. He studied chemistry at the University of Zurich under Alfred Werner and after gaining his Ph.D. in 1911, he spent a further year as assistant in the Chemical Institute. He then took a post as chemist with Paul Ehrlich at the Georg Speyer Haus, Frankfurt-am-Main. In 1919 he became Professor of Chemistry and Director of the Chemical Institute. | 0 | Organic Chemistry |
In 1831, British vinegar merchant Peregrine Phillips patented the contact process, which was a far more economical process for producing sulfur trioxide and concentrated sulfuric acid. Today, nearly all of the world's sulfuric acid is produced using this method. | 7 | Physical Chemistry |
There has been a long debate over the question of whether zero-point fluctuations of quantized vacuum fields are "real" i.e. do they have physical effects that cannot be interpreted by an equally valid alternative theory? Schwinger, in particular, attempted to formulate QED without reference to zero-point fluctuations via his "source theory". From such an approach it is possible to derive the Casimir Effect without reference to a fluctuating field. Such a derivation was first given by Schwinger (1975) for a scalar field, and then generalized to the electromagnetic case by Schwinger, DeRaad, and Milton (1978). in which they state "the vacuum is regarded as truly a state with all physical properties equal to zero". More recently Jaffe (2005) has highlighted a similar approach in deriving the Casimir effect stating "the concept of zero-point fluctuations is a heuristic and calculational aid in the description of the Casimir effect, but not a necessity in QED."
Nevertheless, as Jaffe himself notes in his paper, "no one has shown that source theory or another S-matrix based approach can provide a complete description of QED to all orders." Furthermore, Milonni has shown the necessity of the vacuum field for the formal consistency of QED. In QCD, color confinement has led physicists to abandon the source theory or S-matrix based approach for the strong interactions. The Higgs mechanism, Hawking Radiation and the Unruh effect are also theorized to be dependent on zero-point vacuum fluctuations, the field contribution being an inseparable parts of these theories. Jaffe continues "Even if one could argue away zero-point contributions to the quantum vacuum energy, the problem of spontaneous symmetry breaking remains: condensates [ground state vacua] that carry energy appear at many energy scales in the Standard Model. So there is good reason to be skeptical of attempts to avoid the standard formulation of quantum field theory and the zero-point energies it brings with it." It is difficult to judge the physical reality of infinite zero-point energies that are inherent in field theories, but modern physics does not know any better way to construct gauge-invariant, renormalizable theories than with zero-point energy and they would seem to be a necessity for any attempt at a unified theory. | 7 | Physical Chemistry |
The key feature of the fourth-generation approach was the utilization of bidirectional fragment, vinyl iodide/phosphonium salt. phosphonium salt could be readily obtained as the trisubstituted vinyl iodide is less reactive than alkyl iodide. A Wittig reaction with followed by Suzuki coupling with alkyl iodide efficiently furnished the backbone of (+)-discodermolide. The Smith fourth-generation synthesis of (+)-discodermolide has an overall yield of 9.0% with a longest linear sequence of 17 steps and 36 total steps. | 0 | Organic Chemistry |
In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealized value of approximately 109°. Because of their high strain, the heat of combustion for these small rings is elevated.
Ring strain results from a combination of angle strain, conformational strain or Pitzer strain (torsional eclipsing interactions), and transannular strain, also known as van der Waals strain or Prelog strain. The simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane.
Ring strain energy can be attributed to the energy required for the distortion of bond and bond angles in order to close a ring.
Ring strain energy is believed to be the cause of accelerated rates in altering ring reactions. Its interactions with traditional bond energies change the enthalpies of compounds effecting the kinetics and thermodynamics of ring strain reactions. | 7 | Physical Chemistry |
Used in the measurement of trace amounts of volatile heavy metals such as mercury, cold vapour AFS makes use of the unique characteristic of mercury that allows vapour measurement at room temperature. Free mercury atoms in a carrier gas are excited by a collimated ultraviolet light source at a wavelength of 253.7 nanometres. The excited atoms re-radiate their absorbed energy (fluoresce) at this same wavelength. Unlike the directional excitation source, the fluorescence is omnidirectional and may thus be detected using a photomultiplier tube or UV photodiode.
Gold coated traps may be used to collect mercury in ambient air or other media. The traps are then heated, releasing the mercury from the gold while passing argon through the cartridge. This preconcentrates the mercury, increasing sensitivity, and also transfers the mercury into an inert gas. | 7 | Physical Chemistry |
The vector approach defines flux density as a vector at a point of space and time prescribed by the investigator. To distinguish this approach, one might speak of the full spherical flux density. In this case, nature tells the investigator what is the magnitude, direction, and sense of the flux density at the prescribed point. For the flux density vector, one may write
where denotes the spectral radiance (or specific intensity) at the point at time and frequency , denotes a variable unit vector with origin at the point , denotes an element of solid angle around , and indicates that the integration extends over the full range of solid angles of a sphere.
Mathematically, defined as an unweighted integral over the solid angle of a full sphere, the flux density is the first moment of the spectral radiance (or specific intensity) with respect to solid angle. It is not common practice to make the full spherical range of measurements of the spectral radiance (or specific intensity) at the point of interest, as is needed for the mathematical spherical integration specified in the strict definition; the concept is nevertheless used in theoretical analysis of radiative transfer.
As described below, if the direction of the flux density vector is known in advance because of a symmetry, namely that the radiative field is uniformly layered and flat, then the vector flux density can be measured as the net flux, by algebraic summation of two oppositely sensed scalar readings in the known direction, perpendicular to the layers.
At a given point in space, in a steady-state field, the vector flux density, a radiometric quantity, is equal to the time-averaged Poynting vector, an electromagnetic field quantity.
