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Semiconductors are materials that have an electrical resistivity (and conductivity) between that of metallic conductors and non-metallic insulators. They can be found in the periodic table moving diagonally downward right from boron. They separate the electrical conductors (or metals, to the left) from the insulators (to the right).
Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, etc. Semiconductor devices include the transistor, solar cells, diodes and integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light into electrical energy.
In a metallic conductor, current is carried by the flow of electrons, but in semiconductors, current can be carried either by electrons or by the positively charged "holes" in the electronic band structure of the material. Common semiconductor materials include silicon, germanium and gallium arsenide. | 7 | Physical Chemistry |
To maintain indoor air quality, it may be desirable to expunge return air from the conditioned space and replace it with fresh outdoor air, sometimes called "make-up air." The optimal location to expunge return air from a Cromer cycle system is just after the desiccant (location 2 on Figure 1). At this point, the return air has been loaded with moisture from the desiccant, and expunging it removes additional moisture from the space. Furthermore, this expunge air is cooled below the return air condition by the desiccant's evaporation of the moisture into it. Location 2 (but before the fan) is also the ideal place to bring outdoor air into the system, as the coil can then reduce its temperature and moisture before it enters the space. Also, if heat exchange is provided between the expunged air and the outdoor air, the incoming air can be cooled and brought near to or at saturation before it enters the cooling coil for process 2 to 3, enhancing its dehumidification. | 7 | Physical Chemistry |
FRET efficiencies can also be inferred from the photobleaching rates of the donor in the presence and absence of an acceptor. This method can be performed on most fluorescence microscopes; one simply shines the excitation light (of a frequency that will excite the donor but not the acceptor significantly) on specimens with and without the acceptor fluorophore and monitors the donor fluorescence (typically separated from acceptor fluorescence using a bandpass filter) over time. The timescale is that of photobleaching, which is seconds to minutes, with fluorescence in each curve being given by
where is the photobleaching decay time constant and depends on whether the acceptor is present or not. Since photobleaching consists in the permanent inactivation of excited fluorophores, resonance energy transfer from an excited donor to an acceptor fluorophore prevents the photobleaching of that donor fluorophore, and thus high FRET efficiency leads to a longer photobleaching decay time constant:
where and are the photobleaching decay time constants of the donor in the presence and in the absence of the acceptor respectively. (Notice that the fraction is the reciprocal of that used for lifetime measurements).
This technique was introduced by Jovin in 1989. Its use of an entire curve of points to extract the time constants can give it accuracy advantages over the other methods. Also, the fact that time measurements are over seconds rather than nanoseconds makes it easier than fluorescence lifetime measurements, and because photobleaching decay rates do not generally depend on donor concentration (unless acceptor saturation is an issue), the careful control of concentrations needed for intensity measurements is not needed. It is, however, important to keep the illumination the same for the with- and without-acceptor measurements, as photobleaching increases markedly with more intense incident light. | 1 | Biochemistry |
Fried became a member of the National Academy of Sciences in 1971. He became a member of the American Academy of Arts and Sciences in 1981. He received the Medicinal Chemistry Award in 1974 from the American Chemical Society. He also received the Alfred Burger Award in Medicinal Chemistry in 1996.
He also received the Gregory Pincus Medal from the Worcester Foundation for Experimental Biology and the Roussel Prize from the Roussel Scientific Institute in Paris in 1994.
Bristol-Myers Squibb and the University of Chicago launched in 1990 the first of a series of annual Josef Fried Symposia of Bioorganic Chemistry. Fried is a member of the Medicinal Chemistry Hall of Fame. | 0 | Organic Chemistry |
Heusler compounds are magnetic intermetallics with face-centered cubic crystal structure and a composition of XYZ (half-Heuslers) or XYZ (full-Heuslers), where X and Y are transition metals and Z is in the p-block. The term derives from the name of German mining engineer and chemist Friedrich Heusler, who studied such a compound (CuMnAl) in 1903. Many of these compounds exhibit properties relevant to spintronics, such as magnetoresistance, variations of the Hall effect, ferro-, antiferro-, and ferrimagnetism, half- and semimetallicity, semiconductivity with spin filter ability, superconductivity, topological band structure and are actively studied as thermoelectric materials. Their magnetism results from a double-exchange mechanism between neighboring magnetic ions. Manganese, which sits at the body centers of the cubic structure, was the magnetic ion in the first Heusler compound discovered. (See the Bethe–Slater curve for details of why this happens.) | 8 | Metallurgy |
Borane dimethylsulfide is flammable and reacts readily with water to produce a flammable gas. It also has an unpleasant smell. | 0 | Organic Chemistry |
The core of β-catenin consists of several very characteristic repeats, each approximately 40 amino acids long. Termed armadillo repeats, all these elements fold together into a single, rigid protein domain with an elongated shape – called armadillo (ARM) domain. An average armadillo repeat is composed of three alpha helices. The first repeat of β-catenin (near the N-terminus) is slightly different from the others – as it has an elongated helix with a kink, formed by the fusion of helices 1 and 2. Due to the complex shape of individual repeats, the whole ARM domain is not a straight rod: it possesses a slight curvature, so that an outer (convex) and an inner (concave) surface is formed. This inner surface serves as a ligand-binding site for the various interaction partners of the ARM domains.
The segments N-terminal and far C-terminal to the ARM domain do not adopt any structure in solution by themselves. Yet these intrinsically disordered regions play a crucial role in β-catenin function. The N-terminal disordered region contains a conserved short linear motif responsible for binding of TrCP1 (also known as β-TrCP) E3 ubiquitin ligase – but only when it is phosphorylated. Degradation of β-catenin is thus mediated by this N-terminal segment. The C-terminal region, on the other hand, is a strong transactivator when recruited onto DNA. This segment is not fully disordered: part of the C-terminal extension forms a stable helix that packs against the ARM domain, but may also engage separate binding partners. This small structural element (HelixC) caps the C-terminal end of the ARM domain, shielding its hydrophobic residues. HelixC is not necessary for β-catenin to function in cell–cell adhesion. On the other hand, it is required for Wnt signaling: possibly to recruit various coactivators, such as 14-3-3zeta. Yet its exact partners among the general transcription complexes are still incompletely understood, and they likely involve tissue-specific players. Notably, the C-terminal segment of β-catenin can mimic the effects of the entire Wnt pathway if artificially fused to the DNA binding domain of LEF1 transcription factor.
Plakoglobin (also called γ-catenin) has a strikingly similar architecture to that of β-catenin. Not only their ARM domains resemble each other in both architecture and ligand binding capacity, but the N-terminal β-TrCP-binding motif is also conserved in plakoglobin, implying common ancestry and shared regulation with β-catenin. However, plakoglobin is a very weak transactivator when bound to DNA – this is probably caused by the divergence of their C-terminal sequences (plakoglobin appears to lack the transactivator motifs, and thus inhibits the Wnt pathway target genes instead of activating them). | 1 | Biochemistry |
In the present context, "incomplete transformation" refers to the fact that in the absence of carbide precipitation, the bainite reaction stops well before the austenite reaches its equilibrium or paraequilibrium chemical composition. It stops at the point where the free energies of austenite and ferrite of identical composition become the same, i.e. transformation without a change in chemical composition of the participating phases becomes thermodynamically impossible.
Early research on bainite found that at a given temperature only a certain volume fraction of the austenite would transform to bainite with the remainder decomposing to pearlite after an extended delay. This was the case despite the fact that a complete austenite to pearlite transformation could be achieved at higher temperatures where the austenite was more stable. The fraction of bainite that could form increased as the temperature decreased. This was ultimately explained by accounting for the fact that when the bainitic ferrite formed the supersaturated carbon would be expelled to the surrounding austenite thus thermodynamically stabilising it against further transformation. | 8 | Metallurgy |
Single particle testing is an extensive but powerful laboratory procedure developed by Tomra. Outo of a sample set of multiple hundreds of fragments in the size range 30-60mm are measured individually on each of the available detection technologies. After recording of the raw data, all the fragments are comminute and assayed individually which then allows plotting of the liberation function of the sample set and in addition, the detection efficiency of each detection technology in combination with the calibration method applied. This makes the evaluation of detection and calibration and subsequently the selection of the most powerful combination possible. This analysis is possible to be applied on quarters or half sections of drill core. | 3 | Analytical Chemistry |
Operational backscattering spectrometers at reactors include IN10, IN13, and IN16B at the Institut Laue-Langevin, the High Flux Backscattering Spectrometer (HFBS) at the NIST Center for Neutron Research,
the SPHERES instrument of Forschungszentrum Jülich at FRM II
and EMU at ANSTO. | 7 | Physical Chemistry |
In general, the recovery of fluorescence will not be dominated by either simple isotropic diffusion, or by a single simple unbinding rate. There will be both diffusion and binding, and indeed the diffusion constant may not be uniform in space, and there may be more than one type of binding sites, and these sites may also have a non-uniform distribution in space. Flow processes may also be important. This more complex behavior implies that a model with several parameters is required to describe the data; models with only either a single diffusion constant D or a single off rate constant, k, are inadequate.
There are models with both diffusion and reaction. Unfortunately, a single FRAP curve may provide insufficient evidence to reliably and uniquely fit (possibly noisy) experimental data. Sadegh Zadeh et al. have shown that FRAP curves can be fitted by different pairs of values of the diffusion constant and the on-rate constant, or, in other words, that fits to the FRAP are not unique. This is in three-parameter (on-rate constant, off-rate constant and diffusion constant) fits. Fits that are not unique, are not generally useful.
Thus for models with a number of parameters, a single FRAP experiment may be insufficient to estimate all the model parameters. Then more data is required, e.g., by bleaching areas of different sizes, determining some model parameters independently, etc. | 1 | Biochemistry |
Ketones are classified on the basis of their substituents. One broad classification subdivides ketones into symmetrical and unsymmetrical derivatives, depending on the equivalency of the two organic substituents attached to the carbonyl center. Acetone and benzophenone () are symmetrical ketones. Acetophenone is an unsymmetrical ketone. | 0 | Organic Chemistry |
The concept of reflection can be extended to three-dimensional objects, including the inside parts, even if they are not transparent. The term then relates to structural as well as visual aspects. A three-dimensional object is reversed in the direction perpendicular to the mirror surface. In physics, mirror images are investigated in the subject called geometrical optics. More fundamentally in geometry and mathematics they form the principal objects of Coxeter group theory and reflection groups.
In chemistry, two versions (isomers) of a molecule, one a "mirror image" of the other, are called enantiomers if they are not "superposable" (the correct technical term, though the term "superimposable" is also used) on each other. That is an example of chirality. In general, an object and its mirror image are called enantiomorphs.
If a point of an object has coordinates (x, y, z) then the image of this point (as reflected by a mirror in the y, z plane) has coordinates (−x, y, z). Thus reflection is a reversal of the coordinate axis perpendicular (normal) to the mirrors surface. Although a plane mirror reverses an object only in the direction normal to the mirror surface, this turns the entire three-dimensional image seen in the mirror inside-out, so there is a perception' of a left-right reversal. Hence, the reversal is somewhat misleadingly called a "lateral inversion". The perception of a left-right reversal is geometrically explained by the fact that a three-dimensional object seen in a mirror is an inside-out version of the actual object, like a glove stripped off the left hand and turned into a right-hand glove, but there is still some confusion about the explanation amongst psychologists. The psychology of the perceived left-right reversal is discussed in "Much ado about mirrors" by Professor Michael Corballis (see "external links", below).
