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With the lone exception of the bis(trifluoromethyl) derivative, the dominant reaction of phosphinous acids is tautomerization:
:PROH → OPRH
Even the pentafluorophenyl compound P(CF)OH is unstable with respect to the phosphine oxide.
Although phosphinous acids are rare, their P-bonded coordination complexes are well established, e.g. Mo(CO)P(OH). | 0 | Organic Chemistry |
Suppose an alloy at an equilibrium temperature T consists of mass fraction of element B. Suppose also that at temperature T the alloy consists of two phases, α and β, for which the α consists of , and β consists of . Let the mass of the α phase in the alloy be so that the mass of the β phase is , where is the total mass of the alloy.
By definition, then, the mass of element B in the α phase is , while the mass of element B in the β phase is . Together these two quantities sum to the total mass of element B in the alloy, which is given by . Therefore,
By rearranging, one finds that
This final fraction is the mass fraction of the α phase in the alloy. | 8 | Metallurgy |
Eukaryotic genomes have distinct higher-order chromatin structures that are closely packaged functional relates to gene expression. Chromatin compresses the genome to fit into the cell nucleus, while still ensuring that the gene can be accessed when needed, such as during gene transcription, replication, and DNA repair. The entirety of genome function is based on the underlying relationship between nuclear organization and the mechanisms involved in genome organization, in which there are a number of complex mechanisms and biochemical pathways which can affect the expression of individual genes within the genome. The remaining mitochondrial proteins, metabolic enzymes, DNA and RNA polymerases, ribosomal proteins, and mtDNA regulatory factors are all encoded by nuclear genes. Because nuclear genes constitute the genetic foundation of all eukaryotic organisms, anything that might change their genetic expression has a direct impact on the organism's cellular genotypes and phenotypes. The nucleus also contains a number of distinct subnuclear foci known as nuclear bodies, which are dynamically controlled structures that help numerous nuclear processes run more efficiently. Active genes, for instance, might migrate from chromosomal regions and concentrate into subnuclear foci known as transcription factories. | 1 | Biochemistry |
The thermal pressure of a crystal defines how the unit-cell parameters change as a function of pressure and temperature. Therefore, it also controls how the cell parameters change along an isochore, namely as a function of . Usually, Mie-Grüneisen-Debye and other Quasi harmonic approximation (QHA) based state functions are being used to estimate volumes and densities of mineral phases in diverse applications such as thermodynamic, deep-Earth geophysical models and other planetary bodies. In the case of isotropic (or approximately isotropic) thermal pressure, the unit cell parameter remains constant along the isochore and the QHA is valid. But when the thermal pressure is anisotropic, the unit cell parameter changes so, the frequencies of vibrational modes also change even in constant volume and the QHA is no longer valid.
The combined effect of a change in pressure and temperature is described by the strain tensor :
Where is the volume thermal expansion tensor and is the compressibility tensor. The line in the P-T space which indicates that the strain is constant in a particular direction within the crystal is defined as:
Which is an equivalent definition of the isotropic degree of thermal pressure. | 7 | Physical Chemistry |
Once an attosecond light source is available, one has to drive the pulse towards the sample of interest and, then, measure its dynamics.
The most suitable experimental observables to analyze the electron dynamics in matter are:
* Angular asymmetry in the velocity distribution of molecular photo-fragment.
* Quantum yield of molecular photo-fragments.
* XUV-SXR spectrum transient absorption.
* XUV-SXR spectrum transient reflectivity.
* Photo-electron kinetic energy distribution.
The general strategy is to use a pump-probe scheme to "image" through one of the aforementioned observables the ultra-fast dynamics occurring in the material under investigation. | 7 | Physical Chemistry |
Dioxirane epoxidation is highly versatile, and compares favorably to related peracid oxidations in many respects. Peracids generate acidic byproducts, meaning that acid-labile substrates and products must be avoided. Dioxirane epoxidations using isolated oxidant can be carried out under neutral conditions without the need for aqueous buffering. However, catalytic dioxirane oxidations do require water and are not suitable for hydrolytically unstable substrates.
Some methods are well-suited to the oxidation of electron-rich or electron-poor double bonds, but few are as effective for both classes of substrate as dioxiranes. Weitz-Scheffer conditions (NaOCl, HO/KOH, tBuOH/KOH) work well for oxidations of electron-poor double bonds, and sulfonyl-substituted oxaziridines are effective for electron-rich double bonds.
Metal-based oxidants are often more efficient than dioxirane oxidations in the catalytic mode; however, environmentally unfriendly byproducts are typically generated. In the realm of asymmetric methods, both the Sharpless epoxidation and Jacobsen epoxidation surpass asymmetric dioxirane oxidations in enantioselectivity. Additionally, enzymatic epoxidations are more enantioselective than dioxirane-based methods; however, operational difficulties and low yields are sometimes associated with enzymatic oxidations | 0 | Organic Chemistry |
Evans and his co-workers has developed a strategy that relies heavily on asymmetric aldol methodology for the production of the polypropionate backbone. They used a Claisen condensation reaction to construct the C(13)–C(14) trisubstituted Z-olefin. The Evans synthesis of (+)-discodermolide has an overall yield of 6.4% with a longest linear sequence of 31 steps and 49 total steps. | 0 | Organic Chemistry |
Targets of (p)ppGpp include rRNA operons, of which there are seven in E.coli, all of which have 2 promoters. When (p)ppGpp associates with the promoter it affects the RNA polymerase enzyme's ability to bind and initiate transcription. It is thought that (p)ppGpp may affect the stability of the open complex formed by RNA polymerase on DNA and therefore affect promoter clearance. Its presence also leads to an increase in pausing during transcription elongation and it competes with nucleoside triphosphate substrates.
There is now a consensus that (p)ppGpp is a determinant of growth rate control rather than nucleoside triphosphate (NTP) substrate concentrations. | 1 | Biochemistry |
Diisopinocampheylborane was originally prepared by hydroboration of excess α-pinene with borane,
but it is now more commonly generated from borane-methyl sulfide (BMS).
The compound can be isolated as a solid, but because it is quite sensitive to water and air, it is often generated in situ and used as a solution. The synthesis is complicated by a number of factors, including the tendency of the compound to eliminate pinene.
Diisopinocampheylborane is often represented as a monomer (including in this article), but X-ray crystallography establishes a dimeric structure. | 0 | Organic Chemistry |
A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the union of a line and two half-planes that have this line as a common edge. In higher dimensions, a dihedral angle represents the angle between two hyperplanes.
The planes of a flying machine are said to be at positive dihedral angle when both starboard and port main planes (commonly called "wings") are upwardly inclined to the lateral axis; when downwardly inclined they are said to be at a negative dihedral angle. | 4 | Stereochemistry |
When energetic ions collide with atoms of a target material, an exchange of momentum takes place between them.
These ions, known as "incident ions", set off collision cascades in the target. Such cascades can take many paths; some recoil back toward the surface of the target. If a collision cascade reaches the surface of the target, and its remaining energy is greater than the target's surface binding energy, an atom will be ejected. This process is known as "sputtering". If the target is thin (on an atomic scale), the collision cascade can reach through to its back side; the atoms ejected in this fashion are said to escape the surface binding energy "in transmission".
The average number of atoms ejected from the target per incident ion is called the "sputter yield". The sputter yield depends on several things: the angle at which ions collide with the surface of the material, how much energy they strike it with, their masses, the masses of the target atoms, and the target's surface binding energy. If the target possesses a crystal structure, the orientation of its axes with respect to the surface is an important factor.
The ions that cause sputtering come from a variety of sources—they can come from plasma, specially constructed ion sources, particle accelerators, outer space (e.g. solar wind), or radioactive materials (e.g. alpha radiation).
A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model. An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program TRIM.
Another mechanism of physical sputtering is called "heat spike sputtering". This can occur when the solid is dense enough, and the incoming ion heavy enough, that collisions occur very close to each other. In this case, the binary collision approximation is no longer valid, and the collisional process should be understood as a many-body process. The dense collisions induce a heat spike (also called thermal spike), which essentially melts a small portion of the crystal. If that portion is close enough to its surface, large numbers of atoms may be ejected, due to liquid flowing to the surface and/or microexplosions. Heat spike sputtering is most important for heavy ions (e.g. Xe or Au or cluster ions) with energies in the keV–MeV range bombarding dense but soft metals with a low melting point (Ag, Au, Pb, etc.). The heat spike sputtering often increases nonlinearly with energy, and can for small cluster ions lead to dramatic sputtering yields per cluster of the order of 10,000. For animations of such a process see "Re: Displacement Cascade 1" in the external links section.
Physical sputtering has a well-defined minimum energy threshold, equal to or larger than the ion energy at which the maximum energy transfer from the ion to a target atom equals the binding energy of a surface atom. That is to say, it can only happen when an ion is capable of transferring more energy into the target than is required for an atom to break free from its surface.
This threshold is typically somewhere in the range of ten to a hundred eV.
Preferential sputtering can occur at the start when a multicomponent solid target is bombarded and there is no solid state diffusion. If the energy transfer is more efficient to one of the target components, or it is less strongly bound to the solid, it will sputter more efficiently than the other. If in an AB alloy the component A is sputtered preferentially, the surface of the solid will, during prolonged bombardment, become enriched in the B component, thereby increasing the probability that B is sputtered such that the composition of the sputtered material will ultimately return to AB. | 7 | Physical Chemistry |
The inductive effect can be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to the atom. For example, if an atom has a positive charge and is attached to a -I group its charge becomes amplified and the molecule becomes more unstable. Similarly, if an atom has a negative charge and is attached to a +I group its charge becomes amplified and the molecule becomes more unstable. In contrast, if an atom has a negative charge and is attached to a -I group its charge becomes de-amplified and the molecule becomes more stable than if the I-effect was not taken into consideration. Similarly, if an atom has a positive charge and is attached to a +I group its charge becomes de-amplified and the molecule becomes more stable than if the I-effect was not taken into consideration. The explanation for the above is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability. | 7 | Physical Chemistry |
Shen Kuo's written work of 1088 contains, among other early descriptions of inventions, a method of repeated forging of cast iron under a cold blast similar to the modern Bessemer process.
Chinese metallurgy was widely practiced during the Middle Ages; during the 11th century, the growth of the iron industry caused vast deforestation due to the use of charcoal in the smelting process. To remedy the problem of deforestation, the Song Chinese discovered how to produce coke from bituminous coal as a substitute for charcoal. Although hydraulic-powered bellows for heating the blast furnace had been written about since Du Shi's (d. 38) invention of them in the 1st century CE, the first known illustration of a bellows in operation is found in a book written in 1313 by Wang Zhen (fl. 1290–1333). | 8 | Metallurgy |
TLE uses continuous-wave lasers (typically with a wavelength of around 1000 nm) located outside the vacuum chamber to heat sources of material in order to generate a flux of vapor via evaporation or sublimation. Owing to the localized nature of the heat induced by the laser, a portion of the source may be transformed into a liquid state while the rest remains solid, such that the source acts as its own crucible. The strong absorption of light causes the laser-induced heat to be highly localized via the small diameter of the laser beam, which can also have the effect of confining the heat to the axis of the source. The resulting absorption corresponds to a typical photon penetration depth on the order of 2 nm due to the high absorption coefficients of α ~ 10 cm of many materials. Heat loss via conduction and radiation further localizes the high-temperature region close to the irradiated surface of the source. The localized character of the heating enables many materials to be grown by TLE from freestanding sources without a crucible. Owing to the direct transfer of energy from the laser to the source, TLE is more efficient than other evaporation techniques such as evaporation and molecular beam epitaxy, which typically rely on wire-based Joule heaters to reach high temperatures.
