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* In general, if more than one alkene can be formed during dehalogenation by an elimination reaction, the more stable alkene is the major product. There are two types of elimination reactions, E1 and E2. An E2 reaction is a One step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond. C=C Pi bond. An E1 reaction is the Ionization of the carbon-halogen bond breaking to give a carbocation intermediate, then the Deprotonation of the carbocation.
* For these two reactions, there are 3 possible products, 3-methyl-cyclohexene,1-methyl-cyclohexene, methylene-cyclohexane. The production of each of these occurs at different rates and the ratios of these also change over time. It is well known that the dehydration of the cis isomer is 30 times faster than the trans isomer. It then appears that the reaction proceeds mainly by a trans mechanism and, following the Zaitsev rule, 1-methylcyclohexene is preferentially formed in the early stages of the reaction. Indeed, if only about 10% of the total distillate is collected as the first fraction, one finds that the alkene is about 93% l-methylcyclohexene: at the end of the distillation one finds a value as low as 55% of l-methyl isomer.
* From these results, the phenomenon of the Evelyn effect can be observed and a conclusion can be drawn that a change of mechanism occurs somewhere during the synthesis. | 7 | Physical Chemistry |
Neuropeptidergic means "related to neuropeptides".
A neuropeptidergic agent (or drug) is a chemical which functions to directly modulate the neuropeptide systems in the body or brain. An example is opioidergics. | 1 | Biochemistry |
Microwaves impinging on matter usually interact with charges as well as with spins (via electric and magnetic field components, respectively), with the charge response typically much stronger than the spin response. But in the case of magnetic resonance, spins can be directly probed using microwaves. For paramagnetic materials, this technique is called electron spin resonance (ESR) and for ferromagnetic materials ferromagnetic resonance (FMR).
In the paramagnetic case, such an experiment probes the Zeeman splitting, with a linear relation between the static external magnetic field and the frequency of the probing microwave field. A popular combination, as implemented in commercial X-band ESR spectrometers, is approximately 0.3 T (static field) and 10 GHz (microwave frequency) for a typical material with electron g-factor close to 2. | 7 | Physical Chemistry |
If a compound has a very large specific rotation or a sample is very concentrated, the actual rotation of the sample may be larger than 180°, and so a single polarimeter measurement cannot detect when this has happened (for example, the values +270° and −90° are not distinguishable, nor are the values 361° and 1°). In these cases, measuring the rotation at several different concentrations allows one to determine the true value. Another method would be to use shorter path-lengths to perform the measurements.
In cases of very small or very large angles, one can also use the variation of specific rotation with wavelength to facilitate measurement. Switching wavelength is particularly useful when the angle is small. Many polarimeters are equipped with a mercury lamp (in addition to the sodium lamp) for this purpose. | 4 | Stereochemistry |
Eutrophication is caused by excessive concentrations of nutrients, most commonly phosphates and nitrates, although this varies with location. Prior to their being phasing out in the 1970's, phosphate-containing detergents contributed to eutrophication. Since then, sewage and agriculture have emerged as the dominant phosphate sources. The main sources of nitrogen pollution are from agricultural runoff containing fertilizers and animal wastes, from sewage, and from atmospheric deposition of nitrogen originating from combustion or animal waste.
The limitation of productivity in any aquatic system varies with the rate of supply (from external sources) and removal (flushing out) of nutrients from the body of water. This means that some nutrients are more prevalent in certain areas than others and different ecosystems and environments have different limiting factors. Phosphorus is the limiting factor for plant growth in most freshwater ecosystems, and because phosphate adheres tightly to soil particles and sinks in areas such as wetlands and lakes, due to its prevalence nowadays more and more phosphorus is accumulating inside freshwater bodies. In marine ecosystems, nitrogen is the primary limiting nutrient; nitrous oxide (created by the combustion of fossil fuels) and its deposition in the water from the atmosphere has led to an increase in nitrogen levels, and also the heightened levels of eutrophication in the ocean. | 2 | Environmental Chemistry |
Irma Goldberg (born 1871) was a Russian-born chemist. She was one of the first female organic chemists to have and sustain a successful career, her work even being quoted in her own name in standard textbooks. | 0 | Organic Chemistry |
Van der Waals forces are a subset of electrostatic interactions involving permanent or induced dipoles (or multipoles). These include the following:
* permanent dipole–dipole interactions, alternatively called the Keesom force
* dipole-induced dipole interactions, or the Debye force
* induced dipole-induced dipole interactions, commonly referred to as London dispersion forces
Hydrogen bonding and halogen bonding are typically not classified as Van der Waals forces. | 6 | Supramolecular Chemistry |
In biology, methylene blue is used as a dye for a number of different staining procedures, such as Wrights stain and Jenners stain. Since it is a temporary staining technique, methylene blue can also be used to examine RNA or DNA under the microscope or in a gel: as an example, a solution of methylene blue can be used to stain RNA on hybridization membranes in northern blotting to verify the amount of nucleic acid present. While methylene blue is not as sensitive as ethidium bromide, it is less toxic and it does not intercalate in nucleic acid chains, thus avoiding interference with nucleic acid retention on hybridization membranes or with the hybridization process itself.
It can also be used as an indicator to determine whether eukaryotic cells such as yeast are alive or dead. The methylene blue is reduced in viable cells, leaving them unstained. However dead cells are unable to reduce the oxidized methylene blue and the cells are stained blue. Methylene blue can interfere with the respiration of the yeast as it picks up hydrogen ions made during the process. | 3 | Analytical Chemistry |
Catalytic rate enhancement via dynamic perturbation of surface active sites has been demonstrated experimentally with dynamic electrocatalysis and dynamic photocatalysis. Those results may be explained in the framework of catalytic resonance theory but conclusive evidence is still lacking:
* In 1978, the electro-oxidation of formic acid on a platinum electrode was studied under the application of constant potentials and square-wave pulsed potentials. The latter was found to enhance the current density (and thus catalytic activity) by up to 20 times compared to the potentiostatic conditions, with the optimal wave amplitude and frequency of 600 mV and 2000 Hz, respectively. In 1988, the oxidation of methanol on a platinum electrode was conducted under pulsed potentials between 0.4 and 1.18 V, resulting in an average current almost 100 times higher than the steady-state current at 0.4 V.
