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A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.
A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics.
Weak acids and bases undergo reversible reactions. For example, carbonic acid:
: HCO + HO ⇌ HCO + HO.
The concentrations of reactants and products in an equilibrium mixture are determined by the analytical concentrations of the reagents (A and B or C and D) and the equilibrium constant, K. The magnitude of the equilibrium constant depends on the Gibbs free energy change for the reaction. So, when the free energy change is large (more than about 30 kJ mol), the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although small amounts of the reactants are still expected to be present in the reacting system. A truly irreversible chemical reaction is usually achieved when one of the products exits the reacting system, for example, as does carbon dioxide (volatile) in the reaction
: CaCO + 2HCl → CaCl + HO + CO↑ + HO | 7 | Physical Chemistry |
Sand rammer consists of calibrated sliding weight actuated by cam, a shallow cup to accommodate specimen tube below ram head, a specimen stripper to strip compacted specimen out of specimen tube, a specimen tube to prepare the standard specimen of 50 mm diameter by 50 mm height or 2 inch diameter by 2 inch height for an AFS standard specimen. | 8 | Metallurgy |
A mixing chamber where a mechanical agitator brings in intimate contact the feed solution and the solvent to carry out the transfer of solute(s). The mechanical agitator is equipped with a motor which drives a mixing and pumping turbine. This turbine draws the two phases from the settlers of the adjacent stages, mixes them, and transfers this emulsion to the associated settler.
The mixer may consists of one or multiple stages of mixing tanks. Common laboratory mixers consist of a single mixing stage, whereas industrial scale copper mixers may consist of up to three mixer stages where each stage performs a combined pumping and mixing action. Use of multiple stages allows a longer reaction time and also minimizes the short circuiting of unreacted material through the mixers. | 3 | Analytical Chemistry |
The main components of an XPS system are the source of X-rays, an ultra-high vacuum (UHV) chamber with mu-metal magnetic shielding, an electron collection lens, an electron energy analyzer, an electron detector system, a sample introduction chamber, sample mounts, a sample stage with the ability to heat or cool the sample, and a set of stage manipulators.
The most prevalent electron spectrometer for XPS is the hemispherical electron analyzer. They have high energy resolution and spatial selection of the emitted electrons. Sometimes, however, much simpler electron energy filters - the cylindrical mirror analyzers are used, most often for checking the elemental composition of the surface. They represent a trade-off between the need for high count rates and high angular/energy resolution. This type consists of two co-axial cylinders placed in front of the sample, the inner one being held at a positive potential, while the outer cylinder is held at a negative potential. Only the electrons with the right energy can pass through this setup and are detected at the end. The count rates are high but the resolution (both in energy and angle) is poor.
Electrons are detected using electron multipliers: a single channeltron for single energy detection, or arrays of channeltrons and microchannel plates for parallel acquisition. These devices consists of a glass channel with a resistive coating on the inside. A high voltage is applied between the front and the end. An incoming electron is accelerated to the wall, where it removes more electrons, in such a way that an electron avalanche is created, until a measurable current pulse is obtained. | 7 | Physical Chemistry |
The value is important because it helps determine the Stoichiometry of a system for example in polyurethanes. The value may also be used to calculate equivalent weight and if the functionality is known, the molecular weight also. | 3 | Analytical Chemistry |
A number of microbial metabolisms fractionate sulfur isotopes in distinctive ways, and the sulfur isotopic fingerprints of these metabolisms can be preserved in minerals and ancient organic matter. By measuring the sulfur isotopic composition of these preserved materials, scientists can reconstruct ancient biological processes and the environments where they occurred. δS values in the geologic record have been inferred to reveal the history of microbial sulfate reduction and sulfide oxidation. Paired δS and ΔS records have also been used to show ancient microbial sulfur disproportionation.
Microbial dissimilatory sulfate reduction (MSR), an energy-yielding metabolism performed by bacteria in anoxic environments, is associated with an especially large fractionation factor. The observed ε values range from 0 to −65.6‰. Many factors influence the size of this fractionation, including sulfate reduction rate, sulfate concentration and transport, availability of electron donors and other nutrients, and physiological differences like protein expression. Sulfide produced through MSR may then go on to form the mineral pyrite, preserving the S-depleted fingerprint of MSR in sedimentary rocks. Many studies have investigated the δS values of ancient pyrite in order to understand past biological and environmental conditions. For example, pyrite δS records have been used to reconstruct shifts in primary productivity levels, changing ocean oxygen content, and glacial-interglacial changes in sea level and weathering. Some studies compare sulfur isotopes in pyrite to a second sulfur-containing material, like dissolved sulfate or preserved organic matter. Comparing pyrite to another material gives a fuller picture of how sulfur moved through ancient environments: it provides clues about the size of ancient ε values and the environmental conditions controlling MSR fractionation of sulfur isotopes. | 9 | Geochemistry |
Galvanized steel can last for many decades if other supplementary measures are maintained, such as paint coatings and additional sacrificial anodes. Corrosion in non-salty environments is caused mainly by levels of sulfur dioxide in the air. | 8 | Metallurgy |
William Frohring was brought on as part of the team with Gerstenberger and Ruh, and eventually became chief chemist on the project. The group's formula was based on diluted skimmed milk, with lactose and potassium chloride added to reach the human milk level. Among their novel contributions was to use mixtures of fats and oils rather than cream to duplicate human milk fat.
Experimental batches of SMA distributed to local pediatricians were well received. The group began getting orders for more. Frohring stepped forward as the business leader of the group. His idea was to give the patent of the formula to Babies Dispensary and Children's Hospital, and license the manufacture to Telling-Belle Vernon Dairy.
By 1919, Frohring was director of the laboratory at Telling-Belle Vernon Dairy, and had invented several pieces of laboratory and dairy processing equipment. In 1921, he was made a director and placed in charge of Laboratory Products Company, the company's new subsidiary to manufacture SMA, located in Mason, Michigan.
Laboratory Products Company diversified into the development of research into other newly identified biochemicals. Frohring recruited Albert Fredrick Ottomar Germann to study carotene. The company went on to become the world's major supplier of carotene.
As an employee of the dairy, Frohring accumulated patents on a process for the production of soluble casein, an improved process for lactose production, a vitamin C concentrate from orange and tomato juice for addition to SMA, and a formulation called "Frohs Malted Chocolate Milk". The company was renamed the SMA Corporation, and added carotene concentrate, refined from palm oil, to its product line. Frohring set up a company to process the palm oil. He made his younger brother Paul (formerly sales manager for SMA Corporation), president and general manager. In 1939, SMA, its subsidiary, and all rights to the infant formula were purchased by American Home Products Corporation (AHPC). Frohring stayed on as president of the SMA Corporation, now owned by AHPC. He later became a director of AHPC. | 7 | Physical Chemistry |
Under the split gene theory, an exon is defined by an ORF. It requires a mechanism to recognize an ORF to have originated. As an ORF is defined by a contiguous coding sequence bounded by stop codons, these stop codon ends had to be recognized by the exon-intron gene recognition system. This system could have defined the exons by the presence of a stop codon at the ends of ORFs, which should be included within the ends of the introns and eliminated by the splicing process. Thus, the introns should contain a stop codon at their ends, which would be part of the splice junction sequences.
If this hypothesis was true, the split genes of today's living organisms should contain stop codons exactly at the ends of introns. When Senapathy tested this hypothesis in the splice junctions of eukaryotic genes, he found that the vast majority of splice junctions did contain a stop codon at the end of each intron, outside of the exons. In fact, these stop codons were found to form the “canonical” GT:AG splicing sequence, with the three stop codons occurring as part of the strong consensus signals. Thus, the basic split gene theory for the origin of introns and the split gene structure led to the understanding that the splice junctions originated from the stop codons.
Sequence data for only about 1,000 exon-intron junctions were available when Senapathy thought about this question. He took the data for 1,030 splice junction sequences (donors and acceptors) and counted the codons occurring at
each of the 7- base positions in the donor signal sequence [CAG:GTGAGT] and each of the possible 2-base positions in the acceptor signal [CAG:G] from the GenBank database. He found that the stop codons occurred at high frequency only at the 5th base position in the donor signal and the first base position in the acceptor signal. These positions are the* start of the intron (in fact, one base after the start) and at the end of the intron, as Senapathy had predicted. The codon counts at only these positions are shown. Even when the codons at these positions were not stop
codons, 70% of them began with the first two bases of the stop codons TA and TG [TAT = 75; TAC = 59; TGT = 70].
All three stop codons (TGA, TAA and TAG) were found after one base (G) at the start of introns. These stop codons are shown in the consensus canonical donor splice junction as AG:GT(A/G)GGT, wherein the TAA and TGA are the stop codons, and the additional TAG is also present at this position. Besides the codon CAG, only TAG, which is a stop codon, was found at the ends of introns. The canonical acceptor splice junction is shown as (C/T)AG:GT, in which TAG is the stop codon. These consensus sequences clearly show the presence of the stop codons at the ends of introns bordering the exons in all eukaryotic genes, thus providing a strong corroboration for the split gene theory. Nirenberg again stated that these observations fully supported the split gene theory for the origin of splice junction sequences from stop codons.
Soon after the discovery of introns by Philip Sharp and Richard Roberts, it became known that mutations within splice junctions could lead to diseases. Senapathy showed that mutations in the stop codon bases (canonical bases) caused more diseases than the mutations in non-canonical bases. | 1 | Biochemistry |
Ionic selectivity is defined to evaluate the performance of a nano-channel for ionic flow control. Ionic selectivity is the ratio of the difference in currents of majority and minority carriers to the total current carried by both positive and negative ions, . For a nanochannel with perfect control over cation and anion, the selectivity is unity. For a nanochannel without ionic flow control, the selectivity is zero. | 7 | Physical Chemistry |
In chemical thermodynamics, activity (symbol ) is a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution. The term "activity" in this sense was coined by the American chemist Gilbert N. Lewis in 1907.
By convention, activity is treated as a dimensionless quantity, although its value depends on customary choices of standard state for the species. Activity depends on temperature, pressure and composition of the mixture, among other things. For gases, the activity is the effective partial pressure, and is usually referred to as fugacity.
The difference between activity and other measures of concentration arises because the interactions between different types of molecules in non-ideal gases or solutions are different from interactions between the same types of molecules. The activity of an ion is particularly influenced by its surroundings.
Equilibrium constants should be defined by activities but, in practice, are often defined by concentrations instead. The same is often true of equations for reaction rates. However, there are circumstances where the activity and the concentration are significantly different and, as such, it is not valid to approximate with concentrations where activities are required. Two examples serve to illustrate this point:
*In a solution of potassium hydrogen iodate KH(IO) at 0.02 M the activity is 40% lower than the calculated hydrogen ion concentration, resulting in a much higher pH than expected.
