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The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit time. Reaction rates can vary dramatically. For example, the oxidative rusting of iron under Earths atmosphere is a slow reaction that can take many years, but the combustion of cellulose in a fire is a reaction that takes place in fractions of a second. For most reactions, the rate decreases as the reaction proceeds. A reactions rate can be determined by measuring the changes in concentration over time.
Chemical kinetics is the part of physical chemistry that concerns how rates of chemical reactions are measured and predicted, and how reaction-rate data can be used to deduce probable reaction mechanisms. The concepts of chemical kinetics are applied in many disciplines, such as chemical engineering, enzymology and environmental engineering. | 7 | Physical Chemistry |
The synthesis and degradation of sialic acid are distributed in different compartments of the cell. The synthesis starts in the cytosol, where N-acetylmannosamine 6 phosphate and phosphoenolpyruvate give rise to sialic acid. Later on, Neu5Ac 9 phosphate is activated in the nucleus by a cytidine monophosphate (CMP) residue through CMP-Neu5Ac synthase. Although the linkage between sialic acid and other compounds tends to be a α binding, this specific one is the only one that is a β linkage. CMP-Neu5Ac is then transported to the endoplasmic reticulum or the Golgi apparatus, where it can be transferred to an oligosaccharide chain, becoming a new glycoconjugate. This bond can be modified by O-acetylation or O-methylation. When the glycoconjugate is mature it is transported to the cell surface.
The sialidase is one of the most important enzymes of the sialic acid catabolism. It can cause the removal of sialic acid residues from the cell surface or serum sialoglycoconjugates. Usually, in higher animals, the glycoconjugates that are prone to be degraded are captured by endocytosis. After the fusion of the late endosome with the lysosome, lysosomal sialidases remove sialic acid residues. The activity of these sialidases is based on the removal of O-acetyl groups. Free sialic acid molecules are transported to the cytosol through the membrane of the lysosome. There, they can be recycled and activated again to form another nascent glycoconjugate molecule in the Golgi apparatus. Sialic acids can also be degraded to acylmannosamine and pyruvate with the cytosolic enzyme acylneuraminate lyase.
Some severe diseases can depend on the presence or absence of some enzymes related to the sialic acid metabolism. Sialidosis and Sialic acid deficiency with mutations in the NANS gene (see below) would be examples of this type of disorder. | 0 | Organic Chemistry |
When metals are near the bottom of the reactivity series, their compounds generally decompose easily at high temperatures. This is because stronger bonds form between atoms towards the top of the reactivity series, and strong bonds are difficult to break. For example, copper is near the bottom of the reactivity series, and copper sulfate (CuSO), begins to decompose at about , increasing rapidly at higher temperatures to about . In contrast potassium is near the top of the reactivity series, and potassium sulfate (KSO) does not decompose at its melting point of about , nor even at its boiling point. | 7 | Physical Chemistry |
Rapamycin (Sirolimus) inhibits mTORC1, resulting in the suppression of cellular senescence. This appears to provide most of the beneficial effects of the drug (including life-span extension in animal studies). Suppression of insulin resistance by sirtuins accounts for at least some of this effect. Impaired sirtuin 3 leads to mitochondrial dysfunction.
Rapamycin has a more complex effect on mTORC2, inhibiting it only in certain cell types under prolonged exposure. Disruption of mTORC2 produces the diabetic-like symptoms of decreased glucose tolerance and insensitivity to insulin. | 1 | Biochemistry |
In human bodies, coppers is an important constituent of many essential enzymes, including ceruloplasmin (which carries Cu and oxidizes Fe in human plasma), cytochrome c oxidase, metallothionein and superoxide dismutase 1. Serum in human blood is typically Cu-depleted by ~0.8‰ relative to erythrocytes (i.e., red blood cells). In a study of 49 male and female blood donors, the average δCu value of the donors' blood serum was -0.26 ± 0.40‰, while that of their erythrocytes was +0.56 ± 0.50‰. In a separate study, δCu values of serum in 20 healthy patients ranged from -0.39 to +0.38‰, while the δCu values of their erythrocytes ranged from +0.57 to +1.24‰. To balance Cu loss due to menstruation, a large portion of Cu in the blood of menstruating women comes from their liver. Due to fractionation associated with Cu transport from the liver to the blood, the total blood of pre-menopausal women is generally Cu-depleted relative to that of males and non-menstruating women. The δCu values of healthy human liver tissue in 7 patients ranged from -0.45 to -0.11‰. | 9 | Geochemistry |
Reduction is often expressed as a percentage. The closer it is to 100%, the better.
Letting and be as before, a reduction by % is achieved, where
;Example:
Let, as in the earlier example, the concentration of some contaminant be 580 ppm before and 0.725 ppm after treatment. Then
So this is (better than) a 99% reduction, but not yet quite a 99.9% reduction.
The following table summarizes the most common cases.
In general, if is a whole number, an -log reduction corresponds to a percentage reduction with leading digits "9" in the percentage (provided that it is at least 10%). | 3 | Analytical Chemistry |
Vidarabine phosphate is an adenosine monophosphate nucleotide in which ribose is replaces by an arabinso moiety. It has antiviral and possibly antineoplastic properties. | 1 | Biochemistry |
In humans, about 70% of promoters located near the transcription start site of a gene (proximal promoters) contain a CpG island. CpG islands are generally 200 to 2000 base pairs long, have a C:G base pair content >50%, and have regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide and this occurs frequently in the linear sequence of bases along its 5′ → 3′ direction.
Genes may also have distant promoters (distal promoters) and these frequently contain CpG islands as well. An example is the promoter of the DNA repair gene ERCC1, where the CpG island-containing promoter is located about 5,400 nucleotides upstream of the coding region of the ERCC1 gene. CpG islands also occur frequently in promoters for functional noncoding RNAs such as microRNAs. | 1 | Biochemistry |
The turtapede (Mawup in Navi) is a Pandoran creature that resembles a cross between a turtle, a platypus and a starfish. With its large dorsal fin and long tail, the turtapede is agile underwater. When emerging from the water, mesmerizing colors start to shimmer in the translucent area on its shell. They first appear in the musical Toruk – The First Flight'. | 1 | Biochemistry |
The method was improved by Draine, Flatau, and Goodman, who applied the fast Fourier transform to solve fast convolution problems arising in the discrete dipole approximation (DDA). This allowed for the calculation of scattering by large targets. They distributed an open-source code DDSCAT.
There are now several DDA implementations, extensions to periodic targets, and particles placed on or near a plane substrate. Comparisons with exact techniques have also been published.
Other aspects, such as the validity criteria of the discrete dipole approximation, were published. The DDA was also extended to employ rectangular or cuboid dipoles, which are more efficient for highly oblate or prolate particles. | 7 | Physical Chemistry |
Bromine is present naturally as bromide salts in evaporite deposits. Bromine is also present in soils and in marine algae that synthesize organic bromine compounds. Other natural sources of bromine come from polar regions, salt lakes, and volcanoes.
The primary natural source of bromine to the atmosphere is sea spray aerosols. Smaller fluxes originate from volcanic emissions and biomass burning. The primary atmospheric sinks are sea spray deposition and photochemical reactions, which release gaseous bromine. | 9 | Geochemistry |
Ibuprofen was made available by prescription in the United Kingdom in 1969 and in the United States in 1974.
Ibuprofen is the International nonproprietary name (INN), British Approved Name (BAN), Australian Approved Name (AAN) and United States Adopted Name (USAN). In the United States, it has been sold under the brand-names Motrin and Advil since 1974 and 1984, respectively. Ibuprofen is commonly available in the United States up to the FDA's 1984 dose limit OTC, rarely used higher by prescription.
In 2009, the first injectable formulation of ibuprofen was approved in the United States, under the brand name Caldolor.
Ibuprofen can be taken orally (by mouth) (as a tablet, a capsule, or a suspension) and intravenously. | 4 | Stereochemistry |
Repeated experiments are performed to characterize the mechanism by which the material deforms. The dominant mechanism is the one which dominates the continuous deformation rate (strain rate), however at any given level of stress and temperature, more than one of the creep and plasticity mechanisms may be active. The boundaries between the fields are determined from the constitutive equations of the deformation mechanisms by solving for stress as a function of temperature. Along these boundaries, the deformation rates for the two neighboring mechanisms are equal. The programming code used for many of the published maps is open source
and an archive of its development is online. Many researchers have also written their own codes to make these maps.
The main regions in a typical deformation mechanism map and their constitutive equations are shown in the following subsections. | 8 | Metallurgy |
The Elbs reaction enables the synthesis of condensed aromatic systems. As already demonstrated by Elbs in 1884 it is possible to obtain anthracene through dehydration. Larger aromatic systems like pentacene are also feasible. This reaction does not take place in a single step but leads first to dihydropentacene that is dehydrogenated in a second step with copper as a catalyst.
The acyl compounds required for this reaction can be obtained through a Friedel-Crafts acylation with aluminum chloride.
The Elbs reaction is sometimes accompanied by elimination of substituents and can be unsuited for substituted polyaromatics. | 0 | Organic Chemistry |
*Glycolysis is the process of breaking down glucose into pyruvate, producing two molecules of ATP (per 1 molecule of glucose) in the process. When a cell has a higher concentration of ATP than ADP (i.e. has a high energy charge), the cell cannot undergo glycolysis, releasing energy from available glucose to perform biological work. Pyruvate is one product of glycolysis, and can be shuttled into other metabolic pathways (gluconeogenesis, etc.) as needed by the cell. Additionally, glycolysis produces reducing equivalents in the form of NADH (nicotinamide adenine dinucleotide), which will ultimately be used to donate electrons to the electron transport chain.
*Gluconeogenesis is the opposite of glycolysis; when the cell's energy charge is low (the concentration of ADP is higher than that of ATP), the cell must synthesize glucose from carbon- containing biomolecules such as proteins, amino acids, fats, pyruvate, etc. For example, proteins can be broken down into amino acids, and these simpler carbon skeletons are used to build/ synthesize glucose.
*The citric acid cycle is a process of cellular respiration in which acetyl coenzyme A, synthesized from pyruvate dehydrogenase, is first reacted with oxaloacetate to yield citrate. The remaining eight reactions produce other carbon-containing metabolites. These metabolites are successively oxidized, and the free energy of oxidation is conserved in the form of the reduced coenzymes FADH and NADH. These reduced electron carriers can then be re-oxidized when they transfer electrons to the electron transport chain.
*Ketosis is a metabolic process where the body prioritizes ketone bodies, produced from fat, as its primary fuel source instead of glucose. This shift often occurs when glucose levels are low: during prolonged fasting, strenuous exercise, or specialized diets like ketogenic plans, the body may also adopt ketosis as an efficient alternative for energy production. This metabolic adaptation allows the body to conserve precious glucose for organs that depend on it, like the brain, while utilizing readily available fat stores for fuel.