* Using the formic acid electro-oxidation reaction, oscillation of the applied electrodynamic potential between 0 and 0.8 volts accelerated the formation rate of carbon dioxide more than an order of magnitude higher (20X) than what was achievable on platinum, the best existing catalyst. The maximum catalytic rate was experimentally observed at a frequency of 100 Hz; slower catalytic rates were observed at higher and lower electrodynamic frequencies. The resonant frequency was interpreted as the oscillation between conditions favorable to formic acid decomposition (0 V) and conditions favorable to form CO (0.8 V).
*The concept of implementing periodic illumination to improve the quantum yield of a typical photocatalytic reaction was first introduced in 1964 by Miller et al. In this work, they showed enhanced photosynthetic efficiency in the conversion of CO to O when the algal culture was exposed to periodic illumination in a Taylor vortex reactor. Sczechowski et al. later implemented the same approach for heterogeneous photocatalysis in 1993, where they demonstrated 5-fold increment in photoefficiency of formate decomposition by cycling between light and dark conditions with periods of 72 ms and 1.45 s respectively. They hypothesized that upon illumination of the catalyst, there is a critical illumination time during which absorbed photons generate oxidizing species (h) on the surface of the catalyst. The generated species or their intermediates go on to react with substrates on the surface or in the bulk. During dark period, adsorption, desorption, and diffusion generally occurs in the absence of photons. After a critical recovery period in the dark, the photocatalyst can efficiently use photons again when photons are reintroduced. A summary of work involving “dynamic” photocatalysis was provided by Tokode et al. in 2016.
* Dynamic promotion of methanol decomposition was demonstrated on 2 nm Pt nanoparticles using pulsed light. The rate acceleration to form H relative to static illumination was attributed to the selective weakening of adsorbed carbon monoxide, thereby also increasing the quantum efficiency of applied light.
*In 2021, Sordello et al. experimentally demonstrated a 50% increase of the quantum yield for the Hydrogen Evolution Reaction (HER) over Pt/TiO nanoparticles via formic acid photoreforming under Controlled Period Illumination (CPI).
Implementation of catalyst dynamics has been proposed to occur by additional methods using oscillating light, electric potential, and physical perturbation. | 7 | Physical Chemistry |
tert-Butyl hydroperoxide (tBuOOH) is the organic compound with the formula (CH)COOH. It is one of the most widely used hydroperoxides in a variety of oxidation processes, like the Halcon process. It is normally supplied as a 69–70% aqueous solution. Compared to hydrogen peroxide and organic peracids, tert-butyl hydroperoxide is less reactive and more soluble in organic solvents. Overall, it is renowned for the convenient handling properties of its solutions. Its solutions in organic solvents are highly stable. | 0 | Organic Chemistry |
In an MDS analysis, a stream of liquid containing the particles to be sized is introduced alongside an auxiliary stream in a laminar flow in a microfluidic channel. Because there is no convective mixing of the two streams, the only way particles can move to the auxiliary stream is by diffusion. The rate of this diffusion is dependent on the particle's size, as determined by the Stokes–Einstein equation, so small particles diffuse quicker than large particles.
After a period of diffusion the original and auxiliary streams are split and the degree of diffusion is fixed. The number of particles in each stream can then be detected (in the case of proteins this is achieved by addition of an amine reactive fluorogenic dye). The ratio between the two streams is used to determine the diffusion co-efficient, which is used to calculate the hydrodynamic radius. The sum of particles in both streams can also be used to measure the concentration of the analyte. | 1 | Biochemistry |
Trace amines are an endogenous group of trace amine-associated receptor 1 (TAAR1) agonists – and hence, monoaminergic neuromodulators – that are structurally and metabolically related to classical monoamine neurotransmitters. Compared to the classical monoamines, they are present in trace concentrations. They are distributed heterogeneously throughout the mammalian brain and peripheral nervous tissues and exhibit high rates of metabolism. Although they can be synthesized within parent monoamine neurotransmitter systems, there is evidence that suggests that some of them may comprise their own independent neurotransmitter systems.
Trace amines play significant roles in regulating the quantity of monoamine neurotransmitters in the synaptic cleft of monoamine neurons with . They have well-characterized presynaptic amphetamine-like effects on these monoamine neurons via TAAR1 activation; specifically, by activating TAAR1 in neurons they promote the release and prevent reuptake of monoamine neurotransmitters from the synaptic cleft as well as inhibit neuronal firing. Phenethylamine and amphetamine possess analogous pharmacodynamics in human dopamine neurons, as both compounds induce efflux from vesicular monoamine transporter 2 (VMAT2) and activate TAAR1 with comparable efficacy.
Like dopamine, norepinephrine, and serotonin, the trace amines have been implicated in a vast array of human disorders of affect and cognition, such as ADHD, depression and schizophrenia, among others. Trace aminergic hypo-function is particularly relevant to ADHD, since urinary and plasma phenethylamine concentrations are significantly lower in individuals with ADHD relative to controls and the two most commonly prescribed drugs for ADHD, amphetamine and methylphenidate, increase phenethylamine biosynthesis in treatment-responsive individuals with ADHD. A systematic review of ADHD biomarkers also indicated that urinary phenethylamine levels could be a diagnostic biomarker for ADHD. | 1 | Biochemistry |
The Schoenflies (or Schönflies) notation, named after the German mathematician Arthur Moritz Schoenflies, is a notation primarily used to specify point groups in three dimensions. Because a point group alone is completely adequate to describe the symmetry of a molecule, the notation is often sufficient and commonly used for spectroscopy. However, in crystallography, there is additional translational symmetry, and point groups are not enough to describe the full symmetry of crystals, so the full space group is usually used instead. The naming of full space groups usually follows another common convention, the Hermann–Mauguin notation, also known as the international notation.