*When a 0.1 M hydrochloric acid solution containing methyl green indicator is added to a 5 M solution of magnesium chloride, the color of the indicator changes from green to yellow—indicating increasing acidity—when in fact the acid has been diluted. Although at low ionic strength (< 0.1 M) the activity coefficient approaches unity, this coefficient can actually increase with ionic strength in a high ionic strength regime. For hydrochloric acid solutions, the minimum is around 0.4 M. | 7 | Physical Chemistry |
Acid catalysis is mainly used for organic chemical reactions. Many acids can function as sources for the protons. Acid used for acid catalysis include hydrofluoric acid (in the alkylation process), phosphoric acid, toluenesulfonic acid, polystyrene sulfonate, heteropoly acids, zeolites.
Strong acids catalyze the hydrolysis and transesterification of esters, e.g. for processing fats into biodiesel. In terms of mechanism, the carbonyl oxygen is susceptible to protonation, which enhances the electrophilicity at the carbonyl carbon. | 7 | Physical Chemistry |
When a compound has limited solubility in water it is common practice (in the pharmaceutical industry, for example) to determine pK values in a solvent mixture such as water/dioxane or water/methanol, in which the compound is more soluble. In the example shown at the right, the pK value rises steeply with increasing percentage of dioxane as the dielectric constant of the mixture is decreasing.
A pK value obtained in a mixed solvent cannot be used directly for aqueous solutions. The reason for this is that when the solvent is in its standard state its activity is defined as one. For example, the standard state of water:dioxane mixture with 9:1 mixing ratio is precisely that solvent mixture, with no added solutes. To obtain the pK value for use with aqueous solutions it has to be extrapolated to zero co-solvent concentration from values obtained from various co-solvent mixtures.
These facts are obscured by the omission of the solvent from the expression that is normally used to define pK, but pK values obtained in a given mixed solvent can be compared to each other, giving relative acid strengths. The same is true of pK values obtained in a particular non-aqueous solvent such a DMSO.
A universal, solvent-independent, scale for acid dissociation constants has not been developed, since there is no known way to compare the standard states of two different solvents. | 7 | Physical Chemistry |
Jeremy Munday writes that although "unexpected effects will likely occur" with global PDRC implementation, that "these structures can be removed immediately if needed, unlike methods that involve dispersing particulate matter into the atmosphere, which can last for decades." Wang et al. state that stratospheric aerosol injection "might cause potentially dangerous threats to the Earth’s basic climate operations" that may not be reversible, preferring PDRC. Zevenhoven et al. state that "instead of stratospheric aerosol injection (SAI), cloud brightening or a large number of mirrors in the sky (“sunshade geoengineering”) to block out or reflect incoming (short-wave, SW) solar irradiation, long-wavelength (LW) thermal radiation can be selectively emitted and transferred through the atmosphere into space". | 7 | Physical Chemistry |
Of the various methods of labeling biomolecules, fluorescent labels are advantageous in that they are highly sensitive even at low concentration and non-destructive to the target molecule folding and function.
Green fluorescent protein is a naturally occurring fluorescent protein from the jellyfish Aequorea victoria that is widely
used to tag proteins of interest. GFP emits a photon in the green region of the light spectrum when excited by the absorption of light. The chromophore consists of an oxidized tripeptide -Ser^65-Tyr^66-Gly^67 located within a β barrel. GFP catalyzes the oxidation and
only requires molecular oxygen. GFP has been modified by changing the wavelength of light absorbed to include other colors of fluorescence. YFP or yellow fluorescent protein, BFP or blue fluorescent protein, and CFP or cyan fluorescent protein are examples of GFP variants. These variants are produced by the genetic engineering of the GFP gene.
Synthetic fluorescent probes can also be used as fluorescent labels. Advantages of these labels include a smaller size with more variety in color. They can be used to tag proteins of interest more selectively by various methods including chemical recognition-based labeling, such as utilizing metal-chelating peptide tags, and biological recognition-based labeling utilizing enzymatic reactions. However, despite their wide array of excitation and emission wavelengths as well as better stability, synthetic probes tend to be toxic to the cell and so are not generally used in cell imaging studies.
Fluorescent labels can be hybridized to mRNA to help visualize interaction and activity, such as mRNA localization. An antisense strand labeled with the fluorescent probe is attached to a single mRNA strand, and can then be viewed during cell development to see the movement of mRNA within the cell. | 1 | Biochemistry |
The Liebermann reagent named after Hungarian chemist Leo Liebermann (1852-1926) is used as a simple spot-test to presumptively identify alkaloids as well as other compounds. It is composed of a mixture of potassium nitrite and concentrated sulfuric acid. 1 g of potassium nitrite is used for every 10 mL of sulfuric acid. Potassium nitrite may also be substituted by sodium nitrite. It is used to test for cocaine, morphine, PMA and PMMA.
The test is performed by scraping off a small amount of the substance and adding a drop of the reagent (which is initially clear and colorless). The results are analyzed by viewing the color of the resulting mixture, and by the time taken for the change in color to become apparent. | 3 | Analytical Chemistry |
In jaundice owing to hemolysis (prehepatic, or hemolytic, jaundice), the pathophysiology is that overproduction of bilirubin from the extravascular or intravascular hemolysis overwhelms the capacity of the liver to excrete it. The bilirubin present in the plasma is largely unconjugated in this setting as they haven't been taken up and conjugated by the liver. In this case, total serum bilirubin increases while the ratio of direct bilirubin to indirect bilirubin remains 96 to 4 as up to 96%-99% of bilirubin in the bile are conjugated mentioned above. | 1 | Biochemistry |
Bioleaching is in general simpler and, therefore, cheaper to operate and maintain than traditional processes, since fewer specialists are needed to operate complex chemical plants. And low concentrations are not a problem for bacteria because they simply ignore the waste that surrounds the metals, attaining extraction yields of over 90% in some cases. These microorganisms actually gain energy by breaking down minerals into their constituent elements. The company simply collects the ions out of the solution after the bacteria have finished.
Bioleaching can be used to extract metals from low concentration ores such as gold that are too poor for other technologies. It can be used to partially replace the extensive crushing and grinding that translates to prohibitive cost and energy consumption in a conventional process. Because the lower cost of bacterial leaching outweighs the time it takes to extract the metal.
High concentration ores, such as copper, are more economical to smelt rather bioleach due to the slow speed of the bacterial leaching process compared to smelting. The slow speed of bioleaching introduces a significant delay in cash flow for new mines. Nonetheless, at the largest copper mine of the world, Escondida in Chile the process seems to be favorable.
Economically it is also very expensive and many companies once started can not keep up with the demand and end up in debt. | 8 | Metallurgy |
2-O-methylation refers to the methylation of the 2 hydroxyl group of the ribose within an RNA nucleotide. 2'-O-methylation is found in the five-prime cap of mRNAs in higher eukaryotes. It is involved in differentiating between self and non-self mRNA. Without the 2′-O-methylation mark the immune system triggers higher levels of type 1 interferon activity. While this modification is not currently known to be a response to any particular phenomenon, not everything is fully understood about the mechanisms of this modification due to the difficulty of studying small RNA molecules. However, the effect on RNA stability this modification has could be regulated to modulate transcript levels. | 1 | Biochemistry |
A-values help predict the conformation of cyclohexane rings. The most stable conformation will be the one which has the substituent or substituents equatorial. When multiple substituents are taken into consideration, the conformation where the substituent with the largest A-value is equatorial is favored.
The utility of A-values can be generalized for use outside of cyclohexane conformations. A-values can help predict the steric effect of a substituent. In general, the larger a substituents A-value, the larger the steric effect of that substituent. A methyl group has an A-value of 1.74 while tert-butyl group has an A-value of ~5. Because the A-value of tert-butyl is higher, tert'-butyl has a larger steric effect than methyl. This difference in steric effects can be used to help predict reactivity in chemical reactions. | 4 | Stereochemistry |
A nearly universal rule in copyediting of articles for medical journals and other health science publications is that abbreviations and acronyms must be expanded at first use, to provide a glossing type of explanation. Typically no exceptions are permitted except for small lists of especially well known terms (such as DNA or HIV). Although readers with high subject-matter expertise do not need most of these expansions, those with intermediate or (especially) low expertise are appropriately served by them.
One complication that gene and protein symbols bring to this general rule is that they are not, accurately speaking, abbreviations or acronyms, despite the fact that many were originally coined via abbreviating or acronymic etymology. They are pseudoacronyms (as SAT and KFC also are) because they do not "stand for" any expansion. Rather, the relationship of a gene symbol to the gene name is functionally the relationship of a nickname to a formal name (both are complete identifiers)—it is not the relationship of an acronym to its expansion. In fact, many official gene symbol–gene name pairs do not even share their initial-letter sequences (although some do). Nevertheless, gene and protein symbols "look just like" abbreviations and acronyms, which presents the problem that "failing" to "expand" them (even though it is not actually a failure and there are no true expansions) creates the appearance of violating the spell-out-all-acronyms rule.
One common way of reconciling these two opposing forces is simply to exempt all gene and protein symbols from the glossing rule. This is certainly fast and easy to do, and in highly specialized journals, it is also justified because the entire target readership has high subject matter expertise. (Experts are not confused by the presence of symbols (whether known or novel) and they know where to look them up online for further details if needed.) But for journals with broader and more general target readerships, this action leaves the readers without any explanatory annotation and can leave them wondering what the apparent-abbreviation stands for and why it was not explained. Therefore, a good alternative solution is simply to put either the official gene name or a suitable short description (gene alias/other designation) in parentheses after the first use of the official gene/protein symbol. This meets both the formal requirement (the presence of a gloss) and the functional requirement (helping the reader to know what the symbol refers to). The same guideline applies to shorthand names for sequence variations; AMA says, "In general medical publications, textual explanations should accompany the shorthand terms at first mention." Thus "188del11" is glossed as "an 11-bp deletion at nucleotide 188." This corollary rule (which forms an adjunct to the spell-everything-out rule) often also follows the "abbreviation-leading" style of expansion that is becoming more prevalent in recent years. Traditionally, the abbreviation always followed the fully expanded form in parentheses at first use. This is still the general rule. But for certain classes of abbreviations or acronyms (such as clinical trial acronyms [e.g., ECOG or standardized polychemotherapy regimens [e.g., CHOP), this pattern may be reversed, because the short form is more widely used and the expansion is merely parenthetical to the discussion at hand. The same is true of gene/protein symbols. | 1 | Biochemistry |
The emission of electromagnetic radiation (EMR) during plastic deformation and crack propagation in metals and rocks has been studied. The EMR emissions from metals and alloys have also been explored and confirmed. Molotskii presented a dislocation mechanism for this type of EMR emission. In 2005, Srilakshmi and Misra reported an additional phenomenon of secondary EMR during plastic deformation and crack propagation in uncoated and metal-coated metals and alloys.
