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Heterochromatin in S. cerevisiae is composed of a wide array of proteins and plays several roles. The first stage of heterochromatin formation requires DNA binding proteins, which interact with specific cis DNA sequences at the telomeres, rDNA and HM loci. These proteins, including Rap1p and the origin recognition complex (ORC), serve as a platform for other proteins to bind, condense the DNA, and modify neighboring histones. Some of these proteins, notably Rap1p, also play other roles, including initiation of transcription. The first known step in the formation of dedicated heterochromatin is the binding of Sir4p to Rap1p (Luo, Vega-Palas et al. 2002). Sir4p is one of four ‘Silent Information Regulator’ proteins that also include Sir1p, Sir2p and Sir3p. Of these, Sir2p, Sir3p and Sir4p form the core of heterochromatin. Sir4p serves as a binding site for Sir2p, which is the next to bind. Sir2p deacetylates adjacent histones, which is thought to further condense the chromatin and prevent the binding of other transcription promoting histone modification enzymes. Sir3p binding follows, further condensing the heterochromatin. Sir1p plays a role in the initiation of silencing at the HM loci. A large number of other proteins act in both a synergistic and antagonistic manner.
Early work characterizing Ty5 targeted transposition focused on two fronts: identifying the component of Ty5 responsible for targeting and identifying the factor with which it interacted. Due to the central role of the Sir proteins in heterochromatin formation, they were initially considered as potential targeting signals. Because integration is mediated by the retrotransposon integrase enzyme, it was speculated to contain a component that would recognize heterochromatin.
The C-terminus of the Ty retrotransposon’s integrase contains an extension not seen in the retroviruses. This region is also not conserved between Ty1 and Ty5, whereas the rest of the integrase is, suggesting that this divergence could be responsible for the different targeting of the yeast Ty elements. A mutation was identified in the integrase C-terminus that randomized Ty5 integration, suggesting that this region of integrase was in fact involved in targeted transposition. | 1 | Biochemistry |
The Ostwald–Freundlich equation governs boundaries between two phases; specifically, it relates the surface tension of the boundary to its curvature, the ambient temperature, and the vapor pressure or chemical potential in the two phases.
The Ostwald–Freundlich equation for a droplet or particle with radius is:
: = atomic volume
: = Boltzmann constant
: = surface tension (J m)
: = equilibrium partial pressure (or chemical potential or concentration)
: = partial pressure (or chemical potential or concentration)
: = absolute temperature
One consequence of this relation is that small liquid droplets (i.e., particles with a high surface curvature) exhibit a higher effective vapor pressure, since the surface is larger in comparison to the volume.
Another notable example of this relation is Ostwald ripening, in which surface tension causes small precipitates to dissolve and larger ones to grow. Ostwald ripening is thought to occur in the formation of orthoclase megacrysts in granites as a consequence of subsolidus growth. See rock microstructure for more. | 7 | Physical Chemistry |
DEHPA is used in the solvent extraction of uranium salts from solutions containing the sulfate, chloride, or perchlorate anions. This extraction is known as the “Dapex procedure” (dialkyl phosphoric extraction). Reminiscent of the behaviours of carboxylic acids, DEHPA generally exists as a hydrogen-bonded dimer in the non-polar organic solvents. For practical applications, the solvent, often called a diluent, is typically kerosene. A complex is formed from two equivalents of the conjugate base of DEHPA and one uranyl ion. Complexes of the formula (UO)[(OP(OR)] also form, and at high concentrations of uranium, polymeric complexes may form.
The extractability of Fe is similar to that of uranium, so it must be reduced to Fe before the extraction. | 3 | Analytical Chemistry |
The primary enzyme in the malate–aspartate shuttle is malate dehydrogenase. Malate dehydrogenase is present in two forms in the shuttle system: mitochondrial malate dehydrogenase and cytosolic malate dehydrogenase. The two malate dehydrogenases are differentiated by their location and structure, and catalyze their reactions in opposite directions in this process.
First, in the cytosol, malate dehydrogenase catalyses the reaction of oxaloacetate and NADH to produce malate and NAD. In this process, two electrons generated from NADH, and an accompanying H, are attached to oxaloacetate to form malate.
Once malate is formed, the first antiporter (malate-alpha-ketoglutarate) imports the malate from the cytosol into the mitochondrial matrix and also exports alpha-ketoglutarate from the matrix into the cytosol simultaneously. After malate reaches the mitochondrial matrix, it is converted by mitochondrial malate dehydrogenase into oxaloacetate, during which NAD is reduced with two electrons to form NADH. Oxaloacetate is then transformed into aspartate (since oxaloacetate cannot be transported into the cytosol) by mitochondrial aspartate aminotransferase. Since aspartate is an amino acid, an amino radical needs to be added to the oxaloacetate. This is supplied by glutamate, which in the process is transformed into alpha-ketoglutarate by the same enzyme.
The second antiporter (the glutamate-aspartate antiporter) imports glutamate from the cytosol into the matrix and exports aspartate from the matrix to the cytosol. Once in the cytosol, aspartate is converted by cytosolic aspartate aminotransferase to oxaloacetate.
The net effect of the malate–aspartate shuttle is purely redox: NADH in the cytosol is oxidized to NAD, and NAD in the matrix is reduced to NADH. The NAD in the cytosol can then be reduced again by another round of glycolysis, and the NADH in the matrix can be used to pass electrons to the electron transport chain so ATP can be synthesized.
Since the malate–aspartate shuttle regenerates NADH inside the mitochondrial matrix, it is capable of maximizing the number of ATPs produced in glycolysis (3/NADH), ultimately resulting in a net gain of 38 ATP molecules per molecule of glucose metabolized. Compare this to the glycerol 3-phosphate shuttle, which reduces FAD to produce FADH, donates electrons to the quinone pool in the electron transport chain, and is capable of generating only 2 ATPs per NADH generated in glycolysis (ultimately resulting in a net gain of 36 ATPs per glucose metabolized). (These ATP numbers are prechemiosmotic, and should be reduced in light of the work of Mitchell and many others. Each NADH produces only 2.5 ATPs, and each FADH produces only 1.5 ATPs. Hence, the ATPs per glucose should be reduced to 32 from 38 and 30 from 36. The extra H required to bring in the inorganic phosphate during oxidative-phosphorylation contributes to the 30 and 32 numbers as well). | 1 | Biochemistry |
A primary concern with the use of ionic liquids for carbon capture is their high viscosity compared with that of commercial solvents. Ionic liquids which employ chemisorption depend on a chemical reaction between solute and solvent for CO separation. The rate of this reaction is dependent on the diffusivity of CO in the solvent and is thus inversely proportional to viscosity. The self diffusivity of CO in ionic liquids are generally to the order of 10 m/s, approximately an order of magnitude less than similarly performing commercial solvents used on CO capture. The viscosity of an ionic liquid can vary significantly according to the type of anion and cation, the alkyl chain length, and the amount of water or other impurities in the solvent. Because these solvents can be “designed” and these properties chosen, developing ionic liquids with lowered viscosities is a current topic of research. Supported ionic liquid phases (SILPs) are one proposed solution to this problem. | 7 | Physical Chemistry |
After activation by ATP, once inside the mitochondria, the β-oxidation of a fatty acids occurs via four recurring steps:
#Oxidation by FAD
#Hydration
#Oxidation by NAD
# Thiolysis
# Production of acyl-CoA and acetyl-CoA
The final product of β-oxidation of an even-numbered fatty acid is acetyl-CoA, the entry molecule for the citric acid cycle. If the fatty acid is an odd-numbered chain, the final product of β-oxidation will be propionyl-CoA. This propionyl-CoA will be converted into intermediate methylmalonyl-CoA and eventually succinyl-CoA, which also enters the TCA cycle. | 1 | Biochemistry |
Large portions of the guidelines include basic actions that would normally be included in experiments and publications regardless, such as an item for describing the experimental and control group differences. Other such information includes how many individual units are used in each group in the experiment. These two pieces are defined as essential for any study. This section also includes two desirable points, which are pointing out whether the author's laboratory itself or a core laboratory of the university or organization conducted the qPCR assay and an acknowledgement of any other individuals that contributed to the work. | 1 | Biochemistry |
Each POCIS disk will sample a certain volume of water per day. The volume of water sampled varies from chemical to chemical and is dependent on the physical and chemical properties of the compound as well as the duration of sampling. The sampling rate of POCIS can vary with changes in the water flow, turbulence, temperature, and the buildup of solids on the sampler’s surface. The accumulation of contaminants into a POCIS device is the result of three successive process occurring at the same time. First, the contaminants have to diffuse across the water boundary layer. The thickness of this layer is dependent on water flow and turbulence around the sampler and can significantly alter sampling rates. Second, the contaminant must transport across the membrane either through the water-filled pores or through the membrane itself. Finally, contaminants transfer from the membrane into the sorbent material mainly through adsorption. These last two steps make the modeling, understanding, and prediction of accumulation by a POCIS device challenging. To date, a limited number of chemical sampling rates have been determined.
Accumulation of chemicals by a POCIS device generally follows first order kinetics. The kinetics are characterized by an initial integrative phase, followed by an equilibrium partitioning phase. During the integrative phase of uptake, a passive sampling device accumulates residues linearly relative to time, assuming constant exposure concentrations. Based on current results, the POCIS sampler remains in a linear phase for at least 30 days, and has been observed up to 56 days. Therefore, both laboratory and field data justify the use of a linear uptake model for the calculation of sample rates. In order to estimate the ambient water concentration of contaminants sampled by a POCIS device, there must be available calibration data applicable for in situ conditions regarding the target compound. Currently, this information is limited. | 3 | Analytical Chemistry |
The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in mitochondria and takes the required energy from the proton motive force (PMF). However, chloroplasts rely more on the chemical potential of the PMF to generate the potential energy required for ATP synthesis. The PMF is the sum of a proton chemical potential (given by the proton concentration gradient) and a transmembrane electrical potential (given by charge separation across the membrane). Compared to the inner membranes of mitochondria, which have a significantly higher membrane potential due to charge separation, thylakoid membranes lack a charge gradient. To compensate for this, the 10,000 fold proton concentration gradient across the thylakoid membrane is much higher compared to a 10 fold gradient across the inner membrane of mitochondria. The resulting chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase. As the protons travel back down the gradient through channels in ATP synthase, ADP + P are combined into ATP. In this manner, the light-dependent reactions are coupled to the synthesis of ATP via the proton gradient. | 5 | Photochemistry |
A record-long of human transplant organ preservation with machine perfusion of a liver for 3 days rather than usually <12 hours was reported in 2022. It could possibly be extended to 10 days and prevent substantial cell damage by low temperature preservation methods. Alternative approaches include novel cryoprotectant solvents.
There is a novel organ perfusion system under development that can restore, i.e. on the cellular level, multiple vital (pig) organs one hour after death (during which the body had a prolonged warm ischaemia), and a similar method/system for reviving (pig) brains hours after death. The system for cellular recovery could be used to preserve donor organs or for revival-treatments in medical emergencies. | 1 | Biochemistry |
Hyperphosphatemia, or a high blood level of phosphates, is associated with elevated mortality in the general population. The most common cause of hyperphosphatemia in people, dogs, and cats is kidney failure. In cases of hyperphosphatemia, limiting consumption of phosphate-rich foods, such as some meats and dairy items and foods with a high phosphate-to-protein ratio, such as soft drinks, fast food, processed foods, condiments, and other products containing phosphate-salt additives is advised.