Although Schoenflies notation without superscripts is a pure point group notation, optionally, superscripts can be added to further specify individual space groups. However, for space groups, the connection to the underlying symmetry elements is much more clear in Hermann–Mauguin notation, so the latter notation is usually preferred for space groups. | 7 | Physical Chemistry |
Water body temperature is one of the most ubiquitous variables collected in aquatic biomonitoring. Temperatures at the water surface, through the water column, and in the lowest levels of the water body (benthic zone) can all provide insight into different aspects of an aquatic ecosystem. Water temperature is directly affected by climate change and can have negative affects on many aquatic species, such as salmon. Salmon spawning is temperature dependant: there is a heat accumulation threshold which must be reached before hatching can occur. Post-hatching, salmon live in water within a critical range in temperature, with exposure to temperatures outside of this being potentially lethal. This sensitivity makes them useful indicators of changes in water temperature, hence their use in climate change studies. Similarly, Daphnia populations have been evidenced as being negatively affected by climate change, as earlier springs have caused hatching periods to de-couple from the peak window of food availability. | 2 | Environmental Chemistry |
The most abundant class of deoxyribozymes are ribonucleases, which catalyze the cleavage of a ribonucleotide phosphodiester bond through a transesterification reaction, forming a 23-cyclic phosphate terminus and a 5'-hydroxyl terminus.
Ribonuclease deoxyribozymes typically undergo selection as long, single-stranded oligonucleotides which contain a single ribonucleotide base to act as the cleavage site. Once sequenced, this single-stranded "cis"-form of the deoxyribozyme can be converted to the two-stranded "trans"-form by separating the substrate domain (containing the ribonucleotide cleavage site) and the enzyme domain (containing the catalytic core) into separate strands which can hybridize through two flanking arms consisting of complementary base pairs.
The first known deoxyribozyme was a ribonuclease, discovered in 1994 by Ronald Breaker while a postdoctoral fellow in the laboratory of Gerald Joyce at the Scripps Research Institute.
This deoxyribozyme, later named GR-5,
catalyzes the Pb-dependent cleavage of a single ribonucleotide phosphoester at a rate that is more than 100-fold compared to the uncatalyzed reaction. Subsequently, additional RNA-cleaving deoxyribozymes that incorporate different metal cofactors were developed, including the Mg-dependent E2 deoxyribozyme
and the Ca-dependent Mg5 deoxyribozyme.
These first deoxyribozymes were unable to catalyze a full RNA substrate strand, but by incorporating the full RNA substrate strand into the selection process, deoxyribozymes which functioned with substrates consisting of either full RNA or full DNA with a single RNA base were both able to be utilized.
The first of these more versatile deoxyribozymes, 8-17 and 10–23, are currently the most widely studied deoxyribozymes. In fact, many subsequently discovered deoxyribozymes were found to contain the same catalytic core motif as 8–17, including the previously discovered Mg5, suggesting that this motif represents the "simplest solution for the RNA cleavage problem".
The 10-23 DNAzyme contains a 15-nucleotide catalytic core that is flanked by two substrate recognition domains. This DNAzyme cleaves complementary RNAs efficiently in a sequence specific manner between an unpaired purine and a paired pyrimidine. DNAzymes targeting AU or GU vs. GC or AC are more effective. Furthermore, the RNA cleavage rates have been shown to increase after the introduction of intercalators or the substitution of deoxyguanine with deoxyinosine at the junction of the catalytic loop. Specifically, the addition of 2’-O-methyl modifications to the catalytic proved to significantly increase the cleavage rate both in vitro and in vivo. Additionally, recent studies have focuses on unravelling their kinetics to further understand their performance.
Other notable deoxyribozyme ribonucleases are those that are highly selective for a certain cofactor. Among this group are the metal selective deoxyribozymes such as Pb-specific 17E,
UO-specific 39E,
and Na-specific A43. First crystal structure of a DNAzyme was reported in 2016. 10-23 core based DNAzymes and the respective MNAzymes that catalyse reactions at ambient temperatures were described in 2018 and open doors for use of these nucleic acid based enzymes for many other applications without the need for heating.
A DNA molecule with sequence 5-GGAGAACGCGAGGCAAGGCTGGGAGAAATGTGGATCACGATT-3 acts as a deoxyribozyme that uses light to repair a thymine dimer, using serotonin as cofactor. | 7 | Physical Chemistry |
To answer this question one should know the different and important functions of glycans. The following are some of those functions:
*Glycoproteins and Glycolipids found on the cell surface play a critical role in bacterial and viral recognition.
*They are involved in cellular signaling pathways and modulate cell function.
*They are important in innate immunity.
*They determine cancer development.
*They orchestrate the cellular fate, inhibit proliferation, regulate circulation and invasion.
*They affect the stability and folding of proteins.
*They affect the pathway and fate of glycoproteins.
*There are many glycan-specific diseases, often hereditary diseases.
There are important medical applications of aspects of glycomics:
*Lectins fractionate cells to avoid graft-versus-host disease in hematopoietic stem cell transplantation.
*Activation and expansion of cytolytic CD8 T cells in cancer treatment.
Glycomics is particularly important in microbiology because glycans play diverse roles in bacterial physiology. Research in bacterial glycomics could lead to the development of:
* novel drugs
* bioactive glycans
* glycoconjugate vaccines | 0 | Organic Chemistry |
*Johnson, D. W., Johnson, R. T., & Smith, K. A. (1998). Cooperative learning returns to college what evidence is there that it works?. Change: the magazine of higher learning, 30(4), 26–35.
*Smith, K.A. (2000). Going deeper: Formal small-group learning in large classes. In MacGregor, J., Cooper, J., Smith, K, and Robinson, P., eds. Strategies for Energizing Large Classes: From Small Groups to Learning Communities. New Directions for Teaching and Learning, 81, 25–46. San Francisco: Jossey-Bass.
*Johnson, D.W., Johnson, R.T., and Smith, K.A. (2000). Constructive controversy: The power of intellectual conflict. Change, 32 (1), 28–37.
*Wankat, P. C., Felder, R. M., Smith, K. A., & Oreovicz, F. S. (2002). The scholarship of teaching and learning in engineering. In M.T. Huber & S.P. Morreale, eds., Disciplinary styles in the scholarship of teaching and learning, 217–237.