EMR during the micro-plastic deformation and crack propagation from several metals and alloys and transient magnetic field generation during necking in ferromagnetic metals were reported by Misra (1973–75), which have been confirmed and explored by several researchers. Tudik and Valuev (1980) were able to measure the EMR frequency during tensile fracture of iron and aluminum in the region 100 THz by using photomultipliers. Srilakshmi and Misra (2005a) also reported an additional phenomenon of secondary electromagnetic radiation in uncoated and metal-coated metals and alloys. If a solid material is subjected to stresses of large amplitudes, which can cause plastic deformation and fracture, emissions such as thermal, acoustic, ions, and exo-emissions occur. | 5 | Photochemistry |
Oxanorbornadiene (or another activated alkene) reacts with azides, giving triazoles as a product. However, these product triazoles are not aromatic as they are in the CuAAC or SPAAC reactions, and as a result are not as stable. The activated double bond in oxanobornadiene makes a triazoline intermediate that subsequently spontaneously undergoes a retro Diels-alder reaction to release furan and give 1,2,3- or 1,4,5-triazoles. Even though this reaction is slow, it is useful because oxabornodiene is relatively simple to synthesize. The reaction is not, however, entirely chemoselective. | 0 | Organic Chemistry |
One of the challenges in studying abiogenesis is that the system of reproduction and metabolism utilized by all extant life involves three distinct types of interdependent macromolecules (DNA, RNA, and proteins). This suggests that life could not have arisen in its current form, which has led researchers to hypothesize mechanisms whereby the current system might have arisen from a simpler precursor system. American molecular biologist Alexander Rich was the first to posit a coherent hypothesis on the origin of nucleotides as precursors of life. In an article he contributed to a volume issued in honor of Nobel-laureate physiologist Albert Szent-Györgyi, he explained that the primitive Earth's environment could have produced RNA molecules (polynucleotide monomers) that eventually acquired enzymatic and self-replicating functions.
Further concept of RNA as a primordial molecule can be found in papers by Francis Crick and Leslie Orgel, as well as in Carl Woeses 1967 book The Genetic Code'. Hans Kuhn in 1972 laid out a possible process by which the modern genetic system might have arisen from a nucleotide-based precursor, and this led Harold White in 1976 to observe that many of the cofactors essential for enzymatic function are either nucleotides or could have been derived from nucleotides. He proposed a scenario whereby the critical electrochemistry of enzymatic reactions would have necessitated retention of the specific nucleotide moieties of the original RNA-based enzymes carrying out the reactions, while the remaining structural elements of the enzymes were gradually replaced by protein, until all that remained of the original RNAs were these nucleotide cofactors, "fossils of nucleic acid enzymes". The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on how recent observations of the catalytic properties of various forms of RNA fit with this hypothesis. | 9 | Geochemistry |
RNA activation (RNAa) is a small RNA-guided and Argonaute (Ago)-dependent gene regulation phenomenon in which promoter-targeted short double-stranded RNAs (dsRNAs) induce target gene expression at the transcriptional/epigenetic level. RNAa was first reported in a 2006 PNAS paper by Li et al. who also coined the term "RNAa" as a contrast to RNA interference (RNAi) to describe such gene activation phenomenon. dsRNAs that trigger RNAa have been termed small activating RNA (saRNA). Since the initial discovery of RNAa in human cells, many other groups have made similar observations in different mammalian species including human, non-human primates, rat and mice, plant and C. elegans, suggesting that RNAa is an evolutionarily conserved mechanism of gene regulation.
RNAa can be generally classified into two categories: exogenous and endogenous. Exogenous RNAa is triggered by artificially designed saRNAs which target non-coding sequences such as the promoter and the 3’ terminus of a gene and these saRNAs can be chemically synthesized or expressed as short hairpin RNA (shRNA). Whereas for endogenous RNAa, upregulation of gene expression is guided by naturally occurring endogenous small RNAs such as miRNA in mammalian cells and C. elegans, and 22G RNA in C. elegans. | 1 | Biochemistry |
The reverse anomeric effect, proposed in 1965 by R. U. Lemieux, is the tendency for electropositive groups at the anomeric position to be oriented equatorially. Original publication reported this phenomenon with N-(2,3,4,6-tetra-O-acetyl-α--glucopyranosyl)-4-methylpyridinium bromide. However, further studies have shown the effect to be a solvation and steric issue. It is accepted that there is no generalized reverse anomeric effect. | 0 | Organic Chemistry |
In contrast to benzene ring, pyridine efficiently supports several nucleophilic substitutions. The reason for this is relatively lower electron density of the carbon atoms of the ring. These reactions include substitutions with elimination of a hydride ion and elimination-additions with formation of an intermediate aryne configuration, and usually proceed at the 2- or 4-position.
Many nucleophilic substitutions occur more easily not with bare pyridine but with pyridine modified with bromine, chlorine, fluorine, or sulfonic acid fragments that then become a leaving group. So fluorine is the best leaving group for the substitution with organolithium compounds. The nucleophilic attack compounds may be alkoxides, thiolates, amines, and ammonia (at elevated pressures).
In general, the hydride ion is a poor leaving group and occurs only in a few heterocyclic reactions. They include the Chichibabin reaction, which yields pyridine derivatives aminated at the 2-position. Here, sodium amide is used as the nucleophile yielding 2-aminopyridine. The hydride ion released in this reaction combines with a proton of an available amino group, forming a hydrogen molecule.
Analogous to benzene, nucleophilic substitutions to pyridine can result in the formation of pyridyne intermediates as heteroaryne. For this purpose, pyridine derivatives can be eliminated with good leaving groups using strong bases such as sodium and potassium tert-butoxide. The subsequent addition of a nucleophile to the triple bond has low selectivity, and the result is a mixture of the two possible adducts. | 0 | Organic Chemistry |
The pzc is typically obtained by acid-base titrations of colloidal dispersions while monitoring the electrophoretic mobility of the particles and the pH of the suspension. Several titrations are required to distinguish pzc from iep, using different electrolytes (including varying the electrolyte ionic strength). Once satisfactory graphs are obtained (acid/base amount—pH, and pH—zeta potential), the pzc is established as the common intersection point (cip) of the lines. Therefore, pzc is also sometimes referred to as cip. | 7 | Physical Chemistry |
For ionic compounds, the standard enthalpy of formation is equivalent to the sum of several terms included in the Born–Haber cycle. For example, the formation of lithium fluoride,
may be considered as the sum of several steps, each with its own enthalpy (or energy, approximately):
# , the standard enthalpy of atomization (or sublimation) of solid lithium.
# , the first ionization energy of gaseous lithium.
# , the standard enthalpy of atomization (or bond energy) of fluorine gas.
# , the electron affinity of a fluorine atom.
# , the lattice energy of lithium fluoride.
The sum of these enthalpies give the standard enthalpy of formation () of lithium fluoride:
In practice, the enthalpy of formation of lithium fluoride can be determined experimentally, but the lattice energy cannot be measured directly. The equation is therefore rearranged to evaluate the lattice energy: | 7 | Physical Chemistry |
Supercritical water oxidation (SCWO) is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantage in the destruction of recalcitrant and hazardous wastes such as polychlorinated biphenyls (PCB) or per- and polyfluoroalkyl substances (PFAS). Supercritical water has a density between that of water vapor and liquid at standard conditions, and exhibits high gas-like diffusion rates along with high liquid-like collision rates. In addition, the behavior of water as a solvent is altered (in comparison to that of subcritical liquid water) - it behaves much less like a polar solvent. As a result, the solubility behavior is "reversed" so that oxygen, and organics such as chlorinated hydrocarbons become soluble in the water, allowing single-phase reaction of aqueous waste with a dissolved oxidizer. The reversed solubility also causes salts to precipitate out of solution, meaning they can be treated using conventional methods for solid-waste residuals. Efficient oxidation reactions occur at low temperature (400-650 °C) with reduced NOx production.
SCWO can be classified as green chemistry or as a clean technology. The elevated pressures and temperatures required for SCWO are routinely encountered in industrial applications such as petroleum refining and chemical synthesis.
A unique addition (mostly of academic interest) to the world of supercritical water (SCW) oxidation is generating high-pressure flames inside the SCW medium. The pioneer works on high-pressure supercritical water flames were carried out by professor EU Franck at the German University of Karlsruhe in the late 80s. The works were mainly aimed at anticipating conditions which would cause spontaneous generation of non-desirable flames in the flameless SCW oxidation process. These flames would cause instabilities to the system and its components. ETH Zurich pursued the investigation of hydrothermal flames in continuously operated reactors. The rising needs for waste treatment and destruction methods motivated a Japanese Group in the Ebara Corporation to explore SCW flames as an environmental tool. Research on hydrothermal flames has also begun at NASA Glenn Research Center in Cleveland, Ohio. | 7 | Physical Chemistry |
In ISCR, many reductive processes can take place. There are hydrogenolysis, β-elimination, hydrogenation, α-elimination, and electron transfer. The specific combination of reductive processes that actually take place in the subsurface depends on the species of contaminant that is present and also the type of reduction being used. The natural and biological processes that take place in the substratum also affect the kinds of reductive processes that are found. | 2 | Environmental Chemistry |
If MRL of some medicinal plant is not known it is calculated by the formula:
where SF is the safety factor
*MDI is the mean daily intake
*W is the body weight
*ADI is the acceptable daily intake | 2 | Environmental Chemistry |
Medications can adjust the release of brain neurotransmitters in cases of depression, anxiety disorder, schizophrenia and other mental disorders because an imbalance within neurotransmitter systems can emerge as consistent characteristics in behaviour compromising people's lives. All people have a weaker form of such imbalance in at least one of such neurotransmitter systems that make each of us distinct from one another. The impact of this weak imbalance in neurochemistry can be seen in the consistent features of behaviour in healthy people (temperament). In this sense temperament (as neuro-chemically-based individual differences) and mental illness represents varying degrees along the same continuum of neurotransmitter imbalance in neurophysiological systems of behavioural regulation.
In fact, multiple temperament traits (such as Impulsivity, sensation seeking, neuroticism, endurance, plasticity, sociability or extraversion) have been linked to brain neurotransmitters and hormone systems.
By the end of the 20th century, it became clear that the human brain operates with more than a dozen neurotransmitters and a large number of neuropeptides and hormones. The relationships between these different chemical systems are complex as some of them suppress and some of them induce each other's release during neuronal exchanges. This complexity of relationships devalues the old approach of assigning "inhibitory vs. excitatory" roles to neurotransmitters: the same neurotransmitters can be either inhibitory or excitatory depending on what system they interact with. It became clear that an impressive diversity of neurotransmitters and their receptors is necessary to meet a wide range of behavioural situations, but the links between temperament traits and specific neurotransmitters are still a matter of research. Several attempts were made to assign specific (single) neurotransmitters to specific (single) traits. For example, dopamine was proposed to be a neurotransmitter of the trait of Extraversion, noradrenaline was linked to anxiety, and serotonin was thought to be a neurotransmitter of an inhibition system. These assignments of neurotransmitter functions appeared to be an oversimplification when confronted by the evidence of much more diverse functionality.