*Oxidative phosphorylation and the electron transport chain is the process where reducing equivalents such as NADPH, FADH and NADH can be used to donate electrons to a series of redox reactions that take place in electron transport chain complexes. These redox reactions take place in enzyme complexes situated within the mitochondrial membrane. These redox reactions transfer electrons "down" the electron transport chain, which is coupled to the proton motive force. This difference in proton concentration between the mitochondrial matrix and inner membrane space is used to drive ATP synthesis via ATP synthase.
*Photosynthesis, another major bioenergetic process, is the metabolic pathway used by plants in which solar energy is used to synthesize glucose from carbon dioxide and water. This reaction takes place in the chloroplast. After glucose is synthesized, the plant cell can undergo photophosphorylation to produce ATP. | 1 | Biochemistry |
The 2-norbornyl cation can also be formed via rearrangements of similar ions, such as the 1-norbornyl and 7-norbornyl cations, though these are generally not as well understood. Carbon-14 radioactive isotope labeling experiments have shown that complex scrambling in norbornyl cation systems allow C to be present at all seven positions of the norbornyl system. By cycling between low and high temperatures during the hydrolyses of 1- and 7-choloronorbornanes, a large amount of 2-norbornanol was observed in addition to the expected 1- and 7-norbornanols, respectively. Thus the 1- and 7-norbornyl cations have some mechanism by which they can rearrange to the more stable 2-norbornyl cation on the timescale of solvolysis reactions. | 7 | Physical Chemistry |
In three-dimensional Euclidean space, the densest packing of equal spheres is achieved by a family of structures called close-packed structures. One method for generating such a structure is as follows. Consider a plane with a compact arrangement of spheres on it. Call it A. For any three neighbouring spheres, a fourth sphere can be placed on top in the hollow between the three bottom spheres. If we do this for half of the holes in a second plane above the first, we create a new compact layer. There are two possible choices for doing this, call them B and C. Suppose that we chose B. Then one half of the hollows of B lies above the centers of the balls in A and one half lies above the hollows of A which were not used for B. Thus the balls of a third layer can be placed either directly above the balls of the first one, yielding a layer of type A, or above the holes of the first layer which were not occupied by the second layer, yielding a layer of type C. Combining layers of types A, B, and C produces various close-packed structures.
Two simple arrangements within the close-packed family correspond to regular lattices. One is called cubic close packing (or face-centred cubic, "FCC")—where the layers are alternated in the ABCABC... sequence. The other is called hexagonal close packing ("HCP"), where the layers are alternated in the ABAB... sequence. But many layer stacking sequences are possible (ABAC, ABCBA, ABCBAC, etc.), and still generate a close-packed structure. In all of these arrangements each sphere touches 12 neighboring spheres, and the average density is
In 1611, Johannes Kepler conjectured that this is the maximum possible density amongst both regular and irregular arrangements—this became known as the Kepler conjecture. Carl Friedrich Gauss proved in 1831 that these packings have the highest density amongst all possible lattice packings. In 1998, Thomas Callister Hales, following the approach suggested by László Fejes Tóth in 1953, announced a proof of the Kepler conjecture. Hales proof is a proof by exhaustion involving checking of many individual cases using complex computer calculations. Referees said that they were "99% certain" of the correctness of Hales proof. On 10 August 2014, Hales announced the completion of a formal proof using automated proof checking, removing any doubt. | 3 | Analytical Chemistry |
An Ubbelohde type viscometer or suspended-level viscometer is a measuring instrument which uses a capillary based method of measuring viscosity. It is recommended for higher viscosity cellulosic polymer solutions. The advantage of this instrument is that the values obtained are independent of the total volume. The device was developed by the German chemist Leo Ubbelohde (1877-1964).
ASTM and other test methods are: ISO 3104, ISO 3105, ASTM D445, ASTM D446, ASTM D4020, IP 71, BS 188.
The Ubbelohde viscometer is closely related to the Ostwald viscometer. Both are u-shaped pieces of glassware with a reservoir on one side and a measuring bulb with a capillary on the other. A liquid is introduced into the reservoir then sucked through the capillary and measuring bulb. The liquid is allowed to travel back through the measuring bulb and the time it takes for the liquid to pass through two calibrated marks is a measure for viscosity. The Ubbelohde device has a third arm extending from the end of the capillary and open to the atmosphere. In this way the pressure head only depends on a fixed height and no longer on the total volume of liquid. | 7 | Physical Chemistry |
p75NTR functions in a complex with Nogo-66 receptor (NgR1) to mediate RhoA-dependent inhibition of growth of regenerating axons exposed to inhibitory proteins of CNS myelin, such as Nogo, MAG or OMgP. Without p75NTR, OMgP can activate RhoA and inhibit CNS axon regeneration. Coexpression of p75NTR and OMgP suppress RhoA activation. A complex of NgR1, p75NTR and LINGO1 can activate RhoA. | 1 | Biochemistry |
TRIM33 acts as a tumor suppressor gene preventing the development chronic myelomonocytic leukemia.
TRIM33 regulates also the TRIM28 receptor and promotes physiological aging of hematopoietic stem cells.
TRIM33 acts as an oncogene by preventing apoptosis in B-cell leukemias. | 1 | Biochemistry |
In ancient China, the method was improved by mechanization with the development of the rotary winnowing fan, which used a cranked fan to produce the airstream. This was featured in Wang Zhens book the Nong Shu' of 1313 AD. | 3 | Analytical Chemistry |
The methyl radical has the formula . It exists in dilute gases, but in more concentrated form it readily dimerizes to ethane. It is routinely produced by various enzymes of the radical SAM and methylcobalamin varieties. | 0 | Organic Chemistry |
In the late 19th century, scientists experimentally discovered that and do not absorb infrared radiation (called, at that time, "dark radiation"), while water (both as true vapor and condensed in the form of microscopic droplets suspended in clouds) and and other poly-atomic gaseous molecules do absorb infrared radiation. In the early 20th century, researchers realized that greenhouse gases in the atmosphere made Earths overall temperature higher than it would be without them. The term greenhouse' was first applied to this phenomenon by Nils Gustaf Ekholm in 1901.
During the late 20th century, a scientific consensus evolved that increasing concentrations of greenhouse gases in the atmosphere cause a substantial rise in global temperatures and changes to other parts of the climate system, with consequences for the environment and for human health. | 2 | Environmental Chemistry |
The key elements of tholins are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthesized tholins has confirmed earlier identifications of chemical groups present, including primary amines, nitriles, and alkyl portions such as / forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of : gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952. | 9 | Geochemistry |
Butyrate has been shown to be a critical mediator of the colonic inflammatory response. It is responsible for about 70% of energy from the colonocytes, being a critical SCFA in colon homeostasis. Butyrate possesses both preventive and therapeutic potential to counteract inflammation-mediated ulcerative colitis (UC) and colorectal cancer. It produces different effects in healthy and cancerous cells: this is known as the "butyrate paradox". In particular, butyrate inhibits colonic tumor cells and stimulates proliferation of healthy colonic epithelial cells. The explanation why butyrate is an energy source for normal colonocytes and induces apoptosis in colon cancer cells, is the Warburg effect in cancer cells, which leads to butyrate not being properly metabolized. This phenomenon leads to the accumulation of butyrate in the nucleus, acting as a histone deacetylase (HDAC) inhibitor. One mechanism underlying butyrate function in suppression of colonic inflammation is inhibition of the IFN-γ/STAT1 signalling pathways. It has been shown that butyrate inhibits activity of HDAC1 that is bound to the Fas gene promoter in T cells, resulting in hyperacetylation of the Fas promoter and upregulation of Fas receptor on the T cell surface. It is thus suggested that butyrate enhances apoptosis of T cells in the colonic tissue and thereby eliminates the source of inflammation (IFN-γ production). Butyrate inhibits angiogenesis by inactivating Sp1 transcription factor activity and downregulating vascular endothelial growth factor gene expression.
In summary, the production of volatile fatty acids such as butyrate from fermentable fibers may contribute to the role of dietary fiber in colon cancer. Short-chain fatty acids, which include butyric acid, are produced by beneficial colonic bacteria (probiotics) that feed on, or ferment prebiotics, which are plant products that contain dietary fiber. These short-chain fatty acids benefit the colonocytes by increasing energy production, and may protect against colon cancer by inhibiting cell proliferation.
Conversely, some researchers have sought to eliminate butyrate and consider it a potential cancer driver. Studies in mice indicate it drives transformation of MSH2-deficient colon epithelial cells. | 1 | Biochemistry |
Furthermore, the method showed consistent performance in cancer identification, classification, and treatment effect problems like NSCLC and DLBCL identification, histological classification of subtypes of NSCLC, molecular classification of subtypes of DLBCL, DLBCL COO detection, programmed death-ligand 1 immune-checkpoint inhibition response prediction against advanced NSCLC cases, and prognostic value detection of individual genes. | 1 | Biochemistry |
A monitor in the cockpit displays detailed images in real time, and the system also logs the image and Global Positioning System data at a rate of 30 gigabytes (GB) per hour for later analysis. The on-board data processing system performs numerous real-time processing functions including data acquisition and recording, raw data correction, target detection, cueing and chipping, precision image geo-registration, and display and dissemination of image products and target cue information.
ARCHER has three methods for locating targets:
* signature matching where reflected light is matched to spectral signatures
* anomaly detection using a statistical model of the pixels in the image to determine the probability that a pixel does not match the profile, and
* change detection which executes a pixel-by-pixel comparison of the current image against ground conditions that were obtained in a previous mission over the same area.
In change detection, scene changes are identified, and new, moved or departed targets are highlighted for evaluation. In spectral signature matching, the system can be programmed with the parameters of a missing aircraft, such as paint colors, to alert the operators of possible wreckage. It can also be used to look for specific materials, such as petroleum products or other chemicals released into the environment, or even ordinary items like commonly available blue polyethylene tarpaulins. In an impact assessment role, information on the location of blue tarps used to temporarily repair buildings damaged in a storm can help direct disaster relief efforts; in a counterdrug role, a blue tarp located in a remote area could be associated with illegal activity. | 7 | Physical Chemistry |
In general α-Keggin anions are synthesized in acidic solutions. For example, 12-phosphotungstic acid is formed by condensing phosphate ion with tungstate ions. The heteropolyacid that is formed has the Keggin structure. | 7 | Physical Chemistry |
The deformation field at the slip-band is due to three-dimensional elastic and plastic strains where the concentrated shear of the slip band tip deforms the grain in its vicinity. The elastic strains describe the stress concentration ahead of the slip band, which is important as it can affect the transfer of plastic deformation across grain boundaries. An understanding of this is needed to support the study of yield and inter/intra-granular fracture. The concentrated shear of slip bands can also nucleate cracks in the plane of the slip band, and persistent slip bands that lead to intragranular fatigue crack initiation and growth may also form under cyclic loading conditions. To properly characterise slip bands and validate mechanistic models for their interactions with microstructure, it is crucial to quantify the local deformation fields associated with their propagation. However, little attention has been given to slip bands within grains (i.e., in the absence of grain boundary interaction).