By heating the source, a flux of vapor is produced, the pressure of which frequently has an approximately exponential relation to temperature. The vapor is then deposited onto a substrate, which is heated via a laser. This laser-heated substrate allows the use of adsorption-controlled growth modes, similar to molecular beam epitaxy, ensuring precise control of the stoichiometry and temperature of the deposited film. This precise control is valuable for growing thin-film heterostructures of complex materials, such as high-T superconductors. By positioning all lasers outside of the evaporation chamber, contamination can be reduced compared to using in situ heaters, resulting in highly pure deposited films.
The deposition rate of the vapor impinging upon the substrate is controlled by adjusting the power of the incident source laser. The deposition rate frequently increases exponentially with source temperature, which in turn increases linearly with incident laser power.
The gas in the chamber can be incorporated in the deposition film. With the addition of an oxygen or ozone atmosphere, oxide films can readily be grown with TLE at pressures up to 10 hPa. | 3 | Analytical Chemistry |
A recent trend is to change the method by which the skimmer is fed dirty water from the aquarium as a means to recirculate water within the skimmer multiple times before it is returned to the sump or the aquarium. Aspirating pump skimmers are the most popular type of skimmer to use recirculating designs although other types of skimmers, such as Beckett skimmers, are also available in recirculating versions. While there is a popular belief among some aquarist that this recirculation increases the dwell or contact time of the generated air bubbles within the skimmer there is no authoritative evidence that this is true. Each time water is recirculated within the skimmer any air bubbles in that water sample are destroyed and new bubbles are generated by the recirculating pump venturi apparatus so the air-water contact time begins again for these newly created bubbles. In non-recirculating skimmer designs, a skimmer has one inlet supplied by a pump that pulls water in from the aquarium and injects it with air into the skimmer and releasing the foam or air/water mix into the reaction chamber. With a recirculating design, the one inlet is usually driven by a separate feed pump, or in some cases may be gravity fed, to receive the dirty water to process, while the pump providing the foam or air/water mix into the reaction chamber is set up separately in a closed loop on the side of the skimmer. The recirculating pump pulls water out of the skimmer and injects air to generate the foam or air/water mix before returning it to the skimmer reaction chamber—thus recirculating it. The feed pump in a recirculating design typically injects a smaller amount of dirty water than co/counter-current designs. The separate feed pump allows easy control of the rate of water exchange through the skimmer and for many aquarists this is one of the important attractions of recirculating skimmer designs. Because the pump configuration of these skimmers is similar to that of aspirating pump skimmers, the power consumption advantages are also similar. | 3 | Analytical Chemistry |
During the development of B cells, the immunoglobulin gene undergoes sequences of rearrangements that lead to formation of the antibody repertoire. For example, in the lymphoid cell, a partial rearrangement of the heavy-chain gene occurs which is followed by complete rearrangement of heavy-chain gene. Here at this stage, Pre-B cell, mμ heavy chain and surrogate light chain are formed. The final rearrangement of the light chain gene generates immature B cell and mIgM. The process explained here occurs only in the absence of the antigen. The mature B cell formed as RNA processing changes leaves the bone marrow and is stimulated by the antigen then differentiated into IgM -secreted plasma cells. Also at first, the mature B cell expresses membrane-bound IgD and IgM. These two classes could switch to secretory IgD and IgM during the processing of mRNAs.
Finally, further class switching follows as the cell keep dividing and differentiating. For instance, IgM switches to IgG which switches to IgA that eventually switches to IgE | 1 | Biochemistry |
The term "bootstrap model" is used for a class of theories that use very general consistency criteria to determine the form of a quantum theory from some assumptions on the spectrum of particles. It is a form of S-matrix theory. | 7 | Physical Chemistry |
For solutions, a slightly different equation is used:
In this equation, α (Greek letter "alpha") is the measured rotation in degrees, l is the path length in decimeters, c is the concentration in g/mL, T is the temperature at which the measurement was taken (in degrees Celsius), and λ is the wavelength in nanometers.
For practical and historical reasons, concentrations are often reported in units of g/100mL. In this case, a correction factor in the numerator is necessary:
When using this equation, the concentration and the solvent may be provided in parentheses after the rotation. The rotation is reported using degrees, and no units of concentration are given (it is assumed to be g/100mL). The sign of the rotation (+ or −) is always given. If the wavelength of the light used is 589 nanometer (the sodium D line), the symbol “D” is used. If the temperature is omitted, it is assumed to be at standard room temperature (20 °C).
For example, the specific rotation of a compound would be reported in the scientific literature as:
: (c 1.00, EtOH) | 4 | Stereochemistry |
The National Research Council's 2006 report emphasized that accurate communication of results is essential for the proper use of biomonitoring surveys, but at the same time noted "there is no accepted standard for good biomonitoring communications." In 2007, the Boston University School of Public Health organized a panel on this topic.
An expert panel on Biomonitoring Equivalents has published guidelines for communicating information to the general public and health care providers.
Charles McKay of the Connecticut Poison Control Center is interviewed in a video titled "A Medical Doctor's Perspective on Biomonitoring", which is focused on helping the general public better understand biomonitoring. | 2 | Environmental Chemistry |
Even for systems where only light of optimal wavelengths is passed to the photovoltaic converter, inefficiencies associated with non-radiative recombination and Ohmic losses exist. There are also losses from Fresnel reflections at the PV surface, optimal-wavelength light that passes through the cell unabsorbed, and the energy difference between higher-energy photons and the bandgap energy (though this tends to be less significant than with solar PVs). Non-radiative recombination losses tend to become less significant as the light intensity increases, while they increase with increasing temperature, so real systems must consider the intensity produced by a given design and operating temperature. | 7 | Physical Chemistry |
Evans took a postdoctoral fellowship at the University of Texas at Arlington for the 1970–1971 academic year, followed by second fellowship at the University of Notre Dame in Indiana, where he worked with the organic chemist Ernest L. Eliel studying stereochemistry. Upon the completion of the fellowship, he was invited to be a research instructor at Dartmouth College in 1972, though they did not have the laboratory equipment he required to continue his research. Evans then joined the faculty of the University of North Carolina at Chapel Hill as an assistant professor of chemistry in 1974. He was the first African-American chemistry professor at the university. After 10 years at Chapel Hill, Evans became a full professor, and in 1992 was honored with a Kenan Professor chair.
Evans was a leading researcher in the field of organophosphorus chemistry, authoring more than 85 scientific articles on organosulfur and organophosphorus chemistry. His research led to a deeper understanding of the functions of organophosphate compounds and innovations in methods to produce chemical compounds for pharmaceutical drugs. Evans was inspired by William Standish Knowles, who in 1968 developed a method of asymmetric hydrogenation, which Evans used to develop alternative asymmetric synthesis methods as a way to produce single stereoisomers. Evans started experimenting with organophosphorus chemistry in 1970, developing a process using phosphorus atoms of organophosphate compounds as agents to produce specific stereoisomers. He also devised a method of asymmetric synthesis to synthesize alpha-amino phosphonic acids by adding phosphorus to sulfimides.
At the University of North Carolina Evans assembled a research team of undergraduates, graduate students, and postdoctoral fellows from around the world. In the 1980s, a Ford Foundation Fellowship allowed him to create ties between his research team and a research group at the Paul Sabatier University in France, where he spent a full sabbatical year. Later, with the help of a Fulbright Fellowship, he built ties with groups in Mexico, Poland, Germany, Greece, and Russia.
Evans championed recruiting minority applicants to UNC-Chapel Hill, while on the national front, he served on committees of the American Chemical Society, the National Institutes of Health, the National Science Foundation, and was chair of the U.S. National Committee of the International Union of Pure and Applied Chemistry. He also served on a council that advised the National Institute of General Medical Sciences. | 4 | Stereochemistry |
DMDO is most commonly used for the oxidation of alkenes to epoxides. One advantage of using DMDO is that the only byproduct of oxidation is acetone, a fairly innocuous and volatile compound. DMDO oxidations are particularly mild, sometimes allowing oxidations which might not otherwise be possible.
Despite its high reactivity, DMDO displays good selectivity for electron-rich olefins. DMDO will also oxidize several other functional groups. For example, DMDO will oxidize primary amines to nitro compounds and sulfides to sulfoxides. In some cases, DMDO will even oxidize unactivated C-H bonds:
DMDO can also be used to convert nitro compounds to carbonyl compounds (Nef reaction). | 0 | Organic Chemistry |
Creatinine (; from Ancient Greek: κρέας (kréas) flesh) is a breakdown product of creatine phosphate from muscle and protein metabolism. It is released at a constant rate by the body (depending on muscle mass). | 1 | Biochemistry |
For laboratory use, the N-alkylation reaction is often unselective. A variety of alternative methods have been developed, such as the Delépine reaction, which uses hexamine. The Gabriel synthesis, involving the use of an equivalent to NH, only applies to primary alkyl halides. | 0 | Organic Chemistry |
Linear expansion means change in one dimension (length) as opposed to change in volume (volumetric expansion).
To a first approximation, the change in length measurements of an object due to thermal expansion is related to temperature change by a coefficient of linear thermal expansion (CLTE). It is the fractional change in length per degree of temperature change. Assuming negligible effect of pressure, one may write:
where is a particular length measurement and is the rate of change of that linear dimension per unit change in temperature.
The change in the linear dimension can be estimated to be:
This estimation works well as long as the linear-expansion coefficient does not change much over the change in temperature , and the fractional change in length is small . If either of these conditions does not hold, the exact differential equation (using ) must be integrated. | 7 | Physical Chemistry |
Concerted metalation-deprotonation (CMD) is a mechanistic pathway through which transition-metal catalyzed C–H activation reactions can take place. In a CMD pathway, the C–H bond of the substrate is cleaved and the new C–Metal bond forms through a single transition state. This process does not go through a metal species that is bound to the cleaved hydrogen atom. Instead, a carboxylate or carbonate base deprotonates the substrate. The first proposal of a concerted metalation deprotonation pathway was by S. Winstein and T. G. Traylor in 1955 for the acetolysis of diphenylmercury. It was found to be the lowest energy transition state in a number of computational studies, was experimentally confirmed through NMR experiments, and has been hypothesized to occur in mechanistic studies.