Research groups led by Petra Netter in Germany, Lars Farde in Karolinska Institute in Sweden and Trevor Robbins in Cambridge, UK had most extensive studies of the links between temperament/personality traits or dynamical properties of behavior and groups of neurotransmitters. | 1 | Biochemistry |
N-substituted derivatives are somewhat stable. They are invoked but rarely observed as intermediates in the Mannich reaction. These N,N,N-trisubstituted hexahydro-1,3,5-triazines arise from the condensation of the amine and formaldehyde as illustrated by the route to 1,3,5-trimethyl-1,3,5-triazacyclohexane:
Although adducts generated from primary amines or ammonia are usually unstable, the hemiaminals have been trapped in a cavity. | 0 | Organic Chemistry |
Respiration occurs by both plants and animals throughout the water column, resulting in the destruction, or usage, of organic matter, but photosynthesis can only take place via photosynthetic algae in the presence of light, nutrients and CO. In well-mixed water columns plankton are evenly distributed, but a net production only occurs above the compensation depth. Below the compensation depth there is a net loss of organic matter. The total population of photosynthetic organisms cannot increase if the loss exceeds the net production.
The compensation depth between photosynthesis and respiration of phytoplankton in the ocean must be dependent on some factors: the illumination at the surface, the transparency of the water, the biological character of the plankton present, and the temperature. The compensation point was found nearer to the surface as you move closer to the coast. It is also lower in the winter seasons in the Baltic Sea according to a study that examined the compensation point of multiple photosynthetic species. The blue portion of the visible spectrum, between 455 and 495 nanometers, dominates light at the compensation depth.
A concern regarding the concept of the compensation point is it assumes that phytoplankton remain at a fixed depth throughout a 24-hour period (time frame in which compensation depth is measured), but phytoplankton experience displacement due to isopycnals moving them tens of meters. | 5 | Photochemistry |
Bis(chloromethyl) ether is an organic compound with the chemical formula (ClCH)O. It is a colourless liquid with an unpleasant suffocating odour and it is one of the chloroalkyl ethers. Bis(chloromethyl) ether was once produced on a large scale, but was found to be highly carcinogenic and thus such production has ceased. | 0 | Organic Chemistry |
When covalently attached to a surface, PPGs do not exhibit any surface-induced properties (i.e. they behave like PPGs in solution, and do not exhibit any new properties because of their proximity to a surface). Consequently, PPGs can be patterned on a surface and removed in manner analogous to lithography to create a multifunctionalized surface. This process was first reported by Solas in 1991; protected nucleotides were attached to a surface and spatially-resolved single stranded polynucleotides were generated in a step-wise “grafting from” method. In separate studies, there have been multiple reports of using PPGs to enable the selective separation of blocks within block-copolymers to expose fresh surfaces. Furthermore, this surface patterning method has since been extended to proteins. Caged etching agents (such as hydrogen fluoride protected with 4-hydroxyphenacyl) allows to etch only surfaces exposed to light. | 5 | Photochemistry |
Time-domain diffuse optical sources must have the following characteristics; emission wavelength in the optical window i.e. between 650 and 1350 nanometre (nm); a narrow full width at half maximum (FWHM), ideally a delta function; high repetition rate (>20 MHz) and finally, sufficient laser power (>1 mW) to achieve good signal to noise ratio.
In the past bulky tunable Ti:sapphire Lasers were used. They provided a wide wavelength range of 400 nm, a narrow FWHM (< 1 ps) high average power (up to 1W) and high repetition (up to 100 MHz) frequency. However, they are bulky, expensive and take a long time for wavelength swapping.
In recent years, pulsed fiber lasers based on super continuum generation have emerged. They provide a wide spectral range (400 to 2000 ps), typical average power of 5 to 10 W, a FWHM of < 10ps and a repetition frequency of tens of MHz. However, they are generally quite expensive and lack stability in super continuum generation and hence, have been limited in there use.
The most wide spread sources are the pulsed diode lasers. They have a FWHM of around 100 ps and repetition frequency of up to 100 MHz and an average power of about a few milliwatts. Even though they lack tunability, their low cost and compactness allows for multiple modules to be used in a single system. | 7 | Physical Chemistry |
Aside from its immense environmental impacts, R114, like most chlorofluoroalkanes, forms phosgene gas when exposed to a naked flame. | 2 | Environmental Chemistry |
The Laporte rule is a selection rule formally stated as follows: In a centrosymmetric environment, transitions between like atomic orbitals such as s-s, p-p, d-d, or f-f, transitions are forbidden. The Laporte rule (law) applies to electric dipole transitions, so the operator has u symmetry (meaning ungerade, odd). p orbitals also have u symmetry, so the symmetry of the transition moment function is given by the triple product u×u×u, which has u symmetry. The transitions are therefore forbidden. Likewise, d orbitals have g symmetry (meaning gerade, even), so the triple product g×u×g also has u symmetry and the transition is forbidden.
The wave function of a single electron is the product of a space-dependent wave function and a spin wave function. Spin is directional and can be said to have odd parity. It follows that transitions in which the spin "direction" changes are forbidden. In formal terms, only states with the same total spin quantum number are "spin-allowed". In crystal field theory, d-d transitions that are spin-forbidden are much weaker than spin-allowed transitions. Both can be observed, in spite of the Laporte rule, because the actual transitions are coupled to vibrations that are anti-symmetric and have the same symmetry as the dipole moment operator. | 7 | Physical Chemistry |
Pyruvate produced by glycolysis is converted to lactate. This reaction is catalysed by the enzyme lactate dehydrogenase (LDHA).
pyruvate + NADH + H → lactate + NAD | 1 | Biochemistry |
Trimethylsilyl azide , and tributyltin azide , have all been used, including enantioselective modifications of the reaction are also known. Aminoazides are accessible by the epoxide and aziridine ring cleavage, respectively. | 0 | Organic Chemistry |
For any point in a unit cell, given by fractional coordinates, one can apply a symmetry operation to the point. In some cases it will move to new coordinates, while in other cases the point will remain unaffected. For example, reflecting across a mirror plane will switch all the points left and right of the mirror plane, but points exactly on the mirror plane itself will not move. We can test every symmetry operation in the crystals point group and keep track of whether the specified point is invariant under the operation or not. The (finite) list of all symmetry operations which leave the given point invariant taken together make up another group, which is known as the site symmetry group' of that point. By definition, all points with the same site symmetry group, or a conjugate site symmetry group, are assigned the same Wyckoff position.
The Wyckoff positions are designated by a letter, often preceded by the number of positions that are equivalent to a given position with that letter, in other words the number of positions in the unit cell to which the given position is moved by applying all the elements of the space group. For instance, 2a designates the positions left where they are by a certain subgroup, and indicates that other symmetry elements move the point to a second position in the unit cell. The letters are assigned in alphabetical order with earlier letters indicating positions with fewer equivalent positions, or in other words with larger site symmetry groups. Some designations may apply to a finite number of points per unit cell (such as inversion points, improper rotation points, and intersections of rotation axes with mirror planes or other rotation axes), but other designations apply to infinite sets of points (such as generic points on rotation axes, screw axes, mirror planes, and glide planes, as well as general points not lying on any symmetry axis or plane).
Wyckoff positions are used in calculations of crystal properties. There are two types of positions: general and special.
*General positions are left invariant only for the identity operation (E). Each space group has only one general position.
*Special positions are left invariant by the identity operation and at least one other operation of the space group.
General positions have a site symmetry of the trivial group and all correspond to the same Wyckoff position. Special positions have a non-trivial site symmetry group.
For a particular space group, one can check the Wyckoff positions using International Tables of Crystallography. The table presents the multiplicity, Wyckoff letter and site symmetry for Wyckoff positions. | 3 | Analytical Chemistry |
David Phillips, (born 3 December 1939) is a British chemist specialising in photochemistry and lasers, and was president of the Royal Society of Chemistry from 2010 to 2012. | 5 | Photochemistry |
Level -1 fragments are used to help cloning large level 0 modules. To clone level -1 fragments, blunt-end cloning with restriction ligation can be used. The vector used in cloning level -1 fragments cannot contain Type IIS restriction site BpiI that is used for the following assembly step. Moreover, the vector should also have a different selection marker from the destination vector in next assembly step, for example, if spectinomycin resistance is used in level 0 modules, level -1 fragments should have another antibiotic resistance like ampicillin. | 1 | Biochemistry |
Capnellene is a naturally occurring tricyclic hydrocarbon derived from Capnella imbricata, a species of soft coral found in Indonesia. Since the 1970s, capnellene has been targeted for synthesis by numerous investigators due to its stereochemistry, functionality, and the interesting geometry of the carbon skeleton. Many alcohol derivatives of capnellene have demonstrated potential as a chemotherapeutic agent with antibacterial, anti-inflammatory and anti-tumor properties. | 0 | Organic Chemistry |
A recent example of the Fürst-Plattner rule can be seen from Chrisman et al. where limonene is epoxidized to give a 1:1 mixture of diastereomers. Exposure to a nitrogen nucleophile in water at reflux provides only one ring opened product in 75-85% ee. | 4 | Stereochemistry |
Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface.
The contact angle (θ), as seen in Figure 1, is the angle at which the liquid–vapor interface meets the solid–liquid interface. The contact angle is determined by the balance between adhesive and cohesive forces. As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability.
A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface. Contact angles greater than 90° (high contact angle) generally mean that wetting of the surface is unfavorable, so the fluid will minimize contact with the surface and form a compact liquid droplet.
For water, a wettable surface may also be termed hydrophilic and a nonwettable surface hydrophobic. Superhydrophobic surfaces have contact angles greater than 150°, showing almost no contact between the liquid drop and the surface. This is sometimes referred to as the "Lotus effect". The table describes varying contact angles and their corresponding solid/liquid and liquid/liquid interactions. For nonwater liquids, the term lyophilic is used for low contact angle conditions and lyophobic is used when higher contact angles result. Similarly, the terms omniphobic and omniphilic apply to both polar and apolar liquids. | 7 | Physical Chemistry |
Photosensitization is a process of transferring the energy of absorbed light. After absorption, the energy is transferred to the (chosen) reactants. This is part of the work of photochemistry in general. In particular this process is commonly employed where reactions require light sources of certain wavelengths that are not readily available.
For example, mercury absorbs radiation at 1849 and 2537 angstroms, and the source is often high-intensity mercury lamps. It is a commonly used sensitizer. When mercury vapor is mixed with ethylene, and the compound is irradiated with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.