The long-range stress field (i.e., the elastic strain field) around the tip of a stress concentrator, such as a slip band, can be considered a singularity equivalent to that of a crack. This singularity can be quantified using a path independent integral since it satisfies the conservation laws of elasticity. The conservation laws of elasticity related to translational, rotational, and scaling symmetries were derived initially by Knowles and Sternberg from the Noether's theorem. Budiansky and Rice introduced the J-, M-, L-integral and were the first to give them a physical interpretation as the strain energy-release rates for mechanisms such as cavity propagation, simultaneous uniform expansion, and defect rotation, respectively. When evaluated over a surface that encloses a defect, these conservation integrals represent a configurational force on the defect. That work paved the way for the field of Configurational mechanics of materials, with the path-independent J-integral now widely used to analyse the configurational forces in problems as diverse as dislocation dynamics, misfitting inclusions, propagation of cracks, shear deformation of clays, and co-planar dislocation nucleation from shear loaded cracks. The integrals have been applied to linear elastic and elastic-plastic materials and have been coupled with processes such as thermal and electrochemical loading, and internal tractions. Recently, experimental fracture mechanics studies have used full-field in situ measurements of displacements and elastic strains to evaluate the local deformation field surrounding the crack tip as a J-integral.
Slip bands form due to plastic deformation, and the analysis of the force on a dislocation considers the two-dimensional nature of the dislocation line defect. General definitions of the Peach–Koehler configurational force (𝑃) (or the elastic energy-momentum tensor ) on a dislocation in the arbitrary 𝑥, 𝑥, 𝑥 coordinate system, decompose the Burgers vector (𝑏) to orthogonal components. This leads to the generalised definition of the J-integral in equations below. For a dislocation pile-up, the J-integral is the summation of the Peach–Koehler configurational force of the dislocations in the pile-up (including out-of-plane, 𝑏 ).
𝐽 = ∫ 𝑃 𝑛 𝑑𝑆 = ∫(𝑊 𝑛− 𝑇 𝑢) 𝑑𝑆
𝐽 = 𝑅 𝐽, 𝑖,𝑗,𝑘=1,2,3
where 𝑆 is an arbitrary contour around the dislocation pile-up with unit outward normal 𝑛, 𝑊 is the strain energy density, 𝑇 = 𝜎 𝑛 is the traction on 𝑑𝑆, 𝑢 are the displacement vector components, 𝐽 is 𝐽-integral evaluated along the 𝑥 direction, and 𝑅 is a second-order mapping tensor that maps 𝐽 into 𝑥 direction. This vectorial 𝐽-integral leads to numerical difficulties in the analysis since 𝐽 and, for a three-dimensional slip band or inclined crack, the 𝐽 terms cannot be neglected. | 8 | Metallurgy |
The most common reactions of benzene involve substitution of a proton by other groups. Electrophilic aromatic substitution is a general method of derivatizing benzene. Benzene is sufficiently nucleophilic that it undergoes substitution by acylium ions and alkyl carbocations to give substituted derivatives.
The most widely practiced example of this reaction is the ethylation of benzene.
Approximately 24,700,000 tons were produced in 1999. Highly instructive but of far less industrial significance is the Friedel-Crafts alkylation of benzene (and many other aromatic rings) using an alkyl halide in the presence of a strong Lewis acid catalyst. Similarly, the Friedel-Crafts acylation is a related example of electrophilic aromatic substitution. The reaction involves the acylation of benzene (or many other aromatic rings) with an acyl chloride using a strong Lewis acid catalyst such as aluminium chloride or Iron(III) chloride. | 2 | Environmental Chemistry |
Thermal barrier coatings (TBCs) are used extensively in gas turbine engines to increase component life and engine performance. A coating of about 1-200 µm can reduce the temperature at the superalloy surface by up to 200 K. TBCs are a system of coatings consisting of a bond coat, a thermally grown oxide (TGO), and a thermally insulating ceramic top coat. In most applications, the bond coat is either a MCrAlY (where M=Ni or NiCo) or a Pt modified aluminide coating. A dense bond coat is required to provide protection of the superalloy substrate from oxidation and hot corrosion attack and to form an adherent, slow-growing surface TGO. The TGO is formed by oxidation of the aluminum that is contained in the bond coat. The current (first generation) thermal insulation layer is composed of 7wt % yttria-stabilized zirconia (7YSZ) with a typical thickness of 100–300 µm. Yttria-stabilized zirconia is used due to its low thermal conductivity (2.6W/mK for fully dense material), relatively high coefficient of thermal expansion, and high temperature stability. The electron beam-directed vapor deposition (EB-DVD) process used to apply the TBC to turbine airfoils produces a columnar microstructure with multiple porosity levels. Inter-column porosity is critical to providing strain tolerance (via a low in-plane modulus), as it would otherwise spall on thermal cycling due to thermal expansion mismatch with the superalloy substrate. This porosity reduces the thermal coating's conductivity. | 8 | Metallurgy |
In April the Committee for Medicinal Products for Human Use of the European Medicines Agency endorsed a gene therapy treatment called Strimvelis and the European Commission approved it in June. This treats children born with adenosine deaminase deficiency and who have no functioning immune system. This was the second gene therapy treatment to be approved in Europe.
In October, Chinese scientists reported they had started a trial to genetically modify T cells from 10 adult patients with lung cancer and reinject the modified T cells back into their bodies to attack the cancer cells. The T cells had the PD-1 protein (which stops or slows the immune response) removed using CRISPR-Cas9.
A 2016 Cochrane systematic review looking at data from four trials on topical cystic fibrosis transmembrane conductance regulator (CFTR) gene therapy does not support its clinical use as a mist inhaled into the lungs to treat cystic fibrosis patients with lung infections. One of the four trials did find weak evidence that liposome-based CFTR gene transfer therapy may lead to a small respiratory improvement for people with CF. This weak evidence is not enough to make a clinical recommendation for routine CFTR gene therapy. | 1 | Biochemistry |
In some forms of photosynthetic bacteria, a chromatophore is a pigmented(coloured), membrane-associated vesicle used to perform photosynthesis. They contain different coloured pigments.
Chromatophores contain bacteriochlorophyll pigments and carotenoids. In purple bacteria, such as Rhodospirillum rubrum, the light-harvesting proteins are intrinsic to the chromatophore membranes. However, in green sulfur bacteria, they are arranged in specialised antenna complexes called chlorosomes. | 5 | Photochemistry |
The Permian–Triassic extinction event, triggered by runaway from the Siberian Traps, was marked by ocean deoxygenation.
The boundary between the Ordovician and Silurian periods is marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic periods occurred at a time of low global temperatures (although levels were high), in the midst of a glaciation.
Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water. If the high latitude waters are below , they will be dense enough to sink; as they are cool, oxygen is highly soluble in their waters, and the deep ocean will be oxygenated. If high latitude waters are warmer than , their density is too low for them to sink below the cooler deep waters. Therefore, thermohaline circulation can only be driven by salt-increased density, which tends to form in warm waters where evaporation is high. This warm water can dissolve less oxygen, and is produced in smaller quantities, producing a sluggish circulation with little deep water oxygen. The effect of this warm water propagates through the ocean, and reduces the amount of that the oceans can hold in solution, which makes the oceans release large quantities of into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia. Similar positive feedbacks operate during cold-pole episodes, amplifying their cooling effects.
The periods with cold poles are termed "P-episodes" (short for primo), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events—for example, the Ireviken and Lau events. The inverse is true for the warmer, oxic "S-episodes" (secundo), where deep ocean sediments are typically graptolitic black shales.
A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.
The duration of events is so long compared to their onset because the positive feedbacks must be overwhelmed. Carbon content in the ocean-atmosphere system is affected by changes in weathering rates, which in turn is dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high ) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted the signal of Milankovic cycles, which ultimately trigger the switch between P- and S- episodes.
These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.
The end-Ordovician Hirnantian event may alternatively be a result of algal blooms, caused by sudden supply of nutrients through wind-driven upwelling or an influx of nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation. | 9 | Geochemistry |
The term radical was already in use when radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase "radical" in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie. A radical was identified as the root base of certain acids (The Latin word "radix" meaning "root"). The combination of a radical with oxygen would result in an acid. For example the radical of acetic acid was called "acetic" and that of muriatic acid (hydrochloric acid) was called "muriatic". Joseph Louis Gay-Lussac found evidence for the cyanide radical in 1815 in his work on hydrogen cyanide and a number of cyanide salts he discovered. He also isolated cyanogen ((CN)) not realizing that cyanogen is the cyanide dimer NC-CN. Jean-Baptiste Dumas proposed the ethylene radical from investigations into diethyl ether and ethanol. In his Etherin theory he observed that ether consisted of two equivalents of ethylene and one equivalent of water and that ethylene and ethanol could interconvert in chemical reactions. Ethylene was also the base fragment for a number of other compounds such as ethyl acetate. This Etherin theory was eventually abandoned by Dumas in favor of radical theory. As a radical it should react with an oxide to form the hydrate but it was found that ethylene is resistant to an oxide like calcium oxide. Henri Victor Regnault in 1834 reacted ethylene dichloride (CHCH.Cl) with KOH forming vinyl chloride, water, and KCl. In etherin theory it should not be possible to break up the ethylene fragment in this way.
Radical theory replaced electrochemical dualism which stated that all molecules were to be considered as salts composed of basic and acidic oxides. | 0 | Organic Chemistry |
Arthur John Birch, AC CMG FRS FAA (3 August 1915 – 8 December 1995) was an Australian organic chemist.
Birch developed the Birch reduction of aromatic rings (by treatment with lithium metal and ammonia) which is widely used in synthetic organic chemistry. The Birch Reduction enables the modification of steroids. In 1948 Birch published the first total synthesis of a male sex hormone (19-nortestosterone), as the first member of a new structural series. This series later comprised the first oral contraceptive pill, which was made by others. The Birch reduction also allows for the development of other steroid drugs and antibiotics – he also made the first simple synthesis of the ring A-B structure of cholesterol. Birch published over 440 scientific papers and reports. | 0 | Organic Chemistry |
Sulfuric acid is rarely encountered naturally on Earth in anhydrous form, due to its great affinity for water. Dilute sulfuric acid is a constituent of acid rain, which is formed by atmospheric oxidation of sulfur dioxide in the presence of water – i.e. oxidation of sulfurous acid. When sulfur-containing fuels such as coal or oil are burned, sulfur dioxide is the main byproduct (besides the chief products carbon oxides and water).
Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as pyrite:
The resulting highly acidic water is called acid mine drainage (AMD) or acid rock drainage (ARD).
The can be further oxidized to :
The produced can be precipitated as the hydroxide or hydrous iron oxide:
The iron(III) ion ("ferric iron") can also oxidize pyrite:
When iron(III) oxidation of pyrite occurs, the process can become rapid. pH values below zero have been measured in ARD produced by this process.