*Smith, K. A., Sheppard, S. D., Johnson, D. W., & Johnson, R. T. (2005). Pedagogies of engagement: Classroom‐based practices. Journal of Engineering Education, 94(1), 87–101.
*Johnson, D. W., Johnson, R. T., & Smith, K. (2007). The state of cooperative learning in postsecondary and professional settings. Educational psychology review, 19, 15–29.
*Froyd, J. E., Wankat, P. C., & Smith, K. A. (2012). Five major shifts in 100 years of engineering education. Proceedings of the IEEE, 100(Special Centennial Issue), 1344–1360.
*Singer, S. & Smith, K.A. (2013). Discipline-Based Education Research: Understanding and Improving Learning in Undergraduate Science and Engineering. Guest Editorial. Journal of Engineering Education, 102, 468–471.
*Lichtenstein, G., Chen, H.L., Smith, K.A. & Maldonado, T.A. (2013). Retention and Persistence of Women and Minorities Along the Engineering Pathway in the United States. In A. Johri & B. Olds (Eds), Cambridge Handbook on Engineering Education Research.
*Johnson, D. W., Johnson, R. T., & Smith, K. A. (2014). Cooperative learning: Improving university instruction by basing practice on validated theory. Journal on Excellence in University Teaching, 25(4), 1-26.
*Streveler, R.A. & Smith, K.A. (2020). Opinion: Course Design in the Time of Coronavirus: Put on your Designer's CAP. Advances in Engineering Education, COVID-19 Issue.
*Smith, K.A. & Starfield, A.M. (2023). Reflections on modeling and teaching modeling. The Journal of Undergraduate Mathematics and Its Applications (UMAP), 44(2).
*Smith, K.A. & Felder, R.M. (2023). Cooperative Learning in Engineering Education: The Story of an Ongoing Uphill Climb. In Robyn Gillies, Barbara Millis, and Neil Davidson, eds. Contemporary Global Perspectives on Cooperative Learning. New York: Routledge. | 8 | Metallurgy |
Several azo dyes like Metanil Yellow, Disperse Orange 1, and Acid orange 5 are derivatives of diphenylamine. | 3 | Analytical Chemistry |
A mixture of copper and iron sulfides referred to as matte is treated in converters to oxidize iron in the first stage, and oxidize copper in the second stage. In the first stage oxygen enriched air is blown through the tuyeres to partially convert metal sulfides to oxides:
:FeS + O → FeO + SO
:CuS + O → CuO + SO
Since iron has greater affinity to oxygen, the produced copper oxide reacts with the remaining iron sulfide:
:CuO + FeS → CuS + FeO
The bulk of the copper oxide is turned back into the form of sulfide. In order to separate the obtained iron oxide, flux (mainly silica) is added into the converter. Silica reacts with iron oxide to produce a light slag phase, which is poured off through the hood when the converter is tilted around the rotation axis:
:2 FeO + SiO → FeSiO (sometimes denoted as 2FeO•SiO, fayalite)
After the first portion of slag is poured off the converter, a new portion of matte is added, and the converting operation is repeated many times until the converter is filled with the purified copper sulfide. The converter slag is usually recycled to the smelting stage due to the high content of copper in this by-product. Converter gas contains more than 10% of sulfur dioxide, which is usually captured for the production of sulfuric acid.
The second stage of converting is aimed at oxidizing the copper sulfide phase (purified in the first stage), and produces blister copper. The following reaction takes place in the converter:
:CuS + O → Cu + SO
Copper content in the obtained blister copper is typically more than 95%. Blister copper is the final product of converting. | 8 | Metallurgy |
In materials science, critical resolved shear stress (CRSS) is the component of shear stress, resolved in the direction of slip, necessary to initiate slip in a grain. Resolved shear stress (RSS) is the shear component of an applied tensile or compressive stress resolved along a slip plane that is other than perpendicular or parallel to the stress axis. The RSS is related to the applied stress by a geometrical factor, , typically the Schmid factor:
where is the magnitude of the applied tensile stress, is the angle between the normal of the slip plane and the direction of the applied force, and is the angle between the slip direction and the direction of the applied force. The Schmid factor is most applicable to FCC single-crystal metals, but for polycrystal metals the Taylor factor has been shown to be more accurate. The CRSS is the value of resolved shear stress at which yielding of the grain occurs, marking the onset of plastic deformation. CRSS, therefore, is a material property and is not dependent on the applied load or grain orientation. The CRSS is related to the observed yield strength of the material by the maximum value of the Schmid factor:
CRSS is a constant for crystal families. Hexagonal close-packed crystals, for example, have three main families - basal, prismatic, and pyramidal - with different values for the critical resolved shear stress. | 8 | Metallurgy |
Dendrimers are particularly versatile drug delivery devices due to the wide range of chemical modifications that can be made to increase in vivo suitability and allow for site-specific targeted drug delivery.
Drug attachment to the dendrimer may be accomplished by (1) a covalent attachment or conjugation to the external surface of the dendrimer forming a dendrimer prodrug, (2) ionic coordination to charged outer functional groups, or (3) micelle-like encapsulation of a drug via a dendrimer-drug supramolecular assembly. In the case of a dendrimer prodrug structure, linking of a drug to a dendrimer may be direct or linker-mediated depending on desired release kinetics. Such a linker may be pH-sensitive, enzyme catalyzed, or a disulfide bridge. The wide range of terminal functional groups available for dendrimers allows for many different types of linker chemistries, providing yet another tunable component on the system. Key parameters to consider for linker chemistry are (1) release mechanism upon arrival to the target site, whether that be within the cell or in a certain organ system, (2) drug-dendrimer spacing so as to prevent lipophilic drugs from folding into the dendrimer, and (3) linker degradability and post-release trace modifications on drugs.