Phosphates induce vascular calcification, and a high concentration of phosphates in blood was found to be a predictor of cardiovascular events. | 0 | Organic Chemistry |
SERS can be used to target specific DNA and RNA sequences using a combination of gold and silver nanoparticles and Raman-active dyes, such as Cy3. Specific single nucleotide polymorphisms (SNP) can be identified using this technique. The gold nanoparticles facilitate the formation of a silver coating on the dye-labelled regions of DNA or RNA, allowing SERS to be performed. This has several potential applications: For example, Cao et al. report that gene sequences for HIV, Ebola, Hepatitis, and Bacillus Anthracis can be uniquely identified using this technique. Each spectrum was specific, which is advantageous over fluorescence detection; some fluorescent markers overlap and interfere with other gene markers. The advantage of this technique to identify gene sequences is that several Raman dyes are commercially available, which could lead to the development of non-overlapping probes for gene detection. | 7 | Physical Chemistry |
In granulometry, the particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Significant energy is usually required to disintegrate soil, etc. particles into the PSD that is then called a grain size distribution. | 7 | Physical Chemistry |
*2007 - Recipient of the puRkwa Prize. This is an "international prize for the scientific literacy of the children of the planet" awarded annually by the École nationale supérieure des mines de Saint-Étienne and the French Academy of Sciences.
*2005 - Member of the Academy of Exact, Physical and Natural Sciences of Argentina
*2002 - Medal of the Grand Cross of the Scientific Merit awarded by the President Brazil.
*2001 - Foreign Member of the National Academy of Sciences of the United States of America.
*1993 - Honorary Doctorate from the University of Buenos Aires, Argentina.
*1992 - National "Natural Sciences Award of Chile".
*1990 - Foreign Member of the Institute of Medicine of the National Academy of Sciences of the United States of America.
*1986 to 1988 - "Scholar in Residence" of the Fogarty International Center in the United States.
*1986 - Founding member of the Academy of Sciences of the Third World.
*1983 - Member of the Chilean Academy of Sciences.
*1982 - Founding member of the Academy of Sciences of Latin America.
*1966 - Guggenheim Fellowship | 1 | Biochemistry |
A direct repeat with no intervening nucleotides between the initial sequence and its downstream copy is a Tandem repeat. The nucleotide sequence written in bold characters signifies the repeated sequence.
Linguistically, a typical tandem repeat is comparable to stuttering, or deliberately repeated words, as in "bye-bye". | 1 | Biochemistry |
Although there is sufficient iron in most soils for plant growth, plant iron deficiency is a problem in calcareous soil, due to the low solubility of iron(III) hydroxide. Calcareous soil accounts for 30% of the world's farmland. Under such conditions graminaceous plants (grasses, cereals and rice) secrete phytosiderophores into the soil, a typical example being deoxymugineic acid. Phytosiderophores have a different structure to those of fungal and bacterial siderophores having two α-aminocarboxylate binding centres, together with a single α-hydroxycarboxylate unit. This latter bidentate function provides phytosiderophores with a high selectivity for iron(III). When grown in an iron -deficient soil, roots of graminaceous plants secrete siderophores into the rhizosphere. On scavenging iron(III) the iron–phytosiderophore complex is transported across the cytoplasmic membrane using a proton symport mechanism. The iron(III) complex is then reduced to iron(II) and the iron is transferred to nicotianamine, which although very similar to the phytosiderophores is selective for iron(II) and is not secreted by the roots. Nicotianamine translocates iron in phloem to all plant parts. | 1 | Biochemistry |
Rhazinilam has activity similar to that of colchicine, taxol and vinblastine, acting as a spindle poison. | 0 | Organic Chemistry |
The stopped-flow method is a development of the continuous-flow method used by Hamilton Hartridge and Francis Roughton to study the binding of O to hemoglobin. In the absence of any stopping system the reaction mixture passed to a long tube past an observation system (consisting in 1923 of a simple colorimeter) to waste. By moving the colorimeter along the tube, and knowing the flow rate, Hartridge and Roughton could measure the process after a known time.
In its time this was a revolutionary advance showing an apparently intractable problem (studying a process taking milliseconds with equipment that required seconds for each measurement) could be solved with simple equipment. However, in practice it was limited to reactants available in large quantities: for proteins this effectively limited it to reactions of hemoglobin. For practical purposes this approach is obsolete. | 7 | Physical Chemistry |
Thermogravimetric kinetics may be explored for insight into the reaction mechanisms of thermal (catalytic or non-catalytic) decomposition involved in the pyrolysis and combustion processes of different materials.
Activation energies of the decomposition process can be calculated using Kissinger method.
Though a constant heating rate is more common, a constant mass loss rate can illuminate specific reaction kinetics. For example, the kinetic parameters of the carbonization of polyvinyl butyral were found using a constant mass loss rate of 0.2 wt %/min. | 7 | Physical Chemistry |
The most synthetically useful aminations of enolate anions employ N-acyloxazolidinone substrates. The chiral auxiliaries on these compounds are easily removed after hydrazine formation (with azo compounds) or azidation (with trisyl azide). Azidation using the latter reagent is more efficient than bromination followed by nucleophilic substitution by the azide anion Palladium on carbon and hydrogen gas reduce both azide and hydrazide products (the latter only after conversion to the hydrazine). | 0 | Organic Chemistry |
According to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters". Flocculation
is synonymous with agglomeration and coagulation/coalescence.
Basically, coagulation is a process of addition of coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a mixing technique that promotes agglomeration and assists in the settling of particles. The most common used coagulant is alum, Al(SO)·14HO.
The chemical reaction involved:
:Al(SO) · 14 HO → 2 Al(OH) + 6 H + 3 + 8 HO
During flocculation, gentle mixing accelerates the rate of particle collision, and the destabilized particles are further aggregated and enmeshed into larger precipitates. Flocculation is affected by several parameters, including mixing speeds, mixing intensity, mixing time and pH. The product of the mixing intensity and mixing time is used to describe flocculation processes. | 8 | Metallurgy |
Petroleum is differentiated into types based on its American Petroleum Institute (API) gravity and by how much sulphur it contains. | 9 | Geochemistry |
Propionyl-CoA accumulation can prove toxic to different organisms. Since different cycles have been proposed regarding how propionyl-CoA is transformed into pyruvate, one studied mechanism is the methylcitrate cycle. The initial reaction is beta-oxidation to form the propionyl-CoA which is further broken down by the cycle. This pathway involves the enzymes both related to the methylcitrate cycle as well as the citric acid cycle. These all contribute to the overall reaction to detoxify the bacteria from harmful propionyl-CoA. It is also attributed as a resulting pathway due to the catabolism of fatty acids in mycobacteria. In order to proceed, the prpC gene codes for methylcitrate synthase, and if not present, the methylcitrate cycle will not occur. Instead, catabolism proceeds through propionyl-CoA carboxylase. This mechanism is shown below to the left along with the participating reactants, products, intermediates, and enzymes. | 1 | Biochemistry |
Most sinusitis infections are caused by viruses, for which amoxicillin and amoxicillin-clavulanate are ineffective, and the small benefit gained by amoxicillin may be overridden by the adverse effects.
Amoxicillin is considered the first-line empirical treatment for most cases of uncomplicated bacterial sinusitis in children and adults when culture data is unavailable. Amoxicillin is recommended as the preferred first-line treatment for community-acquired pneumonia in adults by the National Institute for Health and Care Excellence, either alone (mild to moderate severity disease) or in combination with a macrolide. The World Health Organization (WHO) recommends amoxicillin as first-line treatment for pneumonia that is not "severe". Amoxicillin is used in post-exposure inhalation of anthrax to prevent disease progression and for prophylaxis. | 4 | Stereochemistry |
Metabolic suppression is the regulated and reversible reduction of metabolic rate below basal metabolic rate (called standard metabolic rate in ectothermic animals). This reduces the fishs rate of ATP use, which prolongs its survival time at severely hypoxic sub-P P</sub>s by reducing the rate at which the fishs finite anaerobic fuel stores (glycogen) are used. Metabolic suppression also reduces the accumulation rate of deleterious anaerobic end-products (lactate and protons), which delays their negative impact on the fish.
The mechanisms that fish use to suppress metabolic rate occur at behavioral, physiological and biochemical levels. Behaviorally, metabolic rate can be lowered through reduced locomotion, feeding, courtship, and mating. Physiologically, metabolic rate can be lowered through reduced growth, digestion, gonad development, and ventilation efforts. And biochemically, metabolic rate can be further lowered below standard metabolic rate through reduced gluconeogenesis, protein synthesis and degradation rates, and ion pumping across cellular membranes. Reductions in these processes lower ATP use rates, but it remains unclear whether metabolic suppression is induced through an initial reduction in ATP use or ATP supply.
The prevalence of metabolic suppression use among fish species has not been thoroughly explored. This is partly because the metabolic rates of hypoxia-exposed fish, including suppressed metabolic rates, can only be accurately measured using direct calorimetry, and this technique is seldom used for fish. The few studies that have used calorimetry reveal that some fish species employ metabolic suppression in hypoxia/anoxia (e.g., goldfish, tilapia, European eel) while others do not (e.g. rainbow trout, zebrafish). The species that employ metabolic suppression are more hypoxia-tolerant than the species that do not, which suggests that metabolic suppression enhances hypoxia tolerance. Consistent with this, differences in hypoxia tolerance among isolated threespine stickleback populations appear to result from differences in the use of metabolic suppression, with the more tolerant stickleback using metabolic suppression.
Fish that are capable of hypoxia-induced metabolic suppression reduce their metabolic rates by 30% to 80% relative to standard metabolic rates. Because this is not a complete cessation of metabolic rate, metabolic suppression can only prolong hypoxic survival, not sustain it indefinitely. If the hypoxic exposure lasts sufficiently long, the fish will succumb to a depletion of its glycogen stores and/or the over-accumulation of deleterious anaerobic end-products. Furthermore, the severely limited energetic scope that comes with a metabolically suppressed state means that the fish is unable to complete critical tasks such a predator avoidance and reproduction. Perhaps for these reasons, goldfish prioritize their use of aerobic metabolism in most hypoxic environments, reserving metabolic suppression for the extreme case of anoxia. | 9 | Geochemistry |
Perturb-seq couples CRISPR mediated gene knockdowns with single-cell gene expression. Linear models are used to calculate the effect of the knockdown of a single gene on the expression of multiple genes. | 1 | Biochemistry |
To improve the delivery of the new DNA into the cell, the DNA must be protected from damage and positively charged. Initially, anionic and neutral lipids were used for the construction of lipoplexes for synthetic vectors. However, in spite of the facts that there is little toxicity associated with them, that they are compatible with body fluids and that there was a possibility of adapting them to be tissue specific; they are complicated and time-consuming to produce so attention was turned to the cationic versions.