Within the vector approach to the definition, however, there are several specialized sub-definitions. Sometimes the investigator is interested only in a specific direction, for example the vertical direction referred to a point in a planetary or stellar atmosphere, because the atmosphere there is considered to be the same in every horizontal direction, so that only the vertical component of the flux is of interest. Then the horizontal components of flux are considered to cancel each other by symmetry, leaving only the vertical component of the flux as non-zero. In this case some astrophysicists think in terms of the astrophysical flux (density), which they define as the vertical component of the flux (of the above general definition) divided by the number . And sometimes the astrophysicist uses the term Eddington flux to refer to the vertical component of the flux (of the above general definition) divided by the number . | 7 | Physical Chemistry |
Metabolism and Hofmann elimination
Deacetylating vecuronium at position 3 results in a very active metabolite. In the case of rapacuronium the 3-deacylated metabolite is even more potent than rapacuronium. As long as the D-ring acetylcholine moiety is unchanged they retain their muscle relaxing effect. Mono-quaternary aminosteroids produced with deacylation in position 17 on the other hand are generally weak muscle relaxants. In the development of atracurium the main idea was to make use of Hofmann elimination of the muscle relaxant in vivo. When working with bisbenzyl-isoquinolinium types of molecules, inserting proper features into the molecule such as an appropriate electron withdrawing group then Hofmann elimination should occur at conditions in vivo. Atracurium, the resulting molecule, breaks down spontaneously in the body to inactive compounds and being especially useful in patients with kidney or liver failure. Cis-atracurium is very similar to atracurium except it is more potent and has a weaker tendency to cause histamine release.
Structure relations to onset time
The effect of structure on the onset of action is not very well known except that the time of onset appears inversely related to potency. In general mono-quaternary aminosteroids are faster than bis-quaternary compounds, which means they are also of lower potency. A possible explanation for this effect is that drug delivery and receptor binding are of a different timescale. Weaker muscle relaxants are given in larger doses so more molecules in the central compartment must diffuse into the effect compartment, which is the space within the mouth of the receptor, of the body. After delivery to the effect compartment then all molecules act quickly. Therapeutically this relationship is very inconvenient because low potency, often meaning low specificity can decrease the safety margin thus increasing the chances of side-effects. In addition, even though low potency usually accelerates onset of action, it does not guaranty a fast onset. Gallamine, for example, is weak and slow. When fast onset is necessary then succinylcholine or rocuronium are usually preferable.
Elimination
Muscle relaxants can have very different metabolic pathways and it is important that the drug does not accumulate if certain elimination pathways are not active, for example in kidney failure. | 1 | Biochemistry |
An alternative method for measuring the contact angle is the Wilhelmy method, which employs a sensitive force meter of some sort to measure a force that can be translated into a value of the contact angle. In this method, a small plate-shaped sample of the solid in question, attached to the arm of a force meter, is vertically dipped into a pool of the probe liquid (in actuality, the design of a stationary force meter would have the liquid being brought up, rather than the sample being brought down), and the force exerted on the sample by the liquid is measured by the force meter. This force is related to the contact angle by the following equation:
where F is the total force measured by the force meter, F is the force of buoyancy due to the solid sample displacing the liquid, I is the wetted length, and σ is the known surface tension of the liquid. | 7 | Physical Chemistry |
Organic carbonates are not prepared from inorganic carbonate salts.
Two main routes to carbonate esters are practiced: the reaction of an alcohol (or phenol) with phosgene (phosgenation), and the reaction of an alcohol with carbon monoxide and an oxidizer (oxidative carbonylation). Other carbonate esters may subsequently be prepared by transesterification.
In principle carbonate esters can be prepared by direct condensation of methanol and carbon dioxide. The reaction is however thermodynamically unfavorable. A selective membrane can be used to separate the water from the reaction mixture and increase the yield. | 0 | Organic Chemistry |
Hardness can be quantified by instrumental analysis. The total water hardness is the sum of the molar concentrations of Ca and Mg, in mol/L or mmol/L units. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent divalent metal ions), iron, aluminium, and manganese are also present at elevated levels in some locations. The presence of iron characteristically confers a brownish (rust-like) colour to the calcification, instead of white (the colour of most of the other compounds).
Water hardness is often not expressed as a molar concentration, but rather in various units, such as degrees of general hardness (dGH), German degrees (°dH), parts per million (ppm, mg/L, or American degrees), grains per gallon (mpg), English degrees (°e, e, or °Clark), or French degrees (°fH, °f or °HF; lowercase f is used to prevent confusion with degrees Fahrenheit). The table below shows conversion factors between the various units.
The various alternative units represent an equivalent mass of calcium oxide (CaO) or calcium carbonate (CaCO) that, when dissolved in a unit volume of pure water, would result in the same total molar concentration of Mg and Ca. The different conversion factors arise from the fact that equivalent masses of calcium oxide and calcium carbonates differ and that different mass and volume units are used. The units are as follows:
* Parts per million (ppm) is usually defined as 1 mg/L CaCO (the definition used below). It is equivalent to mg/L without chemical compound specified, and to American degree.
* Grain per gallon (gpg) is defined as 1 grain (64.8 mg) of calcium carbonate per U.S. gallon (3.79 litres), or 17.118 ppm.
* a mmol/L is equivalent to 100.09 mg/L CaCO or 40.08 mg/L Ca.
* A degree of General Hardness (dGH or German degree (°dH, deutsche Härte)) is defined as 10 mg/L CaO or 17.848 ppm.
* A Clark degree (°Clark) or English degrees (°e or e) is defined as one grain (64.8 mg) of CaCO per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm.