While there are a number of different possible mechanisms for C–H activation, a CMD pathway is common for high valent, late transition metals like Pd, Rh, Ir, and Ru. The C–H bonds that have been found to undergo C–H activation through CMD include those that are aryl, alkyl, and alkenyl. Investigations into CMD paved the way for the development of many new C–H functionalization reactions, especially in the areas of direct arylation and alkylation by palladium and ruthenium. | 0 | Organic Chemistry |
Charles Moureu attended school in Bayonne. At 17, he apprenticed with his older brother, Félix Moureu, in his brother's pharmacy in Biarritz, in preparation for studies in pharmacy.
From 1884 to 1891 Moureu studied at the École Supérieure de Pharmacie in Paris. He received the school's silver medal in 1886, and both its gold medal and the Laillet Prize in 1887.
In addition, he interned in the Hôpitaux de Paris from 1886 to 1891, where he was given first place rank of the interns in 1887 and in 1889.
Moureu received a degree in 1888 and graduated as a pharmacist first class in 1891, whereupon he was given a position as chief pharmacist of the public asylum of the Seine (des Asiles de la Seine). He remained in this position from 1891 to 1907. This allowed him to continue his research, studying organic chemistry with and Charles Friedel. He received his doctorate in physical sciences in 1893 from the Sorbonne, with the dissertation Contribution à létude de lacide acrylique et de ses dérivés (Contribution to the study of acrylic acid and its derivatives).
He obtained his agrégation in 1899. | 0 | Organic Chemistry |
ITC is a quantitative technique that can determine the binding affinity (), reaction enthalpy (), and binding stoichiometry () of the interaction between two or more molecules in solution. This is achieved by measuring the enthalpies of a series of binding reactions caused by injections of a solution of one molecule to a reaction cell containing a solution of another molecule. The enthalpy values are plotted over the molar ratios resulting from the injections. From the plot, the molar reaction enthalpy , the affinity constant () and the stochiometry are determined by curve fitting. The reaction's Gibbs free energy change () and entropy change () can be determined using the relationship:
(where is the gas constant and is the absolute temperature).
For accurate measurements of binding affinity, the curve of the thermogram must be sigmoidal. The profile of the curve is determined by the c-value, which is calculated using the equation:
where is the stoichiometry of the binding, is the association constant and is the concentration of the molecule in the cell. The c-value must fall between 1 and 1000, ideally between 10 and 100. In terms of binding affinity, it would be approximately from ~ within the limit range. | 7 | Physical Chemistry |
Early diagnosis and analysis of seemingly healthy concrete cover and reinforcement status allows pre-emptive corrosion control measures to reduce unwanted risks to structural safety. Bundesanstalt für Materialforschung und -prüfung (Federal Institute for Materials Research and Testing, Germany) has developed a sensor equipped robotic system to accelerate the collection of several criteria used for diagnostics. Besides ultrasonic, ground-penetrating radar, concrete resistance, potential field, the eddy current method implemented in the Profometer 5 was used to measure the concrete cover. | 8 | Metallurgy |
The Laves graph is a cubic graph, meaning that there are exactly three edges at each vertex. Every pair of a vertex and adjacent edge can be transformed into every other such pair by a symmetry of the graph, so it is a symmetric graph. More strongly, for every two vertices and , every one-to-one correspondence between the three edges incident to and the three edges incident to can be realized by a symmetry. However, the overall structure is chiral: no sequence of translations and rotations can make it coincide with its mirror image. The symmetry group of the Laves graph is the space group .
The girth of this structure is 10—the shortest cycles in the graph have 10 vertices—and 15 of these cycles pass through each vertex. The numbers of vertices at distance 0, 1, 2, ... from any vertex (forming the coordination sequence of the Laves graph) are:
If the surrounding space is partitioned into the regions nearest each vertex—the cells of the Voronoi diagram of this structure—these form heptadecahedra with 17 faces each. They are plesiohedra, polyhedra that tile space isohedrally. Experimenting with the structures formed by these polyhedra led physicist Alan Schoen to discover the gyroid minimal surface, which is topologically equivalent to the surface obtained by thickening the edges of the Laves graph to cylinders and taking the boundary of their union.
The Laves graph is the unique shortest triply-periodic network, in the following sense. Triply-periodic means repeating infinitely in all three dimensions of space, so a triply-periodic network is a connected geometric graph with a three-dimensional lattice of translational symmetries. A fundamental domain is any shape that can tile space with its translated copies under these symmetries. Any lattice has infinitely many choices of fundamental domain, of varying shapes, but they all have the same volume . One can also measure the length of the edges of the network within a single copy of the fundamental domain; call this number . Similarly to , does not depend on the choice of fundamental domain, as long as the domain boundary only crosses the edges, rather than containing parts of their length. The Laves graph has four symmetry classes of vertices (orbits), because the symmetries considered here are only translations, not the rotations needed to map these four classes into each other. Each symmetry class has one vertex in any fundamental domain, so the fundamental domain contains twelve half-edges, with total length . The volume of its fundamental domain is 32. From these two numbers, the ratio (a dimensionless quantity) is therefore . This is in fact the minimum possible value: All triply-periodic networks have with equality only in the case of the Laves graph. | 3 | Analytical Chemistry |
The activity coefficient , which is also a dimensionless quantity, relates the activity to a measured mole fraction (or in the gas phase), molality , mass fraction , molar concentration (molarity) or mass concentration :
The division by the standard molality (usually 1 mol/kg) or the standard molar concentration (usually 1 mol/L) is necessary to ensure that both the activity and the activity coefficient are dimensionless, as is conventional.
The activity depends on the chosen standard state and composition scale; for instance, in the dilute limit it approaches the mole fraction, mass fraction, or numerical value of molarity, all of which are different. However, the activity coefficients are similar.
When the activity coefficient is close to 1, the substance shows almost ideal behaviour according to Henrys law (but not necessarily in the sense of an ideal solution). In these cases, the activity can be substituted with the appropriate dimensionless measure of composition , or . It is also possible to define an activity coefficient in terms of Raoults law: the International Union of Pure and Applied Chemistry (IUPAC) recommends the symbol for this activity coefficient, although this should not be confused with fugacity. | 7 | Physical Chemistry |
Lithotrophic microbes are responsible for the phenomenon known as acid mine drainage. Typically occurring in mining areas, this process concerns the active metabolism of pyrites and other reduced sulfur components to sulfate. One example is the acidophilic bacterial genus, A. ferrooxidans, that use iron(II) sulfide (FeS) to generate sulfuric acid. The acidic product of these specific lithotrophs has the potential to drain from the mining area via water run-off and enter the environment.
Acid mine drainage drastically alters the acidity (pH values of 2 - 3) and chemistry of groundwater and streams, and may endanger plant and animal populations downstream of mining areas. Activities similar to acid mine drainage, but on a much lower scale, are also found in natural conditions such as the rocky beds of glaciers, in soil and talus, on stone monuments and buildings and in the deep subsurface. | 1 | Biochemistry |
Like all chemical reactions, the unimolecular decomposition of ions is subject to thermodynamic versus kinetic reaction control: the kinetic product forms faster, whereas the thermodynamic product is more stable. In the decomposition of ABCD, the reaction to form AD is thermodynamically favored and the reaction to form ABis kinetically favored. This is because the AD reaction has favorable enthalpy and the AB has favorable entropy.
In the reaction depicted schematically in the figure, the rearrangement reaction forms a double bond B=C and a new single bond A-D, which offsets the cleavage of the A-B and C-D bonds. The formation of AB requires bond cleavage without the offsetting bond formation. However, the steric effect makes it more difficult for the molecule to achieve the rearrangement transition state and form AD. The activated complex with strict steric requirements is referred to as a "tight complex" whereas the transition state without such requirements is called a "loose complex". | 7 | Physical Chemistry |
A variety of cleavage agents can be chosen. a desirable agent is one that is sequence neutral, easy to use, and is easy to control. Unfortunately no available agents meet all of these standards, so an appropriate agent can be chosen, depending on your DNA sequence and ligand of interest. The following cleavage agents are described in detail:
DNase I is a large protein that functions as a double-strand endonuclease. It binds the minor groove of DNA and cleaves the phosphodiester backbone. It is a good cleavage agent for footprinting because its size makes it easily physically hindered. Thus is more likely to have its action blocked by a bound protein on a DNA sequence. In addition, the DNase I enzyme is easily controlled by adding EDTA to stop the reaction. There are however some limitations in using DNase I. The enzyme does not cut DNA randomly; its activity is affected by local DNA structure and sequence and therefore results in an uneven ladder. This can limit the precision of predicting a protein’s binding site on the DNA molecule.
Hydroxyl radicals are created from the Fenton reaction, which involves reducing Fe with HO to form free hydroxyl molecules. These hydroxyl molecules react with the DNA backbone, resulting in a break. Due to their small size, the resulting DNA footprint has high resolution. Unlike DNase I they have no sequence dependence and result in a much more evenly distributed ladder. The negative aspect of using hydroxyl radicals is that they are more time consuming to use, due to a slower reaction and digestion time.
Ultraviolet irradiation can be used to excite nucleic acids and create photoreactions, which results in damaged bases in the DNA strand. Photoreactions can include: single strand breaks, interactions between or within DNA strands, reactions with solvents, or crosslinks with proteins. The workflow for this method has an additional step, once both your protected and unprotected DNA have been treated, there is subsequent primer extension of the cleaved products. The extension will terminate upon reaching a damaged base, and thus when the PCR products are run side-by-side on a gel; the protected sample will show an additional band where the DNA was crosslinked with a bound protein. Advantages of using UV are that it reacts very quickly and can therefore capture interactions that are only momentary. Additionally it can be applied to in vivo experiments, because UV can penetrate cell membranes. A disadvantage is that the gel can be difficult to interpret, as the bound protein does not protect the DNA, it merely alters the photoreactions in the vicinity. | 1 | Biochemistry |
Metamerism, in chemistry, is used to define the isomeric relationship between compounds with the same polyvalent functional group with heteroatom but differ in the main carbon chain or any of the side chains. It has rather been an obsolete term for isomerism, which has not been recognised by IUPAC in its publications. When Jöns Jacob Berzelius used the term mesomerism in 1831, he did so to describe those substances which possess the same percentage composition but had different properties. What Berzelius implied to be called metamerism is now considered as isomerism. | 4 | Stereochemistry |
Elasticities coefficient can also be computed numerically, something that is often done in simulation software.