Cationic lipids, due to their positive charge, were first used to condense negatively charged DNA molecules so as to facilitate the encapsulation of DNA into liposomes. Later it was found that the use of cationic lipids significantly enhanced the stability of lipoplexes. Also as a result of their charge, cationic liposomes interact with the cell membrane, endocytosis was widely believed as the major route by which cells uptake lipoplexes. Endosomes are formed as the results of endocytosis, however, if genes can not be released into cytoplasm by breaking the membrane of endosome, they will be sent to lysosomes where all DNA will be destroyed before they could achieve their functions. It was also found that although cationic lipids themselves could condense and encapsulate DNA into liposomes, the transfection efficiency is very low due to the lack of ability in terms of "endosomal escaping". However, when helper lipids (usually electroneutral lipids, such as DOPE) were added to form lipoplexes, much higher transfection efficiency was observed. Later on, it was discovered that certain lipids have the ability to destabilize endosomal membranes so as to facilitate the escape of DNA from endosome, therefore those lipids are called fusogenic lipids. Although cationic liposomes have been widely used as an alternative for gene delivery vectors, a dose dependent toxicity of cationic lipids were also observed which could limit their therapeutic usages.
The most common use of lipoplexes has been in gene transfer into cancer cells, where the supplied genes have activated tumor suppressor control genes in the cell and decrease the activity of oncogenes. Recent studies have shown lipoplexes to be useful in transfecting respiratory epithelial cells. | 1 | Biochemistry |
Based on the sequencing approach used, the 5’ and 3’ adaptor sequences used to tag the cDNA library can be altered as needed. Previously, dual adapter-tagged cDNA libraries have been characterized using Illumina NGS. Low-cycle PCR can also be used to index universal adaptor cDNA libraries following the RT reaction. Alternatively, full-length adaptor sequences of choice can be included in the 5’ and 3’ adaptors used in the initial RT reaction. | 1 | Biochemistry |
Small differences in mass between stable isotopes of the same element can lead to a phenomenon called an "isotope effect," where heavier or lighter isotopes are preferentially incorporated into different natural materials depending on the materials chemical composition or physical state. Isotope effects are divided into two main groups: kinetic isotope effects and equilibrium isotope effects. A kinetic isotope effect occurs when a reaction is irreversible, meaning that the reaction only proceeds in the direction from reactants to products. Kinetic isotope effects cause isotopic fractionation—meaning that they affect the isotopic composition of reactant and product compounds—because the mass differences between stable isotopes can affect the rate of chemical reactions. It takes more energy to reach the transition state of a reaction if the compound has bonds with a heavier isotope, which causes the compound with heavier isotopes to react more slowly. Normal kinetic isotope effects cause the lighter isotope (or isotopes) to be preferentially included in a reactions product. The products are then said to be "depleted" in the heavy isotope relative to the reactant. Rarely, inverse kinetic isotope effects may occur, where the heavier isotope is preferentially included in a reaction's product.
Equilibrium isotope effects cause fractionation because it is more chemically favorable for heavy isotopes to take part in stronger bonds. An equilibrium isotope effect occurs when a reaction is at equilibrium, meaning that the reaction is able to occur in both directions simultaneously. When a reaction is at equilibrium, heavy isotopes will preferentially accumulate where they can form the strongest bonds. For example, when the water in a sealed, half-full bottle is in equilibrium with the vapor above it, the heavier isotopes H and O will accumulate in the liquid, where they form stronger bonds, while the lighter isotopes H and O will accumulate in the vapor. The liquid is then said to be "enriched" in the heavy isotope relative to the vapor. | 9 | Geochemistry |
Thomas and George Cranege (also spelled Cranage), who worked in the ironworking industry in England in the 1760s, are notable for introducing a new method of producing wrought iron from pig iron. | 8 | Metallurgy |
Molecules have various states referred to as energy levels. Fluorescence spectroscopy is primarily concerned with electronic and vibrational states. Generally, the species being examined has a ground electronic state (a low energy state) of interest, and an excited electronic state of higher energy. Within each of these electronic states there are various vibrational states.
In fluorescence, the species is first excited, by absorbing a photon, from its ground electronic state to one of the various vibrational states in the excited electronic state. Collisions with other molecules cause the excited molecule to lose vibrational energy until it reaches the lowest vibrational state from the excited electronic state. This process is often visualized with a Jablonski diagram.
The molecule then drops down to one of the various vibrational levels of the ground electronic state again, emitting a photon in the process. As molecules may drop down into any of several vibrational levels in the ground state, the emitted photons will have different energies, and thus frequencies. Therefore, by analysing the different frequencies of light emitted in fluorescent spectroscopy, along with their relative intensities, the structure of the different vibrational levels can be determined.
For atomic species, the process is similar; however, since atomic species do not have vibrational energy levels, the emitted photons are often at the same wavelength as the incident radiation. This process of re-emitting the absorbed photon is "resonance fluorescence" and while it is characteristic of atomic fluorescence, is seen in molecular fluorescence as well.
In a typical fluorescence (emission) measurement, the excitation wavelength is fixed and the detection wavelength varies, while in a fluorescence excitation measurement the detection wavelength is fixed and the excitation wavelength is varied across a region of interest. An emission map is measured by recording the emission spectra resulting from a range of excitation wavelengths and combining them all together. This is a three dimensional surface data set: emission intensity as a function of excitation and emission wavelengths, and is typically depicted as a contour map. | 7 | Physical Chemistry |
Excessive doses of escitalopram usually cause relatively minor untoward effects, such as agitation and tachycardia. However, dyskinesia, hypertonia, and clonus may occur in some cases. Therapeutic blood levels of escitalopram are usually in the range of 20–80 μg/L but may reach 80–200 μg/L in the elderly, patients with hepatic dysfunction, those who are poor CYP2C19 metabolizers or following acute overdose. Monitoring of the drug in plasma or serum is generally accomplished using chromatographic methods. Chiral techniques are available to distinguish escitalopram from its racemate, citalopram. | 4 | Stereochemistry |
Triazenes are organic compounds that contain the functional group R−N=N−NRR, where the R are each any of various types of substituent groups. Some anti-cancer medications and dyes are triazenes. Formally, the triazenes are related to the unstable chemical triazene, HN−N=NH. | 0 | Organic Chemistry |
Successive stepwise formation constants K in a series such as ML (n = 1, 2, ...) usually decrease as n increases. Exceptions to this rule occur when the geometry of the ML complexes is not the same for all members of the series. The classic example is the formation of the diamminesilver(I) complex [Ag(NH)] in aqueous solution.
In this case, K > K. The reason for this is that, in aqueous solution, the ion written as Ag actually exists as the four-coordinate tetrahedral aqua species [Ag(HO)]. The first step is then a substitution reaction involving the displacement of a bound water molecule by ammonia forming the tetrahedral complex [Ag(NH)(HO)]. In the second step, all the aqua ligands are lost and a linear, two-coordinate product [HN–Ag–NH] is formed. Examination of the thermodynamic data shows that the difference in entropy change is the main contributor to the difference in stability constants for the two complexation reactions.
Other examples exist where the change is from octahedral to tetrahedral, as in the formation of [CoCl] from [Co(HO)]. | 7 | Physical Chemistry |
The a/LCI system has recently been enhanced to allow operation in a clinical setting with the addition of a handheld wand. By carefully controlling the polarization in the delivery fiber, using polarization-maintaining fibers and inline polarizers, the new system allows manipulation of the handheld wand without signal degradation due to birefringence effects. In addition, the new system employed an anti-reflection coated ball lens in the probe tip, which reduces reflections that otherwise limit the depth range of the system.
The portable system uses a 2 ft by 2 ft optical breadboard as the base, with the source, fiber optic components, lens, beamsplitter, and imaging spectrometer mounted to the breadboard. An aluminum cover protects the optics. A fiber probe with a handheld probe enables easy access to tissue samples for testing. On the left side sits a white sample platform, where tissue is placed for testing. The handheld probe is used by the operator to select specific sites on the tissue from which a/LCI readings are acquired. | 7 | Physical Chemistry |
Waters from the modern Pacific and Southern ocean, typically observe an increase in Si/N ratio at intermediate depth, which results in an increase in opal export (~ increase in opal production). In the Southern Ocean and North Pacific, this relationship between opal export and Si/N ratio switches from linear to exponential for Si/N ratios greater than 2. This gradual increase in the importance of silicate (Si) relative to nitrogen (N) has tremendous consequences for the ocean biological production. The change in nutrient ratios contributes to select diatoms as main producers, compared to other (e.g., calcifying) organisms. For example, microcosm experiments have demonstrated that diatoms are DSi supercompetitors and dominate other producers above 2 μM DSi. Consequently, opal vs. carbonate export will be favored, resulting in increasing opal production. The Southern Ocean and the North Pacific also display maximum biogenic silicate/C flux ratios, and consist thus in an enrichment in biogenic silicate, compared to C export flux. This combined increase in opal preservation and export makes the Southern Ocean the most important sink for DSi today. | 1 | Biochemistry |
Herzberg pointed out direct evidence of H molecules in the atmospheres of the outer planets. The atmospheres of the inner planets and of Saturns big moon Titan also show significant CIA in the infrared due to concentrations of nitrogen, oxygen, carbon dioxide and other molecular gases. However, the total CIA contribution of Earths major gases, N and O, to the atmospheres natural greenhouse effect is relatively minor except near the poles. Extrasolar planets have been discovered with hot atmospheres (a thousand kelvin or more) which otherwise resemble Jupiters atmosphere (mixtures of mostly H and He) where relatively strong CIA exists. | 7 | Physical Chemistry |
All dynamic energy budget models follow the energy budget of an individual organism throughout its life cycle; by contrast,"static" energy budget models describe a specific life stage or size of an organism. The main advantage of the DEB-theory based model over most other models is its description of energy assimilation and utilization (reserve dynamics) simultaneously with decoupled processes of growth, development/ maturation, and maintenance. Under constant environmental conditions (constant food and temperature) the standard DEB model can be simplified to the von Bertalanffy (or better, Putter's ) growth model, but its mechanistic process-based setup enables incorporating fluctuating environmental conditions, as well as studying reproduction and maturation in parallel to growth.
DEB theory specifies reserves as separate from structure: these are the two state variables that contribute to physical volume, and (in combination with reproduction buffer of adults) fully define the size of an individual. Maturity (also a state variable of the model) tracks how much energy has been invested into maturation, and therefore determines the life stage of the organism relative to maturity levels at which life stage transitions (birth and puberty) occur. Dynamics of the state variables are given by ordinary differential equations which include the major processes of energy uptake and use: assimilation, mobilization, maintenance, growth, maturation, and reproduction.
* Food is transformed into reserve, which fuels all other metabolic processes. The feeding rate is proportional to the surface area; food handling time and the transformation efficiency from food to reserve are independent of food density.
* A fixed fraction (kappa) of mobilized reserve is allocated to somatic maintenance plus growth (soma), the rest on maturity maintenance plus maturation or reproduction. Maintenance has priority over other processes. Somatic maintenance is proportional to structural body volume, and maturity maintenance to maturity. Heating costs for endotherms and osmotic work (for fresh water organisms) are somatic maintenance costs that are proportional to surface area.
* Stage transitions occur if the cumulated investment into maturation exceeds threshold values. Life stages typically are: embryo, juvenile, and adult. Reserve that is allocated to reproduction is first accumulated in a buffer. The rules for converting the buffer to gametes are species-specific (e.g. spawning can be once per season).