Reflection in a mirror does result in a change in chirality, more specifically from a right-handed to a left-handed coordinate system (or vice versa). If one looks in a mirror two axes (up-down and left-right) coincide with those in the mirror, but the third axis (front-back) is reversed.
If a person stands side-on to a mirror, left and right hands will be reversed directly by the mirror, because the persons left-right axis is then normal to the mirror plane. However, it is important to understand that there are always' only two enantiomorphs, the object and its inside-out image. Therefore, no matter how the object is oriented towards the mirror, all the resulting images are fundamentally identical (as Corballis explains in his paper "Much ado about mirrors", mentioned above).
In the picture of the mountain reflected in the lake (photograph top right), the reversal normal to the reflecting surface is obvious. Notice that there is no obvious front-back or left-right of the mountain. In the example of the urn and mirror (photograph to right), the urn is fairly symmetrical front-back (and left-right). Thus, no obvious reversal of any sort can be seen in the mirror image of the urn.
A mirror image appears more obviously three-dimensional if the observer moves, or if the image is viewed using binocular vision. This is because the relative position of objects changes as the observer's perspective changes, or is differently viewed with each eye.
Looking through a mirror from different positions (but necessarily with the point of observation restricted to the halfspace on one side of the mirror) is like looking at the 3D mirror image of space; without further mirrors only the mirror image of the halfspace before the mirror is relevant; if there is another mirror, the mirror image of the other halfspace is too. | 4 | Stereochemistry |
A 2022 study by researchers from the Mayo Clinic, Maastricht University, and Ethris GmBH, a biotech company that focuses on RNA therapeutics, found that chemically modified mRNA encoding BMP-2 promoted dosage-dependent healing of femoral osteotomies in male rats. The mRNA molecules were complexed within nonviral lipid particles, loaded onto sponges, and surgically implanted into the bone defects. They remained localized around the site of application. Compared to receiving rhBMP-2 directly, bony tissues regenerated after mRNA treatment displayed superior strength and less formation of massive callus. | 1 | Biochemistry |
An ATM designed with a rotated sample is typically a far-field measurement configuration using a time-domain spectroscopy strategy.
A high power infrared laser is typically used. Its beam is split by a beamsplitter into two optical paths: a probe beam and a THz generation beam.
The THz generation beam typically receives the greater fraction of NIR power in order to maximize the power of the THz light commonly generated by a voltage-pulsed photoconductive antenna. The generated THz light is collected through a hyper-hemispherical silicon lens and passed to an off-axis parabolic mirror that collimates the THz beam for polarization by a THz polarizer that is often made of a simple wire-grid. The linearly polarized THz beam is then focused by a second off-axis parabolic mirror onto the sample. The THz beam transmitted through the sample is again collected by a third off-axis parabolic mirror, collimated onto a fourth parabolic mirror that then focuses the beam onto an electro-optic (EO) crystal whose birefringence is perturbed by the strength of the THz beam.
The NIR probe beam is passed through the EO crystal to probe the induced degree of birefringence caused by the THz beam and passed to a detection module that often consists of an NIR quarter wave plate, a Wollaston prism that spatially separates orthogonal polarization states of the probe beam into two optical paths that are individually detected at a balanced detector. The resulting signal reported by the balanced detector is a measure of the difference in magnitude of these two orthogonal components of the NIR probe beam and therefore a direct correlation of the degree of birefringence induced in the EO crystal by the THz beam passed through the sample. | 7 | Physical Chemistry |
Under AC conditions with varying frequency ω, heterogeneous systems and composite materials exhibit a universal dielectric response, in which overall admittance exhibits a region of power law scaling with frequency. . | 7 | Physical Chemistry |
SUI1 is a translation initiation factor that directs the ribosome to the translation start site, helped by eIF2 and the initiator Met-tRNA. SUI1 ensures that translation initiation commences from the correct start codon (usually AUG), by stabilizing the pre-initiation complex around the start codon. SUI1 promotes a high initiation fidelity for the AUG codon, discriminating against non-AUG codons.
In E. coli however, it seems that the SUI1 homolog YciH is an inhibitor of translation during stress instead. | 1 | Biochemistry |
Gene knockout by mutation is commonly carried out in bacteria. An early instance of the use of this technique in Escherichia coli was published in 1989 by Hamilton, et al. In this experiment, two sequential recombinations were used to delete the gene. This work established the feasibility of removing or replacing a functional gene in bacteria. That method has since been developed for other organisms, particularly research animals, like mice. Knockout mice are commonly used to study genes with human equivalents that may have significance for disease. An example of a study using knockout mice is an investigation of the roles of Xirp proteins in Sudden Unexplained Nocturnal Death Syndrome (SUNDS) and Brugada Syndrome in the Chinese Han Population. | 1 | Biochemistry |
Using 2 drugs at the same time can sometimes affect each others fraction unbound. For example, assume that Drug A and Drug B are both protein-bound drugs. If Drug A is given, it will bind to the plasma proteins in the blood. If Drug B is also given, it can displace Drug A from the protein, thereby increasing Drug As fraction unbound. This may increase the effects of Drug A, since only the unbound fraction may exhibit activity.
Note that for Drug A, the % increase in unbound fraction is 100% – hence, Drug A's pharmacological effect can potentially double (depending on whether the free molecules get to their target before they are eliminated by metabolism or excretion). This change in pharmacologic effect could have adverse consequences.
However, this effect is really only noticeable in closed systems where the pool of available proteins could potentially be exceeded by the number of drug molecules. Biological systems, such as humans and animals, are open systems where molecules can be gained, lost or redistributed and where the protein pool capacity is almost never exceeded by the number of drug molecules. A drug that is 99% bound means that 99% of the drug molecules are bound to blood proteins not that 99% of the blood proteins are bound with drug. When two, highly protein-bound drugs (A and B) are added into the same biological system it will lead to an initial small increase in the concentration of free drug A (as drug B ejects some of the drug A from its proteins). However, this free drug A is now more available for redistribution into the body tissues and/or for excretion. This means the total amount of drug in the system will decrease quite rapidly, keeping the free drug fraction (the concentration of free drug divided by the total drug concentration) constant and yielding almost no change in clinical effect.
The effects of drugs displacing each other and changing the clinical effect (though important in some examples) is vastly overestimated usually and a common example incorrectly used to display the importance of this effect is the anticoagulant warfarin. Warfarin is highly protein-bound (>95%) and has a low therapeutic index. Since a low therapeutic index indicates that there is a high risk of toxicity when using the drug, any potential increases in warfarin concentration could be very dangerous and lead to hemorrhage. In horses, it is very true that if warfarin and phenylbutazone are administered concurrently, the horse can develop bleeding issues which can be fatal. This is often explained as being due to the effect of phenylbutazone ejecting warfarin from its plasma protein, thus increasing the concentration of free warfarin and increasing its anticoagulant effect. However, the real problem is that phenylbutazone interferes with the liver's ability to metabolize warfarin so free warfarin cannot be metabolized properly or excreted. This leads to an increase in free warfarin and the resulting bleeding problems. | 1 | Biochemistry |
Many compounds and complexes adopt bitetrahedral structures. In this motif, the two tetrahedra share a common edge. The inorganic polymer silicon disulfide features an infinite chain of edge-shared tetrahedra. In a completely saturated hydrocarbon system, bitetrahedral molecule CH has been proposed as a candidate for the molecule with the shortest possible carbon-carbon single bond. | 4 | Stereochemistry |
Vat leaching involves contacting material, which has usually undergone size reduction and classification, with leach solution in large vats. | 8 | Metallurgy |
Thermal desorption first appeared as an environmental treatment technology in 1985 when it was specified in the Record of Decision for the McKin Company Superfund site within the Royal River watershed in Maine.
It is frequently referred to as "low temp" thermal desorption to differentiate it from high temperature incineration. An early direct fired thermal desorption project was the treatment of 8000 tons of toxaphene (a chlorinated pesticide) contaminated sandy soil at the S&S Flying Services site in Marianna Florida in 1990, with later projects exceeding 170,000 tons at the Cape Fear coal tar site in 1999. A status report from the United States Environmental Protection Agency shows that thermal desorption has been used at 69 Superfund sites through FY2000. In addition, hundreds of remediation projects have been completed using thermal desorption at non-Superfund sites.
For in-situ on-site treatment options, only incineration and stabilization have been used at more Superfund sites. Incineration suffers from poor public acceptance. Stabilization does not provide a permanent remedy, since the contaminants are still on site. Thermal desorption is a widely accepted technology that provides a permanent solution at an economically competitive cost.
The world’s first large-scale thermal desorption for treatment of mercury-containing wastes was erected in Wölsau, for the remediation of the Chemical Factory Marktredwitz (founded in 1788) was considered to be the oldest in Germany. Operation commenced in October 1993 including the first optimising phase. 50,000 tons of mercury-contaminated solid wastes were treated successfully between August 1993 and June 1996. 25 metric tons of mercury had been recovered from soil and rubble. Unfortunately the Marktredwitz plant is often misunderstood in the literature as a pilot-scale plant only. | 2 | Environmental Chemistry |
Ellman's reagent (5,5′-dithiobis-(2-nitrobenzoic acid) or DTNB) is a colorogenic chemical used to quantify the number or concentration of thiol groups in a sample. It was developed by George L. Ellman. | 1 | Biochemistry |
The same capability of natural organic matter that helps with water retention in soil creates problems for current water purification methods. In water, organic matter can still bind to metal ions and minerals. These bound molecules are not necessarily stopped by the purification process, but do not cause harm to any humans, animals, or plants. However, because of the high level of reactivity of organic matter, by-products that do not contain nutrients can be made. These by-products can induce biofouling, which essentially clogs water filtration systems in water purification facilities, as the by-products are larger than membrane pore sizes. This clogging problem can be treated by chlorine disinfection (chlorination), which can break down residual material that clogs systems. However, chlorination can form disinfection by-products.
Water with organic matter can be disinfected with ozone-initiated radical reactions. The ozone (three oxygens) has very strong oxidation characteristics. It can form hydroxyl radicals (OH) when it decomposes, which will react with the organic matter to shut down the problem of biofouling. | 0 | Organic Chemistry |
A real-time polymerase chain reaction (real-time PCR, or qPCR when used quantitatively) is a laboratory technique of molecular biology based on the polymerase chain reaction (PCR). It monitors the amplification of a targeted DNA molecule during the PCR (i.e., in real time), not at its end, as in conventional PCR. Real-time PCR can be used quantitatively and semi-quantitatively (i.e., above/below a certain amount of DNA molecules).
Two common methods for the detection of PCR products in real-time PCR are (1) non-specific fluorescent dyes that intercalate with any double-stranded DNA and (2) sequence-specific DNA probes consisting of oligonucleotides that are labelled with a fluorescent reporter, which permits detection only after hybridization of the probe with its complementary sequence.