Polyethylene glycol (PEG) is a common modification for dendrimers to modify their surface charge and circulation time. Surface charge can influence the interactions of dendrimers with biological systems, such as amine-terminal modified dendrimers which have a propensity to interact with cell membranes with anionic charge. Certain in vivo studies have shown polycationic dendrimers to be cytotoxic through membrane permeabilization, a phenomenon that could be partially mitigated via addition of PEGylation caps on amine groups, resulting in lower cytotoxicity and lower red blood cell hemolysis. Additionally, studies have found that PEGylation of dendrimers results in higher drug loading, slower drug release, longer circulation times in vivo, and lower toxicity in comparison to counterparts without PEG modifications.
Numerous targeting moieties have been used to modify dendrimer biodistribution and allow for targeting to specific organs. For example, folate receptors are overexpressed in tumor cells and are therefore promising targets for localized drug delivery of chemotherapeutics. Folic acid conjugation to PAMAM dendrimers has been shown to increase targeting and decrease off-target toxicity while maintaining on-target cytotoxicity of chemotherapeutics such as methotrexate, in mouse models of cancer.
Antibody-mediated targeting of dendrimers to cell targets has also shown promise for targeted drug delivery. As epidermal growth factor receptors (EGFRs) are often overexpressed in brain tumors, EGFRs are a convenient target for site-specific drug delivery. The delivery of boron to cancerous cells is important for effective neutron capture therapy, a cancer treatment which requires a large concentration of boron in cancerous cells and a low concentration in healthy cells. A boronated dendrimer conjugated with a monoclonal antibody drug that targets EGFRs was used in rats to successfully deliver boron to cancerous cells.
Modifying nanoparticle dendrimers with peptides has also been successful for targeted destruction of colorectal (HCT-116) cancer cells in a co-culture scenario. Targeting peptides can be used to achieve site- or cell-specific delivery, and it has been shown that these peptides increase in targeting specificity when paired with dendrimers. Specifically, gemcitabine-loaded YIGSR-CMCht/PAMAM, a unique kind of dendrimer nanoparticle, induces a targeted mortality on these cancer cells. This is performed via selective interaction of the dendrimer with laminin receptors. Peptide dendrimers may be employed in the future to precisely target cancer cells and deliver chemotherapeutic agents.
The cellular uptake mechanism of dendrimers can also be tuned using chemical targeting modifications. Non-modified PAMAM-G4 dendrimer is taken up into activated microglia by fluid phase endocytosis. Conversely, mannose modification of hydroxyl PAMAM-G4 dendrimers was able to change the mechanism of internalization to mannose-receptor (CD206) mediated endocytosis. Additionally, mannose modification was able to change the biodistribution in the rest of the body in rabbits. | 6 | Supramolecular Chemistry |
Naturally occurring restriction endonucleases are categorized into five groups (Types I, II, III, IV, and V) based on their composition and enzyme cofactor requirements, the nature of their target sequence, and the position of their DNA cleavage site relative to the target sequence. DNA sequence analysis of restriction enzymes however show great variations, indicating that there are more than four types. All types of enzymes recognize specific short DNA sequences and carry out the endonucleolytic cleavage of DNA to give specific fragments with terminal 5'-phosphates. They differ in their recognition sequence, subunit composition, cleavage position, and cofactor requirements, as summarised below:
* Type I enzymes () cleave at sites remote from a recognition site; require both ATP and S-adenosyl-L-methionine to function; multifunctional protein with both restriction digestion and methylase () activities.
* Type II enzymes () cleave within or at short specific distances from a recognition site; most require magnesium; single function (restriction digestion) enzymes independent of methylase.
* Type III enzymes () cleave at sites a short distance from a recognition site; require ATP (but do not hydrolyse it); S-adenosyl-L-methionine stimulates the reaction but is not required; exist as part of a complex with a modification methylase ().
* Type IV enzymes target modified DNA, e.g. methylated, hydroxymethylated and glucosyl-hydroxymethylated DNA
* Type V enzymes utilize guide RNAs (gRNAs) | 1 | Biochemistry |
The equations in the previous subsection reveal that the , , and terms originate through three distinct mechanisms.
The term arises from Zeeman splitting of the ground or excited degenerate states. These field-dependent changes in energies of the magnetic sublevels causes small shifts in the bands to higher/lower energy. The slight offsets result in incomplete cancellation of the positive and negative features, giving a net derivative shape in the spectrum. This intensity mechanism is generally independent of sample temperature.
The term is due to the field-induced mixing of states. Energetic proximity of a third state to either the ground state or excited state gives appreciable Zeeman coupling in the presence of an applied external field. As the strength of the magnetic field increases, the amount of mixing increases to give growth of an absorption band shape. Like the term, the term is generally temperature independent. Temperature dependence of term intensity can sometimes be observed when is particularly low-lying in energy.
The term requires the degeneracy of the ground state, often encountered for paramagnetic samples. This happens due to a change in the Boltzmann population of the magnetic sublevels, which is dependent on the degree of field-induced splitting of the sublevel energies and on the sample temperature. Decrease of the temperature and increase of the magnetic field increases the term intensity until it reaches the maximum (saturation limit). Experimentally, the term spectrum can be obtained from MCD raw data by subtraction of MCD spectra measured in the same applied magnetic field at different temperatures, while and terms can be distinguished via their different band shapes.
The relative contributions of A, B and C terms to the MCD spectrum are proportional to the inverse line width, energy splitting, and temperature:
where is line width and is the zero-field state separation. For typical values of = 1000 cm, = 10,000 cm and = 6 cm (at 10 K), the three terms make relative contributions 1:0.1:150. So, at low temperature the term dominates over and for paramagnetic samples. | 7 | Physical Chemistry |
Chiral analysis refers to the quantification of component enantiomers of racemic drug substances or pharmaceutical compounds. Other synonyms commonly used include enantiomer analysis, enantiomeric analysis, and enantioselective analysis. Chiral analysis includes all analytical procedures focused on the characterization of the properties of chiral drugs. Chiral analysis is usually performed with chiral separation methods where the enantiomers are separated on an analytical scale and simultaneously assayed for each enantiomer.