Parameters of the model are individual specific, but similarities between individuals of the same species yield species-specific parameter estimations. DEB parameters are estimated from several types of data simultaneously. Routines for data entry and parameter estimation are available as free software package [http://www.bio.vu.nl/thb/deb/deblab/debtool/DEBtool_M/manual/index.html DEBtool] implemented in the MATLAB environment, with the process of model construction explained in a [http://www.debtheory.org/wiki/index.php?title=Add-my-pet_Introduction Wiki-style manual] . Estimated parameters are collected in the online library called the [http://www.bio.vu.nl/thb/deb/deblab/add_my_pet/index.html Add-my-pet project]. | 1 | Biochemistry |
In most GTPases, the specificity for the base guanine versus other nucleotides is imparted by the base-recognition motif, which has the consensus sequence [N/T]KXD. The following classification is based on shared features; some examples have mutations in the base-recognition motif that shift their substrate specificity, most commonly to ATP. | 1 | Biochemistry |
Sclerotiorin is an antimicrobial Penicillium frequentans isolate. Sclerotiorin is an aldose reductase inhibitor (IC=0.4 μM) as well as a reversible lipoxygenase inhibitor (IC=4.2 μM). | 0 | Organic Chemistry |
A hypothetical schematic diagram for the transition to an H state by photo excitation is shown in the Figure (After ). An absorbed photon causes an electron from the ground state G to an excited state E (red arrow). State E rapidly relaxes via Franck-Condon relaxation to an intermediate locally reordered state I. Through interactions with others of its kind, this state collectively orders to form a macroscopically ordered metastable state H, further lowering its energy as a result. The new state has a broken symmetry with respect to the G or E state, and may also involve further relaxation compared to the I state. The barrier E prevents state H from reverting to the ground state G. If the barrier is sufficiently large compared to thermal energy kT, where k is the Boltzmann constant, the H state can be stable indefinitely. | 7 | Physical Chemistry |
At the beginning of the 20th century, the entire Russian industry was in a deep crisis, the consequences of which affected the factories of the Urals until 1909. In 1909, the Ural iron and steel plants smelted 34.7 million tons of iron, which is 30.9% less than in 1900. During the crisis years, the share of finished iron increased, new markets were searched for, syndicates and associations were created to fight the competition of factories in Southern Russia. To a lesser extent, the crisis affected the copper smelting industry, thanks to continued demand and an increase in customs duties on copper imports. In the first decade of the 20th century, small technically backward factories with worn-out equipment, which had become unprofitable, were closed. Of the 111 metallurgical plants operating in the Urals in 1900, 35 plants were shut down by 1913. In conditions of tough competition, factories were forced to modernize: blast furnaces with a lightweight casing were erected, hot blast was introduced everywhere, steam engines and ore preparation for smelting, furnaces and puddling furnaces were replaced by open-hearth furnaces, more powerful rolling mills were built, and factories received electricity. In the mountainous districts, the optimization and reorganization of capacities were carried out: the final processing was concentrated, as a rule, at the main plant of the district, the rest of the factories provided supplies of iron. During the Russo-Japanese War, the Izhevsk, Perm, and Zlatoust arms factories sharply increased the production of guns, rifles, and shells.
In 1908, the construction of the Porogi electrometallurgical plant for the production of ferroalloys, and one of the first hydroelectric power plants in Russia to provide the plant with electricity began. Until 1931, the plant was the only producer of ferroalloys in the country.
In 1910, an industrial boom began, which continued until the First World War. From 1910 to 1913, the production of iron increased to 55.3 million poods (by 29.9%), finished metal products - up to 40.8 million poods (by 9.6%). But the share of the Ural factories in the all-Russian iron smelting fell to 21.6%. Commercial banks actively invested in the development of the metallurgy of the Urals. The most important role in the Urals was played by the Azov-Don Commercial Bank, Saint Petersburg International Commercial Bank, and Russo-Asiatic Bank. The volume of investments at the turn of the 20th century was estimated at 10.8 million rubles. Modernization and reconstruction of mountain districts continued. In 1911, a new blast furnace with a volume of 150 m³ and an open-hearth furnace with a capacity of 25 tons were launched at the Nizhniy Tagil plant; two Bessemer converters and two new blast furnaces were installed at the Nizhnesaldinsky plant. The Votkinsk plant was reconstructed for the production of steam locomotives and river vessels. The factories that produced weapons were reconstructed and switched over to the production of civilian products. Also in the pre-war years, the concentration of production at large factories increased: in 1914, out of 49 Ural plants, 16 had the productivity of more than 1 million poods of iron per year and produced 65% of the total volume, including 5 factories with a capacity of more than 2 million poods of iron per year. Nadezhdinsky, Nizhnesaldinsky, Zlatoustovsky, Chusovskoy, and Votkinsky produced 36.1% of the total volume.
Copper smelters of the Urals at the beginning of the 20th century mastered pyrite smelting, which made it possible to process poor sulfur ores. In the pre-war years, the Nizhnekyshtymsky Copper Electrolytic Plant, the Karabashsky, and Kalatinsky plants were launched. Through syndicates formed, British companies owned 65.5% of the copper mined in the Urals. The gold-platinum mining industry underwent mechanization. The first Dutch dredges appeared in 1900 at the Neozhidany Mine on the Is River. By 1913, the number of dredges in the Urals reached 50, they ensured the extraction of 20% of gold and 50% of platinum. Until 1913, the average production of gold in the Urals was 550-650 poods per year, while the average production of platinum was 300-350 poods per year. | 8 | Metallurgy |
The protein encoded by this gene is a transcriptional regulatory protein. It contains paired amphipathic helix (PAH) domains, which are important for protein-protein interactions and may mediate repression by the Mad-Max complex. | 1 | Biochemistry |
The accuracy and precision of this method is supported largely by the confidence level of the results for appropriate liquid/solid combinations (as seen, for example, in fig 6). The Owens/Wendt theory is typically applicable to surfaces with low charge and moderate polarity. Some good examples are polymers that contain heteroatoms, such as PVC, polyurethanes, polyamides, polyesters, polyacrylates, and polycarbonates | 7 | Physical Chemistry |
In special situations, some ligands participate in substitution reactions leading to associative pathways. These ligands can adopt multiple motifs for binding to the metal, each of which involves a different number of electrons "donated." A classic case is the indenyl effect in which an indenyl ligand reversibly "slips' from pentahapto (η) coordination to trihapto (η). Other pi-ligands behave in this way, e.g. allyl (η to η) and naphthalene (η to η). Nitric oxide typically binds to metals to make a linear MNO arrangement, wherein the nitrogen oxide is said to donate 3e linear NO ligand to a 1e bent NO ligand. | 0 | Organic Chemistry |
Pyridine is a Lewis base, donating its pair of electrons to a Lewis acid. Its Lewis base properties are discussed in the ECW model. Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by C-B plots. One example is the sulfur trioxide pyridine complex (melting point 175 °C), which is a sulfation agent used to convert alcohols to sulfate esters. Pyridine-borane (, melting point 10–11 °C) is a mild reducing agent.
Transition metal pyridine complexes are numerous.
Typical octahedral complexes have the stoichiometry and . Octahedral homoleptic complexes of the type are rare or tend to dissociate pyridine. Numerous square planar complexes are known, such as Crabtree's catalyst. The pyridine ligand replaced during the reaction is restored after its completion.
The η coordination mode, as occurs in η benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center. | 0 | Organic Chemistry |
Agosterol A is a bio-active sterol which may have applications in removing multi-drug resistance in various cancers. It was first isolated from marine sponge but has also been produced synthetically. | 0 | Organic Chemistry |
Zero-point energy evolved from historical ideas about the vacuum. To Aristotle the vacuum was , "the empty"; i.e., space independent of body. He believed this concept violated basic physical principles and asserted that the elements of fire, air, earth, and water were not made of atoms, but were continuous. To the atomists the concept of emptiness had absolute character: it was the distinction between existence and nonexistence. Debate about the characteristics of the vacuum were largely confined to the realm of philosophy, it was not until much later on with the beginning of the renaissance, that Otto von Guericke invented the first vacuum pump and the first testable scientific ideas began to emerge. It was thought that a totally empty volume of space could be created by simply removing all gases. This was the first generally accepted concept of the vacuum.
Late in the 19th century, however, it became apparent that the evacuated region still contained thermal radiation. The existence of the aether as a substitute for a true void was the most prevalent theory of the time. According to the successful electromagnetic aether theory based upon Maxwell's electrodynamics, this all-encompassing aether was endowed with energy and hence very different from nothingness. The fact that electromagnetic and gravitational phenomena were easily transmitted in empty space indicated that their associated aethers were part of the fabric of space itself. Maxwell himself noted that:
However, the results of the Michelson–Morley experiment in 1887 were the first strong evidence that the then-prevalent aether theories were seriously flawed, and initiated a line of research that eventually led to special relativity, which ruled out the idea of a stationary aether altogether. To scientists of the period, it seemed that a true vacuum in space might be created by cooling and thus eliminating all radiation or energy. From this idea evolved the second concept of achieving a real vacuum: cool a region of space down to absolute zero temperature after evacuation. Absolute zero was technically impossible to achieve in the 19th century, so the debate remained unsolved. | 7 | Physical Chemistry |
Different tissues of the human body may react differently to changes in telomeres. Telomere length is different in different tissues and cell types of the body. Developing a general telomere lengthening strategy that is effective in all tissues is a complex task; Also, understanding how different types of cells, organs and systems react to telomere manipulation is very important for developing safe and effective interventions. | 1 | Biochemistry |
mCPBA can be prepared by reacting m-chlorobenzoyl chloride with a basic solution of hydrogen peroxide, followed by acidification.
It is sold commercially as a shelf-stable mixture that is less than 72% mCPBA, with the balance made up of m-chlorobenzoic acid (10%) and water. The peroxyacid can be purified by washing the commercial material with a sodium hydroxide and potassium phosphate solution buffered at pH = 7.5. Peroxyacids are generally slightly less acidic than their carboxylic acid counterparts, so the acid impurity can be extracted if the pH is carefully controlled. The purified material is reasonably stable against decomposition if stored at low temperatures in a plastic container.
In reactions where the exact amount of mCPBA must be controlled, a sample can be titrated to determine the exact amount of active oxidant. | 0 | Organic Chemistry |
The closing of atoms into rings may lock particular atoms with distinct substitution by functional groups such that the result is stereochemistry and chirality of the compound, including some manifestations that are unique to rings (e.g., configurational isomers). | 4 | Stereochemistry |
* Palladium precatalyst species are activated under reaction conditions to form a reactive Pd compound, A. The exact identity of the catalytic species depends strongly upon reaction conditions. With simple phosphines, such as PPh (n=2), and in case of bulky phosphines (i.e., ) it was demonstrated that monoligated species (n=1) are formed. Furthermore, some results point to the formation of anionic palladium species, [LPdCl] , which could be the real catalysts in the presence of anions and halides.
* The active Pd catalyst is involved in the oxidative addition step with the aryl or vinyl halide substrate to produce Pd species B. Similar to the above discussion, its structure depends on the employed ligands. This step is believed to be the rate-limiting step of the reaction.
* Complex B reacts with copper acetylide, complex F, in a transmetallation step, yielding complex C and regenerating the copper catalyst.
* The structure of complex C depends on the properties of the ligands. For the facile reductive elimination to occur, the substrate motifs need to be in close vicinity, i.e. cis-orientation, so there can be trans-cis isomerisation involved. In reductive elimination the product tolane is expelled from the complex and the active Pd catalytic species is regenerated. | 0 | Organic Chemistry |
Spoil tips may be conical in shape, and can appear as conspicuous features of the landscape, or they may be much flatter and eroded, especially if vegetation has established itself. In Loos-en-Gohelle, in the former mining area of Pas-de-Calais, France, are a series of five very perfect cones, of which two rise from the plain. | 8 | Metallurgy |
Aluminium smelting is highly energy intensive, and in some countries is economical only if there are inexpensive sources of electricity. In some countries, smelters are given exemptions to energy policy like renewable energy targets.