The Minimum Information for Publication of Quantitative Real-Time PCR Experiments (MIQE) guidelines propose that the abbreviation qPCR be used for quantitative real-time PCR and that RT-qPCR be used for reverse transcription–qPCR. The acronym "RT-PCR" commonly denotes reverse transcription polymerase chain reaction and not real-time PCR, but not all authors adhere to this convention. | 1 | Biochemistry |
If enough mass is used it can create a seasonal advantage. That is, it can heat in the winter and cool in the summer. This is sometimes called passive annual heat storage or PAHS. The PAHS system has been successfully used at 7000 ft. in Colorado and in a number of homes in Montana. The Earthships of New Mexico utilize passive heating and cooling as well as using recycled tires for foundation wall yielding a maximum PAHS/STES. It has also been used successfully in the UK at Hockerton Housing Project. | 7 | Physical Chemistry |
When compounded with fibers, thermosetting resins form fiber-reinforced polymer composites, which are used in the fabrication of factory-finished structural composite OEM or replacement parts, and as site-applied, cured and finished composite repair and protection materials. When used as the binder for aggregates and other solid fillers, they form particulate-reinforced polymer composites, which are used for factory-applied protective coating or component manufacture, and for site-applied and cured construction, or maintenance purposes. | 7 | Physical Chemistry |
* In Thomson scattering light interacts with electrons (this is the low-energy limit of Compton scattering).
* In Rayleigh scattering a medium composed of particles whose sizes are much smaller than the wavelength scatters light sideways. In this scattering process, the energy (and therefore the wavelength) of the incident light is conserved and only its direction is changed. In this case, the scattering intensity is inversely proportional to the fourth power of the reciprocal wavelength of the light. | 7 | Physical Chemistry |
Starting point for the Woodward synthesis was the hydroquinone 1 that was converted to cis-bicycle 2 in a Diels-Alder reaction with butadiene. Conversion to the desired trans isomer 5 was accomplished by synthesis of the sodium enolate salt 4 (benzene, sodium hydride) followed by acidification. Reduction (lithium aluminium hydride) then gave diol 6, a dehydration (HCl/water) gave ketol 7, deoxygenation of its acetate by elemental zinc gave enone 8, formylation (ethyl formate) gave enol 9, Michael ethyl vinyl ketone addition (potassium t-butoxide/t-butanol) gave dione 11 which on reaction with KOH in dioxane gave tricycle 12 in an aldol condensation with elimination of the formyl group. In the next series of steps oxidation (osmium tetroxide) gave diol 13, protection (acetone/copper sulfate) gave acetonide 14, hydrogenation (palladium-strontium carbonate) gave 15, formylation (ethyl formate) gave enol 16 which protected as the enamine 17 (N-methylaniline/methanol) gave via the potassium anion 18, carboxylic acid 19 by reaction with cyanoethylene using triton B as the base.
Acid 19 was converted to lactone 20 (acetic anhydride, sodium acetate) and reaction with methylmagnesium chloride gave tetracyclic ketone 21. Treatment with periodic acid (dioxane) and piperidine acetate (benzene) gave aldehyde 24 through diol 22 (oxidation) and dialdehyde 23 (aldol condensation). Sodium dichromate oxidation gave carboxylic acid 25, Diazomethane treatment gave methyl ester 26 and sodium borohydride the allyl alcohol 27. Chiral resolution of this racemic compound with digitonin produced chiral 28 and on Oppenauer oxidation chiral 29. Hydrogenation (Adams' catalyst) gave alcohol 30, chromic acid oxidation gave ketone 31, sodium borohydride reduction stereoselectively gave alcohol 32, hydrolysis followed by acylation gave acetate 33, thionyl chloride treatment gave acyl chloride 34 and methyl cadmium the ketone 35.
In the final stages reaction of 35 with isohexylmagnesium bromide 36 gave diol 37, acetic acid treatment gave dehydration and then hydrogenation gave acetate 38. Hydrolysis of this ester gave cholestanol 39. The route from cholestanol to cholesterol was already known (see: Robinson synthesis). | 0 | Organic Chemistry |
In materials science, effective medium approximations (EMA) or effective medium theory (EMT) pertain to analytical or theoretical modeling that describes the macroscopic properties of composite materials. EMAs or EMTs are developed from averaging the multiple values of the constituents that directly make up the composite material. At the constituent level, the values of the materials vary and are inhomogeneous. Precise calculation of the many constituent values is nearly impossible. However, theories have been developed that can produce acceptable approximations which in turn describe useful parameters including the effective permittivity and permeability of the materials as a whole. In this sense, effective medium approximations are descriptions of a medium (composite material) based on the properties and the relative fractions of its components and are derived from calculations, and effective medium theory. There are two widely used formulae.
Effective permittivity and permeability are averaged dielectric and magnetic characteristics of a microinhomogeneous medium. They both were derived in quasi-static approximation when the electric field inside a mixture particle may be considered as homogeneous. So, these formulae can not describe the particle size effect. Many attempts were undertaken to improve these formulae. | 7 | Physical Chemistry |
Endoglin has been shown to interact with high affinity to TGF beta receptor 3 and TGF beta receptor 1, and with lower affinity to TGF beta receptor 2. It has high sequence similarity to another TGF beta binding protein, betaglycan, which was one of the first cues that indicated that endoglin is a TGF beta binding proteins. However, it has been shown that TGF beta binds with high affinity to only a small amount of the available endoglin, which suggests that there is another factor regulating this binding.
Endoglin itself doesnt bind the TGF beta ligands, but is present with the TGF beta receptors when the ligand is bound, indicating an important role for endoglin. The full length endoglin will bind to the TGF beta receptor complex whether TGF beta is bound or not, but the truncated forms of endoglin have more specific binding. The amino acid (aa) region 437–558 in the extracellular domain of endoglin will bind to TGF beta receptor II. TGF beta receptor I binds to the 437-588 aa region and to the aa region between 437 and the N-terminus. Unlike TGF beta receptor I which can only bind the cytoplasmic tail when its kinase domain is inactive, TGF beta receptor II can bind endoglin with an inactive and active kinase domain. The kinase is active when it is phosphorylated. Furthermore, TGF beta receptor I will dissociate from endoglin soon after it phosphorylates its cytoplasmic tail, leaving TGF beta receptor I inactive. Endoglin is constituitively phosphorylated at the serine and threonine residues in the cytoplasmic domain. The high interaction between endoglins cytoplasmic and extracellular tail with the TGF beta receptor complexes indicates an important role for endoglin in the modulation of the TGF beta responses, such as cellular localization and cellular migration.
Endoglin can also mediate F-actin dynamics, focal adhesions, microtubular structures, endocytic vesicular transport through its interaction with zyxin, ZRP-1, beta-arrestin and Tctex2beta, LK1, ALK5, TGF beta receptor II, and GIPC. In one study with mouse fibroblasts, the overexpression of endoglin resulted in a reduction of some ECM components, decreased cellular migration, a change in cellular morphology and intercellular cluster formation. | 1 | Biochemistry |
In recent years, more advanced UV-VIS detectors have been increasingly used, based on diode arrays and entire spectrum collection at any given moment of data collection. The are called Diode Array detectors, and they collect entire UV spectra of every point the eluting peaks, while operating as a multi-wavelength UV-vis detector. This way they give additional information, which help understand more about the nature of the substances appearing in the chromatogram and allow their identification. Since the detector facilitates better peak identification it is the preferred detector for HPLC method development.
A schematic of the optical systems is shown in Figure 1. The variable UV-vis absorbance detector's optical bench is showing how the flow cell is positioned after the optical system, including the monochromator, which typically has a physical slit and a moving grating, so it is illuminated by a selected wavelength, reaching a photo-diode. The bench of the diode array detector, however, is configured so that the flow cell is positioned before the optical parts, so that the beam containing the entire spectrum is passing through it. The optical parts consist also with a monochromator and a slit, but with a fixed grating, which disperses the light onto a diode array imaging element. | 3 | Analytical Chemistry |
* Poolman is ISI highly cited researcher in microbiology. He has published over 275 peer-reviewed papers in international scientific journals, which have been cited more than 25,000 times. His H-index (Google Scholar) is 83, and he holds four patents. Poolman shares his findings with wide audiences through newspaper, radio and TV appearances. In 2012 Schrauwers and Poolman wrote the book Synthetische Biologie: de Mens als Schepper (Synthetic Biology: Man as a Creator) to convey the developments in synthetic biology to a lay audience. | 0 | Organic Chemistry |
With fewer ions, the electric field increases, resulting in electrons with energy of about 2 eV, which is enough to excite atoms and produce light. With longer glow discharge tubes, the longer space is occupied by a longer positive column, while the cathode layer remains the same. For example, with a neon sign, the positive column occupies almost the entire length of the tube. | 3 | Analytical Chemistry |
Water gas shift is the most widespread industrial process for the production of dihydrogen, H. It involves the reaction of carbon monoxide and water (syngas) to form hydrogen and carbon dioxide as a byproduct. In many catalytic reaction schemes, one of the elementary reactions is the oxidation of CO with an adsorbed oxygen species. Gold catalysts have been proposed as an alternative for water gas shift at low temperatures, viz. O catalyst has been proven highly active and stable for low-temperature water gas shift. Titania and ceria have also been used as supports for effective catalysts. Unfortunately, Au/CeO is prone to deactivation caused by surface-bound carbonate or formate species.
Although gold catalysts are active at room temperature to CO oxidation, the high amounts of water involved in water gas shift require higher temperatures. At such temperatures, gold is fully reduced to its metallic form. However, the activity of e.g. Au/CeO has been enhanced by CN treatment, whereby metallic gold is leached, leaving behind highly active cations. According to DFT calculations, the presence of such Au cations on the catalyst is allowed by empty, localized nonbonding f states in CeO. On the other hand, STEM studies of Au/CeO have revealed nanoparticles of 3 nm in diameter. Water gas shift has been proposed to occur at the interface of Au nanoparticles and the reduced CeO support. | 7 | Physical Chemistry |
In three dimensions, there are an infinite number of point groups, but all of them can be classified by several families.
* C (for cyclic) has an n-fold rotation axis.
** C is C with the addition of a mirror (reflection) plane perpendicular to the axis of rotation (horizontal plane).
** C is C with the addition of n mirror planes containing the axis of rotation (vertical planes).
* C denotes a group with only mirror plane (for Spiegel, German for mirror) and no other symmetry elements.
* S (for Spiegel, German for mirror) contains only a n-fold rotation-reflection axis. The index, n, should be even because when it is odd an n-fold rotation-reflection axis is equivalent to a combination of an n-fold rotation axis and a perpendicular plane, hence S = C for odd n.
* C has only a rotoinversion axis. This notation is rarely used because any rotoinversion axis can be expressed instead as rotation-reflection axis: For odd n, C = S and C = S = C, and for even n, C = S. Only the notation C (meaning C) is commonly used, and some sources write C, C etc.
* D (for dihedral, or two-sided) has an n-fold rotation axis plus n twofold axes perpendicular to that axis.
** D has, in addition, a horizontal mirror plane and, as a consequence, also n vertical mirror planes each containing the n-fold axis and one of the twofold axes.
** D has, in addition to the elements of D, n vertical mirror planes which pass between twofold axes (diagonal planes).
* T (the chiral tetrahedral group) has the rotation axes of a tetrahedron (three 2-fold axes and four 3-fold axes).
** T includes diagonal mirror planes (each diagonal plane contains only one twofold axis and passes between two other twofold axes, as in D). This addition of diagonal planes results in three improper rotation operations S.
** T includes three horizontal mirror planes. Each plane contains two twofold axes and is perpendicular to the third twofold axis, which results in inversion center i.
* O (the chiral octahedral group) has the rotation axes of an octahedron or cube (three 4-fold axes, four 3-fold axes, and six diagonal 2-fold axes).