* A French degree (°fH or °f) is defined as 10 mg/L CaCO, equivalent to 10 ppm. | 3 | Analytical Chemistry |
Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols), organic compounds with multiple alcohol or hydroxy functional groups, with unsaturated and in some cases saturated dibasic acids. Typical polyols used are glycols including ethylene glycol, propylene glycol, and diethylene glycol; typical acids used are phthalic acid, isophthalic acid, terephthalic acid, and maleic anhydride. Water, a condensation by-product of esterification reactions, is continuously removed by distillation, driving the reaction to completion via Le Chateliers principle. Unsaturated polyesters are generally sold to parts manufacturers as a solution of resin in reactive diluent; styrene is the most common diluent and the industry standard. The diluent allows control over the viscosity of the resin, and is also a participant in the curing reaction. The initially liquid resin is converted to a solid by cross-linking chains. This is done by creating free radicals at unsaturated bonds, which propagate in a chain reaction to other unsaturated bonds in adjacent molecules, linking them in the process. Unsaturation is generally in the form of maleate and fumarate species along the polymer chain. Maleate/fumarate generally does not self-polymerize via radical reactions, but readily reacts with styrene. Maleic anhydride and styrene are known to form alternating copolymers, and are in fact the textbook case of this phenomenon. This is one reason that styrene has been so hard to displace in the market as the industry standard reactive diluent for unsaturated polyester resins, despite increasing efforts to displace the material such as Californias Proposition 65. The initial free radicals are induced by adding a compound that easily decomposes into free radicals. This compound is known as the catalyst within the industry, but initiator is a more appropriate term. Transition metal salts are usually added as a catalyst for the chain-growth crosslinking reaction, and in the industry this type of additive is known as a promoter; the promoter is generally understood to lower the bond dissociation energy of the radical initiator. Cobalt salts are the most common type of promoter used. Common radical initiators used are organic peroxides such as benzoyl peroxide or methyl ethyl ketone peroxide.
Polyester resins are thermosetting and, as with other resins, cure exothermically. The use of excessive initiator especially with a catalyst present can, therefore, cause charring or even ignition during the curing process. Excessive catalyst may also cause the product to fracture or form a rubbery material.
Unsaturated polyesters (UPR) are utilized in many different industrially relevant markets, but in general are used as the matrix material for various types of composites. Glass fiber-reinforced composites comprise the largest segment into which UPRs are used and can be processed via SMC, BMC, pultrusion, cured-in-place pipe (known as relining in Europe), filament winding, vacuum molding, spray-up molding, resin transfer molding (RTM). Wind turbine blades also use them as well as many more processes. UPRs are also used in non-reinforced applications with common examples being gel coats, shirt buttons, mine-bolts, , polymer concrete, and engineered stone/cultured marble. | 7 | Physical Chemistry |
The presence of fecal coliform in aquatic environments may indicate that the water has been contaminated with the fecal material of humans or other animals. Fecal coliform bacteria can enter rivers through direct discharge of waste from mammals and birds, from agricultural and storm runoff, and from human sewage. However, their presence may also be the result of plant material, and pulp or paper mill effluent. | 3 | Analytical Chemistry |
The properties of a photocured material, such as flexibility, adhesion, and chemical resistance, are provided by the functionalized oligomers present in the photocurable composite. Oligomers are typically epoxides, urethanes, polyethers, or polyesters, each of which provide specific properties to the resulting material. Each of these oligomers are typically functionalized by an acrylate. An example shown below is an epoxy oligomer that has been functionalized by acrylic acid. Acrylated epoxies are useful as coatings on metallic substrates and result in glossy hard coatings. Acrylated urethane oligomers are typically abrasion resistant, tough, and flexible, making ideal coatings for floors, paper, printing plates, and packaging materials. Acrylated polyethers and polyesters result in very hard solvent resistant films, however, polyethers are prone to UV degradation and therefore are rarely used in UV curable material. Often formulations are composed of several types of oligomers to achieve the desirable properties for a material.
The monomers used in radiation curable systems help control the speed of cure, crosslink density, final surface properties of the film, and viscosity of the resin. Examples of monomers include styrene, N-Vinylpyrrolidone, and acrylates. Styrene is a low cost monomer and provides a fast cure, N-vinylpyrrolidone results in a material that is highly flexible when cured and has low toxicity, and acrylates are highly reactive, allowing for rapid cure rates, and are highly versatile with monomer functionality ranging from monofunctional to tetrafunctional. Like oligomers, several types of monomers can be employed to achieve the desired properties of the final material. | 5 | Photochemistry |
* Epoxy resin used as the matrix component in many fiber reinforced plastics such as glass-reinforced plastic and graphite-reinforced plastic; casting; electronics encapsulation; construction; protective coatings; adhesives; sealing and joining.
* Polyimides and Bismaleimides used in printed circuit boards and in body parts of modern aircraft, aerospace composite structures, as a coating material and for glass reinforced pipes.
* Cyanate esters or polycyanurates for electronics applications with need for dielectric properties and high glass temperature requirements in aerospace structural composite components.
* Polyester resin fiberglass systems: sheet molding compounds and bulk molding compounds; filament winding; wet lay-up lamination; repair compounds and protective coatings.
* Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers, shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining two bi- or higher functional monomers/oligomers.
* Polyurea/polyurethane hybrids used for abrasion resistant waterproofing coatings.
* Vulcanized rubber.
* Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware.
* Duroplast, light but strong material, similar to Bakelite formerly used in the manufacture of the Trabant automobile, currently used for household objects
* Urea-formaldehyde foam used in plywood, particleboard and medium-density fibreboard.
* Melamine resin used on worktop surfaces.
* Diallyl-phthalate (DAP) used in high temperature and mil-spec electrical connectors and other components. Usually glass filled.
* Epoxy novolac resins used for printed circuit boards, electrical encapsulation, adhesives and coatings for metal.
* Benzoxazines, used alone or hybridised with epoxy and phenolic resins, for structural prepregs, liquid molding and film adhesives for composite construction, bonding and repair.
* Mold or mold runners (the black plastic part in integrated circuits or semiconductors).