To reduce the energy cost of the smelting process, alternative electrolytes such as Na3AlF6 are being investigated that can operate at a lower temperature. However, changing the electrolyte changes the kinetics of the liberated oxygen from the AlO ore. This change in bubble formation can alter the rate the anode reacts with Oxygen or the electrolyte and effectively change the efficiency of the reduction process.
Inert anodes, used in tandem with vertical electrode cells, can also reduce the energy cost of aluminum reduction up to 30% by lowering the voltage needed for reduction to occur. Applying these two technologies at the same times allows the anode-cathode distance to be minimized which decreases restive losses. | 8 | Metallurgy |
Changing the conditions of the reaction solvent can allow separation of phases for product removal, or single phase for reaction. Rapid diffusion accelerates diffusion controlled reactions. Temperature and pressure can tune the reaction down preferred pathways, e.g., to improve yield of a particular chiral isomer. There are also significant environmental benefits over conventional organic solvents. Industrial syntheses that are performed at supercritical conditions include those of polyethylene from supercritical ethene, isopropyl alcohol from supercritical propene, 2-butanol from supercritical butene, and ammonia from a supercritical mix of nitrogen and hydrogen. Other reactions were, in the past, performed industrially in supercritical conditions, including the synthesis of methanol and thermal (non-catalytic) oil cracking. Because of the development of effective catalysts, the required temperatures of those two processes have been reduced and are no longer supercritical. | 7 | Physical Chemistry |
Forces between two or more molecules or atoms, either attractive or repulsive, are called intermolecular forces. Intermolecular forces are experienced by molecules when they are within physical proximity of one another. These forces are very important for properly modeling molecular systems, as to accurately predict the microscopic behavior of molecules in any system, and therefore, are necessary for accurately predicting the physical properties of gases (and liquids) across wide variations in physical conditions.
Arising from the study of physical chemistry, one of the most prominent intermolecular forces throughout physics, are van der Waals forces. Van der Waals forces play a key role in determining nearly all physical properties of fluids such as viscosity, flow rate, and gas dynamics (see physical characteristics section). The van der Waals interactions between gas molecules, is the reason why modeling a "real gas" is more mathematically difficult than an "ideal gas". Ignoring these proximity-dependent forces allows a real gas to be treated like an ideal gas, which greatly simplifies calculation.
The intermolecular attractions and repulsions between two gas molecules are dependent on the amount of distance between them. The combined attractions and repulsions are well-modelled by the Lennard-Jones potential, which is one of the most extensively studied of all interatomic potentials describing the potential energy of molecular systems. The Lennard-Jones potential between molecules can be broken down into two separate components: a long-distance attraction due to the London dispersion force, and a short-range repulsion due to electron-electron exchange interaction (which is related to the Pauli exclusion principle).
When two molecules are relatively distant (meaning they have a high potential energy), they experience a weak attracting force, causing them to move toward each other, lowering their potential energy. However, if the molecules are too far away, then they would not experience attractive force of any significance. Additionally, if the molecules get too close then they will collide, and experience a very high repulsive force (modelled by Hard spheres) which is a much stronger force than the attractions, so that any attraction due to proximity is disregarded.
As two molecules approach each other, from a distance that is neither too-far, nor too-close, their attraction increases as the magnitude of their potential energy increases (becoming more negative), and lowers their total internal energy. The attraction causing the molecules to get closer, can only happen if the molecules remain in proximity for the duration of time it takes to physically move closer. Therefore, the attractive forces are strongest when the molecules move at low speeds. This means that the attraction between molecules is significant when gas temperatures is low. However, if you were to isothermally compress this cold gas into a small volume, forcing the molecules into close proximity, and raising the pressure, the repulsions will begin to dominate over the attractions, as the rate at which collisions are happening will increase significantly. Therefore, at low temperatures, and low pressures, attraction is the dominant intermolecular interaction.
If two molecules are moving at high speeds, in arbitrary directions, along non-intersecting paths, then they will not spend enough time in proximity to be affected by the attractive London-dispersion force. If the two molecules collide, they are moving too fast and their kinetic energy will be much greater than any attractive potential energy, so they will only experience repulsion upon colliding. Thus, attractions between molecules can be neglected at high temperatures due to high speeds. At high temperatures, and high pressures, repulsion is the dominant intermolecular interaction.
Accounting for the above stated effects which cause these attractions and repulsions, real gases, delineate from the ideal gas model by the following generalization:
* At low temperatures, and low pressures, the volume occupied by a real gas, is less than the volume predicted by the ideal gas law.
* At high temperatures, and high pressures, the volume occupied by a real gas, is greater than the volume predicted by the ideal gas law. | 7 | Physical Chemistry |
Pitch, a traditional naval store, was traditionally used to help caulk the seams of wooden sailing vessels (see shipbuilding). Other important historic uses included coating earthenware vessels for the preservation of wine, waterproofing wooden containers, and making torches. Petroleum-derived pitch is black in colour, hence the adjectival phrase, "pitch-black".
The viscoelastic properties of pitch make it well suited for the polishing of high-quality optical lenses and mirrors. In use, the pitch is formed into a lap or polishing surface, which is charged with iron oxide (Jewelers' rouge) or cerium oxide. The surface to be polished is pressed into the pitch, then rubbed against the surface so formed. The ability of pitch to flow, albeit slowly, keeps it in constant uniform contact with the optical surface.
Chasers pitch is a combination of pitch and other substances, used in jewelry making. | 7 | Physical Chemistry |
Sulfonyl iodides, having the general formula RSOI, are quite light-sensitive. Perfluoroalkanesulfonyl iodides, prepared by reaction between silver perfluoroalkanesulfinates and iodine in dichloromethane at −30 °C, react with alkenes to form the normal adducts, RFSOCHCHIR and the adducts resulting from loss of SO, RFCHCHIR. Arenesulfonyl iodides, prepared from reaction of arenesulfinates or arenehydrazides with iodine, can be used as initiators to facilitate the synthesis of poly(methyl methacrylate) containing C–I, C–Br and C–Cl chain ends. | 0 | Organic Chemistry |
In 1942, the Metallurgical Laboratory at the University of Chicago was building the worlds first nuclear reactor called Chicago Pile-1' as part of the Manhattan Project. This would have required an enormous number of graphite blocks and uranium pellets. At the time, there was a limited source of pure uranium. Frank Spedding of Iowa State University was able to produce only two short tons of pure uranium. However, a larger quantity of additional uranium metal was required for the pile to go critical.
Around that time, Westinghouse Lamp Plant was able to supply a small amount of pure uranium metal. By 1941, there was an order for the uranium metal of 10 kilograms which was considered an enormous amount. The plant ramped up the production using makeshift equipment including metal garbage cans from a local market to use in the process. The order was fulfilled within a couple of months. By early 1942, there was another order from Arthur H. Compton of the Metallurgical Laboratory for three short tons of the uranium metal for the Chicago Pile-1.
The project was done in secrecy by not revealing any connection of the work there to the ongoing research on the nuclear reaction. To avoid leaking of information about the project at the plant, it was covered with the program called Tuballoy which was the codename for uranium in programs related to the Manhattan Project, taken from the British atomic weapons effort "Tube Alloys" which had been folded into Manhattan. Due to inadequate equipment and space, the refinement process was done in an ad hoc fashion having some operations done in the basement and some on the roof on a building at the plant. To maintain the confidentiality of the program, many workers who worked on the chemical processes were not familiar with it. Most of them did not know that their work was related to the creation of an atomic weapon.
Although the material and the processes were kept secret, the workers there knew they worked as part of the World War II efforts. They were led to believe that they worked on making conventional bombs. During the height of the production in 1942, there was a war production drive at the plant under the auspices of the labor-management committee to increase its production and warned workers against sabotage. The streets at the plant were renamed to MacArthur Avenue and MacArthur Plaza to promote patriotism.
Finally, the required amount of uranium metal was delivered to Chicago by the Army with containers clearly marked "URANIUM". The Chicago Pile-1 had gone to self-sustaining reaction on December 2, 1942, with the majority of uranium metal from the plant. After that, the plant continued to play a major role in supplying the uranium metal for the Manhattan Project until October 1943 when a better and more economical process was done elsewhere. During the contract, the plant produced 69 short tons of uranium metal in total. | 8 | Metallurgy |
Macromolecules is a peer-reviewed scientific journal that has been published since 1968 by the American Chemical Society. Initially published bimonthly, it became monthly in 1983 and then, in 1990, biweekly. Macromolecules is abstracted and indexed in Scopus, EBSCOhost, PubMed, Web of Science, and SwetsWise. The editor-in-chief is Marc A. Hillmyer.
Its first editor was Dr. Field H. Winslow. | 7 | Physical Chemistry |
Each peroxy group is considered to contain one active oxygen atom. The concept of active oxygen content is useful for comparing the relative concentration of peroxy groups in formulations, which is related to the energy content. In general, energy content increases with active oxygen content, and thus the higher the molecular weight of the organic groups, the lower the energy content and, usually, the lower the hazard.
The term active oxygen is used to specify the amount of peroxide present in any organic peroxide formulation. One of the oxygen atoms in each peroxide group is considered "active". The theoretical amount of active oxygen can be described by the following equation:
where is the number of peroxide groups in the molecule, and is the molecular mass of the pure peroxide.
Organic peroxides are often sold as formulations that include one or more phlegmatizing agents. That is, for safety sake or performance benefits the properties of an organic peroxide formulation are commonly modified by the use of additives to phlegmatize (desensitize), stabilize, or otherwise enhance the organic peroxide for commercial use. Commercial formulations occasionally consist of mixtures of organic peroxides, which may or may not be phlegmatized. | 0 | Organic Chemistry |
Dry spiral separators, capable of distinguishing round particles from nonrounds, are used to sort the feed by shape. The device consists of a tower, around which is wound an inwardly inclined flight. A catchment funnel is placed around this inner flight. Round particles roll at a higher speed than other objects, and so are flung off the inner flight and into the collection funnel. Shapes which are not round enough are collected at the bottom of the flight.
Separators of this type may be used for removing weed seeds from the intended harvest, or to remove deformed lead shot. | 3 | Analytical Chemistry |
Whenever there is a balance between a bulk elastic energy contribution and a surface energy term, surface stresses can be important. Surface contributions are more important at small sizes, so surface stress effects are often important at the nanoscale. | 7 | Physical Chemistry |
Researchers have also proposed early encapsulation in aqueous phase-separated droplets called coacervates. These droplets are driven by the accumulation of macromolecules, producing a distinct dense phase liquid droplet within a more dilute liquid medium. These droplets can propagate, retaining their internal composition, through shear forces and turbulence in the medium, and could have acted as a means of replicating encapsulation for an early protocell. However, replication was highly disordered and droplet fusion is common, calling into question coacervates true potential for distinct compartmentalization leading to competition and early Darwinian-selection. | 9 | Geochemistry |
More recently many research groups have tried to employ enzymes into DKR synthetic routes. Due to the generally high specificity for substrates, enzymes prove to be vital catalysts for binding to only one stereoisomer in the racemic mixture. In 2007 Bäckvall discovered an enzyme-metal coupled reaction that converts allylic acetates to allylic alcohols with excellent stereospecificity.