** O includes horizontal mirror planes and, as a consequence, vertical mirror planes. It contains also inversion center and improper rotation operations.
* I (the chiral icosahedral group) indicates that the group has the rotation axes of an icosahedron or dodecahedron (six 5-fold axes, ten 3-fold axes, and 15 2-fold axes).
** I includes horizontal mirror planes and contains also inversion center and improper rotation operations.
All groups that do not contain more than one higher-order axis (order 3 or more) can be arranged as shown in a table below; symbols in red are rarely used.
In crystallography, due to the crystallographic restriction theorem, n is restricted to the values of 1, 2, 3, 4, or 6. The noncrystallographic groups are shown with grayed backgrounds. D and D are also forbidden because they contain improper rotations with n = 8 and 12 respectively. The 27 point groups in the table plus T, T, T, O and O constitute 32 crystallographic point groups.
Groups with n = ∞ are called limit groups or Curie groups. There are two more limit groups, not listed in the table: K (for Kugel, German for ball, sphere), the group of all rotations in 3-dimensional space; and K, the group of all rotations and reflections. In mathematics and theoretical physics they are known respectively as the special orthogonal group and the orthogonal group in three-dimensional space, with the symbols SO(3) and O(3). | 7 | Physical Chemistry |
The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia. The German chemists Fritz Haber and Carl Bosch developed it in the first decade of the 20th century. The process converts atmospheric nitrogen (N) to ammonia (NH) by a reaction with hydrogen (H) using an iron metal catalyst under high temperatures and pressures. This reaction is slightly exothermic (i.e. it releases energy), meaning that the reaction is favoured at lower temperatures and higher pressures. It decreases entropy, complicating the process. Hydrogen is produced via steam reforming, followed by an iterative closed cycle to react hydrogen with nitrogen to produce ammonia.
The primary reaction is:
Before the development of the Haber process, it had been difficult to produce ammonia on an industrial scale, because earlier methods, such as the Birkeland–Eyde process and the Frank–Caro process, were too inefficient. | 7 | Physical Chemistry |
The nanocar is a molecule designed in 2005 at Rice University by a group headed by Professor James Tour. Despite the name, the original nanocar does not contain a molecular motor, hence, it is not really a car. Rather, it was designed to answer the question of how fullerenes move about on metal surfaces; specifically, whether they roll or slide (they roll).
The molecule consists of an H-shaped chassis with fullerene groups attached at the four corners to act as wheels.
When dispersed on a gold surface, the molecules attach themselves to the surface via their fullerene groups and are detected via scanning tunneling microscopy. One can deduce their orientation as the frame length is a little shorter than its width.
Upon heating the surface to 200 °C the molecules move forward and back as they roll on their fullerene "wheels". The nanocar is able to roll about because the fullerene wheel is fitted to the alkyne "axle" through a carbon-carbon single bond. The hydrogen on the neighboring carbon is no great obstacle to free rotation. When the temperature is high enough, the four carbon-carbon bonds rotate and the car rolls about. Occasionally the direction of movement changes as the molecule pivots. The rolling action was confirmed by Professor Kevin Kelly, also at Rice, by pulling the molecule with the tip of the STM. | 6 | Supramolecular Chemistry |
The cold blob in the North Atlantic (also called the North Atlantic warming hole) describes a cold temperature anomaly of ocean surface waters, affecting the Atlantic Meridional Overturning Circulation (AMOC) which is part of the thermohaline circulation, possibly related to global warming-induced melting of the Greenland ice sheet. | 9 | Geochemistry |
2,4,6-Triisopropylbenzenesulfonyl azide (trisyl azide) is an organic chemical used as a reagent to supply azide for electrophilic amination reactions, such as for the asymmetric synthesis of unnatural amino acids. Introduction of an azide on the α carbon of carboxylic acid derivative using trisyl azide is an efficient alternative to electrophilic halogenation followed by nucleophilic substitution using anionic azide. Using an oxazolidinone as chiral auxiliary typically gives good induction of the stereochemistry at the α position. Subsequent reduction converts the α-azide to an α-amine. | 0 | Organic Chemistry |
In chemistry, the hydrogenation of carbon–nitrogen double bonds is the addition of the elements of dihydrogen (H) across a carbon–nitrogen double bond, forming amines or amine derivatives. Although a variety of general methods have been developed for the enantioselective hydrogenation of ketones, methods for the hydrogenation of carbon–nitrogen double bonds are less general. Hydrogenation of imines is complicated by both syn/anti isomerization and tautomerization to enamines, which may be hydrogenated with low enantioselectivity in the presence of a chiral catalyst. Additionally, the substituent attached to nitrogen affects both the reactivity and spatial properties of the imine, complicating the development of a general catalyst system for imine hydrogenation. Despite these challenges, methods have been developed that address particular substrate classes, such as N-aryl, N-alkyl, and endocyclic imines.
If the complex is chiral and non-racemic and the substrate is prochiral, an excess of a single enantiomer of a chiral product can result. | 0 | Organic Chemistry |
Consider a system composed of chemical species (e.g. water splitting) in thermodynamic equilibrium at constant pressure and thermodynamic temperature T:
:::::HO(l) H(g) + 1/2 O(g) (1)
Equilibrium is displaced to the right only if energy (enthalpy change ΔH for water-splitting) is provided to the system under strict conditions imposed by thermodynamics:
* one fraction must be provided as work, namely the Gibbs free energy change ΔG of the reaction: it consists of "noble" energy, i.e. under an organized state where matter can be controlled, such as electricity in the case of the electrolysis of water. Indeed, the generated electron flow can reduce protons (H) at the cathode and oxidize anions (O) at the anode (the ions exist because of the chemical polarity of water), yielding the desired species.
* the other one must be supplied as heat, i.e. by increasing the thermal agitation of the species, and is equal by definition of the entropy to the absolute temperature T times the entropy change ΔS of the reaction.
Hence, for an ambient temperature T° of 298K (kelvin) and a pressure of 1 atm (atmosphere (unit)) (ΔG° and ΔS° are respectively equal to 237 kJ/mol and 163 J/mol/K, relative to the initial amount of water), more than 80% of the required energy ΔH must be provided as work in order for water-splitting to proceed.
If phase transitions are neglected for simplicity's sake (e.g. water electrolysis under pressure to keep water in its liquid state), one can assume that ΔH et ΔS do not vary significantly for a given temperature change. These parameters are thus taken equal to their standard values ΔH° et ΔS° at temperature T°. Consequently, the work required at temperature T is,
As ΔS° is positive, a temperature increase leads to a reduction of the required work. This is the basis of high-temperature electrolysis. This can also be intuitively explained graphically.
Chemical species can have various excitation levels depending on the absolute temperature T, which is a measure of the thermal agitation. The latter causes shocks between atoms or molecules inside the closed system such that energy spreading among the excitation levels increases with time, and stop (equilibrium) only when most of the species have similar excitation levels (a molecule in a highly excited level will quickly return to a lower energy state by collisions) (Entropy (statistical thermodynamics)).
Relative to the absolute temperature scale, the excitation levels of the species are gathered based on standard enthalpy change of formation considerations; i.e. their stabilities. As this value is null for water but strictly positive for oxygen and hydrogen, most of the excitation levels of these last species are above the ones of water. Then, the density of the excitation levels for a given temperature range is monotonically increasing with the species entropy. A positive entropy change for water-splitting means far more excitation levels in the products. Consequently,
* A low temperature (T°), thermal agitation allow mostly the water molecules to be excited as hydrogen and oxygen levels required higher thermal agitation to be significantly populated (on the arbitrary diagram, 3 levels can be populated for water vs 1 for the oxygen/hydrogen subsystem),
* At high temperature (T), thermal agitation is sufficient for the oxygen/hydrogen subsystem excitation levels to be excited (on the arbitrary diagram, 4 levels can be populated for water vs 8 for the oxygen/hydrogen subsystem). According to the previous statements, the system will thus evolve toward the composition where most of its excitation levels are similar, i.e. a majority of oxygen and hydrogen species.
One can imagine that if T were high enough in Eq.(3), ΔG could be nullified, meaning that water-splitting would occur even without work (thermolysis of water). Though possible, this would require tremendously high temperatures: considering the same system naturally with steam instead of liquid water (ΔH° = 242 kJ/mol; ΔS° = 44 J/mol/K) would hence give required temperatures above 3000K, that make reactor design and operation extremely challenging.
Hence, a single reaction only offers one freedom degree (T) to produce hydrogen and oxygen only from heat (though using Le Chatelier's principle would also allow to slightly decrease the thermolysis temperature, work must be provided in this case for extracting the gas products from the system) | 7 | Physical Chemistry |
Treatment of diisopinocampheylborane with TMEDA give the crystalline adduct of monoisopinocampheylborane. This adduct reacts with boron trifluoride to liberate the monoisopinocampheylborane (IpcBH) in 100% ee. Monoisopinocampheylborane reacts with a variety of alkenes. Two other reagents have been developed for the hydroboration of ketones:
In the above mechanism where G=O and R is Ipc and Cl or 9-Borabicyclononane. Diisopinocampheylchloroborane (IpcBCl) is produced by treating diisopinocampheylborane with hydrogen chloride. The chloride is reported to be more stable that the trialkyl boranes, it works well with aryl alkyl ketones and tert-butyl alkyl ketones. Diisopinocampheylchloroborane is often complementary with diisopinocampheylborane, where one provides the R enantiomer and the other the S, the enantioselectivity is typically very high.
Alpine-borane is produced by hydroborating α-pinene with 9-borabicyclononane. Both of these reagents can be improved upon by using 2-ethylapopinene in place of α-pinene, 2-ethylapopinene has an ethyl group in place of the methyl in α-pinene. The additional steric bulk improves the stereoselectivity of the reduction.
Diisopinocampheylborane reacts with methanol to give diisopinocampheylmethoxyborane, which in turn reacts with an allyl or crotyl Grignard reagent to give B-allyldiisopinocampheylborane. This can then undergo an asymmetric allylboration to give a chiral homologated alcohol, which is a useful building block in a chiral synthesis. | 0 | Organic Chemistry |
Given the short sequences of most promoter elements, promoters can rapidly evolve from random sequences. For instance, in E. coli, ~60% of random sequences can evolve expression levels comparable to the wild-type lac promoter with only one mutation, and that ~10% of random sequences can serve as active promoters even without evolution. | 1 | Biochemistry |
Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be readily deprotonated due to a large difference between the electronegativity of oxygen (3.5) and that of hydrogen (2.1). Hydroxy-containing compounds engage in intermolecular hydrogen bonding increasing the electrostatic attraction between molecules and thus to higher boiling and melting points than found for compounds that lack this functional group. Organic compounds, which are often poorly soluble in water, become water-soluble when they contain two or more hydroxy groups, as illustrated by sugars and amino acid. | 0 | Organic Chemistry |
EVB has been successfully applied to calculating reaction free energies of enzymes. More recently it has been looked at as a tool to study enzyme evolution and to assist in enzyme design. | 7 | Physical Chemistry |
A compact disk/digital versatile disk (CD/DVD) based immunoassay is a method for determining the concentration of a compound in research and diagnostic laboratories by performing the test on an adapted CD/DVD surface using an adapted optical disc drive; these methods have been discussed and prototyped in research labs since 1991. | 1 | Biochemistry |
In chemistry, a template reaction is any of a class of ligand-based reactions that occur between two or more adjacent coordination sites on a metal center. In the absence of the metal ion, the same organic reactants produce different products. The term is mainly used in coordination chemistry. The template effects emphasizes the pre-organization provided by the coordination sphere, although the coordination modifies the electronic properties (acidity, electrophilicity, etc.) of ligands.