ARD can also produce sulfuric acid at a slower rate, so that the acid neutralizing capacity (ANC) of the aquifer can neutralize the produced acid. In such cases, the total dissolved solids (TDS) concentration of the water can be increased from the dissolution of minerals from the acid-neutralization reaction with the minerals.
Sulfuric acid is used as a defense by certain marine species, for example, the phaeophyte alga Desmarestia munda (order Desmarestiales) concentrates sulfuric acid in cell vacuoles. | 7 | Physical Chemistry |
The N-terminus for Caldoramide is N,N-dimethylvaline which is attached to a valine which is attached to an N-Me-valine connected to an N-Me-isoleucine which is attached to the C-terminus. The molecule can also be written as N,N-diMe-Val-Val-N-Me-Val-N-Me-Ile-3-O-Me-4-benzylpyrrolinone. | 1 | Biochemistry |
A primer dimer is formed and amplified in three steps. In the first step, two primers anneal at their respective 3 ends (step I in the figure). If this construct is stable enough, the DNA polymerase will bind and extend the primers according to the complementary sequence (step II in the figure). An important factor contributing to the stability of the construct in step I is a high GC-content at the 3 ends and length of the overlap. The third step occurs in the next cycle, when a single strand of the product of step II is used as a template to which fresh primers anneal leading to synthesis of more PD product. | 1 | Biochemistry |
Robert Marc Mazo, born in 1930 in Brooklyn, New York, is the son of Nathan and Rose Marion (Mazo) Mazo. While in high school in 1948, Mazo won the Seventh Science Talent Search with the project, "Reactions in Liquid Ammonia".
Mazo completed a A.B. at Harvard University in 1952, and an M.S. in Science at Yale University in 1953. With NSF fellowship funding, he earned a Doctor of Philosophy degree at Yale University in 1955, with his dissertation, Theoretical Studies On Low Temperature Phenomena, advised by Lars Onsager and John Gamble Kirkwood.
Mazo and Joan Ruth Spector wed in 1954, and their family includes a daughter and two sons. | 7 | Physical Chemistry |
Lichens may be long-lived, with some considered to be among the oldest living organisms. Lifespan is difficult to measure because what defines the "same" individual lichen is not precise. Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the "same" lichen, and two lichens can merge, then becoming the "same" lichen. One specimen of Rhizocarpon geographicum on East Baffin Island has an estimated age of 9500 years. Thalli of Rhizocarpon geographicum and Rhizocarpon eupetraeoides/inarense in the central Brooks Range of northern Alaska have been given a maximum possible age of 10,000–11,500 years. | 2 | Environmental Chemistry |
Areas that have limited surface water or groundwater may choose to desalinate. RO is an increasingly common method, because of its relatively low energy consumption.
Energy consumption is around , with the development of more efficient energy recovery devices and improved membrane materials. According to the International Desalination Association, for 2011, RO was used in 66% of installed desalination capacity (0.0445 of 0.0674 km/day), and nearly all new plants. Other plants use thermal distillation methods: multiple-effect distillation, and multi-stage flash.
Sea-water RO (SWRO) desalination requires around 3 kWh/m, much higher than those required for other forms of water supply, including RO treatment of wastewater, at 0.1 to 1 kWh/m. Up to 50% of the seawater input can be recovered as fresh water, though lower recovery rates may reduce membrane fouling and energy consumption.
Brackish water reverse osmosis (BWRO) is the desalination of water with less salt than seawater, usually from river estuaries or saline wells. The process is substantially the same as SWRO, but requires lower pressures and less energy. Up to 80% of the feed water input can be recovered as fresh water, depending on feed salinity.
The Ashkelon desalination plant in Israel is the world's largest.
The typical single-pass SWRO system consists of:
* Intake
* Pretreatment
* High-pressure pump (if not combined with energy recovery)
* Membrane assembly
* Energy recovery (if used)
* Remineralisation and pH adjustment
* Disinfection
* Alarm/control panel | 3 | Analytical Chemistry |
A surface in a liquid may be charged by dissociation of surface groups (e.g. silanol groups for glass or silica surfaces) or by adsorption of charged molecules such as polyelectrolyte from the surrounding solution. This results in the development of a wall surface potential which will attract counterions from the surrounding solution and repel co-ions. In equilibrium, the surface charge is balanced by oppositely charged counterions in solution. The region near the surface of enhanced
counterion concentration is called the electrical double layer (EDL). The EDL can be approximated by a sub-division into two regions. Ions in the region closest to the charged wall surface are strongly bound to the surface. This immobile layer is called the Stern or Helmholtz layer. The region adjacent to the Stern layer is called the diffuse layer and contains loosely associated ions that are comparatively mobile. The total electrical double layer due to the formation of the counterion layers results in electrostatic screening of the wall charge and minimizes the Gibbs free energy of EDL formation.
The thickness of the diffuse electric double layer is known as the Debye screening length . At a distance of two Debye screening lengths the electrical potential energy is reduced to 2 percent of the value at the surface wall.
with unit of , where
* is the number density of ion i in the bulk solution,
* is the valency of the ion (for example, H has a valency of +1, and Ca has a valency of +2),
* is the vacuum permittivity, is the relative static permittivity,
* is the Boltzmann constant.
The repulsive free energy per unit area between two planar surfaces is shown as
where
* is the reduced surface potential, ,
* is the potential on the surface.
The interaction free energy between two spheres of radius R is
Combining the van der Waals interaction energy and the double layer interaction energy, the interaction between two particles or two surfaces in a liquid can be expressed as
where W(D) is the repulsive interaction energy due to electric repulsion, and W(D) is the attractive interaction energy due to van der Waals interaction. | 7 | Physical Chemistry |
Members of the BMP family were originally found to induce bone formation, as their name suggests. However, BMPs are very multifunctional and can also regulate apoptosis, cell migration, cell division, and differentiation. They also specify the anterior/posterior axis, induce growth, and regulate homeostasis.
The BMPs bind to the bone morphogenetic protein receptor type II (BMPR2). Some of the proteins of the BMP family are BMP4 and BMP7. BMP4 promotes bone formation, causes cell death, or signals the formation of epidermis, depending on the tissue it is acting on. BMP7 is crucial for kidney development, sperm synthesis, and neural tube polarization. Both BMP4 and BMP7 regulate mature ligand stability and processing, including degrading ligands in lysosomes. BMPs act by diffusing from the cells that create them. | 1 | Biochemistry |
Based on the results of , the IAEA defined the delta scale with SLAP at −55.5‰ for O and −428‰ for H. That is, SLAP was measured to contain approximately 5.55% less oxygen-18 and 42.8% less deuterium than does VSMOW, and these figures were used to anchor the scale at two points. Experimental figures are given below.
* H / H – , , about 1 in 11230 atoms
* H / H – = , measured on 16 September 1976, about 1 in 2.6710 atoms
* O / O – , , about 1 in 528 atoms
* O / O – , about 1 in 3700 atoms | 9 | Geochemistry |
Contaminants that may be in untreated water include microorganisms such as viruses, protozoa and bacteria; inorganic contaminants such as salts and metals; organic chemical contaminants from industrial processes and petroleum use; pesticides and herbicides; and radioactive contaminants. Water quality depends on the local geology and ecosystem, as well as human uses such as sewage dispersion, industrial pollution, use of water bodies as a heat sink, and overuse (which may lower the level of the water).
The United States Environmental Protection Agency (EPA) limits the amounts of certain contaminants in tap water provided by US public water systems. The Safe Drinking Water Act authorizes EPA to issue two types of standards:
* primary standards regulate substances that potentially affect human health;
* secondary standards prescribe aesthetic qualities, those that affect taste, odor, or appearance.
The U.S. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water. Drinking water, including bottled water, may reasonably be expected to contain at least small amounts of some contaminants. The presence of these contaminants does not necessarily indicate that the water poses a health risk.
In urbanized areas around the world, water purification technology is used in municipal water systems to remove contaminants from the source water (surface water or groundwater) before it is distributed to homes, businesses, schools and other recipients. Water drawn directly from a stream, lake, or aquifer and that has no treatment will be of uncertain quality in terms of potability.
The burden of polluted drinking water disproportionally effects under-represented and vulnerable populations. Communities that lack these clean drinking-water services are at risk of contracting water-borne and pollution-related illnesses like Cholera, diarrhea, dysentery, hepatitis A, typhoid, and polio. These communities are often in low-income areas, where human wastewater is discharged into a nearby drainage channel or surface water drain without sufficient treatment, or is used in agricultural irrigation. | 3 | Analytical Chemistry |
A variety of experimental concerns exist for IMHR reactions. Although most of the common Pd(0) catalysts are commercially available (Pd(PPh), Pd(dba), and derivatives), they may also be prepared by simple, high-yielding procedures. Palladium(II) acetate is cheap and may be reduced in situ to palladium(0) with phosphine. Three equivalents of phosphine per equivalent of palladium acetate are commonly used; these conditions generate Pd(PR) as the active catalyst. Bidentate phosphine ligands are common in asymmetric reactions to enhance stereoselectivity.
A wide variety of bases may be used, and the base is often employed in excess. Potassium carbonate is the most common base employed, and inorganic bases are generally used more often than organic bases. A number of additives have also been identified for the Heck reaction—silver salts may be used to drive the reaction down the cationic pathway, and halide salts may be used to convert aryl triflates via the neutral pathway. Alcohols have been shown to enhance catalyst stability in some cases, and acetate salts are beneficial in reactions following the anionic pathway. | 0 | Organic Chemistry |
Nucleosomes are the basic packing unit of genomic DNA built from histone proteins around which DNA is coiled. They serve as a scaffold for formation of higher order chromatin structure as well as for a layer of regulatory control of gene expression. Nucleosomes are quickly assembled onto newly synthesized DNA behind the replication fork. | 1 | Biochemistry |
Hemoglobin has a quaternary structure characteristic of many multi-subunit globular proteins. Most of the amino acids in hemoglobin form alpha helices, and these helices are connected by short non-helical segments. Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, which then causes each polypeptide chain to fold into a specific shape. Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement.
In most vertebrates, the hemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-protein prosthetic heme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement. Such a name is given because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin. This folding pattern contains a pocket that strongly binds the heme group.
A heme group consists of an iron (Fe) ion held in a heterocyclic ring, known as a porphyrin. This porphyrin ring consists of four pyrrole molecules cyclically linked together (by methine bridges) with the iron ion bound in the center. The iron ion, which is the site of oxygen binding, coordinates with the four nitrogen atoms in the center of the ring, which all lie in one plane. The heme is bound strongly (covalently) to the globular protein via the N atoms of the imidazole ring of F8 histidine residue (also known as the proximal histidine) below the porphyrin ring. A sixth position can reversibly bind oxygen by a coordinate covalent bond, completing the octahedral group of six ligands. This reversible bonding with oxygen is why hemoglobin is so useful for transporting oxygen around the body. Oxygen binds in an "end-on bent" geometry where one oxygen atom binds to Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.