In this reaction, a Pd(0) complex is used to interconvert the chirality of the acetate center at a rate fast enough to ensure complete racemization. When this is achieved the CALB enzyme selectively hydrolyzes the (R) substrate because of the low binding affinity for the (S) substrate. This gives almost exclusively the (R) allylic alcohol in 98% ee.
To expand on this chemistry, Bäckvall designed a one-pot, two-reaction system that utilizes the stereochemical outcome of a DKR reaction to undergo a second energetically favorable reaction with high enantioselectivity.
This time a ruthenium complex is used to racemize the allylic alcohol in much the same way as the previous example. The addition of CALB catalyzes the reaction between the (R) isomer and the ester reagent to form a product with a diene and a dienophile. This intermediate can then undergo a tandem Diels-Alder reaction to achieve a decent yield with 97% ee. | 4 | Stereochemistry |
The fundamental process for isolating nucleic acid from starting material of Boom method consists of the following 4 steps (See Fig. 1).
(d) Separating the bonded nucleic acids <br />
Pure nucleic acids are eluted into buffer by decreasing the concentration of chaotropic substance.
Nucleic acids present in the washed (and preferably dried) silica-nucleic
acid complexes is eluted into chosen elution buffer such as TE buffer,
aqua bidest, and so on. The selection of the elution buffer is
co-determined by the contemplated use of the isolated nucleic acid.
</blockquote>
In this way, pure nucleic acids are isolated from the starting material.
By altering the experimental conditions, especially the composition of reagents (chaotropic substance, wash buffer, etc) more specific isolation can be achieved. For example, some compositions of reagents are suitable for obtaining long double-stranded DNA or short single-stranded RNA.
A wide variety of starting biological material are available, including whole blood, blood serum, buffy coat, urine, feces, cerebrospinal fluid, sperm, saliva, tissues, cell cultures, food products, or vaccines. Optimization of procedure is required to maximize yield of nucleic acids from different starting materials or different types of nucleic acids (eg long/short, DNA/RNA, linear/circular, double-stranded/single-stranded).
Today, the assay characterized by using silica coated magnetic beads seems to be the most common. Therefore, in this article, "silica beads" are intended to mean silica coated magnetic beads unless stated otherwise. | 1 | Biochemistry |
In enzymology, the committed step (also known as the first committed step) is an effectively irreversible enzymatic reaction that occurs at a branch point during the biosynthesis of some molecules.
As the name implies, after this step, the molecules are "committed" to the pathway and will ultimately end up in the pathway's final product. The first committed step should not be confused with the rate-limiting step, which is the step with the highest flux control coefficient. It is rare that the first committed step is in fact the rate-determining step. | 1 | Biochemistry |
According to a 2014 review in the New England Journal of Medicine stated that a ferritin level below 30 ng/mL indicates iron deficiency, while a level below 10 ng/mL indicates iron-deficiency anemia. A 2020 World Health Organization guideline states that ferritin indicates iron deficiency below 12 ng/mL in apparently-healthy children under 5 and 15 ng/mL in apparently-healthy individuals of 5 and over.
Some studies suggest that women with fatigue and ferritin below 50 ng/mL see reduced fatigue after iron supplementation.
In the setting of anemia, low serum ferritin is the most specific lab finding for iron-deficiency anemia. However it is less sensitive, since its levels are increased in the blood by infection or any type of chronic inflammation, and these conditions may convert what would otherwise be a low level of ferritin from lack of iron, into a value in the normal range. For this reason, low ferritin levels carry more information than those in the normal range. A falsely low blood ferritin (equivalent to a false positive test) is very uncommon, but can result from a hook effect of the measuring tools in extreme cases.
Low ferritin may also indicate hypothyroidism, vitamin C deficiency or celiac disease.
Low serum ferritin levels are seen in some patients with restless legs syndrome, not necessarily related to anemia, but perhaps due to low iron stores short of anemia.
Vegetarianism is not a cause of low serum ferritin levels, according to the American Dietetic Association's position in 2009: "Incidence of iron-deficiency anemia among vegetarians is similar to that of non-vegetarians. Although vegetarian adults have lower iron stores than non-vegetarians, their serum ferritin levels are usually within the normal range." | 1 | Biochemistry |
In addition to receiving citations from scientific articles and theses, the Bilbao Crystallographic Server also actively publishes research reports in internationally reviewed articles, as well as hosting/participating in international workshops, summer schools and conferences. A list of these publications and events are accessible from the [http://www.cryst.ehu.es/html/doc/bcs_references.html server's web page.]. | 3 | Analytical Chemistry |
It is prepared by electrofluorination of tributylamine using hydrogen fluoride as solvent and source of fluorine: | 2 | Environmental Chemistry |
The term "Reverse electron transfer" is used in regard to the reversibility of the reaction performed by complex I of the mitochondrial or bacterial respiratory chain. Complex I is responsible for the oxidation of NADH generated in catabolism when in the forward reaction electrons from the nucleotide (NADH) are transferred to membrane ubiquinone and energy is saved in the form of proton-motive force. The reversibility of the electron transfer reactions at complex I was first discovered when Chance and Hollunger have shown that the addition of succinate to mitochondria in State 4 leads to an uncoupler-sensitive reduction of the intramitochondrial nucleotides (NAD(P)). When succinate is oxidized by intact mitochondria, complex I can catalyze reverse electron transfer when electrons from ubiquinol (QH, formed during oxidation of succinate) is driven by the proton-motive force to complex I flavin toward the nucleotide-binding site.
Since the discovery of the reverse electron transfer in the 1960s it was regarded as in vitro phenomenon, until the role of RET in the development of ischemia/reperfusion injury has been recognized in the brain and heart. During ischemia substantial amount of succinate is generated in cerebral or cardiac tissue and upon reperfusion it can be oxidized by mitochondria initiating reverse electron transfer reaction. Reverse electron transfer supports the highest rate of mitochondrial Reactive Oxygen Species (ROS) production, and complex I flavin mononucleotide (FMN) has been identified as the site where one-electron reduction of oxygen takes place. | 1 | Biochemistry |
*CTSD can degrade insulin in hepatocytes
*CSTB May protect cell from leaking lysosomes
*LAMP1
*LAMP2
*M6PR | 1 | Biochemistry |
Cations, often carbocations, serve as intermediates in various types of reactions to synthesize new compounds. | 7 | Physical Chemistry |
The degree of dissociation is the fraction of the original solute molecules that have dissociated. It is usually indicated by the Greek symbol . There is a simple relationship between this parameter and the van 't Hoff factor. If a fraction of the solute dissociates into ions, then
For example, the dissociation KCl K + Cl yields ions, so that .
For dissociation in the absence of association, the van 't Hoff factor is: | 7 | Physical Chemistry |
# that a change in DOC or CO (or for that matter Aluminium solubility, but Aluminium solubility is not something that is easily controlled) does NOT have any effect on ANC.
# that once a pH-ANC relation for has been established for a lake the pH-ANC relation can be used to easily calculate the amount of limestone needed to raise lake pH to e.g. 5.5
# not all acid lakes are acid due to human influence since high DOC gives low pH.
# that the concentrations are multiplied with the charge of the species, hence the unit mol charge per liter | 2 | Environmental Chemistry |
In chemistry, a phosphorochloridate is a class of organophosphorus compounds with the formula (RO)P(O)Cl (R = organic substituent). They are tetrahedral in shape, akin to regular phosphates (OP(OR)). They are usually colorless and sensitive toward hydrolysis. They are oxidized derivatives of phosphorochloridites, which have the formula (RO)PCl. A popular example is diethyl phosphorochloridate. | 0 | Organic Chemistry |
The arrangement of leaves on the stem is known as phyllotaxis. A large variety of phyllotactic patterns occur in nature:
;Alternate: One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem.
;Basal: Arising from the base of the plant.
;Cauline: Attached to the aerial stem.
;Opposite: Two leaves, branches, or flower parts attach at each point or node on the stem. Leaf attachments are paired at each node.
;Decussate: An opposite arrangement in which each successive pair is rotated 90° from the previous.
;Whorled, or verticillate: Three or more leaves, branches, or flower parts attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc.). Opposite leaves may appear whorled near the tip of the stem. Pseudoverticillate describes an arrangement only appearing whorled, but not actually so.
;Rosulate: Leaves form a rosette.
;Rows: The term, distichous, literally means two rows. Leaves in this arrangement may be alternate or opposite in their attachment. The term, 2-ranked, is equivalent. The terms, tristichous and tetrastichous, are sometimes encountered. For example, the "leaves" (actually microphylls) of most species of Selaginella are tetrastichous, but not decussate.
In the simplest mathematical models of phyllotaxis, the apex of the stem is represented as a circle. Each new node is formed at the apex, and it is rotated by a constant angle from the previous node. This angle is called the divergence angle. The number of leaves that grow from a node depends on the plant species. When a single leaf grows from each node, and when the stem is held straight, the leaves form a helix.
The divergence angle is often represented as a fraction of a full rotation around the stem. A rotation fraction of 1/2 (a divergence angle of 180°) produces an alternate arrangement, such as in Gasteria or the fan-aloe Kumara plicatilis. Rotation fractions of 1/3 (divergence angles of 120°) occur in beech and hazel. Oak and apricot rotate by 2/5, sunflowers, poplar, and pear by 3/8, and in willow and almond the fraction is 5/13. These arrangements are periodic. The denominator of the rotation fraction indicates the number of leaves in one period, while the numerator indicates the number of complete turns or gyres made in one period. For example:
* 180° (or ): two leaves in one circle (alternate leaves)
* 120° (or ): three leaves in one circle
* 144° (or ): five leaves in two gyres
* 135° (or ): eight leaves in three gyres.
Most divergence angles are related to the sequence of Fibonacci numbers . This sequence begins 1, 1, 2, 3, 5, 8, 13; each term is the sum of the previous two. Rotation fractions are often quotients of a Fibonacci number by the number two terms later in the sequence. This is the case for the fractions 1/2, 1/3, 2/5, 3/8, and 5/13. The ratio between successive Fibonacci numbers tends to the golden ratio . When a circle is divided into two arcs whose lengths are in the ratio , the angle formed by the smaller arc is the golden angle, which is . Because of this, many divergence angles are approximately .
In plants where a pair of opposite leaves grows from each node, the leaves form a double helix. If the nodes do not rotate (a rotation fraction of zero and a divergence angle of 0°), the two helices become a pair of parallel lines, creating a distichous arrangement as in maple or olive trees. More common in a decussate pattern, in which each node rotates by 1/4 (90°) as in the herb basil. The leaves of tricussate plants such as Nerium oleander form a triple helix.
The leaves of some plants do not form helices. In some plants, the divergence angle changes as the plant grows. In orixate phyllotaxis, named after Orixa japonica, the divergence angle is not constant. Instead, it is periodic and follows the sequence 180°, 90°, 180°, 270°. | 5 | Photochemistry |
There are two distinct uniform colorings of a trihexagonal tiling. Naming the colors by indices on the 4 faces around a vertex (3.6.3.6): 1212, 1232. The second is called a cantic hexagonal tiling, h{6,3}, with two colors of triangles, existing in p3m1 (*333) symmetry. | 3 | Analytical Chemistry |
Before entering a tanning unit, the tanner usually applies indoor tanning lotion to the whole body and may use a separate facial-tanning lotion. These lotions are considerably more expensive than drugstore lotions. They contain no sunscreen, but instead moisturize the skin with ingredients such as aloe vera, hempseed oil and sunflower seed oil. They may also contain dihydroxyacetone, a sunless tanner. So-called "tingle" tanning lotions cause vasodilation, increasing blood circulation.