Cadmium; some of the noble gases, for example xenon; zinc; benzophenone; and a large number of organic dyes, are also used as sensitizers.
Photosensitisers are a key component of photodynamic therapy used to treat cancers. | 7 | Physical Chemistry |
Nucleophilic substitution reactions occur when a nucleophilic molecule attacks a positive or partially positive electrophilic center by breaking and creating a new bond. S1 and S2 are two different mechanisms for nucleophilic substitution, and S1 involves a carbocation intermediate. In S1, a leaving group is broken off to create a carbocation reaction intermediate. Then, a nucleophile attacks and forms a new bond with the carbocation intermediate to form the final, substituted product, as shown in the reaction of 2-bromo-2-methylpropane to form 2-methyl-2-propanol.
In this reaction, is the formed carbocation intermediate to form the alcohol product. | 7 | Physical Chemistry |
Methods for the stereoselective synthesis of cyclopropanes from diazocarbonyl compounds and olefins have relied on either the use of pre-formed chiral rhodium catalysts or chiral auxiliaries on the diazocarbonyl compound. For example, Rh[S-DOSP] is a highly effective catalyst for the enantioselective cyclopropanation of alkenes.
Chiral auxiliaries derived from readily available chiral alcohols (such as pantolactone) may be used for diastereoselective cyclopropanations with diazo esters. | 0 | Organic Chemistry |
A simple cubic unit cell, with stacks of atoms arranged as if at the eight corners of a cube would form a single cubic hole or void in the center. If these voids are occupied by ions of opposite charge from the parent lattice, the cesium chloride structure is formed. | 3 | Analytical Chemistry |
Titanium dioxide takes part in self-cleaning glass. Free radicals generated from oxidize organic matter. The rough wedge-like surface can be modified with a hydrophobic monolayer of octadecylphosphonic acid (ODP). surfaces that were plasma etched for 10 seconds and subsequent surface modifications with ODP showed a water contact angle greater than 150◦. The surface was converted into a superhydrophilic surface (water contact angle = 0◦) upon UV illumination, due to rapid decomposition of octadecylphosphonic acid coating resulting from photocatalysis. Due to 's wide band gap, light absorption by the semiconductor material and resulting superhydrophilic conversion of undoped requires ultraviolet radiation (wavelength <390 nm) and thereby restricts self-cleaning to outdoor applications. | 5 | Photochemistry |
In order for euxinic conditions to persist, a positive feedback loop must perpetuate organic matter export to bottom waters and reduction of sulfate under anoxic conditions. Organic matter export is driven by high levels of primary production in the photic zone, supported by a continual supply of nutrients to the oxic surface waters. A natural source of nutrients, such as phosphate (), comes from weathering of rocks and subsequent transport of these dissolved nutrients via rivers. In a nutrient trap, increased input of phosphate from rivers, high rates of recycling of phosphate from sediments, and slow vertical mixing in the water column allow for euxinic conditions to persist. | 9 | Geochemistry |
Nitric acid is normally considered to be a strong acid at ambient temperatures. There is some disagreement over the value of the acid dissociation constant, though the pK value is usually reported as less than −1. This means that the nitric acid in diluted solution is fully dissociated except in extremely acidic solutions. The pK value rises to 1 at a temperature of 250 °C.
Nitric acid can act as a base with respect to an acid such as sulfuric acid:
:;Equilibrium constant: K ≈ 22
The nitronium ion, , is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties, it can undergo an autoprotolysis reaction, similar to the self-ionization of water: | 3 | Analytical Chemistry |
More organic matter is preserved in sediments if there is high primary production, or the sediment is fine-grained. The lack of oxygen helps preservation greatly, and that also is caused by a large supply of organic matter. Soil does not usually preserve organic matter, it would need to be acidified or water logged, as in the bog. Rapid burial ensures the material gets to an oxygen free depth, but also dilutes the organic matter. A low energy environment ensures the sediment is not stirred up and oxygenated. Salt marshes and mangroves meet some of these requirements, but unless the sea level is rising will not have a chance to accumulate much. Coral reefs are very productive, but are well oxygenated, and recycle everything before it is buried. | 9 | Geochemistry |
Typical anions in ionic liquids include the following: tetrafluoroborate (BF), hexafluorophosphate (PF), bis-trifluoromethanesulfonimide (NTf), trifluoromethanesulfonate (OTf), dicyanamide (N(CN)), hydrogen sulphate (HSO), and ethyl sulphate (EtOSO). Magnetic ionic liquids can be synthesized by incorporating paramagnetic anions, illustrated by 1-butyl-3-methylimidazolium tetrachloroferrate. | 7 | Physical Chemistry |
Copepods are known to be the primary consumers of diatoms in the water column and initiate the production of PUA upon grazing. The consumption of PUA-producing diatoms by copepods has been shown to diminish their reproductive success. Specifically, female copepods that consume diatoms spawn eggs with low viabilities and offspring with high teratogenesis rates. The compounds mainly act by preventing cell division and promoting apoptosis in copepod embryos, though the mechanism behind this is still poorly understood. | 1 | Biochemistry |
Alcohols add to electrophilically activated alkenes. The method is atom-economical:
: RC=CR + R–OH → RCH–C(–O–R)–R
Acid catalysis is required for this reaction. Commericially important ethers prepared in this way are derived from isobutene or isoamylene, which protonate to give relatively stable carbocations. Using ethanol and methanol with these two alkenes, four fuel-grade ethers are produced: methyl tert-butyl ether (MTBE), methyl tert-amyl ether (TAME), ethyl tert-butyl ether (ETBE), and ethyl tert-amyl ether (TAEE).
Solid acid catalysts are typically used to promote this reaction. | 0 | Organic Chemistry |
The Newbery-Vautin chlorination process is a method for extracting gold from its ore through the use of chlorination. This process was jointly developed by James Cosmo Newbery and Claude Vautin. | 8 | Metallurgy |
A RHEED system requires an electron source (gun), photoluminescent detector screen and a sample with a clean surface, although modern RHEED systems have additional parts to optimize the technique. The electron gun generates a beam of electrons which strike the sample at a very small angle relative to the sample surface. Incident electrons diffract from atoms at the surface of the sample, and a small fraction of the diffracted electrons interfere constructively at specific angles and form regular patterns on the detector. The electrons interfere according to the position of atoms on the sample surface, so the diffraction pattern at the detector is a function of the sample surface. Figure 1 shows the most basic setup of a RHEED system. | 3 | Analytical Chemistry |
In 1999, the virulence genes associated with Mycobacterium tuberculosis were identified through transposon mutagenesis-mediated gene knockout. A plasmid named pCG113 containing kanamycin resistance genes and the IS1096 insertion sequence was engineered to contain variable 80-base pair tags. The plasmids were then transformed into M. tuberculosis cells by electroporation. Colonies were plated on kanamycin to select for resistant cells. Colonies that underwent random transposition events were identified by BamHI digestion and Southern blotting using an internal IS1096 DNA probe. Colonies were screened for attenuated multiplication to identify colonies with mutations in candidate virulence genes. Mutations leading to an attenuated phenotype were mapped by amplification of adjacent regions to the IS1096 sequences and compared with the published M. tuberculosis genome. In this instance transposon mutagenesis identified 13 pathogenic loci in the M. tuberculosis genome which were not previously associated with disease. This is essential information in understanding the infectious cycle of the bacterium. | 1 | Biochemistry |
Water-splitting photoelectrochemical (PEC) cells use light energy to decompose water into hydrogen and oxygen within a two-electrode cell. In theory, three arrangements of photo-electrodes in the assembly of PECs exist:
* photo-anode made of a n-type semiconductor and a metal cathode
* photo-anode made of a n-type semiconductor and a photo-cathode made of a p-type semiconductor
* photo-cathode made of a p-type semiconductor and a metal anode
There are several requirements for photoelectrode materials in PEC production:
* light absorbance: determined by band gap and appropriate for solar irradiation spectrum
* charge transport: photoelectrodes must be conductive (or semi-conductive) to minimize resistive losses
* suitable band structure: large enough band gap to split water (1.23V) and appropriate positions relative to redox potentials for and
* catalytic activity: high catalytic activity increases efficiency of the water-splitting reaction
* stability: materials must be stable to prevent decomposition and loss of function
In addition to these requirements, materials must be low-cost and earth abundant for the widespread adoption of PEC water splitting to be feasible.
While the listed requirements can be applied generally, photoanodes and photocathodes have slightly different needs. A good photocathode will have early onset of the oxygen evolution reaction (low overpotential), a large photocurrent at saturation, and rapid growth of photocurrent upon onset. Good photoanodes, on the other hand, will have early onset of the hydrogen evolution reaction in addition to high current and rapid photocurrent growth. To maximize current, anode and cathode materials need to be matched together; the best anode for one cathode material may not be the best for another. | 5 | Photochemistry |
Under normal circumstances, the spent chromophore is discharged from the protein by an incoming "recharged" chromophore. However, sometimes the spent chromophore may leave the opsin protein prior to its replacement, when it is bound to the ABCA4 protein (also known as ABCR). At this stage, it is also transformed to all-trans-retinol, and then leaves the photoreceptor outer segment via the IRBP chaperone. It then follows the conventional visual cycle. It is from this pathway that the presence of opsin without a chromophore can be explained. | 1 | Biochemistry |
It is believed that both DXS and DXR are rate-determining enzymes, allowing them to regulate carotenoid levels. This was discovered in an experiment where DXS and DXR were genetically overexpressed, leading to increased carotenoid expression in the resulting seedlings. Also, J-protein (J20) and heat shock protein 70 (Hsp70) chaperones are thought to be involved in post-transcriptional regulation of DXS activity, such that mutants with defective J20 activity exhibit reduced DXS enzyme activity while accumulating inactive DXS protein. Regulation may also be caused by external toxins that affect enzymes and proteins required for synthesis. Ketoclomazone is derived from herbicides applied to soil and binds to DXP synthase. This inhibits DXP synthase, preventing synthesis of DXP and halting the MEP pathway. The use of this toxin leads to lower levels of carotenoids in plants grown in the contaminated soil. Fosmidomycin, an antibiotic, is a competitive inhibitor of DXP reductoisomerase due to its similar structure to the enzyme. Application of said antibiotic prevents reduction of DXP, again halting the MEP pathway. | 5 | Photochemistry |
The ocean can be conceptually divided into a surface layer within which water makes frequent (daily to annual) contact with the atmosphere, and a deep layer below the typical mixed layer depth of a few hundred meters or less, within which the time between consecutive contacts may be centuries. The dissolved inorganic carbon (DIC) in the surface layer is exchanged rapidly with the atmosphere, maintaining equilibrium. Partly because its concentration of DIC is about 15% higher but mainly due to its larger volume, the deep ocean contains far more carbon—it is the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere—but the timescale to reach equilibrium with the atmosphere is hundreds of years: the exchange of carbon between the two layers, driven by thermohaline circulation, is slow.
Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small fraction of which is converted into carbonate. It can also enter the ocean through rivers as dissolved organic carbon. It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the oceans' deeper, more carbon-rich layers as dead soft tissue or in shells as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation.
Oceans are basic (with a current pH value of 8.1 to 8.2). The increase in atmospheric CO shifts the pH of the ocean towards neutral in a process called ocean acidification. Oceanic absorption of CO is one of the most important forms of carbon sequestering. The projected rate of pH reduction could slow the biological precipitation of calcium carbonates, thus decreasing the ocean's capacity to absorb CO. | 5 | Photochemistry |
Alkyl groups form homologous series. The simplest series have the general formula . Alkyls include methyl, (), ethyl (), propyl (), butyl (), pentyl (), and so on. Alkyl groups that contain one ring have the formula , e.g. cyclopropyl and cyclohexyl. The formula of alkyl radicals are the same as alkyl groups, except the free valence "" is replaced by the dot "•" and adding "radical" to the name of the alkyl group (e.g. methyl radical ).
The naming convention is taken from IUPAC nomenclature:
The prefixes taken from IUPAC nomenclature are used to name branched chained structures by their substituent groups, for example 3-methylpentane:
The structure of 3-methylpentane is viewed as consisting of two parts. First, five atoms comprise the longest straight chain of carbon centers. The parent five-carbon compound is named pentane (highlighted blue). The methyl "substituent" or "group" is highlighted red. According to the usual rules of nomenclature, alkyl groups are included in the name of the molecule before the root, as in methylpentane. This name is, however, ambiguous, as the methyl branch could be on various carbon atoms. Thus, the name is 3-methylpentane to avoid ambiguity: The 3- is because the methyl is attached to the third of the five carbon atoms.
If there is more than one of the same alkyl group attached to a chain, then the prefixes are used on the alkyl groups to indicate multiples (i.e., di, tri, tetra, etc.)
This compound is known as 2,3,3-trimethylpentane. Here three identical alkyl groups attached to carbon atoms 2, 3, and 3. The numbers are included in the name to avoid ambiguity about the position of the groups, and "tri" indicates that there are three identical methyl groups. If one of the methyl groups attached to the third carbon atom were instead an ethyl group, then the name would be 3-ethyl-2,3-dimethylpentane. When there are different alkyl groups, they are listed in alphabetical order.
In addition, each position on an alkyl chain can be described according to how many other carbon atoms are attached to it. The terms primary, secondary, tertiary, and quaternary refer to a carbon attached to one, two, three, or four other carbons respectively. | 0 | Organic Chemistry |
inflammation induced by cholesterol loading into immune cells causes heart disease. A class of drugs called statins blocks cholesterol synthesis and is used extensively in treating heart disease. | 1 | Biochemistry |
It is legal under EU law to sell and buy seeds from a composite cross population as an experiment in the period between 2014 and 2018. | 1 | Biochemistry |
A variety of natural and synthetic inhibitors of ATP synthase have been discovered. These have been used to probe the structure and mechanism of ATP synthase. Some may be of therapeutic use. There are several classes of ATP synthase inhibitors, including peptide inhibitors, polyphenolic phytochemicals, polyketides, organotin compounds, polyenic α-pyrone derivatives, cationic inhibitors, substrate analogs, amino acid modifiers, and other miscellaneous chemicals. Some of the most commonly used ATP synthase inhibitors are oligomycin and DCCD. | 5 | Photochemistry |
In an ecological context, phototrophs are often the food source for neighboring heterotrophic life. In terrestrial environments, plants are the predominant variety, while aquatic environments include a range of phototrophic organisms such as algae (e.g., kelp), other protists (such as euglena), phytoplankton, and bacteria (such as cyanobacteria).
Cyanobacteria, which are prokaryotic organisms which carry out oxygenic photosynthesis, occupy many environmental conditions, including fresh water, seas, soil, and lichen. Cyanobacteria carry out plant-like photosynthesis because the organelle in plants that carries out photosynthesis is derived from an endosymbiotic cyanobacterium. This bacterium can use water as a source of electrons in order to perform CO reduction reactions.
A photolithoautotroph is an autotrophic organism that uses light energy, and an inorganic electron donor (e.g., HO, H, HS), and CO as its carbon source. | 5 | Photochemistry |
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to the point group D. Molecules where the three ligands are not identical, such as HCO, deviate from this idealized geometry. Examples of molecules with trigonal planar geometry include boron trifluoride (BF), formaldehyde (HCO), phosgene (COCl), and sulfur trioxide (SO). Some ions with trigonal planar geometry include nitrate (), carbonate (), and guanidinium (). In organic chemistry, planar, three-connected carbon centers that are trigonal planar are often described as having sp hybridization.
Nitrogen inversion is the distortion of pyramidal amines through a transition state that is trigonal planar.
Pyramidalization is a distortion of this molecular shape towards a tetrahedral molecular geometry. One way to observe this distortion is in pyramidal alkenes. | 4 | Stereochemistry |
Hydroboration is typically anti-Markovnikov, i.e. the hydrogen adds to the most substituted carbon of the double bond. That the regiochemistry is reverse of a typical HX addition reflects the polarity of the B-H bonds. Hydroboration proceeds via a four-membered transition state: the hydrogen and the boron atoms added on the same face of the double bond. Granted that the mechanism is concerted, the formation of the C-B bond proceeds slightly faster than the formation of the C-H bond. As a result, in the transition state, boron develops a partially negative charge while the more substituted carbon bears a partially positive charge. This partial positive charge is better supported by the more substituted carbon. Formally, the reaction is an example of a group transfer reaction. However, an analysis of the orbitals involved reveals that the reaction is pseudopericyclic and not subject to the Woodward–Hoffmann rules for pericyclic reactivity.
If BH is used as the hydroborating reagent, reactions typically proceed beyond the monoalkyl borane compounds, especially for less sterically hindered small olefins. Trisubstituted olefins can rapidly produce dialkyl boranes, but further alkylation of the organoboranes is slowed because of steric hindrance. This significant rate difference in producing di- and tri-alkyl boranes is useful in the synthesis of bulky boranes that can enhance regioselectivity. | 0 | Organic Chemistry |
Sometimes modulation of transgene expression may be necessary since strong constitutive expression of a therapeutic gene in retinal tissues could be deleterious for long-term retinal function. Different methods have been utilized for the expression modulation. One way is using exogenously regulatable promoter system in AAV vectors. For example, the tetracycline-inducible expression system uses a silencer/transactivator AAV2 vector and a separate inducible doxycycline-responsive coinjection. When induction occurs by oral doxycycline, this system shows tight regulation of gene expression in both photoreceptor and RPE cells. | 1 | Biochemistry |
To a solution of 1,3-dihydro-1,3-bis(chloromethyl)benzo[c] thiophene 2,2-dioxide (0.584 g, 2.2 mmol) in 50 ml of dry benzene was added 0.80 mL (2.8 mmol) of a 70% benzene solution of NaAlH(OCHCHOCH) via syringe, and the solution was refluxed for 12 hours. The mixture was cooled to 0° and decomposed with 20% sulfuric acid. The benzene layer was separated, washed with 10 mL of water, dried over potassium carbonate, and concentrated to give the product as a yellow oil in 91% yield (0.480 g); IR (film) 770, 1140, and 1320 cm–1; NMR (CDCl) δ 4.22 (q, 2 H), 1.61 and 1.59 (2 d, 6 H, J = 7 Hz), 7.3 (s, 4 H); m/e (rel. intensity) 196 (M+) (14), 132 (M-SO2) (100); MS analysis 196.055796 (calc.), 196.057587 (obs.). | 0 | Organic Chemistry |
How stereoselectivity is achieved in asymmetric nucleophilic epoxidations depends on the method employed. Covered here are various methods for the asymmetric nucleophilic epoxidation of electron-poor olefins. See below for a survey of the substrate scope of the reaction.
When chiral, non-racemic peroxides are used, the two transition states of epoxidation leading to enantiomeric products are diastereomeric. Steric interactions between the peroxide, enone, and templating cation M influence the sense of selectivity observed.
Methods that employ metal peroxides modified by chiral, non-racemic ligands operate by a similar mechanism in which the metal cation plays a templating role. Chiral zinc alkoxides under an oxygen atmosphere have been used to epoxidize some classes of enones (see equation (8) below). The evolution of ethane gas and uptake of oxygen are evidence for ligand exchange followed by oxidation of the intermediate zinc alkoxide species. A catalytic version of this transformation has been achieved using chiral zinc alkylperoxides.
Lithium, magnesium, and calcium alkylperoxides have also been employed as asymmetric nucleophilic epoxidation reagents. Simple tartrate and pseudoephedrine ligands are effective in combination with these metals; however, little detailed information about the precise mechanisms of these systems is known.
In combination with BINOL ligands and cumene hydroperoxide, lanthanide alkoxides can be used to epoxidize both trans and cis enones with high enantioselectivity. Studies of non-linear effects with these catalyst systems suggest that the active catalyst is oligomeric.
Homopolymers of amino acids (polypeptides) can also be used to effect enantioselective epoxidations in the presence of an enone and a peroxide. Structure-reactivity relationships have not emerged, but enantioselectivities in these reactions are often high, and polypeptides can often be used when other methods fail.
Phase-transfer catalysis of nucleophilic epoxidation is also possible using cinchona-based alkaloid catalysts. Phase-transfer methods allow some variability in the oxidant used: hydroperoxides, hydrogen peroxide, and hypochlorites have all been used with some success. | 0 | Organic Chemistry |
* Encyclopædia Britannica, 14th ed.
* J. Day & R. F. Tylecote (eds.), The Industrial Revolution in Metals (1991)
* P. W. King, "The Cupola at Bristol", Somerset Araeology and Natural History 140 (for 1997), 37–52
* P. W. King, "Sir Clement Clerke and the Adoption of coal in metallurgy", Transactions of the Newcomen Society 73(1) (2001–2), 33–53 | 8 | Metallurgy |
Deformation twinning is a response to shear stress. The crystal structure is displaced along successive planes of the crystal, a process also called glide. The twinning is always reflection twinning and the glide plane is also the mirror plane. Deformation twinning can be observed in a calcite cleavage fragment by applying gentle pressure with a knife blade near an edge. This particular glide twinning, {102}, is found almost universally in deformed rock beds containing calcite.
Twinning and slip are competitive mechanisms for crystal deformation. Each mechanism is dominant in certain crystal systems and under certain conditions. In fcc metals, slip is almost always dominant because the stress required is far less than twinning stress.