* Furan resins used in the manufacture of sustainable biocomposite construction, cements, adhesives, coatings and casting/foundry resins.
* Silicone resins used for thermoset polymer matrix composites and as ceramic matrix composite precursors.
* Thiolyte, an electrical insulating thermoset phenolic laminate material.
* Vinyl ester resins used for wet lay-up laminating, molding and fast setting industrial protection and repair materials. | 7 | Physical Chemistry |
The mechanical properties of materials describe characteristics such as their strength and resistance to deformation. For example, steel beams are used in construction because of their high strength, meaning that they neither break nor bend significantly under the applied load.
Mechanical properties include elasticity, plasticity, tensile strength, compressive strength, shear strength, fracture toughness, ductility (low in brittle materials) and indentation hardness. Solid mechanics is the study of the behavior of solid matter under external actions such as external forces and temperature changes.
A solid does not exhibit macroscopic flow, as fluids do. Any degree of departure from its original shape is called deformation. The proportion of deformation to original size is called strain. If the applied stress is sufficiently low, almost all solid materials behave in such a way that the strain is directly proportional to the stress (Hookes law). The coefficient of the proportion is called the modulus of elasticity or Youngs modulus. This region of deformation is known as the linearly elastic region. Three models can describe how a solid responds to an applied stress:
*Elasticity – When an applied stress is removed, the material returns to its undeformed state.
*Viscoelasticity – These are materials that behave elastically, but also have damping. When the applied stress is removed, work has to be done against the damping effects and is converted to heat within the material. This results in a hysteresis loop in the stress–strain curve. This implies that the mechanical response has a time-dependence.
*Plasticity – Materials that behave elastically generally do so when the applied stress is less than a yield value. When the stress is greater than the yield stress, the material behaves plastically and does not return to its previous state. That is, irreversible plastic deformation (or viscous flow) occurs after yield that is permanent.
Many materials become weaker at high temperatures. Materials that retain their strength at high temperatures, called refractory materials, are useful for many purposes. For example, glass-ceramics have become extremely useful for countertop cooking, as they exhibit excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.
In the aerospace industry, high performance materials used in the design of aircraft and/or spacecraft exteriors must have a high resistance to thermal shock. Thus, synthetic fibers spun out of organic polymers and polymer/ceramic/metal composite materials and fiber-reinforced polymers are now being designed with this purpose in mind. | 7 | Physical Chemistry |
Like the closely related progestins hydroxyprogesterone caproate and 19-norprogesterone, gestonorone caproate shows poor activity orally and must be administered parenterally; specifically, via intramuscular injection. Gestonorone caproate is administered by intramuscular injection, and acts as a long-lasting depot by this route. After an intramuscular injection, gestonorone caproate is completely released from the local depot and is highly bioavailable. A single intramuscular injection of 25 to 50 mg gestonorone caproate in oil solution has been found to have a duration of action of 8 to 13 days in terms of clinical biological effect in the uterus in women. At high doses, the duration of action of gestonorone caproate by intramuscular injection has been found to be at least 21 days. Clinical studies have found gestonorone caproate to be satisfactorily effective as a progestogen when injected once a month, whereas it was poorly effective as an injectable contraceptive when it was injected once every two months.
Following a single intramuscular injection of 200 mg radiolabeled gestonorone caproate in 1 mL of solution in men with prostate cancer, maximal levels of gestonorone caproate occurred after 3 ± 1 days and were 420 ± 160 ng/mL. The elimination half-life of gestonorone caproate and its metabolites was 7.5 ± 3.1 days. Approximately 5% of the radioactive steroid content in the blood was unchanged gestonorone caproate. No free gestonorone was observed in circulation or in urine. Gestonorone caproate and its metabolites were eliminated 72% in feces and 28% in urine. Approximately 48 ± 18% of the injected dose had been eliminated after 14 days and approximately 85 ± 12% of the injected dose had been excreted after 30 days.
The metabolism of unesterified gestonorone (17α-hydroxy-19-norprogesterone) is analogous to that of 17α-hydroxyprogesterone, with the corresponding 19-norpregnane metabolites produced. Gestonorone caproate has been found to undergo 5α-reduction similarly to progesterone, 17α-hydroxyprogesterone, and gestonorone, and at a similar rate as these steroids. Conversely however, due to its caproate ester, 5β-reduction of gestonorone caproate is decreased relative to these steroids. As progesterone is metabolized mainly into 5β-pregnanes, decreased 5β-reduction of gestonorone caproate may be involved in its greater potency compared to progesterone. The major metabolites of gestonorone caproate have been reported to be isomers of 19-norpregnanetriol and 19-norpregnanediol-20-one. These metabolites indicate that gestonorone caproate is metabolized mainly by reduction at the C3, C5, and C20 positions. Following an intramuscular injection of 300 mg gestonorone caproate, only a slight increase in urinary pregnanetriol excretion has been observed. Cleavage of the caproate ester of gestonorone caproate is minimal, which indicates that it is not a prodrug of the unesterified steroid. | 4 | Stereochemistry |
Unnatural processes are logically conceivable but do not occur in nature. They would decrease the sum of the entropies if they occurred. | 7 | Physical Chemistry |
The use of iodine-containing solutions as an aid to describing and identifying fungi dates back to the mid-19th century.