For example, a small change (say 5%) can be made to the chosen reactant concentration, and the change in the reaction rate recorded. To illustrate this, assume that the reference reaction rate is , and the reference reactant concentration, . If we increase the reactant concentration by and record the new reaction rate as , then the elasticity can be estimated by using Newton's difference quotient:
A much better estimate for the elasticity can be obtained by doing two separate perturbations in . One perturbation to increase and another to decrease . In each case, the new reaction rate is recorded; this is called the two-point estimation method. For example, if is the reaction rate
when we increase , and is the reaction rate when we decrease , then
we can use the following two-point formula to estimate the elasticity: | 7 | Physical Chemistry |
Although there are many diverse pathogens, many of which are constantly mutating, it is a surprise that a majority of individuals remain free of infections. Thus, maintenance of health requires the body to recognize all pathogens (antigens they present or produce) likely to exist. This is achieved by maintaining a pool of immensely large (about 10) clones of B cells, each of which reacts against a specific epitope by recognizing and producing antibodies against it. However, at any given time very few clones actually remain receptive to their specific epitope. Thus, approximately 10 different epitopes can be recognized by all the B cell clones combined. Moreover, in a lifetime, an individual usually requires the generation of antibodies against very few antigens in comparison with the number that the body can recognize and respond against. | 1 | Biochemistry |
Artificial gene synthesis, or simply gene synthesis, refers to a group of methods that are used in synthetic biology to construct and assemble genes from nucleotides de novo. Unlike DNA synthesis in living cells, artificial gene synthesis does not require template DNA, allowing virtually any DNA sequence to be synthesized in the laboratory. It comprises two main steps, the first of which is solid-phase DNA synthesis, sometimes known as DNA printing. This produces oligonucleotide fragments that are generally under 200 base pairs. The second step then involves connecting these oligonucleotide fragments using various DNA assembly methods. Because artificial gene synthesis does not require template DNA, it is theoretically possible to make a completely synthetic DNA molecule with no limits on the nucleotide sequence or size.
Synthesis of the first complete gene, a yeast tRNA, was demonstrated by Har Gobind Khorana and coworkers in 1972. Synthesis of the first peptide- and protein-coding genes was performed in the laboratories of Herbert Boyer and Alexander Markham, respectively. More recently, artificial gene synthesis methods have been developed that will allow the assembly of entire chromosomes and genomes. The first synthetic yeast chromosome was synthesised in 2014, and entire functional bacterial chromosomes have also been synthesised. In addition, artificial gene synthesis could in the future make use of novel nucleobase pairs (unnatural base pairs). | 1 | Biochemistry |
Photonic crystals allow precise control of electromagnetic wave properties. These materials give rise to the photonic bandgap (PBG). In the spectral range of the PBG, electromagnetic waves cannot propagate. Engineering these materials allows some ability to tailor their emission and absorption properties, allowing for more effective emitter design. Selective emitters with peaks at higher energy than the black body peak (for practical TPV temperatures) allow for wider bandgap converters. These converters are traditionally cheaper to manufacture and less temperature sensitive. Researchers at Sandia Labs predicted a high-efficiency (34% of light emitted converted to electricity) based on TPV emitter demonstrated using tungsten photonic crystals. However, manufacturing of these devices is difficult and not commercially feasible. | 7 | Physical Chemistry |
The Michaelis–Becker reaction is the reaction of a hydrogen phosphonate with a base, followed by a nucleophilic substitution of phosphorus on a haloalkane, to give an alkyl phosphonate. Yields of this reaction are often lower than the corresponding Michaelis–Arbuzov reaction. | 0 | Organic Chemistry |
Accessory pigments are light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll a. They include other forms of this pigment, such as chlorophyll b in green algal and vascular ("higher") plant antennae, while other algae may contain chlorophyll c or d. In addition, there are many non-chlorophyll accessory pigments, such as carotenoids or phycobiliproteins, which also absorb light and transfer that light energy to photosystem chlorophyll. Some of these accessory pigments, in particular the carotenoids, also serve to absorb and dissipate excess light energy, or work as antioxidants. The large, physically associated group of chlorophylls and other accessory pigments is sometimes referred to as a pigment bed.
The different chlorophyll and non-chlorophyll pigments associated with the photosystems all have different absorption spectra, either because the spectra of the different chlorophyll pigments are modified by their local protein environment or because the accessory pigments have intrinsic structural differences. The result is that, in vivo, a composite absorption spectrum of all these pigments is broadened and flattened such that a wider range of visible and infrared radiation is absorbed by plants and algae. Most photosynthetic organisms do not absorb green light well, thus most remaining light under leaf canopies in forests or under water with abundant plankton is green, a spectral effect called the "green window". Organisms such as some cyanobacteria and red algae contain accessory phycobiliproteins that absorb green light reaching these habitats.
In aquatic ecosystems, it is likely that the absorption spectrum of water, along with gilvin and tripton (dissolved and particulate organic matter, respectively), determines phototrophic niche differentiation. The six shoulders in the light absorption of water between wavelengths 400 and 1100 nm correspond to troughs in the collective absorption of at least twenty diverse species of phototrophic bacteria. Another effect is due to the overall trend for water to absorb low frequencies, while gilvin and tripton absorb higher ones. This is why open ocean appears blue and supports yellow species such as Prochlorococcus, which contains divinyl-chlorophyll a and b. Synechococcus, colored red with phycoerythrin, is adapted to coastal bodies, while phycocyanin allows Cyanobacteria to thrive in darker inland waters. | 5 | Photochemistry |
;Cryosurgical systems
A number of medical supply companies have developed cryogen delivery systems for cryosurgery. Most are based on the use of liquid nitrogen, although some employ the use of proprietary mixtures of gases that combine to form the cryogen. | 1 | Biochemistry |
In chemistry, functionality is the presence of functional groups in a molecule. A monofunctional molecule possesses one functional group, a bifunctional (or difunctional) two, a trifunctional three, and so forth. In organic chemistry (and other fields of chemistry), a molecule's functionality has a decisive influence on its reactivity.
In polymer chemistry, the functionality of a monomer refers to its number of polymerizable groups, and affects the formation and the degree of crosslinking of polymers. | 0 | Organic Chemistry |
In February 1862, Lord Kelvin used the existence of the Sun and the stars as an empirical proof that the universe has not achieved thermodynamic equilibrium, as entropy production and free work are still possible, and there are temperature differences between objects. Helmholtz and Rankine expanded Kelvin's work soon after.
Since there are stars and colder objects, the universe is not in thermodynamic equilibrium, so it cannot be infinitely old. | 7 | Physical Chemistry |
The process of transcriptional termination is less understood in eukaryotes, which have extensive post-transcriptional RNA processing, and each of the three types of eukaryotic RNA polymerase have a different termination system.
In RNA polymerase I, Transcription termination factor, RNA polymerase I binds downstream of the pre-rRNA coding regions, causing the dissociation of the RNA polymerase from the template and the release of the new RNA strand.
In RNA polymerase II, the termination occurs via a polyadenylation/cleaving complex. The 3' tail on the ending of the strand is bound at the polyadenylation site, but the strand will continue to code. The newly synthesised ribonucleotides are removed one at a time by the cleavage factors CSTF and CPSF, in a process that is still not fully understood. The remainder of the strand is disengaged by a 5′-exonuclease when the transcription is finished.
RNA polymerase III terminates after a series of uracil polymerization residues in the transcribed mRNA. Unlike in bacteria and in polymerase I, the termination RNA hairpin needs to be upstream to allow for correct cleaving. | 1 | Biochemistry |
Eukaryotic photoautotrophs include red algae, haptophytes, stramenopiles, cryptophytes, chlorophytes, and land plants. These organisms perform photosynthesis through organelles called chloroplasts and are believed to have originated about 2 billion years ago. Comparing the genes of chloroplast and cyanobacteria strongly suggests that chloroplasts evolved as a result of endosymbiosis with cyanobacteria that gradually lost the genes required to be free-living. However, it is difficult to determine whether all chloroplasts originated from a single, primary endosymbiotic event, or multiple independent events. Some brachiopods (Gigantoproductus) and bivalves (Tridacna) also evolved photoautotrophy. | 5 | Photochemistry |
A "strong" ion exchanger will not lose the charge on its matrix once the column is equilibrated and so a wide range of pH buffers can be used. "Weak" ion exchangers have a range of pH values in which they will maintain their charge. If the pH of the buffer used for a weak ion exchange column goes out of the capacity range of the matrix, the column will lose its charge distribution and the molecule of interest may be lost. Despite the smaller pH range of weak ion exchangers, they are often used over strong ion exchangers due to their having greater specificity. In some experiments, the retention times of weak ion exchangers are just long enough to obtain desired data at a high specificity.
Resins (often termed beads) of ion exchange columns may include functional groups such as weak/strong acids and weak/strong bases. There are also special columns that have resins with amphoteric functional groups that can exchange both cations and anions. Some examples of functional groups of strong ion exchange resins are quaternary ammonium cation (Q), which is an anion exchanger, and sulfonic acid (S, -SOOH), which is a cation exchanger. These types of exchangers can maintain their charge density over a pH range of 0–14. Examples of functional groups of Weak ion exchange resins include diethylaminoethyl (DEAE, -CHN(CHH)), which is an anion exchanger, and carboxymethyl (CM, -CH-COOH), which is a cation exchanger. These two types of exchangers can maintain the charge density of their columns over a pH range of 5–9.
In ion chromatography, the interaction of the solute ions and the stationary phase based on their charges determines which ions will bind and to what degree. When the stationary phase features positive groups which attracts anions, it is called an anion exchanger; when there are negative groups on the stationary phase, cations are attracted and it is a cation exchanger. The attraction between ions and stationary phase also depends on the resin, organic particles used as ion exchangers.
Each resin features relative selectivity which varies based on the solute ions present who will compete to bind to the resin group on the stationary phase. The selectivity coefficient, the equivalent to the equilibrium constant, is determined via a ratio of the concentrations between the resin and each ion, however, the general trend is that ion exchangers prefer binding to the ion with a higher charge, smaller hydrated radius, and higher polarizability, or the ability for the electron cloud of an ion to be disrupted by other charges. Despite this selectivity, excess amounts of an ion with a lower selectivity introduced to the column would cause the lesser ion to bind more to the stationary phase as the selectivity coefficient allows fluctuations in the binding reaction that takes place during ion exchange chromatography.
Following table shows the commonly used ion exchangers | 3 | Analytical Chemistry |
Thin-layer chromatography (TLC) is a chromatography technique that separates components in non-volatile mixtures.
It is performed on a TLC plate made up of a non-reactive solid coated with a thin layer of adsorbent material. This is called the stationary phase. The sample is deposited on the plate, which is eluted with a solvent or solvent mixture known as the mobile phase (or eluent). This solvent then moves up the plate via capillary action. As with all chromatography, some compounds are more attracted to the mobile phase, while others are more attracted to the stationary phase. Therefore, different compounds move up the TLC plate at different speeds and become separated. To visualize colourless compounds, the plate is viewed under UV light or is stained. Testing different stationary and mobile phases is often necessary to obtain well-defined and separated spots.