An early example is the dialkylation of a nickel dithiolate:
The corresponding alkylation in the absence of a metal ion would yield polymers. Crown ethers arise from dialkylations that are templated by alkali metals. Other template reactions include the Mannich and Schiff base condensations. The condensation of formaldehyde, ammonia, and tris(ethylenediamine)cobalt(III) to give a clathrochelate complex is one example.
The phosphorus analogue of an aza crown can be prepared by a template reaction. Where it is not possible to isolate the phosphine itself. | 7 | Physical Chemistry |
Eschenmoser began his scientific career as a graduate student in the laboratory of Leopold Ružička, at the Eidgenossische Technische Hochschule (ETH) in Zurich. Ruzicka was a notable organic chemist himself having been awarded the Nobel Prize in Chemistry in 1939 for his work on the synthesis of androsterone and testosterone. Eschenmoser's early work on the cyclization of unsaturated, conjugated hydrocarbons directly contributed to advances in the field of terpene chemistry and provided insight into steroid biosynthesis.
In the early 1960s, having become Professor of General Organic Chemistry at ETH, Eschenmoser began work on what was the most complex natural product synthesized at the time—vitamin B. In a remarkable collaboration with his colleague Robert Burns Woodward at Harvard University, a team of almost one hundred students and postdoctoral workers worked for many years on the total synthesis of this molecule. At the time, a significant obstacle to the synthesis of vitamin B had been the difficulty in the final macrocyclic ring closure necessary to complete the corrin ring structure at the center of the molecule. Eschenmoser and his collaborators discovered methods under which such bonds between corrin ring building blocks could be formed, including a novel photochemical process which established the final junction of rings A and D with a high degree of stereospecificity, the key step in what was dubbed the "A/D variant" of the syntheses. Both the Harvard/ETH "A/B variant" and the ETH "A/D variant" of the syntheses were jointly and concomitantly completed in 1972, and they marked a landmark in the history of organic chemistry.
The Eschenmoser fragmentation, the Eschenmoser sulfide contraction and Eschenmoser's salt are named after him. | 0 | Organic Chemistry |
If there are aryl- groups on phosphorus, transition-metal can not only bind to the phosphorus directly, but also form arene-coordinated products of η-type coordination. Refluxing diphosphene in 1,4-dioxane with the excess of Cr(CO) can generate mono and bis arene tricarbonylchromium(0) complexes. | 0 | Organic Chemistry |
Homologous recombination is the exchange of genes between two DNA strands that include extensive regions of base sequences that are identical to one another. In eukaryotic species, bacteria, and some viruses, homologous recombination happens spontaneously and is a useful tool in genetically engineered. Homologous recombination, which takes place during meiosis in eukaryotes, is essential for the repair of double-stranded DNA breaks and promotes genetic variation by allowing the movement of genetic information during chromosomal crossing. Homologous recombination, a key DNA repair mechanism in bacteria, enables the insertion of genetic material acquired through horizontal transfer of genes and transformation into DNA. Homologous recombination in viruses influences the course of viral evolution. Homologous recombination, a type of gene targeting used in genetic engineering, involves the introduction of an engineered mutation into a particular gene in order to learn more about the function of that gene. This method involves inserting foreign DNA into a cell that has a sequence similar to the target gene while being flanked by sequences that are the same upstream and downstream of the target gene. The target genes DNA is substituted with the foreign DNA sequence during replication when the cell detects the similar flanking regions as homologues. The target gene is "knocked out" by the exchange. By using this technique to target particular alleles in embryonic stem cells in mice, it is possible to create knockout mice. With the aid of gene targeting, numerous mouse genes have been shut down, leading to the creation of hundreds of distinct mouse models of various human diseases, such as cancer, diabetes, cardiovascular diseases, and neurological disorders. Mario Capecchi, Sir Martin J. Evans, and Oliver Smithies performed groundbreaking research on homologous recombination in mouse stem cells, and they shared the 2007 Nobel Prize in Physiology or Medicine for their findings. Traditionally, homologous recombination was the main method for causing a gene knockout. This method involves creating a DNA construct containing the desired mutation. For knockout purposes, this typically involves a drug resistance marker in place of the desired knockout gene. The construct will also contain a minimum of 2kb of homology to the target sequence. The construct can be delivered to stem cells either through microinjection or electroporation. This method then relies on the cells own repair mechanisms to recombine the DNA construct into the existing DNA. This results in the sequence of the gene being altered, and most cases the gene will be translated into a nonfunctional protein, if it is translated at all. However, this is an inefficient process, as homologous recombination accounts for only 10 to 10 of DNA integrations. Often, the drug selection marker on the construct is used to select for cells in which the recombination event has occurred.
These stem cells now lacking the gene could be used in vivo, for instance in mice, by inserting them into early embryos. If the resulting chimeric mouse contained the genetic change in their germline, this could then be passed on offspring.
In diploid organisms, which contain two alleles for most genes, and may as well contain several related genes that collaborate in the same role, additional rounds of transformation and selection are performed until every targeted gene is knocked out. Selective breeding may be required to produce homozygous knockout animals. | 1 | Biochemistry |
Ampicillin is in the penicillin group of beta-lactam antibiotics and is part of the aminopenicillin family. It is roughly equivalent to amoxicillin in terms of activity. Ampicillin is able to penetrate gram-positive and some gram-negative bacteria. It differs from penicillin G, or benzylpenicillin, only by the presence of an amino group. This amino group, present on both ampicillin and amoxicillin, helps these antibiotics pass through the pores of the outer membrane of gram-negative bacteria, such as Escherichia coli, Proteus mirabilis, Salmonella enterica, and Shigella.
Ampicillin acts as an irreversible inhibitor of the enzyme transpeptidase, which is needed by bacteria to make the cell wall. It inhibits the third and final stage of bacterial cell wall synthesis in binary fission, which ultimately leads to cell lysis; therefore, ampicillin is usually bacteriolytic. | 4 | Stereochemistry |
A variety of compounds are added to stabilize the foams. These additives include pine oil, various alcohols (methyl isobutyl carbinol (MIBC)), polyglycols, xylenol (cresylic acid). | 8 | Metallurgy |
Cilazapril is an angiotensin-converting enzyme inhibitor (ACE inhibitor) used for the treatment of hypertension and congestive heart failure.
It was patented in 1982 and approved for medical use in 1990. | 4 | Stereochemistry |
* Benzaldehyde, for bees
* Butopyronoxyl (trade name Indalone). Widely used in a "6-2-2" mixture (60% Dimethyl phthalate, 20% Indalone, 20% Ethylhexanediol) during the 1940s and 1950s before the commercial introduction of DEET
* DEET (N,N-diethyl-m-toluamide)
* Dimethyl carbate
* Dimethyl phthalate, not as common as it once was but still occasionally an active ingredient in commercial insect repellents
* Ethyl butylacetylaminopropionate (IR3535 or 3-[N-Butyl-N-acetyl]-aminopropionic acid, ethyl ester)
* Ethylhexanediol, also known as Rutgers 612 or "6–12 repellent," discontinued in the US in 1991 due to evidence of causing developmental defects in animals
* Icaridin, also known as picaridin, Bayrepel, and KBR 3023
* Methyl anthranilate and other anthranilate-based insect repellents
* Metofluthrin
* Permethrin is a contact insecticide rather than a repellent
* SS220 is a repellent being researched that has shown promise to provide significantly better protection than DEET
* Tricyclodecenyl allyl ether, a compound often found in synthetic perfumes | 1 | Biochemistry |
In geometry, a figure is chiral (and said to have chirality) if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. An object that is not chiral is said to be achiral.
A chiral object and its mirror image are said to be enantiomorphs. The word chirality is derived from the Greek (cheir), the hand, the most familiar chiral object; the word enantiomorph stems from the Greek (enantios) opposite + (morphe) form. | 4 | Stereochemistry |
Thermogalvanic cells are a kind of heat engine. Ultimately the driving force behind them is the transport of entropy from the high temperature source to the low temperature sink. Therefore, these cells work thanks to a thermal gradient established between different parts of the cell. Because the rate and enthalpy of chemical reactions depend directly on the temperature, different temperatures at the electrodes imply different chemical equilibrium constants. This translates into unequal chemical equilibrium conditions on the hot side and on the cold side. The thermocell tries to approach an homogeneous equilibrium and, in doing so, produces a flow of chemical species and electrons. The electrons flow through the path of least resistance (the outer circuit) making it possible to extract power from the cell. | 7 | Physical Chemistry |
Molecules whose framework forms a closed cage, like dodecahedrane and buckminsterfullerene, can encapsulate atoms and small molecules in the hollow space within. Those insertions are not chemically bonded to the caging compound, but merely mechanically trapped in it.
Cross, Saunders and Prinzbach succeeded in encapsulating helium atoms in dodecahedrane by shooting He ions at a film of the compound. They obtained microgram quantities of He@ (the "@" being the standard notation for encapsulation), which they described as a quite stable substance. The molecule has been described as "the world's smallest helium balloon". | 0 | Organic Chemistry |
In biochemistry, the glutamate–glutamine cycle is a cyclic metabolic pathway which maintains an adequate supply of the neurotransmitter glutamate in the central nervous system. Neurons are unable to synthesize either the excitatory neurotransmitter glutamate, or the inhibitory GABA from glucose. Discoveries of glutamate and glutamine pools within intercellular compartments led to suggestions of the glutamate–glutamine cycle working between neurons and astrocytes. The glutamate/GABA–glutamine cycle is a metabolic pathway that describes the release of either glutamate or GABA from neurons which is then taken up into astrocytes (non-neuronal glial cells). In return, astrocytes release glutamine to be taken up into neurons for use as a precursor to the synthesis of either glutamate or GABA. | 1 | Biochemistry |
Setting up for transcription in mammals is regulated by many cis-regulatory elements, including core promoter and promoter-proximal elements that are located near the transcription start sites of genes. Core promoters combined with general transcription factors are sufficient to direct transcription initiation, but generally have low basal activity. Other important cis-regulatory modules are localized in DNA regions that are distant from the transcription start sites. These include enhancers, silencers, insulators and tethering elements. Among this constellation of elements, enhancers and their associated transcription factors have a leading role in the initiation of gene transcription. An enhancer localized in a DNA region distant from the promoter of a gene can have a very large effect on gene transcription, with some genes undergoing up to 100-fold increased transcription due to an activated enhancer.
Enhancers are regions of the genome that are major gene-regulatory elements. Enhancers control cell-type-specific gene transcription programs, most often by looping through long distances to come in physical proximity with the promoters of their target genes. While there are hundreds of thousands of enhancer DNA regions, for a particular type of tissue only specific enhancers are brought into proximity with the promoters that they regulate. In a study of brain cortical neurons, 24,937 loops were found, bringing enhancers to their target promoters. Multiple enhancers, each often at tens or hundred of thousands of nucleotides distant from their target genes, loop to their target gene promoters and can coordinate with each other to control transcription of their common target gene.