Melzers reagent was first described in 1924 and takes its name from its inventor, the mycologist Václav Melzer, who modified an older chloral hydrate-containing IKI solution developed by botanist Arthur Meyer. Melzer was a specialist in Russula', a genus in which the amyloidy on the spore ornamentation or entire spore is of great taxonomic significance. | 3 | Analytical Chemistry |
The small family of proteins that are sometimes referred to as polyphenolic proteins are produced by some marine invertebrates like the blue mussel, Mytilus edulis by some algae, and by the polychaete Phragmatopoma californica. These proteins contain a high level of a post-translationally modified—oxidized—form of tyrosine, L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA) as well as the disulfide (oxidized) form of cysteine (cystine). In the zebra mussel (Dreissena polymorpha'), two such proteins, Dpfp-1 and Dpfp-2, localize in the juncture between byssus threads and adhesive plaque. The presence of these proteins appear, generally, to contribute to stiffening of the materials functioning as bioadhesives. The presence of the dihydroxyphenylalanine-moiety arises from action of a tyrosine hydroxylase-type of enzyme; in vitro, it has been shown that the proteins can be cross-linked (polymerized) using a mushroom tyrosinase. | 1 | Biochemistry |
Sequence comparisons indicated a huge similarity between cytochromes P450 identified in man and bacteria, and suggested that the superfamily cytochrome P450 first originated from a common ancestral gene some three billion years ago.
The superfamily cytochrome P450 was named in 1961, because of the 450-nm spectral peak pigment that cytochrome P450 has when reduced and bound to carbon monoxide. In the early 1960s, P450 was thought to be one enzyme, and by the mid 1960s it was associated with drug and steroid metabolism.
However, the membrane-associated and hydrophobic nature of the enzyme system impeded purification, and the number of proteins involved could not be accurately counted. Advances in mRNA purification in the early 1980s allowed to isolate the first cDNA encoding a complete cytochrome P450 (CYP) protein, and thereafter, results of many cloning studies have revealed a large number of different enzymes.
Advances in molecular biology and genomics facilitated the biochemical characterisation of individual P450 enzymes:
* The cytochromes P450 act on many endogenous substrates, introducing oxidative, peroxidative, and reductive changes into small molecules of widely different chemical structures. Substrates identified to date include saturated and unsaturated fatty acids, eicosanoids, sterols and steroids, bile acids, vitamin D3 derivatives, retinoids, and uroporphyrinogens.
* Many cytochrome P450 enzymes can metabolise various exogenous compounds including drugs, environmental chemicals and pollutants, and natural plant products.
* Metabolism of foreign chemicals frequently results in successful detoxication of the irritant; However, the actions of P450 enzymes can also generate toxic metabolites that contribute to increased risks of cancer, birth defects, and other toxic effects.
* The expression of many P450 enzymes is often induced by accumulation of a substrate.
* The ability of one P450 substrate to affect the concentrations of another in this manner is the basis for so-called drug-drug interactions, which complicate treatment. | 1 | Biochemistry |
Full article: Biological pump
Particulate organic carbon, created through biological production, can be exported from the upper ocean in a flux commonly termed the biological pump, or respired (equation 6) back into inorganic carbon. In the former, dissolved inorganic carbon is biologically converted into organic matter by photosynthesis (equation 5) and other forms of autotrophy that then sinks and is, in part or whole, digested by heterotrophs. Particulate organic carbon can be classified, based on how easily organisms can break them down for food, as labile, semilabile, or refractory. Photosynthesis by phytoplankton is the primary source for labile and semilabile molecules, and is the indirect source for most refractory molecules. Labile molecules are present at low concentrations outside of cells (in the picomolar range) and have half-lives of only minutes when free in the ocean. They are consumed by microbes within hours or days of production and reside in the surface oceans, where they contribute a majority of the labile carbon flux. Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized. Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin. Refractory DOM can reach depths greater than 1000 m and circulates through the oceans over thousands of years. Over the course of a year, approximately 20 gigatons of photosynthetically-fixed labile and semilabile carbon is taken up by heterotrophs, whereas fewer than 0.2 gigatons of refractory carbon is consumed. Marine dissolved organic matter (DOM) can store as much carbon as the current atmospheric CO supply, but industrial processes are altering the balance of this cycle. | 9 | Geochemistry |
Bases: adenine (A), cytosine (C), guanine (G) and thymine (T) or uracil (U).
Amino acids: Alanine (Ala, A), Arginine (Arg, R), Asparagine (Asn, N), Aspartic acid (Asp, D), Cysteine (Cys, C), Glutamic acid (Glu, E), Glutamine (Gln, Q), Glycine (Gly, G), Histidine (His, H), Isoleucine (Ile, I), Leucine (Leu, L), Lysine (Lys, K), Methionine (Met, M), Phenylalanine (Phe, F), Proline (Pro, P), Serine (Ser, S), Threonine (Thr, T), Tryptophan (Trp, W), Tyrosine (Tyr, Y), Valine (Val, V) | 1 | Biochemistry |
The evolution of early reproductive proteins and enzymes is attributed in modern models of evolutionary theory to ultraviolet radiation. UVB causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand that reproductive enzymes cannot copy. This leads to frameshifting during genetic replication and protein synthesis, usually killing the cell. Before formation of the UV-blocking ozone layer, when early prokaryotes approached the surface of the ocean, they almost invariably died out. The few that survived had developed enzymes that monitored the genetic material and removed thymine dimers by nucleotide excision repair enzymes. Many enzymes and proteins involved in modern mitosis and meiosis are similar to repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome DNA damages caused by UV. | 5 | Photochemistry |
Warts, moles, skin tags, solar keratoses, molluscum, Mortons neuroma and small skin cancers are candidates for cryosurgical treatment. Several internal disorders are also treated with cryosurgery, including liver cancer, prostate cancer, lung cancer, oral cancers, cervical disorders and, more commonly in the past, hemorrhoids. Soft tissue conditions such as plantar fasciitis (joggers heel) and fibroma (benign excrescence of connective tissue) can be treated with cryosurgery.
Cryosurgery works by taking advantage of the destructive force of freezing temperatures on cells. When their temperature sinks beyond a certain level ice crystals begin forming inside the cells and, because of their lower density, eventually tear apart those cells. Further harm to malignant growth will result once the blood vessels supplying the affected tissue begin to freeze.