Even though carbon dioxide is carried by hemoglobin, it does not compete with oxygen for the iron-binding positions but is bound to the amine groups of the protein chains attached to the heme groups.
The iron ion may be either in the ferrous Fe or in the ferric Fe state, but ferrihemoglobin (methemoglobin) (Fe) cannot bind oxygen. In binding, oxygen temporarily and reversibly oxidizes (Fe) to (Fe) while oxygen temporarily turns into the superoxide ion, thus iron must exist in the +2 oxidation state to bind oxygen. If superoxide ion associated to Fe is protonated, the hemoglobin iron will remain oxidized and incapable of binding oxygen. In such cases, the enzyme methemoglobin reductase will be able to eventually reactivate methemoglobin by reducing the iron center.
In adult humans, the most common hemoglobin type is a tetramer (which contains four subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as αβ. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 16,000 daltons, for a total molecular weight of the tetramer of about 64,000 daltons (64,458 g/mol). Thus, 1 g/dL = 0.1551 mmol/L. Hemoglobin A is the most intensively studied of the hemoglobin molecules.
In human infants, the fetal hemoglobin molecule is made up of 2 α chains and 2 γ chains. The γ chains are gradually replaced by β chains as the infant grows.
The four polypeptide chains are bound to each other by salt bridges, hydrogen bonds, and the hydrophobic effect. | 7 | Physical Chemistry |
In the 17th century, Johann Rudolf Glauber devised a process to obtain nitric acid by distilling potassium nitrate with sulfuric acid. In 1776 Antoine Lavoisier cited Joseph Priestley's work to point out that it can be converted from nitric oxide (which he calls "nitrous air"), "combined with an approximately equal volume of the purest part of common air, and with a considerable quantity of water." In 1785 Henry Cavendish determined its precise composition and showed that it could be synthesized by passing a stream of electric sparks through moist air. In 1806, Humphry Davy reported the results of extensive distilled water electrolysis experiments concluding that nitric acid was produced at the anode from dissolved atmospheric nitrogen gas. He used a high voltage battery and non-reactive electrodes and vessels such as gold electrode cones that doubled as vessels bridged by damp asbestos.
The industrial production of nitric acid from atmospheric air began in 1905 with the Birkeland–Eyde process, also known as the arc process. This process is based upon the oxidation of atmospheric nitrogen by atmospheric oxygen to nitric oxide with a very high temperature electric arc. Yields of up to approximately 4–5% nitric oxide were obtained at 3000 °C, and less at lower temperatures. The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to nitrogen dioxide, and this was subsequently absorbed in water in a series of packed column or plate column absorption towers to produce dilute nitric acid. The first towers bubbled the nitrogen dioxide through water and non-reactive quartz fragments. About 20% of the produced oxides of nitrogen remained unreacted so the final towers contained an alkali solution to neutralize the rest. The process was very energy intensive and was rapidly displaced by the Ostwald process once cheap ammonia became available.
Another early production method was invented by French engineer Albert Nodon around 1913. His method produced nitric acid from electrolysis of calcium nitrate converted by bacteria from nitrogenous matter in peat bogs. An earthenware pot surrounded by limestone was sunk into the peat and staked with tarred lumber to make a compartment for the carbon anode around which the nitric acid is formed. Nitric acid was pumped out from an earthenware pipe that was sunk down to the bottom of the pot. Fresh water was pumped into the top through another earthenware pipe to replace the fluid removed. The interior was filled with coke. Cast iron cathodes were sunk into the peat surrounding it. Resistance was about 3 ohms per cubic meter and the power supplied was around 10 volts. Production from one deposit was 800 tons per year.
Once the Haber process for the efficient production of ammonia was introduced in 1913, nitric acid production from ammonia using the Ostwald process overtook production from the Birkeland–Eyde process. This method of production is still in use today. | 3 | Analytical Chemistry |
In addition to being used for their traditional properties, bioactive ceramics have seen specific use for due to their biological activity. Calcium phosphates, oxides, and hydroxides are common examples. Other natural materials — generally of animal origin — such as bioglass and other composites feature a combination of mineral-organic composite materials such as HAP, alumina, or titanium dioxide with the biocompatible polymers (polymethylmethacrylate): PMMA, poly(L-lactic) acid: PLLA, poly(ethylene). Composites can be differentiated as bioresorbable or non-bioresorbable, with the latter being the result of the combination of a bioresorbable calcium phosphate (HAP) with a non-bioresorbable polymer (PMMA, PE). These materials may become more widespread in the future, on account of the many combination possibilities and their aptitude at combining a biological activity with mechanical properties similar to those of the bone. | 7 | Physical Chemistry |
In mass spectrometry, an ion funnel is a device used to focus a beam of ions using a series of stacked ring electrodes with decreasing inner diameter. A combined radio frequency and fixed electrical potential is applied to the grids. In electrospray ionization-mass spectrometry (ESI-MS), ions are created at atmospheric pressure, but are analyzed at subsequently lower pressures. Ions can be lost while they are shuttled from areas of higher to lower pressure due to the transmission process caused by a phenomenon called joule expansion or “free-jet expansion.” These ion clouds expand outward, which limits the amount of ions that reach the detector, so fewer ions are analyzed. The ion funnel refocuses and transmits ions efficiently from those areas of high to low pressure. | 7 | Physical Chemistry |
*Stress corrosion cracking Stress corrosion (NACE term)
*Corrosion fatigue
*Caustic cracking (ASTM term)
*Caustic embrittlement (ASM term)
*Sulfide stress cracking (ASM, NACE term)
*Stress-accelerated Corrosion (NACE term)
*Hydrogen stress cracking (ASM term)
*Hydrogen-assisted stress corrosion cracking (ASM term) | 8 | Metallurgy |
The (tert-butyl)cyclopentadiene is prepared by alkylation of cyclopentadiene with tert-butyl bromide in the presence of sodium hydride and dibenzo-18-crown-6. The intermediate in this synthesis is di-tert-butylcyclopentadiene. This compound is conveniently prepared by alkylation of cyclobutadiene with tert-butyl bromide under phase-transfer conditions.
Illustrative of the unusual complexes made possible with these bulky ligands is molecular iron nitrido complex (BuCH)FeN. In contrast to (CMe)IrCl, (BuCH)IrCl is monomeric. | 0 | Organic Chemistry |
There are multiple pseudouridine detection methods beginning with the addition of N-cyclohexyl-N′-b-(4-methylmorpholinium) ethylcarbodiimide metho-p-toluene-sulfonate (CMCT; also known as CMC), since its reaction with pseudouridine produces CMC-Ψ. CMC-Ψ causes reverse transcriptase to stall one nucleotide in the 3’ direction. These methods have single-nucleotide resolution.
In an optimisation step, azido-CMC can confer the ability to add biotinylation; subsequent biotin pulldown will enrich Ψ-containing transcripts, allowing identification of even low-abundance transcripts. | 1 | Biochemistry |
The three main side reactions that produce impurities have in common that they decompose urea.
Urea hydrolyzes back to ammonium carbamate in the hottest stages of the synthesis plant, especially in the stripper, so residence times in these stages are designed to be short.
Biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia.
Normally this reaction is suppressed in the synthesis reactor by maintaining an excess of ammonia, but after the stripper, it occurs until the temperature is reduced. Biuret is undesirable in urea fertilizer because it is toxic to crop plants to varying degrees, but it is sometimes desirable as a nitrogen source when used in animal feed.
Isocyanic acid HNCO and ammonia results from the thermal decomposition of ammonium cyanate , which is in chemical equilibrium with urea:
This decomposition is at its worst when the urea solution is heated at low pressure, which happens when the solution is concentrated for prilling or granulation (see below). The reaction products mostly volatilize into the overhead vapours, and recombine when these condense to form urea again, which contaminates the process condensate. | 0 | Organic Chemistry |
* Water: water has the highest volumetric heat capacity of all commonly used material. Typically, it is placed in large container(s), acrylic tubes for example, in an area with direct sunlight. It may also be used to saturate other types material such as soil to increase heat capacity.
* Concrete, clay bricks and other forms of masonry: the thermal conductivity of concrete depends on its composition and curing technique. Concretes with stones are more thermally conductive than concretes with ash, perlite, fibers, and other insulating aggregates. Concrete's thermal mass properties save 5–8% in annual energy costs compared to softwood lumber.
* Insulated concrete panels consist of an inner layer of concrete to provide the thermal mass factor. This is insulated from the outside by a conventional foam insulation and then covered again with an outer layer of concrete. The effect is a highly efficient building insulation envelope.
* Insulating concrete forms are commonly used to provide both thermal mass and insulation to building structures. The concrete mass provides the specific heat capacity required for good thermal inertia. Insulating layers created on the side or interior surfaces of the form provide good thermal resistance.
* Clay brick, adobe brick or mudbrick: see brick and adobe.
* Earth, mud and sod: dirt's heat capacity depends on its density, moisture content, particle shape, temperature, and composition. Early settlers to Nebraska built houses with thick walls made of dirt and sod because wood, stone, and other building materials were scarce. The extreme thickness of the walls provided some insulation, but mainly served as thermal mass, absorbing thermal energy during the day and releasing it during the night. Nowadays, people sometimes use earth sheltering around their homes for the same effect. In earth sheltering, the thermal mass comes not only from the walls of the building, but from the surrounding earth that is in physical contact with the building. This provides a fairly constant, moderating temperature that reduces heat flow through the adjacent wall.
* Rammed earth: rammed earth provides excellent thermal mass because of its high density, and the high specific heat capacity of the soil used in its construction.
* Natural rock and stone: see stonemasonry.
* Logs are used as a building material to create the exterior, and perhaps also the interior, walls of homes. Log homes differ from some other construction materials listed above because solid wood has both moderate R-value (insulation) and also significant thermal mass. In contrast, water, earth, rocks, and concrete all have low R-values. This thermal mass allows a log home to hold heat better in colder weather, and to better retain its cooler temperature in hotter weather.
* Phase-change materials | 7 | Physical Chemistry |
Products of the reaction include not only biodiesel, but also the byproducts soap, glycerol, excess alcohol, and trace amounts of water. All of these byproducts must be removed to meet the standards, but the order of removal is process-dependent.