TLC is quick, simple, and gives high sensitivity for a relatively low cost. It can monitor reaction progress, identify compounds in a mixture, determine purity, or purify small amounts of compound. | 3 | Analytical Chemistry |
Polarity also has an effect on allylic strain. In terms of stereoselectivity, polar groups act like large, bulky groups. Even though two groups may have approximately the same A values the polar group will act as though it were much bulkier. This is due to the donor character of the polar group. Polar groups increase the HOMO energy of the σ-system in the transition state. This causes the transition state to be in a much more favorable position when the polar group is not interacting in a 1,3 allylic strain. | 4 | Stereochemistry |
Current trends in organic chemistry include (as of 2020):
*Biocatalysis
*Catalysis
*Chemosensors
*Chiral synthesis
* Flow chemistry
* Green chemistry
*Mechanochemistry
*Photoredox catalysis | 0 | Organic Chemistry |
By the time Loewi began his experiments there was much discussion among scientists whether communication between nerves and muscles was chemical or electrical by nature. Experiments by Luigi Galvani in the 18th century had demonstrated that electrical stimulation of the frog sciatic nerve resulted in twitching of the leg muscles, and from this he developed the concept of bioelectricity. This led to the idea that direct electrical contact between nerves and muscles mediated transmission of excitation. However, work by John Newport Langley had suggested that in the autonomic nervous system communication in the ciliary ganglion was chemical. Loewi's experiments, published in 1921, finally settled the issue, proving that synaptic transmission was chemical.
Loewi performed a very simple yet elegant experiment. Using an isolated frog heart he had previously found that stimulation of the vagus nerve resulted in a slowing of the heart rate, while stimulation of the sympathetic nerve caused the heart rate to speed up (Figure 1). He reasoned that stimulation of either the vagus or sympathetic nerve would cause the nerve terminal to release a substance which would either slow or accelerate the heart rate. To prove this, he took a frog heart, which had been cannulated in order to perfuse the fluid surrounding the heart, and electrically stimulated the vagus nerve until the heart rate slowed. He then collected the fluid surrounding the heart and added it to a second frog heart which had been stripped of its vagal and sympathetic nerves. By adding the fluid surrounding the first heart to the second heart, he caused the heart rate of the second heart to slow down. This proved that stimulation of the vagus nerve caused the release of a substance which acted upon the heart tissue and directly caused the heart rate to slow down. (Figure 2) This substance was called vagusstoff. Vagustoff was later confirmed to be acetylcholine and was found to be the principal neurotransmitter in the parasympathetic nervous system.
In an interesting aside, Loewi apparently had the idea for his experiment in a dream. He wrote it down in the middle of the night but the next morning could not decipher his writing. He eventually had the same dream on another night, and decided to run to the laboratory to perform the experiment in the middle of the night. About this incident, Loewi writes:
:On mature consideration, in the cold light of the morning, I would not have done it. After all, it was an unlikely enough assumption that the vagus should secrete an inhibitory substance; it was still more unlikely that a chemical substance that was supposed to be effective at very close range between nerve terminal and muscle be secreted in such large amounts that it would spill over and, after being diluted by the perfusion fluid, still be able to inhibit another heart. (Loewi 1921)
Loewi was fortunate in his choice of experimental preparation. In the species of frog used (Rana esculenta), the vagus contains both inhibitory and stimulatory fibers. In the winter, inhibitory fibers predominate, so Loewi was also fortunate to have performed his experiments in February or March. Additionally, acetylcholinesterase activity (the enzyme that degrades acetylcholine) is low, particularly in an unheated laboratory, allowing the neurotransmitter to remain long enough to be collected and applied to a second heart. Thanks to this confluence of events, Loewi was able to describe the existence of vagusstoff and prove the existence of chemical synaptic transmission. | 1 | Biochemistry |
Under homeostasis, the reactivity of heme is controlled by its insertion into the "heme pockets" of hemoproteins. Under oxidative stress however, some hemoproteins, e.g. hemoglobin, can release their heme prosthetic groups. The non-protein-bound (free) heme produced in this manner becomes highly cytotoxic, most probably due to the iron atom contained within its protoporphyrin IX ring, which can act as a Fenton's reagent to catalyze in an unfettered manner the production of free radicals. It catalyzes the oxidation and aggregation of protein, the formation of cytotoxic lipid peroxide via lipid peroxidation and damages DNA through oxidative stress. Due to its lipophilic properties, it impairs lipid bilayers in organelles such as mitochondria and nuclei. These properties of free heme can sensitize a variety of cell types to undergo programmed cell death in response to pro-inflammatory agonists, a deleterious effect that plays an important role in the pathogenesis of certain inflammatory diseases such as malaria and sepsis. | 1 | Biochemistry |
The isotopic composition of the components in a biochemical system can be defined in different ways depending on the definition of isotopic ratio. Three definitions are described here: | 7 | Physical Chemistry |
Cyanosulfidic chemistry has several limitations. While the products are all formed from the same starting materials, many of the reactions require the periodic delivery of new reagents which complicates the syntheses. The chemical synthesis is therefore not truly “one-pot” chemistry which would require all reactants to be provided at the beginning which no further alterations. Sutherland and colleagues argue that a “flow-chemistry” approach featuring the movement of compounds through a stream experiencing different geochemical conditions makes their proposed system plausible. | 9 | Geochemistry |
L-(+)-tartaric acid, can participate in several reactions. As shown the reaction scheme below, dihydroxymaleic acid is produced upon treatment of L-(+)-tartaric acid with hydrogen peroxide in the presence of a ferrous salt.
:HOCCH(OH)CH(OH)COH + HO → HOCC(OH)C(OH)COH + 2 HO
Dihydroxymaleic acid can then be oxidized to tartronic acid with nitric acid. | 4 | Stereochemistry |
For molecular biology research different types of markers may be used based on the selection sought. These include:
*Positive or selection markers are selectable markers that confer selective advantage to the host organism. An example would be antibiotic resistance, which allows the host organism to survive antibiotic selection.
*Negative or counterselectable markers are selectable markers that eliminate or inhibit growth of the host organism upon selection. An example would be thymidine kinase, which makes the host sensitive to ganciclovir selection.
*Positive and negative selectable markers can serve as both a positive and a negative marker by conferring an advantage to the host under one condition, but inhibits growth under a different condition. An example would be an enzyme that can complement an auxotrophy (positive selection) and be able to convert a chemical to a toxic compound (negative selection). | 1 | Biochemistry |
A disadvantage of AFM compared with the scanning electron microscope (SEM) is the single scan image size. In one pass, the SEM can image an area on the order of square millimeters with a depth of field on the order of millimeters, whereas the AFM can only image a maximum scanning area of about 150×150 micrometers and a maximum height on the order of 10–20 micrometers. One method of improving the scanned area size for AFM is by using parallel probes in a fashion similar to that of millipede data storage.
The scanning speed of an AFM is also a limitation. Traditionally, an AFM cannot scan images as fast as an SEM, requiring several minutes for a typical scan, while an SEM is capable of scanning at near real-time, although at relatively low quality. The relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image making the AFM less suited for measuring accurate distances between topographical features on the image. However, several fast-acting designs were suggested to increase microscope scanning productivity including what is being termed videoAFM (reasonable quality images are being obtained with videoAFM at video rate: faster than the average SEM). To eliminate image distortions induced by thermal drift, several methods have been introduced.
AFM images can also be affected by nonlinearity, hysteresis, and creep of the piezoelectric material and cross-talk between the x, y, z axes that may require software enhancement and filtering. Such filtering could "flatten" out real topographical features. However, newer AFMs utilize real-time correction software (for example, feature-oriented scanning) or closed-loop scanners, which practically eliminate these problems. Some AFMs also use separated orthogonal scanners (as opposed to a single tube), which also serve to eliminate part of the cross-talk problems.
As with any other imaging technique, there is the possibility of image artifacts, which could be induced by an unsuitable tip, a poor operating environment, or even by the sample itself, as depicted on the right. These image artifacts are unavoidable; however, their occurrence and effect on results can be reduced through various methods.
Artifacts resulting from a too-coarse tip can be caused for example by inappropriate handling or de facto collisions with the sample by either scanning too fast or having an unreasonably rough surface, causing actual wearing of the tip.
Due to the nature of AFM probes, they cannot normally measure steep walls or overhangs. Specially made cantilevers and AFMs can be used to modulate the probe sideways as well as up and down (as with dynamic contact and non-contact modes) to measure sidewalls, at the cost of more expensive cantilevers, lower lateral resolution and additional artifacts. | 6 | Supramolecular Chemistry |
A Soxhlet extractor is a piece of laboratory apparatus invented in 1879 by Franz von Soxhlet. It was originally designed for the extraction of a lipid from a solid material. Typically, Soxhlet extraction is used when the desired compound has a limited solubility in a solvent, and the impurity is insoluble in that solvent. It allows for unmonitored and unmanaged operation while efficiently recycling a small amount of solvent to dissolve a larger amount of material. | 3 | Analytical Chemistry |
Azlocillin is considered a broad spectrum antibiotic and can be used against a number of Gram positive and Gram negative bacteria. The following represents MIC susceptibility data for a few medically significant organisms.
* Escherichia coli 1 μg/mL – 32 μg/mL
* Haemophilus spp. 0.03 μg/mL – 2 μg/mL
* Pseudomonas aeruginosa 4 μg/mL – 6.25 μg/mL | 4 | Stereochemistry |
* Sodium: 15 mmol/L
* Potassium: 9 mmol/L
* Magnesium: 4 mmol/L
* Calcium: 0.015 mmol/L
* Ketoglutarate/glutamic acid: 1 mmol/L
* Histidine: 198 mmol/L
* Mannitol: 30 mmol/L
* Tryptophan: 2 mmol/L | 1 | Biochemistry |
The formation of a complex between a metal ion, M, and a ligand, L, is in fact usually a substitution reaction. For example, in aqueous solutions, metal ions will be present as aqua ions, so the reaction for the formation of the first complex could be written as
The equilibrium constant for this reaction is given by
[L] should be read as "the concentration of L" and likewise for the other terms in square brackets. The expression can be greatly simplified by removing those terms which are constant. The number of water molecules attached to each metal ion is constant. In dilute solutions the concentration of water is effectively constant. The expression becomes
Following this simplification a general definition can be given, for the general equilibrium
The definition can easily be extended to include any number of reagents. The reagents need not always be a metal and a ligand but can be any species which form a complex. Stability constants defined in this way, are association constants. This can lead to some confusion as pK values are dissociation constants. In general purpose computer programs it is customary to define all constants as association constants. The relationship between the two types of constant is given in association and dissociation constants. | 7 | Physical Chemistry |
Dye completed a Bachelor of Arts degree in Natural Sciences in 1997 at the University of Cambridge, followed by a PhD on the Mechanical effects of welding superalloys in 2000, supervised by Roger Reed. | 8 | Metallurgy |
Common side effects include rash, loss of appetite, nausea, diarrhea, and low blood white blood cell levels. Other serious side effects include liver problems, obliterative bronchiolitis, and myasthenia gravis. It is not recommended in people with lupus erythematosus. Use during pregnancy may result in harm to the baby. Penicillamine works by binding heavy metals; the resulting penicillamine–metal complexes are then removed from the body in the urine.
Bone marrow suppression, dysgeusia, anorexia, vomiting, and diarrhea are the most common side effects, occurring in ~20–30% of the patients treated with penicillamine.