Cryosurgery is used to treat a variety of benign skin lesions including:
* Acne
* Dermatofibroma
* Hemangioma
* Keloid (hypertrophic scar)
* Molluscum contagiosum
* Myxoid cyst
* Pyogenic granuloma
* Seborrheic keratoses
* Skin tags
* Warts (including anogenital warts)
Cryosurgery may also be used to treat low risk skin cancers such as basal cell carcinoma and squamous cell carcinoma but a biopsy should be obtained first to confirm the diagnosis, determine the depth of invasion and characterize other high risk histologic features. | 1 | Biochemistry |
The gallium antimonide (GaSb) PV cell, invented in 1989, is the basis of most PV cells in modern TPV systems. GaSb is a III-V semiconductor with the zinc blende crystal structure. The GaSb cell is a key development owing to its narrow bandgap of 0.72 eV. This allows GaSb to respond to light at longer wavelengths than silicon solar cell, enabling higher power densities in conjunction with manmade emission sources. A solar cell with 35% efficiency was demonstrated using a bilayer PV with GaAs and GaSb, setting the solar cell efficiency record.
Manufacturing a GaSb PV cell is quite simple. Czochralski tellurium-doped n-type GaSb wafers are commercially available. Vapor-based zinc diffusion is carried out at elevated temperatures (~450 °C) to allow for p-type doping. Front and back electrical contacts are patterned using traditional photolithography techniques and an anti-reflective coating is deposited. Efficiencies are estimated at ~20% using a 1000 °C black body spectrum. The radiative limit for efficiency of the GaSb cell in this setup is 52%. | 7 | Physical Chemistry |
Aside from chemical requirements several key factors influence extraction efficiency:
* Retention time - refers to the time spent in the leaching system by the solids. This is calculated as the total volumetric capacity of the leach tank/s divided by the volumetric throughput of the solid/liquid slurry. Retention time is commonly measured in hours for precious metals recovery. A sequence of leach tanks is referred to as a leach "train", and retention time is measured considering the total volume of the leach train. The desired retention time is determined during the testing phase, and the system is then designed to achieve this.
* Size - The ore must be ground to a size that exposes the desired mineral to the leaching agent (referred to as “liberation”), and in tank leaching this must be a size that can be suspended by the agitator. In vat leaching this is the size that is the most economically viable, where the recovery achieved as ore is ground finer is balanced against the increased cost of processing the material.
* Slurry density - The slurry density (percent solids) determines retention time. The settling rate and viscosity of the slurry are functions of the slurry density. The viscosity, in turn, controls the gas mass transfer and the leaching rate.
* Numbers of tanks - Agitated tank leach circuits are typically designed with no less than four tanks and preferably more to prevent short-circuiting of the slurry through the tanks.
* Dissolved gas - Gas is often injected below the agitator or into the vat to obtain the desired dissolved gas levels – typically oxygen, in some base metal plants sulphur dioxide may be required.
* Reagents - Adding and maintaining the appropriate amount of reagents throughout the leach circuit is critical to a successful operation. Adding insufficient quantities of reagents reduces the metal recovery but adding excess reagents increases the operating costs without recovering enough additional metal to cover the cost •of the reagents.
The tank leaching method is commonly used to extract gold and silver from ore, such as with the Sepro Leach Reactor. | 8 | Metallurgy |
The antiviral drug ritonavir exists as two polymorphs, which differ greatly in efficacy. Such issues were solved by reformulating the medicine into gelcaps and tablets, rather than the original capsules. | 3 | Analytical Chemistry |
Genetically modified fish are used for scientific research, as pets and as a food source. Aquaculture is a growing industry, currently providing over half the consumed fish worldwide. Through genetic engineering it is possible to increase growth rates, reduce food intake, remove allergenic properties, increase cold tolerance and provide disease resistance. Fish can also be used to detect aquatic pollution or function as bioreactors.
Several groups have been developing zebrafish to detect pollution by attaching fluorescent proteins to genes activated by the presence of pollutants. The fish will then glow and can be used as environmental sensors. The GloFish is a brand of genetically modified fluorescent zebrafish with bright red, green, and orange fluorescent color. It was originally developed by one of the groups to detect pollution, but is now part of the ornamental fish trade, becoming the first genetically modified animal to become publicly available as a pet when in 2003 it was introduced for sale in the USA.
GM fish are widely used in basic research in genetics and development. Two species of fish, zebrafish and medaka, are most commonly modified because they have optically clear chorions (membranes in the egg), rapidly develop, and the one-cell embryo is easy to see and microinject with transgenic DNA. Zebrafish are model organisms for developmental processes, regeneration, genetics, behavior, disease mechanisms and toxicity testing. Their transparency allows researchers to observe developmental stages, intestinal functions and tumour growth. The generation of transgenic protocols (whole organism, cell or tissue specific, tagged with reporter genes) has increased the level of information gained by studying these fish.
GM fish have been developed with promoters driving an over-production of growth hormone for use in the aquaculture industry to increase the speed of development and potentially reduce fishing pressure on wild stocks. This has resulted in dramatic growth enhancement in several species, including salmon, trout and tilapia. AquaBounty Technologies, a biotechnology company, have produced a salmon (called AquAdvantage salmon) that can mature in half the time as wild salmon. It obtained regulatory approval in 2015, the first non-plant GMO food to be commercialized. As of August 2017, GMO salmon is being sold in Canada. Sales in the US started in May 2021. | 1 | Biochemistry |
Bicycloaromaticity in chemistry is an extension of the concept of homoaromaticity with two aromatic ring currents situated in a non-planar molecule and sharing the same electrons. The concept originates with Melvin Goldstein who first reported about it in 1967. It is of some importance in academic research. Using MO theory the bicyclo[3.2.2]nonatrienyl cation was predicted to be destabilised and the corresponding anion predicted to be stabilised by bicycloaromaticity.