The density of glycerol is greater than that of biodiesel, and this property difference is exploited to separate the bulk of the glycerol coproduct. Residual methanol is typically recovered by distillation and reused. Soaps can be removed or converted into acids. Residual water is also removed from the fuel. | 0 | Organic Chemistry |
The concept of microemulsion is applied in this process. The transesterification involves sucrose and fatty acid methyl ester in a solvent, propylene glycol. A basic catalyst, such as anhydrous potassium carbonate, and soap, or a fatty acid salt, are added. The reaction is carried out at 130-135 °C. Propylene glycol is removed through distillation under vacuum at above 120 °C. The purified product is achieved by filtration. The yield of the reaction is 96%. 85% of sucrose esters is monosubstituted and 15% is disubstituted. | 0 | Organic Chemistry |
The term supermolecule (or supramolecule) was introduced by Karl Lothar Wolf et al. (Übermoleküle) in 1937 to describe hydrogen-bonded acetic acid dimers. The study of non-covalent association of complexes of molecules has since developed into the field of supramolecular chemistry. The term supermolecule is sometimes used to describe supramolecular assemblies, which are complexes of two or more molecules (often macromolecules) that are not covalently bonded. The term supermolecule is also used in biochemistry to describe complexes of biomolecules, such as peptides and oligonucleotides composed of multiple strands. | 6 | Supramolecular Chemistry |
Minze Stuiver (25 October 1929 – 26 December 2020) was a Dutch geochemist who was at the forefront of geoscience research from the 1960s until his retirement in 1998. He helped transform radiocarbon dating from a simple tool for archaeology and geology to a precise technique with applications in solar physics, oceanography, geochemistry, and carbon dynamics. Minze Stuiver's research encompassed the use of radiocarbon (C) to understand solar cycles and radiocarbon production, ocean circulation, lake carbon dynamics and archaeology as well as the use of stable isotopes to document past climate changes. | 9 | Geochemistry |
Photoprotection of the human skin is achieved by extremely efficient internal conversion of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV photon into small, harmless amounts of heat. If the energy of the UV photon were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).
In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life. The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage and indirect DNA damage. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10s)—this way the excited DNA does not have enough time to react with other molecules.
For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat.
This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals. | 5 | Photochemistry |
Ethanol precipitation is a method used to purify and/or concentrate RNA, DNA, and polysaccharides such as pectin and xyloglucan from aqueous solutions by adding ethanol as an antisolvent. | 1 | Biochemistry |
Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into outer space from Earth's surface, or from the skin of a human observer. The effect is well-known among amateur astronomers.
The effect can be experienced by comparing skin temperature from looking straight up into a cloudless night sky for several seconds, to that after placing a sheet of paper between the face and the sky. Since outer space radiates at about a temperature of , and the sheet of paper radiates at about (around room temperature), the sheet of paper radiates more heat to the face than does the darkened cosmos. The effect is blunted by Earth's surrounding atmosphere, and particularly the water vapor it contains, so the apparent temperature of the sky is far warmer than outer space. The sheet does not block the cold, but instead reflects heat to the face and radiates the heat of the face that it just absorbed.
The same radiative cooling mechanism can cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing. | 7 | Physical Chemistry |
Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as CO is added). A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. As CO is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow. Thus, BTB is commonly used in science classes to demonstrate that the more that muscles are used, the greater the CO output.
Bromothymol blue has been used in conjunction with phenol red to monitor the fungal asparaginase enzyme activity with phenol red turning pink and bromothymol blue turning blue indicating an increase in pH and therefore enzyme activity. However, a recent study suggests that methyl red is more useful in determining activity due to the bright yellow ring formed in the zone of enzyme activity.
It may also be used in the laboratory as a biological slide stain. At this point, the bromothymol is already blue, and a few drops of BTB are used on a water slide. The specimen is mixed with blue BTB solution and fixed to a slide by a cover slip. It is sometimes used to define cell walls or nuclei under the microscope.
Bromothymol is used in obstetrics for detecting premature rupture of membranes. Amniotic fluid typically has a pH > 7.2, bromothymol will therefore turn blue when brought in contact with fluid leaking from the amnion. As vaginal pH normally is acidic, the blue color indicates the presence of amniotic fluid. The test may be false-positive in the presence of other alkaline substances such as blood or semen, or in the presence of bacterial vaginosis. | 3 | Analytical Chemistry |
Electron paramagnetic resonance (EPR) is a technique that allows the detection of free radicals formed in chemical or biological systems. In addition, it studies the symmetry and electronic distribution of paramagnetic ions. This is a highly specific technique because the magnetic parameters are characteristic of each ion or free radical. The physical principles of this technique are analogous to those of NMR, but in the case of EPR, electronic spins are excited instead of nuclear, that is interesting in certain electrode reactions. | 7 | Physical Chemistry |
Symptoms of M2DS include infantile hypotonia and failure to thrive, delayed psychomotor development, impaired speech, abnormal or absent gait, epilepsy, spasticity, gastrointestinal motility problems, recurrent infections, and genitourinary abnormalities. Many of those affected by M2DS also fit diagnostic criteria for autism. M2DS can be associated with syndromic facies, namely an abnormally flat back of the head, underdevelopment of the midface, ear anomalies, deep-set eyes, prominent chin, pointed nose, and a flat nasal bridge. | 1 | Biochemistry |
A description of any thermodynamic system employs the four laws of thermodynamics that form an axiomatic basis. The first law specifies that energy can be transferred between physical systems as heat, as work, and with transfer of matter. The second law defines the existence of a quantity called entropy, that describes the direction, thermodynamically, that a system can evolve and quantifies the state of order of a system and that can be used to quantify the useful work that can be extracted from the system.
In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of the thermodynamic system and its surroundings. A system is composed of particles, whose average motions define its properties, and those properties are in turn related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.
With these tools, thermodynamics can be used to describe how systems respond to changes in their environment. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, corrosion engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, materials science, and economics, to name a few.
This article is focused mainly on classical thermodynamics which primarily studies systems in thermodynamic equilibrium. Non-equilibrium thermodynamics is often treated as an extension of the classical treatment, but statistical mechanics has brought many advances to that field. | 7 | Physical Chemistry |
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle furan, but the furanose ring does not have double bonds. | 0 | Organic Chemistry |
Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O/min or joule per hour per kg body mass J/(h·kg). Proper measurement requires a strict set of criteria to be met. These criteria include being in a physically and psychologically undisturbed state and being in a thermally neutral environment while in the post-absorptive state (i.e., not actively digesting food). In bradymetabolic animals, such as fish and reptiles, the equivalent term standard metabolic rate (SMR) applies. It follows the same criteria as BMR, but requires the documentation of the temperature at which the metabolic rate was measured. This makes BMR a variant of standard metabolic rate measurement that excludes the temperature data, a practice that has led to problems in defining "standard" rates of metabolism for many mammals.
Metabolism comprises the processes that the body needs to function. Basal metabolic rate is the amount of energy per unit of time that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate affects the rate that a person burns calories and ultimately whether that individual maintains, gains, or loses weight. The basal metabolic rate accounts for about 60 to 75% of the daily calorie expenditure by individuals. It is influenced by several factors. In humans, BMR typically declines by 1–2% per decade after age 20, mostly due to loss of fat-free mass, although the variability between individuals is high. | 1 | Biochemistry |
Supramolecular containers do not only have an application in catalysis but also in the opposite, namely, inhibition. A container molecule could encapsulate a guest molecule and thus subsequently renders the guest unreactive. A mechanism of inhibition could either be that the substrate is completely isolated from the reagent or that the container molecule destabilize the transition state of the reaction.
Nitschke and coworkers invented a self-assembly ML supramolecular host with a tetrahedral hydrophobic cavity that can encapsulate white phosphorus. Pyrophoric phosphorus, which could self-combust upon contact with air, is rendered air-stable within the cavity. Even though the hole in the cavity is large enough for an oxygen molecule to enter, the transition state of the combustion is too large to fit within the small cage cavity. | 6 | Supramolecular Chemistry |
Bacterial DNA is packed with the help of polyamines and proteins called nucleoid-associated proteins. Protein-associated DNA occupies about 1/4 of the intracellular volume forming a concentrated viscous phase with liquid crystalline properties, called the nucleoid. Other research also indicated that the genome of bacteria occupies approximately 10-15% of the bacteria's volume. Similar DNA packaging exists also in chloroplasts and mitochondria. Bacterial DNA is sometimes referred to as the bacterial chromosome. Bacterial nucleoid evolutionary represents an intermediate engineering solution between the protein-free DNA packing in viruses and protein-determined packing in eukaryotes.
Sister chromosomes in the bacterium Escherichia coli are induced by stressful conditions to condense and undergo pairing. Stress-induced condensation occurs by a non-random, zipper-like convergence of sister chromosomes. This convergence appears to depend on the ability of identical double-stranded DNA molecules to specifically identify each other, a process that culminates in the proximity of homologous sites along the paired chromosomes. Diverse stress conditions appear to prime bacteria to effectively cope with severe DNA damages such as double-strand breaks. The apposition of homologous sites associated with stress-induced chromosome condensation helps explain how repair of double-strand breaks and other damages occurs. | 1 | Biochemistry |
(+)-Discodermolide is a polyketide natural product found to stabilize microtubules. (+)-discodermolide was isolated by Gunasekera and his co-workers at the Harbor Branch Oceanographic Institute from the deep-sea sponge Discodermia dissoluta in 1990. (+)-Discodermolide was found to be a potent inhibitor of tumor cell growth in several MDR cancer cell lines. (+)-discodermolide also shows some unique characters, including a linear backbone structure, immunosuppressive properties both in vitro and in vivo, potent induction of an accelerated senescence phenotype, and synergistic antiproliferative activity in combination with paclitaxel. Discodermolide was recognized as one of the most potent natural promoters of tubulin assembly. A large number of efforts toward the total synthesis of (+)-discodermolide were directed by its interesting biological activities and extreme scarcity of natural sources (0.002% w/w from frozen marine sponge). The compound supply necessary for complete clinical trials cannot be met by harvesting, isolation, and purification. As of 2005, attempts at synthesis or semi-synthesis by fermentation have proven unsuccessful. As a result, all discodermolide used in preclinical studies and clinical trials has come from large-scale total synthesis. | 0 | Organic Chemistry |
Laser spray ionization refers to one of several methods for creating ions using a laser interacting with a spray of neutral particles or ablating material to create a plume of charged particles. The ions thus formed can be separated by m/z with mass spectrometry. Laser spray is one of several ion sources that can be coupled with liquid chromatography-mass spectrometry for the detection of larger molecules. | 3 | Analytical Chemistry |
In a living cell, ultrasensitive modules are embedded in a bigger network with upstream and downstream components. This components may constrain the range of inputs that the module will receive as well as the range of the module's outputs that network will be able to detect. Altszyler et al. (2014) studied how the effective ultrasensitivity of a modular system is affected by these restrictions. They found for some ultrasensitive motifs that dynamic range limitations imposed by downstream components can produce effective sensitivities much larger than that of the original module when considered in isolation. | 1 | Biochemistry |
Ouabain or (from Somali waabaayo, "arrow poison" through French ouabaïo) also known as g-strophanthin, is a plant derived toxic substance that was traditionally used as an arrow poison in eastern Africa for both hunting and warfare. Ouabain is a cardiac glycoside and in lower doses, can be used medically to treat hypotension and some arrhythmias. It acts by inhibiting the Na/K-ATPase, also known as the sodium–potassium ion pump. However, adaptations to the alpha-subunit of the -ATPase via amino acid substitutions, have been observed in certain species, namely some herbivore- insect species, that have resulted in toxin resistance.