Goggles (eye protection) should be worn to avoid eye damage. In one 2004 study, tanners said they avoided goggles to prevent leaving pale skin around the eyes. In the US, CFR Title 21 requires that new tanning equipment come with eye protection and most states require that commercial tanning operators provide eye protection for their clients. Laws in other countries are similar. | 5 | Photochemistry |
In heterocyclic chemistry, organic reactions are classified by the type of heterocycle formed with respect to ring-size and type of heteroatom. See for instance the chemistry of indoles. Reactions are also categorized by the change in the carbon framework. Examples are ring expansion and ring contraction, homologation reactions, polymerization reactions, insertion reactions, ring-opening reactions and ring-closing reactions.
Organic reactions can also be classified by the type of bond to carbon with respect to the element involved. More reactions are found in organosilicon chemistry, organosulfur chemistry, organophosphorus chemistry and organofluorine chemistry. With the introduction of carbon-metal bonds the field crosses over to organometallic chemistry. | 0 | Organic Chemistry |
The Hall–Petch relation predicts that as the grain size decreases the yield strength increases. The Hall–Petch relation was experimentally found to be an effective model for materials with grain sizes ranging from 1 millimeter to 1 micrometer. Consequently, it was believed that if average grain size could be decreased even further to the nanometer length scale the yield strength would increase as well. However, experiments on many nanocrystalline materials demonstrated that if the grains reached a small enough size, the critical grain size which is typically around , the yield strength would either remain constant or decrease with decreasing grains size. This phenomenon has been termed the reverse or inverse Hall–Petch relation. A number of different mechanisms have been proposed for this relation. As suggested by Carlton et al., they fall into four categories: (1) dislocation-based, (2) diffusion-based, (3) grain-boundary shearing-based, (4) two-phase-based.
There have been several works done to investigate the mechanism behind the inverse Hall–Petch relationship on numerous materials. In Han’s work, a series of molecular dynamics simulations were done to investigate the effect of grain size on the mechanical properties of nanocrystalline graphene under uniaxial tensile loading, with random shapes and random orientations of graphene rings. The simulation was run at grain sizes of nm and at room temperature. It was found that in the grain size of range 3.1 nm to 40 nm, inverse Hall–Petch relationship was observed. This is because when the grain size decreases at nm scale, there is an increase in the density of grain boundary junctions which serves as a source of crack growth or weak bonding. However, it was also observed that at grain size below 3.1 nm, a pseudo Hall–Petch relationship was observed, which results an increase in strength. This is due to a decrease in stress concentration of grain boundary junctions and also due to the stress distribution of 5-7 defects along the grain boundary where the compressive and tensile stress are produced by the pentagon and heptagon rings, etc. Chen at al. have done research on the inverse HallPetch relations of high-entropy CoNiFeAlCu alloys. In the work, polycrystalline models of FCC structured CoNiFeAlCu with grain sizes ranging from 7.2 nm to 18.8 nm were constructed to perform uniaxial compression using molecular dynamic simulations. All compression simulations were done after setting the periodic boundary conditions across the three orthogonal directions. It was found that when the grain size is below 12.1 nm the inverse Hall–Petch relation was observed. This is because as the grain size decreases partial dislocations become less prominent and so as deformation twinning. Instead, it was observed that there is a change in the grain orientation and migration of grain boundaries and thus cause the growth and shrinkage of neighboring grains. These are the mechanisms for inverse Hall–Petch relations. Sheinerman et al. also studied inverse Hall–Petch relation for nanocrystalline ceramics. It was found that the critical grain size for the transition from direct Hall–Petch to inverse Hall–Petch fundamentally depends on the activation energy of grain boundary sliding. This is because in direct Hall–Petch the dominant deformation mechanism is intragrain dislocation motion while in inverse Hall–Petch the dominant mechanism is grain boundary sliding. It was concluded that by plotting both the volume fraction of grain boundary sliding and volume fraction of intragrain dislocation motion as a function of grain size, the critical grain size could be found where the two curves cross.
Other explanations that have been proposed to rationalize the apparent softening of metals with nanosized grains include poor sample quality and the suppression of dislocation pileups.
The pileup of dislocations at grain boundaries is a hallmark mechanism of the Hall–Petch relationship. Once grain sizes drop below the equilibrium distance between dislocations, though, this relationship should no longer be valid. Nevertheless, it is not entirely clear what exactly the dependency of yield stress should be on grain sizes below this point. | 8 | Metallurgy |
Thermodynamics has an intricate etymology.
By a surface-level analysis, the word consists of two parts that can be traced back to Ancient Greek. Firstly, ("of heat"; used in words such as thermometer) can be traced back to the root θέρμη therme, meaning "heat". Secondly, the word ("science of force [or power]") can be traced back to the root δύναμις dynamis, meaning "power".
In 1849, the adjective thermo-dynamic is used by William Thomson.
In 1854, the noun thermo-dynamics is used by Thomson and William Rankine to represent the science of generalized heat engines.
Pierre Perrot claims that the term thermodynamics was coined by James Joule in 1858 to designate the science of relations between heat and power, however, Joule never used that term, but used instead the term perfect thermo-dynamic engine in reference to Thomson's 1849 phraseology. | 7 | Physical Chemistry |
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.
In contrast, hydrophobes are not attracted to water and may seem to be repelled by it. Hygroscopics are attracted to water, but are not dissolved by water. | 6 | Supramolecular Chemistry |
The first time that A15 structure was observed was in 1931 when an electrolytically deposited layer of tungsten was examined. Discussion of whether the β-tungsten structure is an allotrope of tungsten or the structure of a tungsten suboxide was long-standing, but since the 1950s there has been many publications showing that the material is a true allotrope of tungsten.
The first intermetallic compound discovered with typical AB composition was chromium silicide CrSi, discovered in 1933. Several other compounds with A15 structure were discovered in following years. No large interest existed in research on those compounds. This changed with the discovery that vanadium silicide VSi showed superconductivity at around 17 K in 1953. In following years, several other AB superconductors were found. Niobium-germanium held the record for the highest temperature of 23.2 K from 1973 until the discovery of the cuprate superconductors in 1986. It took time for the method to produce wires from the very brittle A15 phase materials to be established. This method is still complicated. Though some A15 phase materials can withstand higher magnetic field intensity and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most applications due to easier manufacturing.
NbSn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers.
A relaxed form of the Voronoi diagram of the A15 phase seems to have the least surface area among all the possible partitions of three-dimensional Euclidean space in regions of equal volume. This partition, also known as the Weaire–Phelan structure, is often present in clathrate hydrates. | 8 | Metallurgy |
Similar to other N-linked glycan types, the biosynthesis of paucimannosidic proteins across most species has been documented to be facilitated by the actions of a limited set of glyco-enzymes including beta-N-acetylhexosaminidases (Hex) and alpha-mannosidases, through GnT-I-dependent and -independent truncation pathways. | 1 | Biochemistry |
Activity coefficients of electrolyte solutions may be calculated theoretically, using the Debye–Hückel equation or extensions such as the Davies equation, Pitzer equations or TCPC model. Specific ion interaction theory (SIT) may also be used.
For non-electrolyte solutions correlative methods such as UNIQUAC, NRTL, MOSCED or UNIFAC may be employed, provided fitted component-specific or model parameters are available. COSMO-RS is a theoretical method which is less dependent on model parameters as required information is obtained from quantum mechanics calculations specific to each molecule (sigma profiles) combined with a statistical thermodynamics treatment of surface segments.
For uncharged species, the activity coefficient γ mostly follows a salting-out model:
This simple model predicts activities of many species (dissolved undissociated gases such as CO, HS, NH, undissociated acids and bases) to high ionic strengths (up to 5 mol/kg). The value of the constant b for CO is 0.11 at 10 °C and 0.20 at 330 °C.
For water as solvent, the activity a can be calculated using:
where ν is the number of ions produced from the dissociation of one molecule of the dissolved salt, b is the molality of the salt dissolved in water, φ is the osmotic coefficient of water, and the constant 55.51 represents the molality of water. In the above equation, the activity of a solvent (here water) is represented as inversely proportional to the number of particles of salt versus that of the solvent. | 7 | Physical Chemistry |
Pathogens synthesize proteins that can serve as "recognizable" antigens; they may express the molecules on their surface or release them into the surroundings (body fluids). What makes these substances recognizable is that they bind very specifically and somewhat strongly to certain host proteins called antibodies. The same antibodies can be anchored to the surface of cells of the immune system, in which case they serve as receptors, or they can be secreted in the blood, known as soluble antibodies. On a molecular scale, the proteins are relatively large, so they cannot be recognized as a whole; instead, their segments, called epitopes, can be recognized. An epitope comes in contact with a very small region (of 15–22 amino acids) of the antibody molecule; this region is known as the paratope. In the immune system, membrane-bound antibodies are the B-cell receptor (BCR). Also, while the T-cell receptor is not biochemically classified as an antibody, it serves a similar function in that it specifically binds to epitopes complexed with major histocompatibility complex (MHC) molecules. The binding between a paratope and its corresponding antigen is very specific, owing to its structure, and is guided by various noncovalent bonds, not unlike the pairing of other types of ligands (any atom, ion or molecule that binds with any receptor with at least some degree of specificity and strength). The specificity of binding does not arise out of a rigid lock and key type of interaction, but rather requires both the paratope and the epitope to undergo slight conformational changes in each other's presence. | 1 | Biochemistry |
Canadian Harold Harvey was among the first to research a "dead" lake. In 1971, he and R. J. Beamish published a report, "Acidification of the La Cloche Mountain Lakes", documenting the gradual deterioration of fish stocks in 60 lakes in Killarney Park in Ontario, which they had been studying systematically since 1966.
In the 1970s and 80s, acid rain was a major topic of research at the Experimental Lakes Area (ELA) in Northwestern Ontario, Canada. Researchers added sulfuric acid to whole lakes in controlled ecosystem experiments to simulate the effects of acid rain. Because its remote conditions allowed for whole-ecosystem experiments, research at the ELA showed that the effect of acid rain on fish populations started at concentrations much lower than those observed in laboratory experiments. In the context of a food web, fish populations crashed earlier than when acid rain had direct toxic effects to the fish because the acidity led to crashes in prey populations (e.g. mysids). As experimental acid inputs were reduced, fish populations and lake ecosystems recovered at least partially, although invertebrate populations have still not completely returned to the baseline conditions. This research showed both that acidification was linked to declining fish populations and that the effects could be reversed if sulfuric acid emissions decreased, and influenced policy in Canada and the United States.