Many compounds of biological and pharmacological interest are chiral. Pharmacodynamic, pharmacokinetic, and toxicological properties of the enantiomers of racemic chiral drugs has expanded significantly and become a key issue for both the pharmaceutical industry and regulatory agencies. Typically one of the enantiomers is more active pharmacologically (eutomer). In several cases, unwanted side effects or even toxic effects may occur with the inactive enantiomer (distomer). Even if the side effects are not that serious, the inactive enantiomer has to be metabolized, this puts an unnecessary burden on the already stressed out system of the patient. Large differences in activity between enantiomers reveal the need to accurate assessment of enantiomeric purity of pharmaceutical, agrochemicals, and other chemical entities like fragrances and flavors become very important. Moreover, the moment a racemic therapeutic is placed in a biological system, a chiral environment, it is no more 50:50 due enantioselective absorption, distribution, metabolism, and elimination (ADME) process. Hence to track the individual enantiomeric profile there is a need for chiral analysis tool.
Chiral technology is an active subject matter related to asymmetric synthesis and enantioselective analysis, particularly in the area of chiral chromatography. As a consequence of the advances in chiral technology, a number of pharmaceuticals currently marketed as racemic drugs are undergoing re-assessment as chiral specific products or chiral switches. Despite the choice to foster either a single enantiomer or racemic drug, in the current regulatory environment, there will be a need for enantioselective investigations. This poses a big challenge to pharmaceutical analysts and chromatographers involved in drug development process. In pharmaceutical research and development stereochemical analytical methodology may be required to comprehend enantioselective drug action and disposition, chiral purity assessment, study stereochemical stability during formulation and production, assess dosage forms, enantiospecific bioavailability and bioequivalence investigations of chiral drugs. Besides pharmaceutical applications chiral analysis plays a major role in the study of biological and environmental samples and also in the forensic field. Chiral analysis methods and applications between the period 2010 and 2020 are exhaustively reviewed recently. There are number of articles, columns, and interviews in [https://www.chromatographyonline.com/ LCGC] relating to emerging trends in chiral analysis and its application in drug discovery and development process.
For chiral examination there is a need to have the right chiral environment. This could be provided as a plane polarized light, an additional chiral compound or by exploiting the inborn chirality of nature. The chiral analytical strategies incorporate physical, biological, and separation science techniques. Recently an optical-based absolute chiral analysis has been reported. The most frequently employed technique in enantioselective analysis involve the separation science techniques, in particular chiral chromatographic methods or chiral chromatography. Today wide range of CSPs are available commercially based on various chiral selectors including polysaccharides, cyclodextrins, glycopeptide antibiotics, proteins, Pirkle, crown ethers, etc. to achieve analysis of chiral molecules. | 4 | Stereochemistry |
In biotechnology, polymersomes are a class of artificial vesicles, tiny hollow spheres that enclose a solution. Polymersomes are made using amphiphilic synthetic block copolymers to form the vesicle membrane, and have radii ranging from 50 nm to 5 µm or more. Most reported polymersomes contain an aqueous solution in their core and are useful for encapsulating and protecting sensitive molecules, such as drugs, enzymes, other proteins and peptides, and DNA and RNA fragments. The polymersome membrane provides a physical barrier that isolates the encapsulated material from external materials, such as those found in biological systems.
Synthosomes are polymersomes engineered to contain channels (transmembrane proteins) that allow certain chemicals to pass through the membrane, into or out of the vesicle. This allows for the collection or enzymatic modification of these substances.
The term "polymersome" for vesicles made from block copolymers was coined in 1999. Polymersomes are similar to liposomes, which are vesicles formed from naturally occurring lipids. While having many of the properties of natural liposomes, polymersomes exhibit increased stability and reduced permeability. Furthermore, the use of synthetic polymers enables designers to manipulate the characteristics of the membrane and thus control permeability, release rates, stability and other properties of the polymersome. | 1 | Biochemistry |
*CJ Ferguson, RJ Hughes, BTT Pham, BS Hawkett, RG Gilbert, AK Serelis, CH Such. Aqueous dispersions of polymer particles. PCT/AU02/01735 (2002)
*S Peach, BR Morrison, RG Gilbert. Finely divided polymer dispersions, their production and use. Ger. Offen. DE 19929395 (2000)
*N Subramaniam, R Balic, RG Gilbert. Modified rubber polymer latex. PCT/AU98/00191 (1998)
*D Kukulj, TP Davis, RG Gilbert. Polymerization reactions under miniemulsion conditions. PCT PN6696 (1997) | 0 | Organic Chemistry |
As a strong base, TBD fully deprotonates most phenols, carboxylic acids, and some C-acids. It catalyzes a variety of reactions including Michael reactions, Henry reactions (nitroaldol reactions), transesterification reactions, and Knoevenagel condensations.