Other possible adverse effects include:
* Nephropathy
* Hepatotoxicity
* Membranous glomerulonephritis
* Aplastic anemia (idiosyncratic)
* Antibody-mediated myasthenia gravis and Lambert–Eaton myasthenic syndrome, which may persist even after its withdrawal
* Drug-induced systemic lupus erythematosus
* Elastosis perforans serpiginosa
* Toxic myopathies
* Unwanted breast growth
* Oligospermia | 4 | Stereochemistry |
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
Glow discharges are used as a source of light in devices such as neon lights, cold cathode fluorescent lamps and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry. They are also used in the surface treatment technique called sputtering. | 3 | Analytical Chemistry |
A large group of organocatalysts incorporate the urea or the thiourea moiety. These catalytically effective (thio)urea derivatives termed (thio)urea organocatalysts provide explicit double hydrogen-bonding interactions to coordinate and activate H-bond accepting substrates.
Their current uses are restricted to asymmetric multicomponent reactions, including those involving Michael addition, asymmetric multicomponent reactions for the synthesis of spirocycles, asymmetric multicomponent reactions involving acyl Strecker reactions, asymmetric Petasis reactions, asymmetric Biginelli reactions, asymmetric Mannich reactions, asymmetric aza-Henry reactions, and asymmetric reductive coupling reactions. | 0 | Organic Chemistry |
A thermal diode in this sense is a device whose thermal resistance is different for heat flow in one direction than for heat flow in the other direction. I.e., when the thermal diode's first terminal is hotter than the second, heat will flow easily from the first to the second, but when the second terminal is hotter than the first, little heat will flow from the second to the first.
Such an effect was first observed in a copper–cuprous-oxide interface by Chauncey Starr in the 1930s. Beginning in 2002, theoretical models were proposed to explain this effect. In 2006 the first microscopic solid-state thermal diodes were built. In April 2015 Italian researchers at CNR announced development of a working thermal diode, publishing results in Nature Nanotechnology.
Thermal siphons can act as a one-way heat flow.
Heat pipes operating in [https://www.1-act.com/resources/heat-pipe-fundamentals/different-types-of-heat-pipes/diode-heat-pipes/ gravity] may also have this effect. | 7 | Physical Chemistry |
Its name is derived from the word “pigment” (P) and the presence of a major bleaching band centered around 695-700 nm in the flash-induced absorbance difference spectra of P700/ P700+•. | 5 | Photochemistry |
Importantly, the rate of change in ocean acidification is much higher than in the geological past. This faster change prevents organisms from gradually adapting, and prevents climate cycle feedbacks from kicking in to mitigate ocean acidification. Ocean acidification is now on a path to reach lower pH levels than at any other point in the last 300 million years. The rate of ocean acidification (i.e. the rate of change in pH value) is also estimated to be unprecedented over that same time scale. These expected changes are considered unprecedented in the geological record. In combination with other ocean biogeochemical changes, this drop in pH value could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean, beginning as early as 2100.
The extent of further ocean chemistry changes, including ocean pH, will depend on climate change mitigation efforts taken by nations and their governments. Different scenarios of projected socioeconomic global changes are modelled by using the Shared Socioeconomic Pathways (SSP) scenarios.
Under a very high emission scenario (SSP5-8.5), model projections estimate that surface ocean pH could decrease by as much as 0.44 units by the end of this century, compared to the end of the 19th century. This would mean a pH as low as about 7.7, and represents a further increase in H+ concentrations of two to four times beyond the increase to date. | 9 | Geochemistry |
The combined RT-PCR and qPCR technique has been described as quantitative RT-PCR or real-time RT-PCR (sometimes even called quantitative real-time RT-PCR), has been variously abbreviated as qRT-PCR, RT-qPCR, RRT-PCR, and rRT-PCR. In order to avoid confusion, the following abbreviations will be used consistently throughout this article:
Not all authors, especially earlier ones, use this convention and the reader should be cautious when following links. RT-PCR has been used to indicate both real-time PCR (qPCR) and reverse transcription PCR (RT-PCR). | 1 | Biochemistry |
Scheme 1 shows the synthesis of the oxetane D ring from the C ring starting from the (+) enantiomer of the Wieland-Miescher ketone (1). Reduction of this diketone with sodium borohydride provided unsaturated ketoalcohol 2, which was protected as an acetate. Formation of the ketal was accompanied by alkene rearrangement. The acetyl group was replaced by a tert-butyldimethylsilyl protecting group. Hydroboration followed by oxidation with hydrogen peroxide gave alcohol 5. The hydroxyl group was then oxidized to a carbonyl group giving ketone 6 by action of pyridinium dichromate. With all the sensitive functional groups protected, the methylene group required for the oxetane ring D was then provided by the Corey-Chaykovsky reagent, which converted the carbonyl group to an epoxide (7). Treatment of this epoxide with aluminium isopropoxide gave allylic alcohol 8. Two more hydroxyl groups were added by oxidation of the newly formed double bond with a catalytic amount of osmium tetroxide in the presence of N-methylmorpholine N-oxide. This reaction lacked stereospecificity and the yield of triol 9 with the correct stereochemistry was therefore reduced. The primary alcohol was protected as a silyl ether and the secondary alcohol was activated as a triflate (11). Heating this trimethylsilyl protected triflate in refluxing ethlyene glycol closed the ring to give oxetane 12. | 0 | Organic Chemistry |
The karyorelictid nuclear code (translation table 27) is a genetic code used by the nuclear genome of the Karyorelictea ciliate Parduczia sp. This code, along with translation tables 28 and 31, is remarkable in that every one of the 64 possible codons can be a sense codon. Translation termination probably relies on context, specifically proximity to the poly(A) tail. | 1 | Biochemistry |
Respiratory, allergic, or immune effects in infants or children are associated with man-made VOCs and other indoor or outdoor air pollutants.
Some VOCs, such as styrene and limonene, can react with nitrogen oxides or with ozone to produce new oxidation products and secondary aerosols, which can cause sensory irritation symptoms. VOCs contribute to the formation of tropospheric ozone and smog.
Health effects include eye, nose, and throat irritation; headaches, loss of coordination, nausea; and damage to the liver, kidney, and central nervous system. Some organics can cause cancer in animals; some are suspected or known to cause cancer in humans. Key signs or symptoms associated with exposure to VOCs include conjunctival irritation, nose and throat discomfort, headache, allergic skin reaction, dyspnea, declines in serum cholinesterase levels, nausea, vomiting, nose bleeding, fatigue, dizziness.
The ability of organic chemicals to cause health effects varies greatly from those that are highly toxic to those with no known health effects. As with other pollutants, the extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. Eye and respiratory tract irritation, headaches, dizziness, visual disorders, and memory impairment are among the immediate symptoms that some people have experienced soon after exposure to some organics. At present, not much is known about what health effects occur from the levels of organics usually found in homes. | 0 | Organic Chemistry |
The Overlapping distribution method was introduced by Charles H. Bennett for estimating chemical potential. | 7 | Physical Chemistry |
The eigenstate thermalization hypothesis (or ETH) is a set of ideas which purports to explain when and why an isolated quantum mechanical system can be accurately described using equilibrium statistical mechanics. In particular, it is devoted to understanding how systems which are initially prepared in far-from-equilibrium states can evolve in time to a state which appears to be in thermal equilibrium. The phrase "eigenstate thermalization" was first coined by Mark Srednicki in 1994, after similar ideas had been introduced by Josh Deutsch in 1991. The principal philosophy underlying the eigenstate thermalization hypothesis is that instead of explaining the ergodicity of a thermodynamic system through the mechanism of dynamical chaos, as is done in classical mechanics, one should instead examine the properties of matrix elements of observable quantities in individual energy eigenstates of the system. | 7 | Physical Chemistry |
The triazine family of herbicides, which includes atrazine, was introduced in the 1950s; they have the current distinction of being the herbicide family of greatest concern regarding groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied to soils of above-neutral pH. Under alkaline soil conditions, atrazine may be carried into the soil profile as far as the water table by soil water following rainfall causing the aforementioned contamination. Atrazine is thus said to have "carryover", a generally undesirable property for herbicides.
Glyphosate had been first prepared in the 1950s but its herbicidal activity was only recognized in the 1960s. It was marketed as Roundup in 1971. The development of glyphosate-resistant crop plants, it is now used very extensively for selective weed control in growing crops. The pairing of the herbicide with the resistant seed contributed to the consolidation of the seed and chemistry industry in the late 1990s.
Many modern herbicides used in agriculture and gardening are specifically formulated to degrade within a short period after application. | 2 | Environmental Chemistry |
Typically FbFPs have an excitation maximum at a wavelength of approximately 450 nm (blue light) and a second distinct excitation peak around 370 nm (UV-A light). The main emission peak is at approx. 495 nm, with a shoulder around 520 nm. One variant of Pp2FbFP (Q116V) exhibits a 10 nm blue shift in both the excitation and emission spectra. Rationally designed variants of iLOV and CagFbFP exhibit 6 and 7 nm red shifts, respectively. | 1 | Biochemistry |
The 1993 Keuhne synthesis concerns racemic strychnine. Starting compounds tryptamine 1 and 4,4-dimethoxy acrolein 2 were reacted together with boron trifluoride to acetal 3 as a single diastereomer in an amine-carbonyl condensation / sigmatropic rearrangement sequence.
Hydrolysis with perchloric acid afforded aldehyde 4. A Johnson–Corey–Chaykovsky reaction (trimethylsulfonium iodide / n-butyllithium) converted the aldehyde into an epoxide which reacted in situ with the tertiary amine to ammonium salt 5 (contaminated with other cyclization products). Reduction (palladium on carbon/hydrogen) removed the benzyl group to alcohol 6, more reduction (sodium cyanoborohydride) and acylation (acetic anhydride / pyridine) produced 7 as a mixture of epimers (at C17). Ring closure of ring III to 8 was then accomplished with an aldol reaction using lithium bis(trimethylsilyl)amide (using only the epimer with correct configuration). Even more reduction (sodium borohydride) and acylation resulted in epimeric di-acetate 9.
A DBU mediated elimination reaction formed olefinic alcohol 10 and subsequent Swern oxidation have an unstable amino ketone 11. In the final steps a Horner–Wadsworth–Emmons reaction (methyl 2-(diethy1phosphono)acetate) give acrylate ester 12 as a mixture of cis and trans isomers which could be coached into the right (trans) direction by application of light in a photochemical rearrangement, the ester group was reduced (DIBAL / boron trifluoride) to isostrychnine 13 and racemic strychnine 14 was formed by base-catalyzed ring closure as in the Woodward synthesis.
In the 1998 Keuhne synthesis of chiral (−)-strychnine the starting material was derived from chiral tryptophan. | 0 | Organic Chemistry |
The technological features of FSL Kode constructs and the koding process can be summarized as follows:
* Rapid and simple – simple contact for 10–120 minutes and constructs spontaneously and stably incorporate – no washing required.
* Replicable – same variables (time, temperature, concentration) equals the same result.
* Toxicity – FSL constructs are biocompatible, disperse into biological solutions without solvents, detergents. They label non-covalently and are non-genetic. Normal vitality and functionality is maintained in modified cells/virions/organisms. Toxicity/vitality experiments in small laboratory animals, zebrafish, cell cultures, spermatozoa and embryos find no toxic effects within physiological ranges.