Bicycloaromaticity has been studied by others in relation to the bicyclo[3.2.2]nonatrienyl cation and in relation to specific carbanions. In 2017 experimental evidence was reported for bicycloaromaticity (dual aromaticity) to exist in a bicyclic porphyrinoid. This system has been described as aromatic with two ring systems of 26 (n=6) and 34 (n=8) electrons. By oxidation, another system was described as a triplet-state biradical, again considered aromatic by application of Baird's rule. | 7 | Physical Chemistry |
There have been recalls for various medications due to the presence of nitrosamine impurities. There have been recalls for angiotensin II receptor blockers, ranitidine, valsartan and others.
The US Food and Drug Administration published guidance about the control of nitrosamine impurities in medicines. Health Canada published guidance about nitrosamine impurities in medications and a list of established acceptable intake limits of nitrosamine impurities in medications. | 0 | Organic Chemistry |
The Flippin–Lodge (FL) angle, is the latter-derived of two angles that fully define the geometry of "attack" (approach via collision) of a nucleophile on a trigonal unsaturated center of an electrophilic molecule (the second being the Bürgi–Dunitz angle, , see below). Theory and application of these angles falls into the area of synthetic, and of physical organic chemistry (in the specializations of chemical structure and reaction mechanism), in the latter, within a sub-specialty called structure correlation. Studies of and can be theoretical, based on calculations, or experimental (either quantitative, based on X-ray crystallography, or inferred and semiquantitative, rationalizing results of particular chemical reactions), or a combination of these.
Nucleophiles in this addition reaction may range from single atoms (hydride, chloride), to polar organic functional groups (amines, alcohols), to complex systems (nucleophilic enolates with chiral catalysts, amino acid side chains in enzyme active sites; see below). Planar electrophiles include aldehydes and ketones, carboxylic acid-derivatives such as esters, and amides, and the carbon-carbon double bonds of particular alkenes (olefins). In the example of nucleophilic attack at a carbonyl, is a measure of the "offset" of the nucleophiles approach to the electrophile, toward one or the other of the two substituents attached to the carbonyl carbon. The relative values of angles for pairs of reactions can be inferred and semiquantitative, based on rationalizations of the products of the reactions; alternatively, as noted in the figure, values may be formally derived from crystallographic coordinates by geometric calculations, or graphically, e.g., after projection of Nu onto the carbonyl plane and measuring the angle supplementary to LNu-C-O (where Nu is the projected atom). This often overlooked angle of the nucleophiles trajectory was named the Flippin-Lodge angle by Clayton H. Heathcock after his contributing collaborators Lee A. Flippin and Eric P. Lodge. The second angle defining the geometry, the more well known Bürgi–Dunitz angle, , describes the Nu-C-O bond angle and was named after crystallographers Hans-Beat Bürgi and Jack D. Dunitz, its first senior investigators (see that related article).
The Flippin–Lodge angle has been abbreviated variously by the symbols φ, ψ, θ, and or ; the latter pair to closely associate the Flippin–Lodge angle with its sister angle, the Bürgi–Dunitz, which was originally abbreviated as by its discoverers/formulators (e.g., see Bürgi et al., 1974.). The symbols and are used here, respectively, to refer to the Flippin-Lodge and Bürgi-Dunitz concepts and measured values. | 7 | Physical Chemistry |
Compounds like allicin and ajoene are responsible for the odor of garlic. Lenthionine contributes to the flavor of shiitake mushrooms. Volatile organosulfur compounds also contribute subtle flavor characteristics to wine, nuts, cheddar cheese, chocolate, coffee, and tropical fruit flavors. Many of these natural products also have important medicinal properties such as preventing platelet aggregation or fighting cancer.
Humans and other animals have an exquisitely sensitive sense of smell toward the odor of low-valent organosulfur compounds such as thiols, sulfides, and disulfides. Malodorous volatile thiols are protein-degradation products found in putrid food, so sensitive identification of these compounds is crucial to avoiding intoxication. Low-valent volatile sulfur compounds are also found in areas where oxygen levels in the air are low, posing a risk of suffocation.
Copper is required for the highly sensitive detection of certain volatile thiols and related organosulfur compounds by olfactory receptors in mice. Whether humans, too, require copper for sensitive detection of thiols is not yet known. | 9 | Geochemistry |
The simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure (0.1–10 torr; about 1/10000th to 1/100th of atmospheric pressure). A low pressure is used to increase the mean free path; for a fixed electric field, a longer mean free path allows a charged particle to gain more energy before colliding with another particle. The cell is typically filled with neon, but other gases can also be used. An electric potential of several hundred volts is applied between the two electrodes. A small fraction of the population of atoms within the cell is initially ionized through random processes, such as thermal collisions between atoms or by gamma rays. The positive ions are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential. The initial population of ions and electrons collides with other atoms, exciting or ionizing them. As long as the potential is maintained, a population of ions and electrons remains. | 3 | Analytical Chemistry |
The surface tension profiles of the branched copolymers solutions were
performed using a HTS surface tensiometer as a function polymer concentration to produce pH-triggered aggregation emulsion droplets. | 7 | Physical Chemistry |
The characteristics of steel, in particular the carbon and chromium content, can be controlled by adjusting the oxygen/argon ratio during the manufacturing process. The oxygen/argon ratio is also important in the creation of thin films used in the manufacture of lithium ion batteries. | 3 | Analytical Chemistry |
Hydrocolloids describe certain chemicals (mostly polysaccharides and proteins) that are colloidally dispersible in water. Thus becoming effectively "soluble" they change the rheology of water by raising the viscosity and/or inducing gelation. They may provide other interactive effects with other chemicals, in some cases synergistic, in others antagonistic. Using these attributes hydrocolloids are very useful chemicals since in many areas of technology from foods through pharmaceuticals, personal care and industrial applications, they can provide stabilization, destabilization and separation, gelation, flow control, crystallization control and numerous other effects. Apart from uses of the soluble forms some of the hydrocolloids have additional useful functionality in a dry form if after solubilization they have the water removed - as in the formation of films for breath strips or sausage casings or indeed, wound dressing fibers, some being more compatible with skin than others. There are many different types of hydrocolloids each with differences in structure function and utility that generally are best suited to particular application areas in the control of rheology and the physical modification of form and texture. Some hydrocolloids like starch and casein are useful foods as well as rheology modifiers, others have limited nutritive value, usually providing a source of fiber.