Deprotonation at the 7-position gives a particularly electron-rich ligand as manifested in the redox properties of ditungsten tetra(hpp). | 0 | Organic Chemistry |
Although the Interferometric microscopy has been demonstrated only for optical images (visible light), this technique may find application in high resolution atom optics, or optics of neutral atom beams (see Atomic de Broglie microscope), where the Numerical aperture is usually very limited | 7 | Physical Chemistry |
SERS-based immunoassays can be used for detection of low-abundance biomarkers. For example, antibodies and gold particles can be used to quantify proteins in serum with high sensitivity and specificity. | 7 | Physical Chemistry |
The lac repressor (LacI) is a DNA-binding protein that inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. These genes are repressed when lactose is not available to the cell, ensuring that the bacterium only invests energy in the production of machinery necessary for uptake and utilization of lactose when lactose is present. When lactose becomes available, it is firstly converted into allolactose by β-Galactosidase (lacZ) in bacteria. The DNA binding ability of lac repressor bound with allolactose is inhibited due to allosteric regulation, thereby genes coding for proteins involved in lactose uptake and utilization can be expressed. | 1 | Biochemistry |
E. coli has a protein to protect other periplasmic proteins from low pH environments called the Asr protein. The gene responsible for this protein is PhoB-dependent, and can only be turned on when the Pho regulon is activated by low Pi concentration. Synthesis of the Asr protein imparts acid shock resistance to E. coli enabling it to survive in environments like the stomach which has a low pH. Many acid tolerance genes are induced by more than just the low pH environment and require other environmental signals to be present in order to be activated. These specific nutrients being present or in low concentrations, anaerobiosis, and host-produced factors. | 1 | Biochemistry |
Usually the endosymbiosis event is considered to have occurred in the Archaeplastida, within which the glaucophyta being the possible earliest diverging lineage. The glaucophyte chloroplast group is the smallest of the three primary chloroplast lineages, being found in only 13 species, and is thought to be the one that branched off the earliest. Glaucophytes have chloroplasts that retain a peptidoglycan wall between their double membranes, like their cyanobacterial parent. For this reason, glaucophyte chloroplasts are also known as muroplasts (besides cyanoplasts or cyanelles). Glaucophyte chloroplasts also contain concentric unstacked thylakoids, which surround a carboxysome – an icosahedral structure that glaucophyte chloroplasts and cyanobacteria keep their carbon fixation enzyme RuBisCO in. The starch that they synthesize collects outside the chloroplast. Like cyanobacteria, glaucophyte and rhodophyte chloroplast thylakoids are studded with light collecting structures called phycobilisomes. For these reasons, glaucophyte chloroplasts are considered a primitive intermediate between cyanobacteria and the more evolved chloroplasts in red algae and plants. | 5 | Photochemistry |
Fuzzy regions modulate the conformational equilibrium or flexibility of the binding interface via transient interactions. Dynamic regions can also compete with binding sites or tether them to the target. Modifications of fuzzy regions by further interactions, or posttranslational modifications impact binding affinity or specificity. Alternative splicing can modulate the length of fuzzy regions resulting in context-dependent binding (e.g. tissue-specificity) on the complex. EGF/MAPK, TGF-β and WNT/Wingless signaling pathways employ tissue-specific fuzzy regions. | 4 | Stereochemistry |
Powder metallurgy is a class of modern processing techniques in which metals are first powdered, and then formed into the desired shape by heating below the melting point. This is in contrast to casting, which occurs with molten metal. Superalloy manufacturing often employs powder metallurgy because of its material efficiency - typically much less waste metal must be machined away from the final product—and its ability to facilitate mechanical alloying. Mechanical alloying is a process by which reinforcing particles are incorporated into the superalloy matrix material by repeated fracture and welding. | 8 | Metallurgy |
Mathematically, the stress at some point in the material is a plane stress if one of the three principal stresses (the eigenvalues of the Cauchy stress tensor) is zero. That is, there is Cartesian coordinate system in which the stress tensor has the form
For example, consider a rectangular block of material measuring 10, 40 and 5 cm along the , , and , that is being stretched in the direction and compressed in the direction, by pairs of opposite forces with magnitudes 10 N and 20 N, respectively, uniformly distributed over the corresponding faces. The stress tensor inside the block will be
More generally, if one chooses the first two coordinate axes arbitrarily but perpendicular to the direction of zero stress, the stress tensor will have the form
and can therefore be represented by a 2 × 2 matrix, | 8 | Metallurgy |
In chemistry, materials science, and physics, the liquidus temperature specifies the temperature above which a material is completely liquid, and the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. The solidus is the locus of temperatures (a curve on a phase diagram) below which a given substance is completely solid (crystallized). The solidus temperature, specifies the temperature below which a material is completely solid, and the minimum temperature at which a melt can co-exist with crystals in thermodynamic equilibrium.
Liquidus and solidus are mostly used for impure substances (mixtures) such as glasses, metal alloys, ceramics, rocks, and minerals. Lines of liquidus and solidus appear in the phase diagrams of binary solid solutions, as well as in eutectic systems away from the invariant point. | 7 | Physical Chemistry |
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments. In other words, it is a biologically mediated process which results in the sequestering of carbon in the deep ocean away from the atmosphere and the land. The biological pump is the biological component of the "marine carbon pump" which contains both a physical and biological component. It is the part of the broader oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).
Budget calculations of the biological carbon pump are based on the ratio between sedimentation (carbon export to the ocean floor) and remineralization (release of carbon to the atmosphere).
The biological pump is not so much the result of a single process, but rather the sum of a number of processes each of which can influence biological pumping. Overall, the pump transfers about 10.2 gigatonnes of carbon every year into the ocean's interior and a total of 1300 gigatonnes carbon over an average 127 years. This takes carbon out of contact with the atmosphere for several thousand years or longer. An ocean without a biological pump would result in atmospheric carbon dioxide levels about 400 ppm higher than the present day. | 9 | Geochemistry |
In theoretical physics, the Mandelstam variables are numerical quantities that encode the energy, momentum, and angles of particles in a scattering process in a Lorentz-invariant fashion. They are used for scattering processes of two particles to two particles. The Mandelstam variables were first introduced by physicist Stanley Mandelstam in 1958.
If the Minkowski metric is chosen to be , the Mandelstam variables are then defined by
where p and p are the four-momenta of the incoming particles and p and p are the four-momenta of the outgoing particles.
is also known as the square of the center-of-mass energy (invariant mass) and as the square of the four-momentum transfer. | 7 | Physical Chemistry |
Flutamide and hydroxyflutamide have been found in vitro to inhibit CYP17A1 (17α-hydroxylase/17,20-lyase), an enzyme which is required for the biosynthesis of androgens. In accordance, flutamide has been found to slightly but significantly lower androgen levels in GnRH analogue-treated male prostate cancer patients and women with polycystic ovary syndrome. In a directly comparative study of flutamide monotherapy (375mg once daily) versus bicalutamide monotherapy (80mg once daily) in Japanese men with prostate cancer, after 24weeks of treatment flutamide decreased dehydroepiandrosterone (DHEA) levels by about 44% while bicalutamide increased them by about 4%. As such, flutamide is a weak inhibitor of androgen biosynthesis. However, the clinical significance of this action may be limited when flutamide is given without a GnRH analogue to non-castrated men, as the medication markedly elevates testosterone levels into the high normal male range via prevention of AR activation-mediated negative feedback on the hypothalamic–pituitary–gonadal axis in this context. | 4 | Stereochemistry |
Although other cells lines, such as African green monkey kidney (COS) and baby hamster kidney (BHK), can be used for recombinant protein production, the most commonly employed host system in transient expression of mammalian cells involves derivatives of the HEK293 cell line, which is based on the human embryonic kidney cell line established in 1977 by Graham et al. The HEK293 cell line was created via transformation with sheared Adenovirus 5 DNA. Advantages of using this cell line include their high rates of transfection and ability to grow in a serum-free medium, which results in reduced cost and lowered risk of contamination with animal-derived material typically found in serum.