In 1985, seven Canadian provinces (all except British Columbia, Alberta, and Saskatchewan) and the federal government signed the Eastern Canada Acid Rain Program. The provinces agreed to limit their combined sulfur dioxide emissions to 2.3 million tonnes by 1994. The Canada-US Air Quality Agreement was signed in 1991. In 1998, all federal, provincial, and territorial Ministers of Energy and Environment signed The Canada-Wide Acid Rain Strategy for Post-2000, which was designed to protect lakes that are more sensitive than those protected by earlier policies. | 2 | Environmental Chemistry |
In chemistry, pentagonal pyramidal molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are arranged around a central atom, at the vertices of a pentagonal pyramid. It is one of the few molecular geometries with uneven bond angles. | 4 | Stereochemistry |
The region of space enclosed by open system boundaries is usually called a control volume. It may or may not correspond to physical walls. It is convenient to define the shape of the control volume so that all flow of matter, in or out, occurs perpendicular to its surface. One may consider a process in which the matter flowing into and out of the system is chemically homogeneous. Then the inflowing matter performs work as if it were driving a piston of fluid into the system. Also, the system performs work as if it were driving out a piston of fluid. Through the system walls that do not pass matter, heat () and work () transfers may be defined, including shaft work.
Classical thermodynamics considers processes for a system that is initially and finally in its own internal state of thermodynamic equilibrium, with no flow. This is feasible also under some restrictions, if the system is a mass of fluid flowing at a uniform rate. Then for many purposes a process, called a flow process, may be considered in accord with classical thermodynamics as if the classical rule of no flow were effective. For the present introductory account, it is supposed that the kinetic energy of flow, and the potential energy of elevation in the gravity field, do not change, and that the walls, other than the matter inlet and outlet, are rigid and motionless.
Under these conditions, the first law of thermodynamics for a flow process states: the increase in the internal energy of a system is equal to the amount of energy added to the system by matter flowing in and by heating, minus the amount lost by matter flowing out and in the form of work done by the system. Under these conditions, the first law for a flow process is written:
where and respectively denote the average internal energy entering and leaving the system with the flowing matter.
There are then two types of work performed: flow work described above, which is performed on the fluid in the control volume (this is also often called work), and shaft work, which may be performed by the fluid in the control volume on some mechanical device with a shaft. These two types of work are expressed in the equation:
Substitution into the equation above for the control volume cv yields:
The definition of enthalpy, , permits us to use this thermodynamic potential to account jointly for internal energy and work in fluids for a flow process:
During steady-state operation of a device (see turbine, pump, and engine), any system property within the control volume is independent of time. Therefore, the internal energy of the system enclosed by the control volume remains constant, which implies that in the expression above may be set equal to zero. This yields a useful expression for the power generation or requirement for these devices with chemical homogeneity in the absence of chemical reactions:
This expression is described by the diagram above. | 7 | Physical Chemistry |
In the geometry of crystal nets, one can treat edges as line segments. For example, in a crystal net, it is presumed that edges do not “collide” in the sense that when treating them as line segments, they do not intersect. Several polyhedral constructions can be derived from crystal nets. For example, a vertex figure can be obtained by subdividing each edge (treated as a line segment) by the insertion of subdividing points, and then the vertex figure of a given vertex is the convex hull of the adjacent subdividing points (i.e., the convex polyhedron whose vertices are the adjacent subdividing points).
Another polyhedral construction is to determine the neighborhood of a vertex in the crystal net. One application is to define an energy function as a (possibly weighted) sum of squares of distances from vertices to their neighbors, and with respect to this energy function, the net is in equilibrium (with respect to this energy function) if each vertex is positioned at the centroid of its neighborhood, this is the basis of the crystal net identification program SYSTRE. (mathematicians
use the term ``harmonic realiaztions" instead of ``crystal nets in equilibrium positions" because the positions are characterized by the discrete Laplace equation; they also introduced the notion of standard realizations which are special harmonic realizations characterized by a certain minimal principle as well;see ). Some crystal nets are isomorphic to crystal nets in equilibrium positions, and since an equilibrium position is a normal form, the crystal net isomorphism problem (i.e., the query whether two given crystal nets are isomorphic as graphs; not to be confused with crystal isomorphism) is readily computed even though, as a subsumption of the graph isomorphism problem, it is apparently computationally difficult in general. | 3 | Analytical Chemistry |
In synthetic allosteric systems there are typically two or more conformers with stability differences due to strain contributions. Positive cooperativity for example results from increased binding of a substrate A to a conformer C2 which is produced by binding of an effector molecule E. If the conformer C2 has a similar stability as another equilibrating conformer C1 a fit induced by the substrate A will lead to binding of A to C2 also in absence of the effector E. Only if the stability of the conformer C2 is significantly smaller, meaning that in absence of an effector E the population of C2 is much smaller than that of C1, the ratio K2/K1 which measures the efficiency of the allosteric signal will increase. The ratio K2/K1 can be related directly to the strain energy difference between the conformers C1 and C2; if it is small higher concentrations of A will directly bind to C2 and make the effector E inefficient. In addition, the response time of such allosteric switches depends on the strain of the conformer interconversion transitions state. | 4 | Stereochemistry |
In thermodynamics, adiabatic changes are those that do not increase the entropy. They occur slowly in comparison to the other characteristic timescales of the system of interest and allow heat flow only between objects at the same temperature. For isolated systems, an adiabatic change allows no heat to flow in or out. | 7 | Physical Chemistry |
Stone wool or rock wool is a spun mineral fibre used as an insulation product and in hydroponics. It is manufactured in a blast furnace fed with diabase rock which contains very low levels of metal oxides. The resultant slag is drawn off and spun to form the rock wool product. Very small amounts of metals are also produced which are an unwanted by-product. | 8 | Metallurgy |
Scientists found a variety of rock types in the Columbia Hills, and they placed them into six different categories. The six are: Adirondack, Clovis, Wishstone, Peace, Watchtower, Backstay, and Independence. They are named after a prominent rock in each group. Their chemical compositions, as measured by APXS, are significantly different from each other. Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids.
They are enriched in the elements phosphorus, sulfur, chlorine, and bromine—all of which can be carried around in water solutions. The Columbia Hills' rocks contain basaltic glass, along with varying amounts of olivine and sulfates.
The olivine abundance varies inversely with the amount of sulfates. This is exactly what is expected because water destroys olivine but helps to produce sulfates.
The Clovis group is especially interesting because the Mossbauer spectrometer (MB) detected goethite in it. Goethite forms only in the presence of water, so its discovery is the first direct evidence of past water in the Columbia Hills's rocks. In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence, although the rocks probably once contained much olivine. Olivine is a marker for the lack of water because it easily decomposes in the presence of water. Sulfate was found, and it needs water to form.
Wishstone contained a great deal of plagioclase, some olivine, and anhydrate (a sulfate). Peace rocks showed sulfur and strong evidence for bound water, so hydrated sulfates are suspected. Watchtower class rocks lack olivine consequently they may have been altered by water. The Independence class showed some signs of clay (perhaps montmorillonite a member of the smectite group). Clays require fairly long term exposure to water to form.
One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron. Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe form, which would happen if water had been present.
Towards the middle of the six-year mission (a mission that was supposed to last only 90 days), large amounts of pure silica were found in the soil. The silica could have come from the interaction of soil with acid vapors produced by volcanic activity in the presence of water or from water in a hot spring environment.
After Spirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."
In summary, Spirit found evidence of slight weathering on the plains of Gusev, but no evidence that a lake was there. However, in the Columbia Hills there was clear evidence for a moderate amount of aqueous weathering. The evidence included sulfates and the minerals goethite and carbonates which only form in the presence of water. It is believed that Gusev crater may have held a lake long ago, but it has since been covered by igneous materials. All the dust contains a magnetic component which was identified as magnetite with some titanium. Furthermore, the thin coating of dust that covers everything on Mars is the same in all parts of the planet. | 9 | Geochemistry |
Efforts to elucidate the excited states of diphosphenes is important and valuable to realize the application of PP double bonds in molecular electronics. In triplets trans-HPPH, the P-P bond length is predicted to be 2.291 Å. It is not only longer than the P-P double bond in ground state trans-bis(2,4,6-tri-tert-butylphenyl)diphosphene, but also longer than that of P-P single bond in HPPH. Calculation of the dihedral angle of trans-HPPH suggests that it is almost 90 degree, which means the formation of and P-P bonds is forbidden and σ bond is enhanced. | 0 | Organic Chemistry |
Solid materials are formed from densely packed atoms, which interact intensely. These interactions produce the mechanical (e.g. hardness and elasticity), thermal, electrical, magnetic and optical properties of solids. Depending on the material involved and the conditions in which it was formed, the atoms may be arranged in a regular, geometric pattern (crystalline solids, which include metals and ordinary water ice) or irregularly (an amorphous solid such as common window glass).
The bulk of solid-state physics, as a general theory, is focused on crystals. Primarily, this is because the periodicity of atoms in a crystal — its defining characteristic — facilitates mathematical modeling. Likewise, crystalline materials often have electrical, magnetic, optical, or mechanical properties that can be exploited for engineering purposes.
The forces between the atoms in a crystal can take a variety of forms. For example, in a crystal of sodium chloride (common salt), the crystal is made up of ionic sodium and chlorine, and held together with ionic bonds. In others, the atoms share electrons and form covalent bonds. In metals, electrons are shared amongst the whole crystal in metallic bonding. Finally, the noble gases do not undergo any of these types of bonding. In solid form, the noble gases are held together with van der Waals forces resulting from the polarisation of the electronic charge cloud on each atom. The differences between the types of solid result from the differences between their bonding. | 8 | Metallurgy |
A prominent enol ether is phosphoenol pyruvate.
The enzyme chorismate mutase catalyzes the Claisen rearrangement of the enol ether called chorismate to prephenate, an intermediate in the biosynthesis of phenylalanine and tyrosine.
Batyl alcohol and related glycyl ethers are susceptible to dehydrogenation catalyzed unsaturases to give the vinyl ethers called plasmalogens: | 0 | Organic Chemistry |
Molecular recognition takes place in a noisy, crowded biological environment and the recognizer often has to cope with the task of selecting its target among a variety of similar competitors. For example, the ribosome has to select the correct tRNA that matches the mRNA codon among many structurally similar tRNAs. If the recognizer and its correct target match perfectly like a lock and a key, then the binding probability will be high since no deformation is required upon binding. At the same time, the recognizer might also bind to a competitor with a similar structure with high probability. Introducing an energy barrier, in particular, structural mismatch between the recognizer (lock) and the key, reduces the binding probability to the correct target but reduces even more the binding probability to a similar wrong target and thus improves the specificity. Yet, introducing too much deformation drastically reduces binding probability to the correct target. Therefore, the optimal balance between maximizing the correct binding probability and minimizing the incorrect binding probability is achieved when the recognizer is slightly off target. This suggests that conformational changes during molecular recognition processes, such as the induced fit mechanism, are advantageous for enhancing the specificity of recognition. Such conformational changes may be fine-tuned by mutations that affect the mechanical response of the recognizer, also at positions far from the binding site. | 1 | Biochemistry |
He began his professional career in 1954 as a professor of Applied Physics at Waseda University in Tokyo. He held this position until 1960 when he came to the United States on an exchange program with Duke University. At Duke, he taught polymer rheology. In the early 1960s Tokita joined the U. S. Rubber Company in New Jersey, later Uniroyal, working there for 30 years on elastomer processing. He later joined Uniroyal Goodrich Tire Company in Akron in a research role. He joined Cabot Corporation in Billerica in 1990. During his career he produced 9 U.S. Patents. His most cited scientific article treated the subject of morphology formation in elastomer blends. | 7 | Physical Chemistry |
Subsets and Splits