* Amphiphilic – the amphiphilic nature of the FSL Kode construct makes them water dispersible (clear solution of micelles), yet once interacted with a membrane they insert/coat and become water resistant
* Variable design – a single F can be presented in more than 100 ways by varying the spacer and lipid.
* High biovisibility – as the spacer holds the F moiety away for the membrane it is able to achieve increased sensitivity, specificity and reactivity can be optimized by use of multiple and variable biomarker presentations on the same surface.
* Additive – FSL modification is compatible with other technologies allowing users to add additional features to cells/viruses/organisms/surfaces already modified by more traditional methods. Multiple FSL constructs may be added to a surface simultaneously by simply creating a mix of FSL Kode constructs. Constructs insert into living or fixed cell (glutaraldehyde) membranes.
* Simple FSL peptide synthesis – there is a reactive-functional-group FSL Kode construct with maleimide as its functional group which can be used for preparation of FSLs from cysteine-containing peptides, proteins or any other thiols of biological interest. The effective synthetic approach is based on the well-known Michael nucleophilic addition to maleimides (Fig. 7).
* Synthetic "Gylcolipids" – one family of the FSL constructs are synthetic glycolipids with well-defined hydrophobic tails and carbohydrate head groups | 1 | Biochemistry |
In a multistep reaction, the rate-determining step does not necessarily correspond to the highest Gibbs energy on the reaction coordinate diagram. If there is a reaction intermediate whose energy is lower than the initial reactants, then the activation energy needed to pass through any subsequent transition state depends on the Gibbs energy of that state relative to the lower-energy intermediate. The rate-determining step is then the step with the largest Gibbs energy difference relative either to the starting material or to any previous intermediate on the diagram.
Also, for reaction steps that are not first-order, concentration terms must be considered in choosing the rate-determining step. | 7 | Physical Chemistry |
hair cell - half-life - halobacteria - halotolerance - haploid - heat of fusion - heat of vaporization - heat shock protein - Hsp70 (70 kDa heat shock proteins) - Hsp90 (90 kDa heat shock proteins) - heavy-chain immunoglobulin - Hela cell - helminth protein - helper T cell - hemopexin - hemoglobin - herpes simplex virus protein vmw65 - heterocyclic compound - heterotroph - heterozygote - Hfr cell - Hill reaction - His tag - histamine H1 receptor - histamine H2 receptor - histamine receptor - histidine - histone - history of science and technology - HIV receptor - holoenzyme - homeobox - homeodomain protein - homology - homoserine - homozygote - homunculus - hormone - housekeeping gene - Human Genome Project - hybridization - hydrocarbon - hydrogen - hydrogen bond - hydrogenation - hydrogen-deuterium exchange - hydrolysis - hydrolytic enzyme - hydrophilic - hydrophobe - hydrophobic - hydrophobicity analysis - hydroxyl | 1 | Biochemistry |
Radiation emitted by a moving source is subject to Doppler shift due to a finite line-of-sight velocity projection. If different parts of the emitting body have different velocities (along the line of sight), the resulting line will be broadened, with the line width proportional to the width of the velocity distribution. For example, radiation emitted from a distant rotating body, such as a star, will be broadened due to the line-of-sight variations in velocity on opposite sides of the star (this effect usually referred to as rotational broadening). The greater the rate of rotation, the broader the line. Another example is an imploding plasma shell in a Z-pinch. | 7 | Physical Chemistry |
Plants in temperate and polar regions adapt to winter and sub zero temperatures by relocating nutrients from leaves and shoots to storage organs. Freezing temperatures induce dehydrative stress on plants, as water absorption in the root and water transport in the plant decreases. Water in and between cells in the plant freezes and expands, causing tissue damage. Cold hardening is a process in which a plant undergoes physiological changes to avoid, or mitigate cellular injuries caused by sub-zero temperatures. Non-acclimatized individuals can survive −5 °C, while an acclimatized individual in the same species can survive −30 °C. Plants that originated in the tropics, like tomato or maize, don't go through cold hardening and are unable to survive freezing temperatures. The plant starts the adaptation by exposure to cold yet still not freezing temperatures. The process can be divided into three steps. First the plant perceives low temperature, then converts the signal to activate or repress expression of appropriate genes. Finally, it uses these genes to combat the stress, caused by sub-zero temperatures, affecting its living cells. Many of the genes and responses to low temperature stress are shared with other abiotic stresses, like drought or salinity.
When temperature drops, the membrane fluidity, RNA and DNA stability, and enzyme activity change. These, in turn, affect transcription, translation, intermediate metabolism, and photosynthesis, leading to an energy imbalance. This energy imbalance is thought to be one of the ways the plant detects low temperature. Experiments on arabidopsis show that the plant detects the change in temperature, rather than the absolute temperature. The rate of temperature drop is directly connected to the magnitude of calcium influx, from the space between cells, into the cell. Calcium channels in the cell membrane detect the temperature drop, and promotes expression of low temperature responsible genes in alfalfa and arabidopsis. The response to the change in calcium elevation depends on the cell type and stress history. Shoot tissue will respond more than root cells, and a cell that already is adapted to cold stress will respond more than one that has not been through cold hardening before. Light doesn't control the onset of cold hardening directly, but shortening of daylight is associated with fall, and starts production of reactive oxygen species and excitation of photosystem 2, which influences low-temp signal transduction mechanisms. Plants with compromised perception of day length have compromised cold acclimation.
Cold increases cell membrane permeability and makes the cell shrink, as water is drawn out when ice is formed in the extracellular matrix between cells. To retain the surface area of the cell membrane so it will be able to regain its former volume when temperature rises again, the plant forms more and stronger Hechtian strands. These are tubelike structures that connect the protoplast with the cell wall. When the intracellular water freezes, the cell will expand, and without cold hardening the cell would rupture. To protect the cell membrane from expansion induced damage, the plant cell changes the proportions of almost all lipids in the cell membrane, and increases the amount of total soluble protein and other cryoprotecting molecules, like sugar and proline.
Chilling injury occurs at 0–10 degrees Celsius, as a result of membrane damage, metabolic changes, and toxic buildup. Symptoms include wilting, water soaking, necrosis, chlorosis, ion leakage, and decreased growth. Freezing injury may occur at temperatures below 0 degrees Celsius. Symptoms of extracellular freezing include structural damage, dehydration, and necrosis. If intracellular freezing occurs, it will lead to death. Freezing injury is a result of lost permeability, plasmolysis, and post-thaw cell bursting.
When spring comes, or during a mild spell in winter, plants de-harden, and if the temperature is warm for long enough – their growth resumes. | 1 | Biochemistry |
Abortive initiation is a normal process of transcription and occurs both in vitro and in vivo. After each nucleotide-addition step in initial transcription, RNA polymerase, stochastically, can proceed on the pathway toward promoter escape (productive initiation) or can release the RNA product and revert to the RNA polymerase-promoter open complex (abortive initiation). During this early stage of transcription, RNA polymerase enters a phase during which dissociation of the transcription complex energetically competes with the elongation process. Abortive cycling is not caused by strong binding between the initiation complex and the promoter. | 1 | Biochemistry |
Planar extension tests are carried out on thin specimens which are constrained from deforming in one direction. For planar extension in the directions with the direction constrained, the principal stretches are . From incompressibility . Hence .
Therefore,
The left Cauchy–Green deformation tensor can then be expressed as
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have
The engineering strain is . The engineering stress is | 7 | Physical Chemistry |
The Membership comprises almost 140 member companies in Europe as well as from the rest of the world. Members are:
* Coil coaters companies
* Paint, film and pre-treatment chemical suppliers
* Service centres/Stockholders
* Raw material suppliers
* Equipment manufacturers
* Technology providers
* Related associations/institutes
Member companies are e.g. Euramax Coated Products, ArcelorMittal, Tata Steel, Novelis, Hydro Aluminium, Becker Industrial Coatings, Akzo Nobel Industrial Finishes, BASF, Shingels, Henkel, Bronx… | 8 | Metallurgy |
Levetiracetam does not undergo extensive metabolism, and the metabolites formed are not active and do not exert pharmacological activity. Metabolism of levetiracetam is not by liver cytochrome P450 enzymes, but through other metabolic pathways such as hydrolysis and hydroxylation. | 4 | Stereochemistry |
Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)NiCl:
:2 Ni(CO) + 2 ClCHCH=CH → Ni(μ-Cl)(η-CH) + 8 CO
A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl. Oxidative addition route has been used for Mo(II) allyl complexes as well:
Other methods of synthesis involve addition of nucleophiles to η-diene complexes and hydride abstraction from alkene complexes. For example, palladium(II) chloride attacks alkenes to give first an alkene complex, but then abstracts hydrogen to give a dichlorohydridopalladium alkene complex, and then eliminates hydrogen chloride:
:PdCl + >C=CHCHPd–(η-(>CCHCHPd(H)⚟(>CCHC<) → ClPd⚟(>CCHC<) + HCl
One allyl complex can transfer an allyl ligand to another complex. An anionic metal complex can displace a halide, to give an allyl complex. However, if the metal center is coordinated to 6 or more other ligands, the allyl may end up "trapped" as a σ (η-) ligand. In such circumstances, heating or irradiation can dislocate another ligand to free up space for the alkene-metal bond.
In principle, salt metathesis reactions can adjoin an allyl ligand from an allylmagnesium bromide or related allyl lithium reagent. However, the carbanion salt precursors require careful synthesis, as allyl halides readily undergo Wurtz coupling. Mercury and tin allyl halides appear to avoid this side-reaction. | 0 | Organic Chemistry |
An induction period in chemical kinetics is an initial slow stage of a chemical reaction; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions.
In some catalytic reactions, a pre-catalyst needs to undergo a transformation to form the active catalyst, before the catalyst can take effect. Time is required for this transformation, hence the induction period. For example, with Wilkinson's catalyst, one triphenylphosphine ligand must dissociate to give the coordinatively unsaturated 14-electron species which can participate in the catalytic cycle:
Similarly, for an autocatalytic reaction, where one of the reaction products catalyzes the reaction itself, the rate of reaction is low initially until sufficient products have formed to catalyze the reaction.
Reactions generally accelerate when heat is applied. Where a reaction is exothermic, the rate of the reaction may initially be low. As the reaction proceeds, heat is generated, and the rate of reaction increases. This type of reaction often exhibits an induction period as well.
The reactions to form Grignard reagents are notorious for having induction periods. This is usually due to two reasons: Firstly, the thin film of oxide on the magnesium reagent must be removed before the bulk magnesium can react. Secondly, Grignard reactions, while exothermic, are typically conducted at low temperature for better selectivity. For these two reasons, Grignard reactions often can have a long induction period, followed by a thermal runaway, even causing the reaction solvent to boil-off. | 7 | Physical Chemistry |
In theory, any complexation reaction can be used as a volumetric technique provided that:
# The reaction reaches equilibrium rapidly after each portion of titrant is added.
# Interfering situations do not arise. For instance, the stepwise formation of several different complexes of the metal ion with the titrant, resulting in the presence of more than one complex in solution during the titration process.