The schematic illustration in this section shows an enhancer looping around to come into close physical proximity with the promoter of a target gene. The loop is stabilized by a dimer of a connector protein (e.g. dimer of CTCF or YY1), with one member of the dimer anchored to its binding motif on the enhancer and the other member anchored to its binding motif on the promoter (represented by the red zigzags in the illustration). Several cell function specific transcription factors (there are about 1,600 transcription factors in a human cell) generally bind to specific motifs on an enhancer and a small combination of these enhancer-bound transcription factors, when brought close to a promoter by a DNA loop, govern level of transcription of the target gene. Mediator (a complex usually consisting of about 26 proteins in an interacting structure) communicates regulatory signals from enhancer DNA-bound transcription factors directly to the RNA polymerase II (pol II) enzyme bound to the promoter.
Enhancers, when active, are generally transcribed from both strands of DNA with RNA polymerases acting in two different directions, producing two enhancer RNAs (eRNAs) as illustrated in the Figure. An inactive enhancer may be bound by an inactive transcription factor. Phosphorylation of the transcription factor may activate it and that activated transcription factor may then activate the enhancer to which it is bound (see small red star representing phosphorylation of transcription factor bound to enhancer in the illustration). An activated enhancer begins transcription of its RNA before activating transcription of messenger RNA from its target gene. | 1 | Biochemistry |
In polymer physics, the coil–globule transition is the collapse of a macromolecule from an expanded coil state through an ideal coil state to a collapsed globule state, or vice versa. The coil–globule transition is of importance in biology due to the presence of coil-globule transitions in biological macromolecules such as proteins and DNA. It is also analogous with the swelling behavior of a crosslinked polymer gel and is thus of interest in biomedical engineering for controlled drug delivery. A particularly prominent example of a polymer possessing a coil-globule transition of interest in this area is that of Poly(N-isopropylacrylamide) (PNIPAAm). | 7 | Physical Chemistry |
Human chorionic gonadotropin injection is extensively used for final maturation induction in lieu of luteinizing hormone. In the presence of one or more mature ovarian follicles, ovulation can be triggered by the administration of HCG. As ovulation will happen between 38 and 40 hours after a single HCG injection, procedures can be scheduled to take advantage of this time sequence, such as intrauterine insemination or sexual intercourse. Also, patients that undergo IVF, in general, receive HCG to trigger the ovulation process, but have an oocyte retrieval performed at about 34 to 36 hours after injection, a few hours before the eggs actually would be released from the ovary.
As hCG supports the corpus luteum, administration of HCG is used in certain circumstances to enhance the production of progesterone.
In the male, hCG injections are used to stimulate the Leydig cells to synthesize testosterone. The intratesticular testosterone is necessary for spermatogenesis from the sertoli cells. Typical uses for hCG in men include hypogonadism and fertility treatment, including during testosterone replacement therapy to restore or maintain fertility and prevent testicular atrophy.
Several vaccines against human chorionic gonadotropin (hCG) for the prevention of pregnancy are currently in clinical trials. | 1 | Biochemistry |
This aspect of agricultural chemistry deals with the role of molecular chemistry in agriculture as well as the negative consequences. | 1 | Biochemistry |
The mouse sperm genome is 80–90% methylated at its CpG sites in DNA, amounting to about 20 million methylated sites. After fertilization, early in the first day of embryogenesis, the paternal chromosomes are almost completely demethylated in six hours by an active TET-dependent process, before DNA replication begins (blue line in Figure).
Demethylation of the maternal genome occurs by a different process. In the mature oocyte, about 40% of its CpG sites in DNA are methylated. In the pre-implantation embryo up to the blastocyst stage (see Figure), the only methyltransferase present is an isoform of DNMT1 designated DNMT1o. It appears that demethylation of the maternal chromosomes largely takes place by blockage of the methylating enzyme DNMT1o from entering the nucleus except briefly at the 8 cell stage (see DNA demethylation). The maternal-origin DNA thus undergoes passive demethylation by dilution of the methylated maternal DNA during replication (red line in Figure). The morula (at the 16 cell stage), has only a small amount of DNA methylation (black line in Figure). | 1 | Biochemistry |
Michael Philip Hartshorn (10 September 1936 – 15 December 2017) was a British-born New Zealand organic chemist. He was awarded the Hector Memorial Medal by the Royal Society of New Zealand in 1973. | 0 | Organic Chemistry |
The ash content of a sample is a measure of the amount of inorganic noncombustible material it contains. The residues after a sample is completely burnt - in contrast to the ash remaining after incomplete combustion - typically consist of oxides of the inorganic elements present in the original sample. Ash is one of the components in the proximate analysis of biological materials, consisting mainly of salty, inorganic constituents. It includes metal salts which are important for processes requiring ions such as Na (sodium), K (potassium), and Ca (calcium). It also includes trace minerals which are required for unique molecules, such as chlorophyll and hemoglobin.
Procedures for ash content determination are similar to procedures for loss on ignition. Typically, the term ash is used for primarily organic material such as fuels and foodstuffs, while the term loss on ignition is used for primarily inorganic material such as rocks and combusted ash.
A crucible can be used to determine the percentage of ash contained in a sample of material such as coal, wood, oil, rubber, plastics, foodstuffs, or any burnable material. The appropriate method for ash determination varies depending upon the type of sample analyzed. Each method may vary in parameters such as furnace temperature, residence time in the furnace, number of heating steps, and sample preparation procedures.
The ISO mandates ash content determination for most foodstuffs. Examples include
* ISO 2171: Cereals, pulses and by-products — Determination of ash yield by incineration;
* ISO 3593: Starch — Determination of ash;
* ISO 928: Spices and condiments - Determination of total ash; and
* ISO 936: Meat and meat products - Determination of total ash.
Examples of ash content methods for the determination of ash in other solids include
* ASTM D482: Standard Test Method for Ash from Petroleum Products;
* ISO 6245: Petroleum products — Determination of ash;
* ASTM D874: Standard Test Method for Sulfated Ash from Lubricating Oils and Additives;
* ASTM D3174: Standard Test Method for Ash in the Analysis Sample of Coal and Coke from Coal;
* ISO 1171: Solid mineral fuels — Determination of ash;
* ISO 18122: Solid biofuels — Determination of ash content;
* ASTM D1102: Standard Test Method for Ash in Wood;
* ASTM D2974: Standard Test Methods for Determining the Water (Moisture) Content, Ash Content, and Organic Material of Peat and Other Organic Soils;
* ASTM D2866: Standard Test Method for Total Ash Content of Activated Carbon;
* ISO 3451: Plastics — Determination of ash — Part 1: General methods;
* ASTM D2584: Standard Test Method for Ignition Loss of Cured Reinforced Resins; and
* ASTM D5630: Standard Test Method for Ash Content in Plastics. | 3 | Analytical Chemistry |
As early as the 1860s, experiments demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not straightforward. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments. Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible products. Since life tends to use whatever is available, an explanation is needed for why the set used is so small. Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks. | 9 | Geochemistry |
This method adds to the dilution to threshold method by considering the perceived intensity of the compounds as well. Assessors can report this based on a predetermined scale.
The posterior intensity method measures the maximum intensity perceived for each eluting compound. A panel of assessors is recommended to be used to obtain an averaged signal. On the other hand, the dynamic time-intensity method measures
the intensity at different points in time starting from the time of elution, allowing a continuous measurement of onset, maximum, and decline of the odour intensity. This is used in the Osme (Greek word for odour) method developed in 1992 by Da Silva. An aromagram can then be constructed in a similar way as an FID chromatogram whereby intensity is plotted as a function of retention time. The peak height corresponds to the maximum intensity perceived whereas the peak width corresponds to the duration of the odour perceived.
The time requirement maybe high for this particular method regarding the essentials of assessor training, as lack of training may result in inconsistencies in scale usage. However, with a trained panel of assessors, the analysis can be done in a relatively short amount of time with high precision. | 3 | Analytical Chemistry |
If a differentiable function is one-to-one (injective) for each independent variable, e.g., is one-to-one for at a fixed while it is not necessarily one-to-one for , then the following total differentials exist because each independent variable is a differentiable function for the other variables, e.g., .
Substituting the first equation into the second and rearranging, we obtain
Since and are independent variables, and may be chosen without restriction. For this last equation to generally hold, the bracketed terms must be equal to zero. The left bracket equal to zero leads to the reciprocity relation while the right bracket equal to zero goes to the cyclic relation as shown below. | 7 | Physical Chemistry |
Carbohydrate macromolecules (polysaccharides) are formed from polymers of monosaccharides. Because monosaccharides have multiple functional groups, polysaccharides can form linear polymers (e.g. cellulose) or complex branched structures (e.g. glycogen). Polysaccharides perform numerous roles in living organisms, acting as energy stores (e.g. starch) and as structural components (e.g. chitin in arthropods and fungi). Many carbohydrates contain modified monosaccharide units that have had functional groups replaced or removed.
Polyphenols consist of a branched structure of multiple phenolic subunits. They can perform structural roles (e.g. lignin) as well as roles as secondary metabolites involved in signalling, pigmentation and defense. | 7 | Physical Chemistry |
Retention uniformity is calculated from the following formula:
where n is the number of compounds separated, R are the Retention factor of the compounds sorted in non-descending order. | 3 | Analytical Chemistry |
The system is the part of the universe being studied, while the surroundings is the remainder of the universe that lies outside the boundaries of the system. It is also known as the environment or the reservoir. Depending on the type of system, it may interact with the system by exchanging mass, energy (including heat and work), momentum, electric charge, or other conserved properties. The environment is ignored in the analysis of the system, except in regards to these interactions. | 7 | Physical Chemistry |
The field of surface chemistry started with heterogeneous catalysis pioneered by Paul Sabatier on hydrogenation and Fritz Haber on the Haber process. Irving Langmuir was also one of the founders of this field, and the scientific journal on surface science, Langmuir, bears his name. The Langmuir adsorption equation is used to model monolayer adsorption where all surface adsorption sites have the same affinity for the adsorbing species and do not interact with each other. Gerhard Ertl in 1974 described for the first time the adsorption of hydrogen on a palladium surface using a novel technique called LEED. Similar studies with platinum, nickel, and iron followed. Most recent developments in surface sciences include the 2007 Nobel prize of Chemistry winner Gerhard Ertl's advancements in surface chemistry, specifically
his investigation of the interaction between carbon monoxide molecules and platinum surfaces. | 7 | Physical Chemistry |
The following are examples of topics in food physical chemistry that are of interest to both the food industry and food science:
* Water in foods
** Local structure in liquid water
** Micro-crystallization in ice cream emulsions
* Dispersion and surface-adsorption processes in foods
* Water and protein activities
* Food hydration and shelf-life
* Hydrophobic interactions in foods
* Hydrogen bonding and ionic interactions in foods
* Disulfide bond breaking and formation in foods
* Food dispersions
* Structure-functionality in foods
* Food micro- and nano- structure
* Food gels and gelling mechanisms
* Cross-linking in foods
* Starch gelatinization and retrogradation
* Physico-chemical modification of carbohydrates
* Physico-chemical interactions in food formulations
* Freezing effects on foods and freeze concentration of liquids
* Glass transition in wheat gluten and wheat doughs
* Drying of foods and crops
* Rheology of wheat doughs, cheese and meat
* Rheology of extrusion processes
* Food enzyme kinetics
* Immobilized enzymes and cells
* Microencapsulation
* Carbohydrates structure and interactions with water and proteins
* Maillard browning reactions
* Lipids structures and interactions with water and food proteins
* Food proteins structure, hydration and functionality in foods
* Food protein denaturation
* Food enzymes and reaction mechanisms
* Vitamin interactions and preservation during food processing
* Interaction of salts and minerals with food proteins and water
* Color determinations and food grade coloring
* Flavors and sensorial perception of foods
* Properties of food additives | 7 | Physical Chemistry |
Upon treatment with a base, such as triethylamine, methanesulfonyl chloride will undergo an elimination to form sulfene. Sulfene can undergo cycloadditions to form various heterocycles. α-Hydroxyketones react with sulfene to form five-membered sultones. | 0 | Organic Chemistry |
Planar chirality, also known as 2D chirality, is the special case of chirality for two dimensions.