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. | 0 | Organic Chemistry |
Initiation involves the small subunit of the ribosome binding to the 5 end of mRNA with the help of initiation factors (IF). In bacteria and a minority of archaea, initiation of protein synthesis involves the recognition of a purine-rich initiation sequence on the mRNA called the Shine–Dalgarno sequence. The Shine–Dalgarno sequence binds to a complementary pyrimidine-rich sequence on the 3 end of the 16S rRNA part of the 30S ribosomal subunit. The binding of these complementary sequences ensures that the 30S ribosomal subunit is bound to the mRNA and is aligned such that the initiation codon is placed in the 30S portion of the P-site. Once the mRNA and 30S subunit are properly bound, an initiation factor brings the initiator tRNA–amino acid complex, f-Met-tRNA, to the 30S P site. The initiation phase is completed once a 50S subunit joins the 30S subunit, forming an active 70S ribosome. Termination of the polypeptide occurs when the A site of the ribosome is occupied by a stop codon (UAA, UAG, or UGA) on the mRNA, creating the primary structure of a protein. tRNA usually cannot recognize or bind to stop codons. Instead, the stop codon induces the binding of a release factor protein (RF1 & RF2) that prompts the disassembly of the entire ribosome/mRNA complex by the hydrolysis of the polypeptide chain from the peptidyl transferase center of the ribosome. Drugs or special sequence motifs on the mRNA can change the ribosomal structure so that near-cognate tRNAs are bound to the stop codon instead of the release factors. In such cases of translational readthrough, translation continues until the ribosome encounters the next stop codon. | 1 | Biochemistry |
The discoverers were reluctant to assert an oxidation state of the iron centers in the compound, instead deferring the details of the electronic structure to computational studies.
The crystal structure reveals that the three iron centers arrange in an equilateral triangle (nearly ideal; Fe1 = 59.67°, Fe2 = 60.15°, and Fe3 = 60.18°) The corresponding bond lengths are similar to one another, (Fe1–Fe2 = 2.829 Å, Fe1–Fe3 = 2.815 Å, and Fe2–Fe3 = 2.830 Å), and reflective of Fe-Fe single bonds. As a trinuclear cluster, it would be thought to have a stable 48-electron closed-shell configuration (24 electrons from the three iron atoms and 24 electrons from the three COT rings).
In the original depiction, each COT ligand acts as an η and η donor, and thus, some degree of π-allylic and pentadienyl bonding modes can be inferred – though the degree of metal-to-ligand electron transfer is uncertain. Computational models suggest the binding mode to lie between η and η, as small shifts in geometry make each mode effectively equivalent (see section on fluxional behavior). Furthermore, DFT calculations with the BLYP functional using a TZP basis set for iron and DZP for carbon and hydrogen estimate a Hirshfeld charge of 0.08 on the iron centers (and Voronoi deformation density of 0.00). Interestingly, all of the bond orders of the C-C ring lie between 1.26 and 1.33, sharply contrasting the discrete single and double bonds of free cyclooctatetraene, or COT complexes with non-bound olefins.
Doubly reduced COT (dianion) is known to adopt a planar (aromatic) comformation to metal centers, which is not observed in Fe(COT). However, arguments also exist that such comformations are more related to binding efficiency than aromaticity.
In computational studies (BP86), when Fe(CH) is optimized as a singlet (gas phase), the iron centers are arranged in an ideal equilateral triangle, as experimentally observed in the crystal structure. In such an electronic configuration, each iron atom achieves an 18-electron configuration through pseudo η and η coordination to alternating COT ligands. However, if the compound is optimized as a triplet structure, the iron centers instead are a scalene triangle, featuring significant Jahn-Teller distortions.
Additional NBO analysis of the singlet structure reveals Wiberg Bond Indices of 0.22 for the Fe-Fe bonds, closely reminiscent of that of D Fe(CO) (0.18). | 7 | Physical Chemistry |
Estimates in 2023 found that the current carbon dioxide concentration in the atmosphere may be the highest it has been in the last 14 million years. However the IPCC Sixth Assessment Report estimated similar levels 3 to 3.3 million years ago in the mid-Pliocene warm period. This period can be a proxy for likely climate outcomes with current levels of .
Carbon dioxide is believed to have played an important effect in regulating Earths temperature throughout its 4.54 billion year history. Early in the Earths life, scientists have found evidence of liquid water indicating a warm world even though the Suns output is believed to have only been 70% of what it is today. Higher carbon dioxide concentrations in the early Earths atmosphere might help explain this faint young sun paradox. When Earth first formed, Earth's atmosphere may have contained more greenhouse gases and concentrations may have been higher, with estimated partial pressure as large as , because there was no bacterial photosynthesis to reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas, may have been more prevalent as well.
Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods. Carbon dioxide concentrations have varied widely over the Earths history. It is believed to have been present in Earths first atmosphere, shortly after Earth's formation. The second atmosphere, consisting largely of nitrogen and was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids. A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.
The production of free oxygen by cyanobacterial photosynthesis eventually led to the oxygen catastrophe that ended Earths second atmosphere and brought about the Earths third atmosphere (the modern atmosphere) 2.4 billion years ago. Carbon dioxide concentrations dropped from 4,000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million 20,000 years ago . | 2 | Environmental Chemistry |
Consider a column filled with a transporting medium and a balloon filled with a working fluid. Due to the hydrostatic pressure of the transporting medium, the pressure inside the column increases along the z axis (see figure). Initially, the balloon is inflated by the working fluid at temperature T and pressure P and located on top of the column. A thermogravitational cycle is decomposed into four ideal steps:
* 1→2: Descent of the balloon towards the bottom of the column. The working fluid undergoes adiabatic compression with its temperature increasing and its pressure reaching value P at the bottom (P>P).
* 2→3: While the ballon lays at the bottom, the working fluid receives heat from the hot source at temperature T and undergoes isobaric expansion at pressure P.
* 3→4: The balloon rises towards the column top. The working fluid undergoes adiabatic expansion with a drop in temperature and reaches pressure P after expansion when the balloon is on top.
* 4→1: Once arrived on top, the working fluid supplies heat to the cold source at temperature T while undergoing isobaric compression at pressure P.
For a thermogravitational cycle to occur, the balloon has to be denser than the transporting medium during 1→2 step and less dense during 3→4 step. If these conditions are not naturally satisfied by the working fluid, a weight can be attached to the balloon to increase its effective mass density. | 7 | Physical Chemistry |
Early considerations of the geometry of hypervalent molecules returned familiar arrangements that were well explained by the VSEPR model for atomic bonding. Accordingly, AB and AB type molecules would possess a trigonal bi-pyramidal and octahedral geometry, respectively. However, in order to account for the observed bond angles, bond lengths and apparent violation of the Lewis octet rule, several alternative models have been proposed.
In the 1950s an expanded valence shell treatment of hypervalent bonding was adduced to explain the molecular architecture, where the central atom of penta- and hexacoordinated molecules would utilize d AOs in addition to s and p AOs. However, advances in the study of ab initio calculations have revealed that the contribution of d-orbitals to hypervalent bonding is too small to describe the bonding properties, and this description is now regarded as much less important. It was shown that in the case of hexacoordinated SF, d-orbitals are not involved in S-F bond formation, but charge transfer between the sulfur and fluorine atoms and the apposite resonance structures were able to account for the hypervalency (See below).
Additional modifications to the octet rule have been attempted to involve ionic characteristics in hypervalent bonding. As one of these modifications, in 1951, the concept of the 3-center 4-electron (3c-4e) bond, which described hypervalent bonding with a qualitative molecular orbital, was proposed. The 3c-4e bond is described as three molecular orbitals given by the combination of a p atomic orbital on the central atom and an atomic orbital from each of the two ligands on opposite sides of the central atom. Only one of the two pairs of electrons is occupying a molecular orbital that involves bonding to the central atom, the second pair being non-bonding and occupying a molecular orbital composed of only atomic orbitals from the two ligands. This model in which the octet rule is preserved was also advocated by Musher. | 4 | Stereochemistry |
Since its formal statement in 1972, Clars rule has received a vast amount of experimental evidence. The dependence of the color and reactivity of some small polycyclic aromatic hydrocarbons on the number of π-sextets in their structures is reported by Clar himself in his seminal contribution. Similarly, it was shown that the HOMO-LUMO gap, and therefore the color, of a series of heptacatafusenes depends on the number of π-sextets. Clars rule has also been supported by experimental results about the distribution of π-electrons in polycyclic aromatic hydrocarbons, valence bond calculations, and nucleus independent chemical shift studies.
Clars rule is widely applied in the fields of chemistry and materials science. For instance, Clars rule can be used to predict several properties of graphene nanoribbons. Aromatic π-sextets play an important part in the determination of the ground state of open shell biradical-type structures. , Clar's rule can rationalize the observed a decrease of the bandgap of holey graphenes with increasing size. | 7 | Physical Chemistry |
Historically, small muffle ovens were often used for a second firing of porcelain at a relatively low temperature to fix overglaze enamels; these tend to be called muffle kilns. The pigments for most enamel colours discoloured at the high temperatures required for the body and glaze of the porcelain. They were used for painted enamels on metal for the same reason.
Like other types of muffle furnaces, the design isolates the objects from the flames producing the heat (with electricity this is not so important). For historical overglaze enamels the kiln was generally far smaller than that for the main firing, and produced firing temperatures in the approximate range of 750 to 950 °C, depending on the colours used. Typically, wares were fired for between five and twelve hours and then cooled over twelve hours. | 8 | Metallurgy |
In nearly all species of eukaryotic algae (Chloromonas being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the pyrenoid. Carbon dioxide is concentrated in this compartment using a combination of CO pumps, bicarbonate pumps, and carbonic anhydrases. The pyrenoid is not a membrane bound compartment, but is found within the chloroplast, often surrounded by a starch sheath (which is not thought to serve a function in the CCM). | 5 | Photochemistry |
Nitrilimines or nitrile amides are a class of organic compounds sharing a common functional group with the general structure R-CN-NR corresponding to the conjugate base of an amine bonded to the N-terminus of a nitrile. The dominant structure for the parent compound nitrilimine is that of the propargyl-like in scheme 1 with a C-N triple bond and with a formal positive charge on nitrogen and two lone pairs and a formal negative charge on the terminal nitrogen. Other structures such as hypervalent , allene-like , allylic and carbene are of lesser relevance.