# A complexometric indicator capable of locating equivalence point with fair accuracy is available.
In practice, the use of EDTA as a titrant is well established. | 3 | Analytical Chemistry |
Electroporation can also be used to help deliver drugs or genes into the cell by applying short and intense electric pulses that transiently permeabilize cell membrane, thus allowing transport of molecules otherwise not transported through a cellular membrane. This procedure is referred to as electrochemotherapy when the molecules to be transported are chemotherapeutic agents or gene electrotransfer when the molecule to be transported is DNA. Scientists from Karolinska Institutet and the University of Oxford use electroporation of exosomes to deliver siRNAs, antisense oligonucleotides, chemotherapeutic agents and proteins specifically to neurons after inject them systemically (in blood). Because these exosomes are able to cross the blood brain barrier, this protocol could solve the problem of poor delivery of medications to the central nervous system, and potentially treat Alzheimers disease, Parkinsons disease, and brain cancer, among other conditions.
Bacterial transformation is generally the easiest way to make large amounts of a particular protein needed for biotechnology purposes or in medicine. Since gene electrotransfer is very simple, rapid and highly effective technique it first became very convenient replacement for other transformation procedures.
Recent research has shown that shock waves could be used for pre-treating the cell membrane prior to electroporation. This synergistic strategy has shown to reduce external voltage requirement and create larger pores. Also application of shock waves allow scope to target desired membrane site. This procedure allows to control the size of the pore. | 1 | Biochemistry |
Methods to study interpolymer complexes could be classified into:
:(1) approaches to demonstrate the fact of the complex formation and to determine the composition of IPCs in solutions;
:(2) approaches to study the structure of IPCs formed;
:(3) methods to characterize IPCs in solid state. | 7 | Physical Chemistry |
Bronze disease is the chloride corrosion of cuprous (copper-based) artifacts. It was originally thought to be caused by bacteria. It is contagious in that the chlorides which cause it can spread the condition if they are brought into contact with another cuprous object. Despite its name, bronze disease can affect any copper-bearing alloy, not just bronze. It is not reserved for antique objects but can affect contemporary metals like modern cupro-nickel coins.
Bronze disease ranges from vivid green to pastel green. It is commonly present in all colors in this range due to the series of reactions that cause it and there may also be tiny, possibly microscopic, blue crystals. Bronze disease typically affects isolated patches of the object in severe cases being a visibly and tactilely raised bloom of microscopic crystals as well as being associated with pitting. The patches of bronze disease can be scraped off the surface using a fingernail or a wooden pick. These properties are all in comparison with verdigris, which is normally a duller shade, uniform across the whole of the affected object, and cannot be scratched off with wood or fingernails. Unlike bronze disease, verdigris serves to protect the metal.
As it relies upon the presence of chlorides, water, and oxygen, the absence of one of these three halts the progress, although any damage done is irreversible. Treatment for the condition typically involves physical removal of the chlorides (through scrubbing), chemical or electrochemical removal, and then isolating the object from oxygen, water, and future chloride contamination using an airtight container or a wax coating. These treatments may also remove any patina, loss of which is often seen as undesirable to collectors and conservators but is preferable to loss of the object.
Bronze disease is common or even ubiquitous on artefacts recovered from a marine environment due to the presence of chlorides in seawater. Coastal areas may also be hazardous due to salt carried in the atmosphere as well as the humidity. Absence of dissolved chlorides and oxygen in the soil means buried objects may not be affected while interred (similarly, lack of soluble salts and oxygen means that buried metals may not develop a patina or that oxidation of the metal may be reversed). When an artefact is recovered, surface encrustations may hide and/or protect bronze disease.
Chlorides may occur in or on the metal due to contamination from soil, water (especially seawater), the atmosphere, human sweat, or be present as impurities when the object was created. In many cases chlorides may be present within the interior of the artefact; the disease may reoccur if not isolated from water and/or oxygen. | 8 | Metallurgy |
Biochemical receptors are large protein molecules that can be activated by the binding of a ligand such as a hormone or a drug. Receptors can be membrane-bound, as cell surface receptors, or inside the cell as intracellular receptors, such as nuclear receptors including those of the mitochondrion. Binding occurs as a result of non-covalent interactions between the receptor and its ligand, at locations called the binding site on the receptor. A receptor may contain one or more binding sites for different ligands. Binding to the active site on the receptor regulates receptor activation directly. The activity of receptors can also be regulated by the binding of a ligand to other sites on the receptor, as in allosteric binding sites. Antagonists mediate their effects through receptor interactions by preventing agonist-induced responses. This may be accomplished by binding to the active site or the allosteric site. In addition, antagonists may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity to exert their effects.
The term antagonist was originally coined to describe different profiles of drug effects. The biochemical definition of a receptor antagonist was introduced by Ariens and Stephenson in the 1950s. The current accepted definition of receptor antagonist is based on the receptor occupancy model. It narrows the definition of antagonism to consider only those compounds with opposing activities at a single receptor. Agonists were thought to turn "on" a single cellular response by binding to the receptor, thus initiating a biochemical mechanism for change within a cell. Antagonists were thought to turn "off" that response by blocking the receptor from the agonist. This definition also remains in use for physiological antagonists, substances that have opposing physiological actions, but act at different receptors. For example, histamine lowers arterial pressure through vasodilation at the histamine H receptor, while adrenaline raises arterial pressure through vasoconstriction mediated by alpha-adrenergic receptor activation.
Our understanding of the mechanism of drug-induced receptor activation and receptor theory and the biochemical definition of a receptor antagonist continues to evolve. The two-state model of receptor activation has given way to multistate models with intermediate conformational states. The discovery of functional selectivity and that ligand-specific receptor conformations occur and can affect interaction of receptors with different second messenger systems may mean that drugs can be designed to activate some of the downstream functions of a receptor but not others. This means efficacy may actually depend on where that receptor is expressed, altering the view that efficacy at a receptor is receptor-independent property of a drug. | 1 | Biochemistry |
Continuing her interest in the f-elements, Soderholm shifted her focus from solid-state materials to nanoparticles and solutions, taking advantage of advances in X-ray structural probes made available by synchrotron facilities. Building on her earlier work using neutron scattering, her team became the first to discover that plutonium exists in solution as tiny, well-defined nanoparticles. This work solved a longstanding problem in understanding transport of plutonium in the environment and resulted in the development of a new, patented approach to separating plutonium during nuclear reprocessing. | 7 | Physical Chemistry |
A bond angle is the geometric angle between two adjacent bonds. Some common shapes of simple molecules include:
* Linear: In a linear model, atoms are connected in a straight line. The bond angles are set at 180°. For example, carbon dioxide and nitric oxide have a linear molecular shape.
* Trigonal planar: Molecules with the trigonal planar shape are somewhat triangular and in one plane (flat). Consequently, the bond angles are set at 120°. For example, boron trifluoride.
* Angular: Angular molecules (also called bent or V-shaped) have a non-linear shape. For example, water (HO), which has an angle of about 105°. A water molecule has two pairs of bonded electrons and two unshared lone pairs.
* Tetrahedral: Tetra- signifies four, and -hedral relates to a face of a solid, so "tetrahedral" literally means "having four faces". This shape is found when there are four bonds all on one central atom, with no extra unshared electron pairs. In accordance with the VSEPR (valence-shell electron pair repulsion theory), the bond angles between the electron bonds are arccos(−) = 109.47°. For example, methane (CH) is a tetrahedral molecule.
* Octahedral: Octa- signifies eight, and -hedral relates to a face of a solid, so "octahedral" means "having eight faces". The bond angle is 90 degrees. For example, sulfur hexafluoride (SF) is an octahedral molecule.
* Trigonal pyramidal: A trigonal pyramidal molecule has a pyramid-like shape with a triangular base. Unlike the linear and trigonal planar shapes but similar to the tetrahedral orientation, pyramidal shapes require three dimensions in order to fully separate the electrons. Here, there are only three pairs of bonded electrons, leaving one unshared lone pair. Lone pair – bond pair repulsions change the bond angle from the tetrahedral angle to a slightly lower value. For example, ammonia (NH). | 4 | Stereochemistry |
Protein adsorption influences the interactions that occur at the tissue-implant interface. Protein adsorption can lead to blood clots, the foreign-body response and ultimately the degradation of the device. In order to counter-act the effects of protein adsorption, implants are often coated with a polymer coating to decrease protein adsorption.
Polyethylene glycol (PEG) coatings have been shown to minimize protein adsorption in the body. The PEG coating consists of hydrophilic molecules that are repulsive to protein adsorption. Proteins consist of hydrophobic molecules and charge sites that want to bind to other hydrophobic molecules and oppositely charged sites. By applying a thin monolayer coating of PEG, protein adsorption is prevented at the device site. Furthermore, the device's resistance to protein adsorption, fibroblast adhesion and bacteria adhesion are increased. | 7 | Physical Chemistry |
An example of a transversely isotropic material is the so-called on-axis unidirectional fiber composite lamina where the fibers are circular in cross section. In a unidirectional composite, the plane normal to the fiber direction can be considered as the isotropic plane, at long wavelengths (low frequencies) of excitation. In the figure to the right, the fibers would be aligned with the axis, which is normal to the plane of isotropy.
In terms of effective properties, geological layers of rocks are often interpreted as being transversely isotropic. Calculating the effective elastic properties of such layers in petrology has been coined Backus upscaling, which is described below. | 3 | Analytical Chemistry |
Single crystal growth starts with a seed crystal that is used to template growth of a larger crystal. The overall process is lengthy, and machining is necessary after the single crystal is grown. | 8 | Metallurgy |
The Streptamer staining principle combines the classic method of T cell isolation by MHC-multimers with the Strep-tag/Strep-Tactin technology. The Strep-tag is a short peptide sequence that displays moderate binding affinity for the biotin-binding site of a mutated streptavidin molecule, called Strep-Tactin. For the Streptamer technology, the Strep-Tactin molecules are multimerized and form the "backbone", thus creating a platform for binding to strep-tagged proteins. Additionally, the Strep-Tactin backbone has a fluorescent label to allow flow cytometry analysis. Incubation of MHC-Strep-tag fusion proteins with the Strep-Tactin backbone results in the formation of a MHC-multimer, which is capable for antigen-specific staining of T cells. | 1 | Biochemistry |
Robinson was an undergraduate student at the University of Cambridge, where she studied natural sciences. She moved to the University of Oxford for her graduate studies, where she investigated pleistocene climate chronology. After completing her doctorate, Robinson moved to California. She was appointed a postdoctoral fellow at California Institute of Technology. At Caltech, worked alongside Jess Adkins on deep sea corals. The research took her on a cruise in the North Atlantic ocean, where she journeyed in a submarine to undersea mountains. On this trip she collected fossils from the sea floor. She studied 16,000 year old coral fossils from the Southern Ocean. This experience inspired her to explore how the Atlantic Ocean changed during climate transitions. She moved to the Woods Hole Oceanographic Institution, where she was made Associate Scientist. | 9 | Geochemistry |
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