Most fundamentally, planar chirality is a mathematical term, finding use in chemistry, physics and related physical sciences, for example, in astronomy, optics and metamaterials. Recent occurrences in latter two fields are dominated by microwave and terahertz applications as well as micro- and nanostructured planar interfaces for infrared and visible light. | 4 | Stereochemistry |
In nanobiotechnology, a peptoid nanosheet is a synthetic protein structure made from peptoids. Peptoid nanosheets have a thickness of about three nanometers and a length of up to 100 micrometers, meaning that they have a two-dimensional, flat shape that resembles paper on the nanoscale.
This makes them one of the thinnest known two-dimensional organic crystalline materials with an area to thickness ratio of greater than 10 nm. Peptoid nanosheets were discovered in the laboratory of Dr. Ron Zuckermann at the Lawrence Berkeley National Laboratory in 2010. Due to the ability to customize peptoids and therefore the properties of the peptoid nanosheet, it has possible applications in the areas of drug and small molecule delivery and biosensing. | 0 | Organic Chemistry |
For very high temperatures there are relativistic corrections to this formula, that is, additional terms of the order of | 7 | Physical Chemistry |
Because of the relatively long residence time of the ocean's thermohaline circulation, carbon transported as marine snow into the deep ocean by the biological pump can remain out of contact with the atmosphere for more than 1000 years. That is, when the marine snow is finally decomposed to inorganic nutrients and dissolved carbon dioxide, these are effectively isolated from the surface ocean for relatively long time scales related to ocean circulation. Consequently, enhancing the quantity of marine snow that reaches the deep ocean is the basis of several geoengineering schemes to enhance carbon sequestration by the ocean. Ocean nourishment and iron fertilisation seek to boost the production of organic material in the surface ocean, with a concomitant rise in marine snow reaching the deep ocean. These efforts have not yet produced a sustainable fertilization that effectively transports carbon out of the system.
Increases in ocean temperatures, a projected indicator of climate change, may result in a decrease in the production of marine snow due to the enhanced stratification of the water column. Increasing stratification decreases the availability of phytoplankton nutrients such as nitrate, phosphate and silicic acid, and could lead to a decrease in primary production and, thus, marine snow.
The microbial communities associated with marine snow are also interesting to microbiologists. Recent research indicates transported bacteria may exchange genes with previously thought to be isolated populations of bacteria inhabiting the breadth of the ocean floor. In such an immense area there may be as yet undiscovered species tolerant of high pressures and extreme cold, perhaps finding use in bioengineering and pharmacy. | 9 | Geochemistry |
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal. | 7 | Physical Chemistry |
Holmes was elected Fellow of the Royal Society (FRS) in 2000, and Fellow of the Australian Academy of Science in 2006. In 2003, he received the Descartes Prize and in 2012 the Royal Medal of the Royal Society. His formal titles include Chemistry alumnus, Laureate Professor of Chemistry, University of Melbourne; CSIRO Fellow, CSIRO Division of Materials Science and Engineering; Emeritus Professor and Distinguished Research Fellow, Imperial College London; Fellow of the Royal Society; Fellow of the Australian Academy of Science; Fellow of the Australian Academy of Technological Sciences and Engineering; and Foreign Secretary and (as of 2014) President of the Australian Academy of Science. In 2011, he received the Royal Society of Chemistry's John B Goodenough Award.
In 2004 he was appointed a Member of the Order of Australia "for service to science through research and development, particularly in the fields of organic synthesis and polymer chemistry"; and in 2017 was appointed Companion of the Order of Australia for eminent service to science through developments in the field of organic and polymer chemistry as a researcher, editor and academic, and through the governance of nationally recognised, leading scientific organisations. He was awarded the 2021 Matthew Flinders Medal and Lecture. | 0 | Organic Chemistry |
The Hellmann–Feynman theorem is actually a direct, and to some extent trivial, consequence of the variational principle (the Rayleigh-Ritz variational principle) from which the Schrödinger equation may be derived. This is why the Hellmann–Feynman theorem holds for wave-functions (such as the Hartree–Fock wave-function) that, though not eigenfunctions of the Hamiltonian, do derive from a variational principle. This is also why it holds, e.g., in density functional theory, which is not wave-function based and for which the standard derivation does not apply.
According to the Rayleigh–Ritz variational principle, the eigenfunctions of the Schrödinger equation are stationary points of the functional (which is nicknamed Schrödinger functional for brevity):
The eigenvalues are the values that the Schrödinger functional takes at the stationary points:
where satisfies the variational condition:
By differentiating Eq. (3) using the chain rule, the following equation is obtained:
Due to the variational condition, Eq. (4), the second term in Eq. (5) vanishes. In one sentence, the Hellmann–Feynman theorem states that the derivative of the stationary values of a function(al) with respect to a parameter on which it may depend, can be computed from the explicit dependence only, disregarding the implicit one. On account of the fact that the Schrödinger functional can only depend explicitly on an external parameter through the Hamiltonian, Eq. (1) trivially follows. | 6 | Supramolecular Chemistry |
Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. Selenium exerts its biological functions through selenoproteins, which contain the amino acid selenocysteine. Twenty-five selenoproteins are encoded in the human genome. | 1 | Biochemistry |
A thermodynamic system can be identified or described in various ways. Most directly, it can be identified by a suitable set of state variables. Less directly, it can be described by a suitable set of quantities that includes state variables and state functions.
The primary or original identification of the thermodynamic state of a body of matter is by directly measurable ordinary physical quantities. For some simple purposes, for a given body of given chemical constitution, a sufficient set of such quantities is volume and pressure.
Besides the directly measurable ordinary physical variables that originally identify a thermodynamic state of a system, the system is characterized by further quantities called state functions, which are also called state variables, thermodynamic variables, state quantities, or functions of state. They are uniquely determined by the thermodynamic state as it has been identified by the original state variables. There are many such state functions. Examples are internal energy, enthalpy, Helmholtz free energy, Gibbs free energy, thermodynamic temperature, and entropy. For a given body, of a given chemical constitution, when its thermodynamic state has been fully defined by its pressure and volume, then its temperature is uniquely determined. Thermodynamic temperature is a specifically thermodynamic concept, while the original directly measureable state variables are defined by ordinary physical measurements, without reference to thermodynamic concepts; for this reason, it is helpful to regard thermodynamic temperature as a state function.
A passage from a given initial thermodynamic state to a given final thermodynamic state of a thermodynamic system is known as a thermodynamic process; usually this is transfer of matter or energy between system and surroundings. In any thermodynamic process, whatever may be the intermediate conditions during the passage, the total respective change in the value of each thermodynamic state variable depends only on the initial and final states. For an idealized continuous or quasi-static process, this means that infinitesimal incremental changes in such variables are exact differentials. Together, the incremental changes throughout the process, and the initial and final states, fully determine the idealized process.
In the most commonly cited simple example, an ideal gas, the thermodynamic variables would be any three variables out of the following four: amount of substance, pressure, temperature, and volume. Thus, the thermodynamic state would range over a three-dimensional state space. The remaining variable, as well as other quantities such as the internal energy and the entropy, would be expressed as state functions of these three variables. The state functions satisfy certain universal constraints, expressed in the laws of thermodynamics, and they depend on the peculiarities of the materials that compose the concrete system.
Various thermodynamic diagrams have been developed to model the transitions between thermodynamic states. | 7 | Physical Chemistry |
Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozymes demonstrated that RNA can be both genetic material (like DNA) and a biological catalyst (like protein enzymes), and contributed to the RNA world hypothesis, which suggests that RNA may have been important in the evolution of prebiotic self-replicating systems.
The most common activities of natural or in vitro evolved ribozymes are the cleavage (or ligation) of RNA and DNA and peptide bond formation. For example, the smallest ribozyme known (GUGGC-3) can aminoacylate a GCCU-3 sequence in the presence of PheAMP. Within the ribosome, ribozymes function as part of the large subunit ribosomal RNA to link amino acids during protein synthesis. They also participate in a variety of RNA processing reactions, including RNA splicing, viral replication, and transfer RNA biosynthesis. Examples of ribozymes include the hammerhead ribozyme, the VS ribozyme, leadzyme, and the hairpin ribozyme.
Researchers who are investigating the origins of life through the RNA world hypothesis have been working on discovering a ribozyme with the capacity to self-replicate, which would require it to have the ability to catalytically synthesize polymers of RNA. This should be able to happen in prebiotically plausible conditions with high rates of copying accuracy to prevent degradation of information but also allowing for the occurrence of occasional errors during the copying process to allow for Darwinian evolution to proceed.
Attempts have been made to develop ribozymes as therapeutic agents, as enzymes which target defined RNA sequences for cleavage, as biosensors, and for applications in functional genomics and gene discovery. | 7 | Physical Chemistry |
Rate constant can be calculated for elementary reactions by molecular dynamics simulations.
One possible approach is to calculate the mean residence time of the molecule in the reactant state. Although this is feasible for small systems with short residence times, this approach is not widely applicable as reactions are often rare events on molecular scale.
One simple approach to overcome this problem is Divided Saddle Theory. Such other methods as the Bennett Chandler procedure, and Milestoning have also been developed for rate constant calculations. | 7 | Physical Chemistry |
It was found that the luciferase enzyme produced in fireflies is localized to the peroxisome within the photocytes. When mammalian cells were modified to produce the enzyme, it was found that they were targeted to the mammalian peroxisome as well. Because protein targeting to peroxisomes is not well understood, this finding is valuable for its potential to aid in the determination of peroxisome targeting mechanisms. If the cell produces a large amount of luciferase, some of the protein ends up in the cytoplasm. It is unknown what feature of the luciferase enzyme causes it to be targeted to the peroxisome since no particular protein sequences related to peroxisome targeting have been discovered. | 1 | Biochemistry |
Magnesium monoperoxyphthalate (MMPP) is a water-soluble peroxy acid used as an oxidant in organic synthesis. Its main areas of use are the conversion of ketones to esters (Baeyer-Villiger oxidation), epoxidation of alkenes (Prilezhaev reaction), oxidation of sulfides to sulfoxides and sulfones, oxidation of amines to produce amine oxides, and in the oxidative cleavage of hydrazones.
Due to its insolubility in non-polar solvents MMPP has seen less use than the more widely used meta-chloroperoxybenzoic acid (mCPBA). Although work up procedures are more simply handled in polar solvents, usage of MMPP to oxidize nonpolar substrates in biphasic media combined with a phase transfer catalyst have been inefficient. Despite this MMPP has certain advantages over mCPBA including a lower cost of production and increased stability.