Twinning can occur by cooperative displacement of atoms along the face of the twin boundary. This displacement of a large quantity of atoms simultaneously requires significant energy to perform. Therefore, the theoretical stress required to form a twin is quite high. It is believed that twinning is associated with dislocation motion on a coordinated scale, in contrast to slip, which is caused by independent glide at several locations in the crystal.
Compared to slip, twinning produces a deformation pattern that is more heterogeneous in nature. This deformation produces a local gradient across the material and near intersections between twins and grain boundaries. The deformation gradient can lead to fracture along the boundaries, particularly in bcc transition metals at low temperatures.
Of the three common crystalline structures bcc, fcc, and hcp, the hcp structure is the most likely to form deformation twins when strained, because they rarely have a sufficient number of slip systems for an arbitrary shape change. High strain rates, low stacking-fault energy and low temperatures facilitate deformation twinning.
If a metal with face-centered cubic (fcc) structure, like Al, Cu, Ag, Au, etc., is subjected to stress, it will experience twinning. The formation and migration of twin boundaries is partly responsible for ductility and malleability of fcc metals.
Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel. In certain types of high strength steels, very fine deformation twins act as primary obstacles against dislocation motion. These steels are referred to as TWIP steels, where TWIP stands for twinning-induced plasticity. | 3 | Analytical Chemistry |
While mouse and human antibodies are structurally similar, the differences between them were sufficient to invoke an immune response when murine monoclonal antibodies were injected into humans, resulting in their rapid removal from the blood, as well as systemic inflammatory effects and the production of human anti-mouse antibodies (HAMA).
Recombinant DNA has been explored since the late 1980s to increase residence times. In one approach called "CDR grafting", mouse DNA encoding the binding portion of a monoclonal antibody was merged with human antibody-producing DNA in living cells. The expression of this "chimeric" or "humanised" DNA through cell culture yielded part-mouse, part-human antibodies. | 1 | Biochemistry |
The decay energy, Q (also called the Q-value of the reaction), corresponds to a disappearance of mass.
For the alpha decay nuclear reaction: , (where P is the parent nuclide and D the daughter).
, or to put in the more commonly used units: Q (MeV) = -931.5 ΔM (Da), (where ΔM = ΣM - ΣM).
When the daughter nuclide and alpha particle formed are in their ground states (common for alpha decay), the total decay energy is divided between the two in kinetic energy (T):
The size of T is dependent on the ratio of masses of the products and due to the conservation of momentum (the parent's momentum = 0 at the moment of decay) this can be calculated:
and ,
The alpha particle, or He nucleus, is an especially strongly bound particle. This combined with the fact that the binding energy per nucleon has a maximum value near A=56 and systematically decreases for heavier nuclei, creates the situation that nuclei with A>150 have positive Q-values for the emission of alpha particles.
For example, one of the heaviest naturally occurring isotopes, (ignoring charges):
: Q = -931.5 (234.043 601 + 4.002 603 254 13 - 238.050 788 2) = 4.2699 MeV
Note that the decay energy will be divided between the alpha-particle and the heavy recoiling daughter so that the kinetic energy of the alpha particle (T) will be slightly less:
T = (234.043 601 / 238.050 788 2) 4.2699 = 4.198 MeV, (note this is for the U to Th reaction, which in this case has the branching ratio of 79%). The kinetic energy of the recoiling Th daughter nucleus is T = (m / m) Q = (4.002 603 254 13 / 238.050 788 2) 4.2699 = 0.0718 MeV or 71.8 keV, which whilst much smaller is still substantially bigger than that of chemical bonds (<10 eV) meaning the daughter nuclide will break away from whatever chemical environment the parent had been in.
The recoil energy is also the reason that alpha spectrometers, whilst run under reduced pressure, are not operated at too low a pressure so that the air helps stop the recoiling daughter from moving completely out of the original alpha-source and cause serious contamination problems if the daughters are themselves radioactive.
The Qα‐values generally increase with increasing atomic number but the variation in the mass surface due to shell effects can overwhelm the systematic increase. The sharp peaks near A = 214 are due to the effects of the N = 126 shell. | 7 | Physical Chemistry |
A property of a physical system, such as the entropy of a gas, that stays approximately constant when changes occur slowly is called an adiabatic invariant. By this it is meant that if a system is varied between two end points, as the time for the variation between the end points is increased to infinity, the variation of an adiabatic invariant between the two end points goes to zero.
In thermodynamics, an adiabatic process is a change that occurs without heat flow; it may be slow or fast. A reversible adiabatic process is an adiabatic process that occurs slowly compared to the time to reach equilibrium. In a reversible adiabatic process, the system is in equilibrium at all stages and the entropy is constant. In the 1st half of the 20th century the scientists that worked in quantum physics used the term "adiabatic" for reversible adiabatic processes and later for any gradually changing conditions which allow the system to adapt its configuration. The quantum mechanical definition is closer to the thermodynamical concept of a quasistatic process and has no direct relation with adiabatic processes in thermodynamics.
In mechanics, an adiabatic change is a slow deformation of the Hamiltonian, where the fractional rate of change of the energy is much slower than the orbital frequency. The area enclosed by the different motions in phase space are the adiabatic invariants.
In quantum mechanics, an adiabatic change is one that occurs at a rate much slower than the difference in frequency between energy eigenstates. In this case, the energy states of the system do not make transitions, so that the quantum number is an adiabatic invariant.
The old quantum theory was formulated by equating the quantum number of a system with its classical adiabatic invariant. This determined the form of the Bohr–Sommerfeld quantization rule: the quantum number is the area in phase space of the classical orbit. | 7 | Physical Chemistry |
The terms glycans and polysaccharides are defined by IUPAC as synonyms meaning "compounds consisting of a large number of monosaccharides linked glycosidically". However, in practice the term glycan may also be used to refer to the carbohydrate portion of a glycoconjugate, such as a glycoprotein, glycolipid, or a proteoglycan, even if the carbohydrate is only an oligosaccharide. Glycans usually consist solely of O-glycosidic linkages of monosaccharides. For example, cellulose is a glycan (or, to be more specific, a glucan) composed of β-1,4-linked -glucose, and chitin is a glycan composed of β-1,4-linked N-acetyl--glucosamine. Glycans can be homo- or heteropolymers of monosaccharide residues, and can be linear or branched. | 0 | Organic Chemistry |
Cultural or anthropogenic eutrophication is the process that causes eutrophication because of human activity. The problem became more apparent following the introduction of chemical fertilizers in agriculture (green revolution of the mid-1900s). Phosphorus and nitrogen are the two main nutrients that cause cultural eutrophication as they enrich the water, allowing for some aquatic plants, especially algae to grow rapidly and bloom in high densities. Algal blooms can shade out benthic plants thereby altering the overall plant community. When algae die off, their degradation by bacteria removes oxygen, potentially, generating anoxic conditions. This anoxic environment kills off aerobic organisms (e.g. fish and invertebrates) in the water body. This also affects terrestrial animals, restricting their access to affected water (e.g. as drinking sources). Selection for algal and aquatic plant species that can thrive in nutrient-rich conditions can cause structural and functional disruption to entire aquatic ecosystems and their food webs, resulting in loss of habitat and species biodiversity.
There are several sources of excessive nutrients from human activity including run-off from fertilized fields, lawns, and golf courses, untreated sewage and wastewater and internal combustion of fuels creating nitrogen pollution. Cultural eutrophication can occur in fresh water and salt water bodies, shallow waters being the most susceptible. In shore lines and shallow lakes, sediments are frequently resuspended by wind and waves which can result in nutrient release from sediments into the overlying water, enhancing eutrophication. The deterioration of water quality caused by cultural eutrophication can therefore negatively impact human uses including potable supply for consumption, industrial uses and recreation. | 2 | Environmental Chemistry |
Ammonium carbamate solutions are highly corrosive to metallic construction materials – even to resistant forms of stainless steel – especially in the hottest parts of the synthesis plant such as the stripper. Historically corrosion has been minimized (although not eliminated) by continuous injection of a small amount of oxygen (as air) into the plant to establish and maintain a passive oxide layer on exposed stainless steel surfaces. Highly corrosion resistant materials have been introduced to reduce the need for passivation oxygen, such as specialized duplex stainless steels in the 1990s, and zirconium or zirconium-clad titanium tubing in the 2000s. | 0 | Organic Chemistry |
Alkali metals also dissolve in some small primary amines, such as methylamine and ethylamine and hexamethylphosphoramide, forming blue solutions. THF dissolves alkali metal, but a Birch reduction (see ) analogue does not proceed without a diamine ligand. Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to intercalate graphite with these metals. | 0 | Organic Chemistry |
Certain titanium and zirconium complexes catalyze intermolecular hydroamination of alkynes and allenes. Both stoichiometric and catalytic variants were initially examined with zirconocene bis(amido) complexes. Titanocene amido and sulfonamido complexes catalyze the intra-molecular hydroamination of aminoalkenes via a [[2+2 Photocycloaddition|[2+2] cycloaddition]] that forms the corresponding azametallacyclobutane, as illustrated in the figure below. Subsequent protonolysis by incoming substrate gives the α-vinyl-pyrrolidine (1) or tetrahydropyridine (2) product. Experimental and theoretical evidence support the proposed imido intermediate and mechanism with neutral group IV catalysts. | 0 | Organic Chemistry |
To perform microcrystallization, a small piece of lichen is extracted using acetone or other solvents, filtered, and evaporated to yield a residue. The residue is transferred to a microscope slide, and a drop of microcrystallization reagent is added before capping with a cover glass. Commonly used reagents include GAW (HO/glycerol/ethanol 1:1:1, v/v/v) and GE (acetic acid/glycerol 1:3). Slides using GE or GAW are gently heated and then allowed to cool, promoting the crystallization process. Once formed, crystals are best observed under polarized light with a 200–1,000-fold magnification.
This method requires basic laboratory equipment, including a microscope equipped for polarized light, test tubes, pipettes, a micro spirit-lamp or micro Bunsen burner, spatula or scalpel, and microscope slides and cover glasses. Lichen substances can be identified based on the distinctive shape and color of their crystals. | 3 | Analytical Chemistry |
Torovirus is a genus of viruses within the family Coronaviridae, subfamily Torovirinae that primarily infect vertebrates and include Berne virus of horses and Breda virus of cattle. They cause gastroenteritis in mammals, including humans but rarely. | 1 | Biochemistry |
Stereochemical control within [4+4] reactions is not common, however it does exist. Enol-exo stereoselectivity is possible using certain cross-linking techniques and has been met with moderate success.
Control over the configuration at newly established bridgehead centers by use of preexisting stereocenters has been largely unsuccessful, apart from one specialized example. | 5 | Photochemistry |
As in the standard code, initiation is most efficient at AUG. In addition, GUG and UUG starts are documented in archaea and bacteria. In Escherichia coli, UUG is estimated to serve as initiator for about 3% of the bacteriums proteins. CUG is known to function as an initiator for one plasmid-encoded protein (RepA) in E. coli. In addition to the NUG initiations, in rare cases bacteria can initiate translation from an AUU codon as e.g. in the case of poly(A) polymerase PcnB and the InfC gene that codes for translation initiation factor IF3. The internal assignments are the same as in the standard code though UGA codes at low efficiency for tryptophan in Bacillus subtilis and, presumably, in Escherichia coli'.