The term hydrocolloids also refers to a type of dressing designed to lock moisture in the skin and help the natural healing process of skin to reduce scarring, itching and soreness. | 7 | Physical Chemistry |
Photo-induced Rydberg ionization (PIRI) was developed following REMPI experiments on electronic autoionization of low-lying Rydberg states of carbon dioxide. In REMPI photoelectron experiments, it was determined that a two-photon ionic core photoabsorption process (followed by prompt electronic autoionization) could dominate the direct single photon absorption in the ionization of some Rydberg states of carbon dioxide. These sorts of two excited electron systems had already been under study in the atomic physics, but there the experiments involved high order Rydberg states. PIRI works because electronic autoionization can dominate direct photoionization (photoionization). The circularized near-threshold Rydberg state is more likely to undergo a core photoabsorption than to absorb a photon and directly ionize the Rydberg state. PIRI extends the near-threshold spectroscopic techniques to allow access to the electronic states (including dissociative molecular states and other hard to study systems) as well as the vibrational states of molecular ions. | 7 | Physical Chemistry |
The central component of GEOTRACES is a series of [http://www.geotraces.org/cruises/cruise-summary cruises spanning the global ocean] and sampling the full water column. These dedicated GEOTRACES cruises collect seawater for analysis of a wide range of trace element and isotopes. This strategy is guided by the principle that more will be learned through complementary investigation of multiple trace elements than can be achieved in an exhaustive study of one element in isolation. | 9 | Geochemistry |
The equilibrium constant for the reaction is related to ΔG° by the relation:
where T is the absolute temperature and R is the gas constant. A positive value of ΔG° therefore implies
so that starting from molar stoichiometric quantities such a reaction would move backwards toward equilibrium, not forwards.
Nevertheless, endergonic reactions are quite common in nature, especially in biochemistry and physiology. Examples of endergonic reactions in cells include protein synthesis, and the Na/K pump which drives nerve conduction and muscle contraction. | 7 | Physical Chemistry |
Air pollutants generated by aluminium smelters include carbonyl sulfide, hydrogen fluoride, polycyclic compounds, lead, nickel, manganese, polychlorinated biphenyls, and mercury. Copper smelter emissions include arsenic, beryllium, cadmium, chromium, lead, manganese, and nickel. Lead smelters typically emit arsenic, antimony, cadmium and various lead compounds. | 8 | Metallurgy |
In polymers the glass transition temperature, T, is often expressed as the temperature at which the Gibbs free energy is such that the activation energy for the cooperative movement of 50 or so elements of the polymer is exceeded . This allows molecular chains to slide past each other when a force is applied. From this definition, we can see that the introduction of relatively stiff chemical groups (such as benzene rings) will interfere with the flowing process and hence increase T.
The stiffness of thermoplastics decreases due to this effect (see figure.) When the glass temperature has been reached, the stiffness stays the same for a while, i.e., at or near E, until the temperature exceeds T, and the material melts. This region is called the rubber plateau.
In ironing, a fabric is heated through this transition so that the polymer chains become mobile. The weight of the iron then imposes a preferred orientation. T can be significantly decreased by addition of plasticizers into the polymer matrix. Smaller molecules of plasticizer embed themselves between the polymer chains, increasing the spacing and free volume, and allowing them to move past one another even at lower temperatures. Addition of plasticizer can effectively take control over polymer chain dynamics and dominate the amounts of the associated free volume so that the increased mobility of polymer ends is not apparent. The addition of nonreactive side groups to a polymer can also make the chains stand off from one another, reducing T. If a plastic with some desirable properties has a T that is too high, it can sometimes be combined with another in a copolymer or composite material with a T below the temperature of intended use. Note that some plastics are used at high temperatures, e.g., in automobile engines, and others at low temperatures.
In viscoelastic materials, the presence of liquid-like behavior depends on the properties of and so varies with rate of applied load, i.e., how quickly a force is applied. The silicone toy Silly Putty behaves quite differently depending on the time rate of applying a force: pull slowly and it flows, acting as a heavily viscous liquid; hit it with a hammer and it shatters, acting as a glass.
On cooling, rubber undergoes a liquid-glass transition, which has also been called a rubber-glass transition. | 7 | Physical Chemistry |
Photocatalytic water splitting is a process that uses photocatalysis for the dissociation of water (HO) into hydrogen () and oxygen (). The inputs are light energy (photons), water, and a catalyst(s). The process is inspired by Photosynthesis, which converts water and carbon dioxide into oxygen and carbohydrates. Water splitting using solar radiation has not been commercialized. Photocatalytic water splitting is done by dispersing photocatalyst particles in water or depositing them on a substrate, unlike Photoelectrochemical cell, which are assembled into a cell with a photoelectrode. Hydrogen fuel production using water and light (photocatalytic water splitting), instead of petroleum, is an important renewable energy strategy. | 5 | Photochemistry |
p-Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both monomers used in the production of polyethylene terephthalate (PET) plastic bottles and polyester clothing. 98% of p-xylene production, and half of all xylenes produced is consumed in this manner. o-Xylene is an important precursor to phthalic anhydride. The demand for isophthalic acid is relatively modest, so m-xylene is rarely sought (and hence the utility of its conversion to the o- and p-isomers). | 2 | Environmental Chemistry |
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