MMPP is also used as the active ingredient in certain surface disinfectants such as Dismozon Pur. As a surface disinfectant MMPP exhibits a broad spectrum biocidal effect including inactivation of endospores. Its wide surface compatibility enables its use on sensitive materials, such as plastic and rubber equipment used in hospitals. Additionally MMPP has been investigated as a potential antibacterial agent for mouthwashes and toothpaste. | 0 | Organic Chemistry |
The saturation state (known as Ω) of seawater for a mineral is a measure of the thermodynamic potential for the mineral to form or to dissolve, and for calcium carbonate is described by the following equation:
Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral (Ca and CO), divided by the apparent solubility product at equilibrium (K), that is, when the rates of precipitation and dissolution are equal. In seawater, dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon. Above this saturation horizon, Ω has a value greater than 1, and does not readily dissolve. Most calcifying organisms live in such waters. Below this depth, Ω has a value less than 1, and will dissolve. The carbonate compensation depth is the ocean depth at which carbonate dissolution balances the supply of carbonate to sea floor, therefore sediment below this depth will be void of calcium carbonate. Increasing levels, and the resulting lower pH of seawater, decreases the concentration of CO and the saturation state of therefore increasing dissolution.
Calcium carbonate most commonly occurs in two common polymorphs (crystalline forms): aragonite and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon, and aragonite compensation depth, is always nearer to the surface than the calcite saturation horizon. This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite. Ocean acidification and the resulting decrease in carbonate saturation states raise the saturation horizons of both forms closer to the surface. This decrease in saturation state is one of the main factors leading to decreased calcification in marine organisms because the inorganic precipitation of is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with lower saturation states. | 9 | Geochemistry |
Both bulk and depth analysis of solids may be performed with glow discharge. Bulk analysis assumes that the sample is fairly homogeneous and averages the emission or mass spectrometric signal over time. Depth analysis relies on tracking the signal in time, therefore, is the same as tracking the elemental composition in depth.
Depth analysis requires greater control over operational parameters. For example, conditions (current, potential, pressure) need to be adjusted so that the crater produced by sputtering is flat bottom (that is, so that the depth analyzed over the crater area is uniform). In bulk measurement, a rough or rounded crater bottom would not adversely impact analysis. Under the best conditions, depth resolution in the single nanometer range has been achieved (in fact, within-molecule resolution has been demonstrated).
The chemistry of ions and neutrals in vacuum is called gas phase ion chemistry and is part of the analytical study that includes glow discharge. | 3 | Analytical Chemistry |
Senapathy proposed a plausible mechanistic and functional rationale why the eukaryotic nucleus originated, a major question in biology. If the transcripts of the split genes and the spliced mRNAs were present in a cell without a nucleus, the ribosomes would try to bind to both the un-spliced primary RNA transcript and the spliced mRNA, which would result in chaos. A boundary that separates the RNA splicing process from the mRNA translation avoids this problem. The nuclear boundary provides a clear separation of the primary RNA splicing and the mRNA translation.
These investigations thus led to the possibility that primordial DNA with essentially random sequence gave rise to the complex structure of the split genes with exons, introns and splice junctions. Cells that harbored split genes had to be complex with a nuclear cytoplasmic boundary, and must have a spliceosomal machinery. Thus, it was possible that the earliest cell was complex and eukaryotic. Surprisingly, findings from extensive comparative genomics research from several organisms since 2007 overwhelmingly show that the earliest organisms could have been highly complex and eukaryotic, and could have contained complex proteins, as predicted by Senapathy's theory.
The spliceosome is a highly complex mechanism, containing ~200 proteins and several SnRNPs. Collins and Penny stated, “We begin with the hypothesis that ... the spliceosome has increased in complexity throughout eukaryotic evolution. However, examination of the distribution of spliceosomal components indicates that not only was a spliceosome present in the eukaryotic ancestor but it also contained most of the key components found in today's eukaryotes. ... the last common ancestor of extant eukaryotes appears to show much of the molecular complexity seen today.” This suggests that the earliest eukaryotic organisms were complex and contained sophisticated genes and proteins. | 1 | Biochemistry |
A major advantage of the SELDI process is the chromatographic separation step. While liquid chromatography-mass spectrometry (LC-MS) is based on the elution of analytes in the separated sample, separation in SELDI is based on retention. Any sample components that interfere with analytical measurements, such as salts, detergents, and buffers, are washed away before analysis with mass spectrometry. Only the analytes that are bound to the surface are analyzed, reducing the overall complexity of the sample. As a result, there is an increased probability of detecting analytes that are present in lower concentrations. Because of the initial separation step, protein profiles can be obtained from samples of as few as 25-50 cells.
In biological applications, SELDI-TOF-MS has a major advantage in that the technique does not require the use of radioactive isotopes. Furthermore, an assay can be sampled at multiple time points during an experiment. Additionally, in proteomics, the biomarker discovery, identification, and validation steps can all be done on the SELDI surface. | 1 | Biochemistry |
One common pair fluorophores for biological use is a cyan fluorescent protein (CFP) – yellow fluorescent protein (YFP) pair. Both are color variants of green fluorescent protein (GFP). Labeling with organic fluorescent dyes requires purification, chemical modification, and intracellular injection of a host protein. GFP variants can be attached to a host protein by genetic engineering which can be more convenient. Additionally, a fusion of CFP and YFP ("tandem-dimer") linked by a protease cleavage sequence can be used as a cleavage assay. | 1 | Biochemistry |
The electrochemical regeneration of activated carbon based adsorbents involves the removal of molecules adsorbed onto the surface of the adsorbent with the use of an electric current in an electrochemical cell restoring the carbon's adsorptive capacity. Electrochemical regeneration represents an alternative to thermal regeneration commonly used in waste water treatment applications. Common adsorbents include powdered activated carbon (PAC), granular activated carbon (GAC) and activated carbon fibre. | 7 | Physical Chemistry |
Primer dimers may be visible after gel electrophoresis of the PCR product. PDs in ethidium bromide-stained gels are typically seen as a 30-50 base-pair (bp) band or smear of moderate to high intensity and distinguishable from the band of the target sequence, which is typically longer than 50 bp.
In quantitative PCR, PDs may be detected by melting curve analysis with intercalating dyes, such as SYBR Green I, a nonspecific dye for detection of double-stranded DNA. Because they usually consist of short sequences, the PDs denature at lower temperature than the target sequence and hence can be distinguished by their melting-curve characteristics. | 1 | Biochemistry |
Up to here, an ideal surface was considered, but going beyond the idealized case there are several effects which influence premelting:
*Curvature: When the surface considered is not planar but exhibits a curvature premelting is affected. The rule is that whenever the surface is concave, viewed from the solid's perspective, then premelting is advanced. The fraction by which the thickness of the liquid film increases is given by , where r is the local radius of the curved surface. Therefore, it is also plausible that premelting starts in scratches or in corners of steps and hence has a flattening effect.
*Disordered solids: As disorder in the solid increases its local free energy, the local chemical potential of the disordered solid lies above the chemical potential of the ordered solid. In thermodynamic equilibrium the chemical potential of the premelted liquid film has to be equal to that of the disordered solid, so it can be concluded that disorder in the solid phase causes the effect of premelting to increase.
*Impurities: Consider the case of the depression of the ice melting temperature due to dissolved salt. For premelting the situation is far more difficult than one would expect from that simple statement. It starts with the Lifshitz theory which was roughly sketched above. But now the impurities cause screening in the liquid, they adsorb on the border between solid and liquid phase and all those effects make a general derivation of impurity effects impossible to state here. But it can be said that impurities have a great effect on the temperature from which on premelting can be observed and they especially affect the thickness of the layer. This however does not mean that the thickness is a monotone function in the concentration. | 7 | Physical Chemistry |
VMAT1 also has effects on the modulation of gastrin processing in G cells. These intestinal endocrine cells process amine precursors, and VMAT1 pulls them into vesicles for storage. The activity of VMAT1 in these cells has a seemingly inhibitory effect on the processing of gastrin. Essentially, this means that certain compounds in the gut can be taken into these G cells and either amplify or inhibit the function of VMAT1, which will impact gastrin processing (conversion from G34 to G17).
Additionally, VMAT1 is known to play a role in the uptake and secretion of serotonin in the gut. Enterochromaffin cells in the intestines will secrete serotonin in response to the activation of certain mechanosensors. The regulation of serotonin in the gut is critically important, as it modulates appetite and controls intestinal contraction. | 1 | Biochemistry |
Tantalum alkylidene complexes arise by treating trialkyltantalum dichloride with alkyl lithium reagents. This reaction initially forms a thermally unstable tetraalkyl-monochloro-tantalum complex, which undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride.
Tantalum alkylidene complexes are nucleophilic. They effect a number of reactions including: olefinations, olefin metathesis, hydroaminoalkylation of olefins, and conjugate allylation of enones.
Ethylene, propylene, and styrene react with tantalum alkylidene complexes to yield olefin metathesis products. | 0 | Organic Chemistry |
Despite the difficulty of theoretical interpretation, measured conductivity is a good indicator of the presence or absence of conductive ions in solution, and measurements are used extensively in many industries. For example, conductivity measurements are used to monitor quality in public water supplies, in hospitals, in boiler water and industries that depend on water quality such as brewing. This type of measurement is not ion-specific; it can sometimes be used to determine the amount of total dissolved solids (TDS) if the composition of the solution and its conductivity behavior are known. Conductivity measurements made to determine water purity will not respond to non conductive contaminants (many organic compounds fall into this category), therefore additional purity tests may be required depending on application.
Applications of TDS measurements are not limited to industrial use; many people use TDS as an indicator of the purity of their drinking water. Additionally, aquarium enthusiasts are concerned with TDS, both for freshwater and salt water aquariums. Many fish and invertebrates require quite narrow parameters for dissolved solids. Especially for successful breeding of some invertebrates normally kept in freshwater aquariums—snails and shrimp primarily—brackish water with higher TDS, specifically higher salinity, water is required. While the adults of a given species may thrive in freshwater, this is not always true for the young and some species will not breed at all in non-brackish water.
Sometimes, conductivity measurements are linked with other methods to increase the sensitivity of detection of specific types of ions. For example, in the boiler water technology, the boiler blowdown is continuously monitored for "cation conductivity", which is the conductivity of the water after it has been passed through a cation exchange resin. This is a sensitive method of monitoring anion impurities in the boiler water in the presence of excess cations (those of the alkalizing agent usually used for water treatment). The sensitivity of this method relies on the high mobility of H in comparison with the mobility of other cations or anions. Beyond cation conductivity, there are analytical instruments designed to measure Degas conductivity, where conductivity is measured after dissolved carbon dioxide has been removed from the sample, either through reboiling or dynamic degassing.
Conductivity detectors are commonly used with ion chromatography. | 7 | Physical Chemistry |
Preparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.
* Oxidation of primary alcohols or aldehydes with strong oxidants such as potassium dichromate, Jones reagent, potassium permanganate, or sodium chlorite. The method is more suitable for laboratory conditions than the industrial use of air, which is "greener" because it yields less inorganic side products such as chromium or manganese oxides.
* Oxidative cleavage of olefins by ozonolysis, potassium permanganate, or potassium dichromate.
* Hydrolysis of nitriles, esters, or amides, usually with acid- or base-catalysis.
* Carbonation of a Grignard reagent and organolithium reagents:
* Halogenation followed by hydrolysis of methyl ketones in the haloform reaction
* Base-catalyzed cleavage of non-enolizable ketones, especially aryl ketones: | 0 | Organic Chemistry |
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