The NCBI raw format is as follows, with UUG, CUG, AUU, AUC, AUA, AUG, and GUG marked as possible initiators:
AAs = FFLLSSSSYY**CC*WLLLLPPPPHHQQRRRRIIIMTTTTNNKKSSRRVVVVAAAADDEEGGGG
Starts = ---M------**--*----M------------MMMM---------------M------------
Base1 = TTTTTTTTTTTTTTTTCCCCCCCCCCCCCCCCAAAAAAAAAAAAAAAAGGGGGGGGGGGGGGGG
Base2 = TTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGG
Base3 = TCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAG
Initiation at AUC and AUA is not addressed in the NCBI description text, but both are indeed known to occur in E. coli. | 1 | Biochemistry |
* In the 2016 film Spectral, the US military battles mysterious enemy creatures fashioned out of Bose–Einstein condensates.
* In the 2003 novel Blind Lake, scientists observe sentient life on a planet 51 light-years away using telescopes powered by Bose–Einstein condensate-based quantum computers.
* The video game franchise Mass Effect has cryonic ammunition whose flavour text describes it as being filled with Bose–Einstein condensates. Upon impact, the bullets rupture and spray super-cold liquid on the enemy. | 7 | Physical Chemistry |
Making a distinction between homogeneous and heterogeneous mixtures is a matter of the scale of sampling. On a coarse enough scale, any mixture can be said to be homogeneous, if the entire article is allowed to count as a "sample" of it. On a fine enough scale, any mixture can be said to be heterogeneous, because a sample could be as small as a single molecule. In practical terms, if the property of interest of the mixture is the same regardless of which sample of it is taken for the examination used, the mixture is homogeneous.
Gy's sampling theory quantitatively defines the heterogeneity of a particle as:
where , , , , and are respectively: the heterogeneity of the th particle of the population, the mass concentration of the property of interest in the th particle of the population, the mass concentration of the property of interest in the population, the mass of the th particle in the population, and the average mass of a particle in the population.
During sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero.
Pierre Gy derived, from the Poisson sampling model, the following formula for the variance of the sampling error in the mass concentration in a sample:
in which V is the variance of the sampling error, N is the number of particles in the population (before the sample was taken), q is the probability of including the ith particle of the population in the sample (i.e. the first-order inclusion probability of the ith particle), m is the mass of the ith particle of the population and a is the mass concentration of the property of interest in the ith particle of the population.
The above equation for the variance of the sampling error is an approximation based on a linearization of the mass concentration in a sample.
In the theory of Gy, correct sampling is defined as a sampling scenario in which all particles have the same probability of being included in the sample. This implies that q no longer depends on i, and can therefore be replaced by the symbol q. Gy's equation for the variance of the sampling error becomes:
where a is that concentration of the property of interest in the population from which the sample is to be drawn and M is the mass of the population from which the sample is to be drawn. | 7 | Physical Chemistry |
The carbon-silicon bond is highly electron-releasing and can stabilize a positive charge in the β position through hyperconjugation. Electrophilic additions to allyl- and vinylsilanes take advantage of this, and site selectivity generally reflects this property—electrophiles become bound to the carbon γ to the silyl group. The electron-donating strength of the carbon-silicon bond is similar to that of an acetamide substituent and equal to roughly two alkyl groups. After formation of the carbon-electrophile bond, silicon elimination is assisted by a nucleophile.
A model of the most likely reactive conformation of the allylsilane (see below) suggests that the new double bond that forms will predominantly possess the (E) configuration. However, addition of a nucleophile (such as the counterion of the electrophile) to the intermediate silyl-stabilized carbocation complicates this picture. Because the elimination to generate the double bond is stereospecifically anti, nucleophilic addition to either face of the silyl-stabilized carbanion leads to the formation of mixtures of double bond isomers. Diastereomeric mixtures of double bond isomers are common when Lewis acids are used to activate the electrophile.
Under conditions of nucleophilic catalysis, any intermediate along the reaction pathway may incorporate a silicon-nucleophile bond. This factor does not affect the outcome unless nucleophilic attack liberates free anions or allylic transposition occurs. The latter is known to occur for hypervalent allylsilanes incorporating fluoride. | 0 | Organic Chemistry |
Levobupivacaine, the S(-)-enantiomer of bupivacaine has been developed as an alternative to the racemic mixture, as it has been shown to have a lower cardiotoxicity than bupivacaine. Under European Union advice, it can be applied for minor and major surgical anaesthesia, as well as (post-operative) pain management. Particularly, it has been found suitable for multiple procedures, such as epidural block. It is effective for human patients who receive elective Caesarean section or lower body surgery, as it does not diverge dramatically in terms of sensory and/or motor block duration in comparison to bupivacaine. Deserving of consideration is the fact that its enhanced motor blocking can be a downside for patients receiving an epidural injection during childbirth, as a certain level of movement may still be required.
Other than childbirth, possible applications of levobupivacaine include upper and lower limb surgery, as well as eye surgery, where it blocks the extraocular muscle, highly efficient and convenient for patients undergoing vitreoretinal anterior segment or cataract surgery.
Levobupivacaine can be combined with other analgesics, including opioids, for postoperative pain management. | 4 | Stereochemistry |
A number of industrial processes produce combustible dust as a by-product. The most common being wood dust. Combustible dust has been defined as: a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations. In addition to wood, combustible dusts include metals, especially magnesium, titanium and aluminum, as well as other carbon-based dusts. There are at least 140 known substances that produce combustible dust. While the particles in a combustible dusts may be of any size, normally they have a diameter of less than 420 μm. , the United States Occupational Safety and Health Administration has yet to adopt a comprehensive set of rules on combustible dust.
When suspended in air (or any oxidizing environment), the fine particles of combustible dust present a potential for explosions. Accumulated dust, even when not suspended in air, remains a fire hazard. The National Fire Protection Association (U.S.) specifically addresses the prevention of fires and dust explosions in agricultural and food products facilities in NFPA Code section 61, and other industries in NFPA Code sections 651–664. Collectors designed to reduce airborne dust account for more than 40 percent of all dust explosions. Other important processes are grinding and pulverizing, transporting powders, filing silos and containers (which produces powder), and the mixing and blending of powders.
Investigation of 200 dust explosions and fires, between 1980 and 2005, indicated approximately 100 fatalities and 600 injuries. In January 2003, a polyethylene powder explosion and fire at the West Pharmaceutical Services plant in Kinston, North Carolina resulted in the deaths of six workers and injuries to 38 others. In February 2008 an explosion of sugar dust rocked the Imperial Sugar Company's plant at Port Wentworth, Georgia, resulting in thirteen deaths. | 7 | Physical Chemistry |
Pulse labelling is a biochemistry technique of identifying the presence of a target molecule by labeling a sample with a radioactive compound. This is mainly done to identify the stage at which the messenger RNA is being produced in a cell. | 1 | Biochemistry |
Isomerism was first observed in 1827, when Friedrich Wöhler prepared silver cyanate and discovered that, although its elemental composition of was identical to silver fulminate (prepared by Justus von Liebig the previous year), its properties were distinct. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be distinct only when their elemental compositions differ. (We now know that the bonding structures of fulminate and cyanate can be approximately described as ≡ and , respectively.)
Additional examples were found in succeeding years, such as Wöhlers 1828 discovery that urea has the same atomic composition () as the chemically distinct ammonium cyanate. (Their structures are now known to be and , respectively.) In 1830 Jöns Jacob Berzelius introduced the term isomerism' to describe the phenomenon.
In 1848, Louis Pasteur observed that tartaric acid crystals came into two kinds of shapes that were mirror images of each other. Separating the crystals by hand, he obtained two version of tartaric acid, each of which would crystallize in only one of the two shapes, and rotated the plane of polarized light to the same degree but in opposite directions. In 1860, Pasteur explicitly hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms. | 4 | Stereochemistry |
RNA polymerase II holoenzyme is a form of eukaryotic RNA polymerase II that is recruited to the promoters of protein-coding genes in living cells. It consists of RNA polymerase II, a subset of general transcription factors, and regulatory proteins known as SRB proteins.
Part of the assembly of the holoenzyme is referred to as the preinitiation complex, because its assembly takes place on the gene promoter before the initiation of transcription. The mediator complex acts as a bridge between RNA polymerase II and the transcription factors. | 1 | Biochemistry |
Similar to the draw spinning technique the direct ink writing technique utilized reversible physical solidification to produce hydrogel fibers. The pregel solution was liqufied through shear thinning process which can be generated by adding microscopic particles such as mircrogel. After leaving the nuzzle, the hydrogel will solidify and retain their shape, and network will be made perment after crosslinking.
An example would be the production of the fiber developed by Lewis et al. Where Silk fibroin was used to generate the desired shear-thinning properties. And the network was formed when the solvent was subsequently changed. | 7 | Physical Chemistry |
An alteration in copy number state with respect to a single-copy reference locus is referred to as a “copy number variation” (CNV) if it appears in germline cells, or a copy number alteration (CNA) if it appears in somatic cells. A CNV or CNA could be due to a deletion or amplification of a locus with respect to the number of copies of the reference locus present in the cell, and together, they are major contributors to variability in the human genome. They have been associated with cancers; neurological, psychiatric, and autoimmune diseases; and adverse drug reactions. However, it is difficult to measure these allelic variations with high precision using other methods such as qPCR, thus making phenotypic and disease associations with altered CNV status challenging.
The large number of “digitized,” endpoint measurements made possible by sample partitioning enables dPCR to resolve small differences in copy number with better accuracy and precision when compared to other methods such as SNP-based microarrays or qPCR. qPCR is limited in its ability to precisely quantify gene amplifications in several diseases, including Crohn’s disease, HIV-1 infection, and obesity.
dPCR was designed to measure the concentration of a nucleic acid target in copies per unit volume of the sample. When operating in dilute reactions where less than ~10% of the partitions contain a desired target (referred to as “limiting dilution”), copy number can be estimated by comparing the number of fluorescent droplets arising from a target CNV with the number of fluorescent droplets arising from an invariant single-copy reference locus. In fact, both at these lower target concentrations and at higher ones where multiple copies of the same target can co-localize to a single partition, Poisson statistics are used to correct for these multiple occupancies to give a more accurate value for each target’s concentration.
Digital PCR has been used to uncover both germline and somatic variation in gene copy number between humans and to study the link between amplification of HER2 (ERBB2) and breast cancer progression. | 1 | Biochemistry |
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