Nitrilimines were first observed in the thermal decomposition of 2-tetrazoles releasing nitrogen:
Nitrilimines are linear 1,3-dipoles represented by structures and . A major use is in heterocyclic synthesis. E.g. with alkynes they generate pyrazoles in a 1,3-dipolar cycloaddition. Due to their high energy, they are usually generated in situ as a reactive intermediate. | 0 | Organic Chemistry |
A receptor antagonist is any given ligand that binds to a receptor in some way without causing any immediate or downstream response, essentially neutralizing the receptor until something with a stronger affinity removes the antagonist or the antagonist itself unbinds. Generally, antagonists can act one of two ways: 1) they can either block the receptors directly, preventing the usual ligand from binding, such as in the case of atropine when it blocks specific acetylcholine receptors to provide important medical benefits. This is competitive antagonism, as they are competing for the same binding sites on the receptor. The other is by binding to a receptor in a site other than the designated receptor site, inducing a conformational change to prevent the usual ligand(s) from binding and activating a downstream cascade. A commonly-seen and used receptor antagonist is naloxone, another opioid competitive antagonist typically used to treat opioid overdoses by blocking receptors outright. Further elaboration can be found in "Orthosteric v. Allosteric Modulators." | 1 | Biochemistry |
Chestnut blight was first noticed in American Chestnut trees that were growing in what is now known as the Bronx Zoo in the year 1904. For years following this incident, it was argued as to what the identity of the pathogen was, as well as the appropriate approach to its control. The earliest attempts to fix the problem on the chestnut involved chemical solutions or physical ones. They attempted to use fungicides, cut limbs off of trees to stop the infection, and completely remove infected trees from habitations to not allow them to infect the others. All of these strategies ended up unsuccessful. Even quarantine measures were put into place which were helped by the passage of Plant Quarantine Act. Chestnut blight still proved to be a huge problem as it rapidly moved through the densely populated forests of chestnut trees. In 1914, the idea was considered to induce blight resistance to the trees through various different means and breeding mechanisms. | 1 | Biochemistry |
Methyl Orange is an azobenzene derivative that can be formed from dimethylaniline and sulfanilic acid through first, a diazonium salt formation with the sulfanilic acid, followed by a nucleophilic attack from the dimethylaniline and rearomatization. | 3 | Analytical Chemistry |
* Cyanofullerenes are a class of modified fullerenes in which cyano- groups are attached to a fullerene skeleton. These have the formula , where n takes the values 1 to 9. | 0 | Organic Chemistry |
Due to the constant perfusion of the microdialysis probe with fresh perfusate, a total equilibrium cannot be established. This results in dialysate concentrations that are lower than those measured at the distant sampling site. In order to correlate concentrations measured in the dialysate with those present at the distant sampling site, a calibration factor (recovery) is needed. The recovery can be determined at steady-state using the constant rate of analyte exchange across the microdialysis membrane. The rate at which an analyte is exchanged across the semipermeable membrane is generally expressed as the analyte’s extraction efficiency. The extraction efficiency is defined as the ratio between the loss/gain of analyte during its passage through the probe (C−C) and the difference in concentration between perfusate and distant sampling site (C−C).
In theory, the extraction efficiency of a microdialysis probe can be determined by: 1) changing the drug concentrations while keeping the flow rate constant or 2) changing the flow rate while keeping the respective drug concentrations constant. At steady-state, the same extraction efficiency value is obtained, no matter if the analyte is enriched or depleted in the perfusate. Microdialysis probes can consequently be calibrated by either measuring the loss of analyte using drug-containing perfusate or the gain of analyte using drug-containing sample solutions. To date, the most frequently used calibration methods are the low-flow-rate method, the no-net-flux method, the dynamic (extended) no-net-flux method, and the retrodialysis method. The proper selection of an appropriate calibration method is critically important for the success of a microdialysis experiment. Supportive in vitro experiments prior to the use in animals or humans are therefore recommended. In addition, the recovery determined in vitro may differ from the recovery in humans. Its actual value therefore needs to be determined in every in vivo experiment. | 1 | Biochemistry |
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see next section) or other quantitative and qualitative characteristics of the reaction. It also describes each reactive intermediate, activated complex, and transition state, which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also explain the reason for the reactants and catalyst used, the stereochemistry observed in reactants and products, all products formed and the amount of each.
The electron or arrow pushing method is often used in illustrating a reaction mechanism; for example, see the illustration of the mechanism for benzoin condensation in the following examples section.
A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single-step conversion is in fact a multistep reaction. | 7 | Physical Chemistry |
Tetramethylguanidine is mainly used as a strong, non-nucleophilic base for alkylations, often as a substitute for the more expensive DBU and DBN. Since it is highly water-soluble, it is easily removed from mixtures in organic solvents. It is also used as a base-catalyst in the production of polyurethane. | 0 | Organic Chemistry |
There is overwhelming evidence for production of quark–gluon plasma in relativistic heavy ion collisions.
The important classes of experimental observations are
* Strangeness production
* Elliptic flow
* Jet quenching
* J/ψ melting
* Hanbury Brown and Twiss effect and Bose–Einstein correlations
* Single particle spectra (thermal photons and thermal dileptons) | 7 | Physical Chemistry |
An active metabolite results when a drug is metabolized by the body into a modified form which produces effects in the body. Usually these effects are similar to those of the parent drug but weaker, although they can still be significant (see e.g. 11-hydroxy-THC, morphine-6-glucuronide). Certain drugs such as codeine and tramadol have metabolites (morphine and O-desmethyltramadol respectively) that are stronger than the parent drug and in these cases the metabolite may be responsible for much of the therapeutic action of the parent drug. Sometimes, however, metabolites may produce toxic effects and patients must be monitored carefully to ensure they do not build up in the body. This is an issue with some well-known drugs, such as pethidine (meperidine) and dextropropoxyphene. | 1 | Biochemistry |
Many other methods for beam-deflection measurements exist.
* Piezoelectric detection – Cantilevers made from quartz (such as the qPlus configuration), or other piezoelectric materials can directly detect deflection as an electrical signal. Cantilever oscillations down to 10pm have been detected with this method.
* Laser Doppler vibrometry – A laser Doppler vibrometer can be used to produce very accurate deflection measurements for an oscillating cantilever (thus is only used in non-contact mode). This method is expensive and is only used by relatively few groups.
* Scanning tunneling microscope (STM) — The first atomic microscope used an STM complete with its own feedback mechanism to measure deflection. This method is very difficult to implement, and is slow to react to deflection changes compared to modern methods.
* Optical interferometry – Optical interferometry can be used to measure cantilever deflection. Due to the nanometre scale deflections measured in AFM, the interferometer is running in the sub-fringe regime, thus, any drift in laser power or wavelength has strong effects on the measurement. For these reasons optical interferometer measurements must be done with great care (for example using index matching fluids between optical fibre junctions), with very stable lasers. For these reasons optical interferometry is rarely used.
* Capacitive detection – Metal coated cantilevers can form a capacitor with another contact located behind the cantilever. Deflection changes the distance between the contacts and can be measured as a change in capacitance.
* Piezoresistive detection – Cantilevers can be fabricated with piezoresistive elements that act as a strain gauge. Using a Wheatstone bridge, strain in the AFM cantilever due to deflection can be measured. This is not commonly used in vacuum applications, as the piezoresistive detection dissipates energy from the system affecting Q of the resonance. | 6 | Supramolecular Chemistry |
The Zener ratio is a dimensionless number that is used to quantify the anisotropy for cubic crystals. It is sometimes referred as anisotropy ratio and is named after Clarence Zener. Conceptually, it quantifies how far a material is from being isotropic (where the value of 1 means an isotropic material).
Its mathematical definition is
where refers to Elastic constants in Voigt notation. | 3 | Analytical Chemistry |
Source: [https://www.biochemistry.org/grants-and-awards/awards/the-portland-press-excellence-in-science-award/ Biochemical Society]
; Novartis Medal and Prize
* 2021: Bart Vanhaesebroeck
* 2019: Caroline Dean
* 2018: Laurence Pearl
* 2017: Doreen Cantrell
* 2014: Jeff Errington
* 2013: Tony Kouzarides
* 2010: D. Grahame Hardie
* 2009: Louise Johnson
* 2008: Stephen C. West
* 2007: Adrian Bird
* 2006: James Barber
* 2005: Alan Hall
* 2004: Jean D. Beggs
* 2003: Iain D. Campbell
* 2002: Michael S. Neuberger
* 2000: Kiyoshi Nagai
* 1999: Christopher J. Marshall
* 1998: Richard N. Perham
* 1997: Ronald Laskey
* 1996: John E. Walker
* 1995: Christopher F. Higgins
* 1993: T. Rabbitts
* 1992: Philip Cohen
* 1991: Paul Nurse
* 1987: Thomas L. Blundell
* 1985: E. A. Barnard
* 1984: Philip J. Randle
* 1983: George K. Radda
* 1981: I. Helen Muir
* 1980: Sydney Brenner
* 1979: J. B. Gurdon
* 1978: J. Rodney Quayle
* 1977: César Milstein
* 1976: Samuel V. Perry
* 1973: Peter D. Mitchell
* 1972: R. T. Williams
* 1971: D. H. Northcote
* 1970: D. C. Phillips
* 1968: William J. Whelan
* 1967: D. M. Blow
* 1966: R. R. Porter
* 1965: J. W. Cornforth and
Source: | 1 | Biochemistry |
Many hormones and their structural and functional analogs are used as medication. The most commonly prescribed hormones are estrogens and progestogens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.
A "pharmacologic dose" or "supraphysiological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful, though not without potentially adverse side effects. An example is the ability of pharmacologic doses of glucocorticoids to suppress inflammation. | 1 | Biochemistry |
Thioesters hydrolyze to thiols and the carboxylic acid:
:RC(O)SR' + HO → RCOH + RSH
The carbonyl center in thioesters is more reactive toward amine than oxygen nucleophiles, giving amides:
This reaction is exploited in native chemical ligation, a protocol for peptide synthesis.
In a related reaction, thioesters can be converted into esters. Thioacetate esters can also be cleaved with methanethiol in the presence of stoichiometric base, as illustrated in the preparation of pent-4-yne-1-thiol:
A reaction unique to thioesters is the Fukuyama coupling, in which the thioester is coupled with an organozinc halide by a palladium catalyst to give a ketone. | 0 | Organic Chemistry |
Biosynthesis of FeMoco is a complicated process that requires several Nif gene products, specifically those of nifS, nifQ, nifB, nifE, nifN, nifV, nifH, nifD, and nifK (expressed as the proteins NifS, NifU, etc.). FeMoco assembly is proposed to be initiated by NifS and NifU which mobilize Fe and sulfide into small Fe-S fragments. These fragments are transferred to the NifB scaffold and arranged into a FeMoSC cluster before transfer to the NifEN protein (encoded by nifE and nifN) and rearranged before delivery to the MoFe protein. Several other factors participate in the biosynthesis. For example, NifV is the homocitrate synthase that supplies homocitrate to FeMoco. NifV, a protein factor, is proposed to be involved in the storage and/or mobilization of Mo. Fe protein is the electron donor for MoFe protein. These biosynthetic factors have been elucidated and characterized with the exact functions and sequence confirmed by biochemical, spectroscopic, and structural analyses. | 7 | Physical Chemistry |
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