Several engineered sublines were later developed by incorporating viral elements derived from mammalian viruses, such as SV40 virus or Epstein–Barr virus (EBV), which are notable for their high retention of plasmid DNA in an episomal state and their capacity to increase transcription and translation via specific viral properties. These later sublines were consequently identified to have two interacting components: the SV40 large T-antigen binding to the SV20 origin of replication (SV40ori) and the EBV-derived nuclear antigen-1 (EBNA-1) protein to its associated origin of replication (oriP).
Typical historical yields of transient expression in HEK293 cells transfected using PEI-25kDa was 20-40 mg/L of recombinant antibody protein. In 2008, Backliwal et. al reported for the first time yields crossing 1 g/L of recombinant antibody protein. | 1 | Biochemistry |
To discuss the visibility of defects in topographic images according to theory, consider the exemplary case of a single dislocation: It will give rise to contrast in topography only if the lattice planes involved in diffraction are distorted in some way by the existence of the dislocation. This is true in the case of an edge dislocation if the scattering vector of the Bragg reflection used is parallel to the Burgers vector of the dislocation, or at least has a component in the plane perpendicular to the dislocation line, but not if it is parallel to the dislocation line. In the case of a screw dislocation, the scattering vector has to have a component along the Burgers vector, which is now parallel to dislocation line. As a rule of thumb, a dislocation will be invisible in a topograph if the vector product
is zero.
(A more precise rule will have to distinguish between screw and edge dislocations and to also take the direction of the dislocation line into account – see e.g. [http://www.msel.nist.gov/practiceguides/SP960_10.pdf].)
If a defect is visible, often there occurs not only one, but several distinct images of it on the topograph. Theory predicts three images of single defects: The so-called direct image, the kinematical image, and the intermediary image.
For details see e.g. (Authier 2003). | 3 | Analytical Chemistry |
Deconvolution can be used to apparently improve spectral resolution. In the case of NMR spectra, the process is relatively straight forward, because the line shapes are Lorentzian, and the convolution of a Lorentzian with another Lorentzian is also Lorentzian. The Fourier transform of a Lorentzian is an exponential. In the co-domain (time) of the spectroscopic domain (frequency) convolution becomes multiplication. Therefore, a convolution of the sum of two Lorentzians becomes a multiplication of two exponentials in the co-domain. Since, in FT-NMR, the measurements are made in the time domain division of the data by an exponential is equivalent to deconvolution in the frequency domain. A suitable choice of exponential results in a reduction of the half-width of a line in the frequency domain. This technique has been rendered all but obsolete by advances in NMR technology. A similar process has been applied for resolution enhancement of other types of spectra, with the disadvantage that the spectrum must be first Fourier transformed and then transformed back after the deconvoluting function has been applied in the spectrum's co-domain. | 7 | Physical Chemistry |
Cellular metabolism is represented by a large number of metabolic reactions involving the conversion of the carbon source (usually glucose) into the building blocks needed for macromolecular biosynthesis. These reactions form metabolic networks within cells. These networks can then be used to study metabolism within cells.
To allow these networks to interact, a tight connection between them is necessary. This connection is provided by usage of common cofactors such as ATP, ADP, NADH and NADPH. In addition to this, sharing of some metabolites between the different networks further tightens the connections between the different networks. | 1 | Biochemistry |
Steel is an alloy composed of between 0.2 and 2.0 percent carbon, with the balance being iron. From prehistory through the creation of the blast furnace, iron was produced from iron ore as wrought iron, 99.82–100 percent Fe, and the process of making steel involved adding carbon to iron, usually in a serendipitous manner, in the forge, or via the cementation process. The introduction of the blast furnace reversed the problem. A blast furnace produces pig iron — an alloy of approximately 90 percent iron and 10 percent carbon. When the process of steel-making is started with pig iron, instead of wrought iron, the challenge is to remove a sufficient amount of carbon to reduce it to the 0.2 to 2 percentage for steel.
Before about 1860, steel was an expensive product, made in small quantities and used mostly for swords, tools and cutlery; all large metal structures were made of wrought or cast iron. Steelmaking was centered in Sheffield and Middlesbrough, Britain, which supplied the European and American markets. The introduction of cheap steel was due to the Bessemer and the open hearth processes, two technological advances made in England. In the Bessemer process, molten pig iron is converted to steel by blowing air through it after it was removed from the furnace. The air blast burned the carbon and silicon out of the pig iron, releasing heat and causing the temperature of the molten metal to rise. Henry Bessemer demonstrated the process in 1856 and had a successful operation going by 1864. By 1870 Bessemer steel was widely used for ship plate. By the 1850s, the speed, weight, and quantity of railway traffic was limited by the strength of the wrought iron rails in use. The solution was to turn to steel rails, which the Bessemer process made competitive in price. Experience quickly proved steel had much greater strength and durability and could handle the increasingly heavy and faster engines and cars.
After 1890 the Bessemer process was gradually supplanted by open-hearth steelmaking and by the middle of the 20th century was no longer in use. The open-hearth process originated in the 1860s in Germany and France. The usual open-hearth process used pig iron, ore, and scrap, and became known as the Siemens-Martin process. Its process allowed closer control over the composition of the steel; also, a substantial quantity of scrap could be included in the charge. The crucible process remained important for making high-quality alloy steel into the 20th century. By 1900 the electric arc furnace was adapted to steelmaking and by the 1920s, the falling cost of electricity allowed it to largely supplant the crucible process for specialty steels. | 8 | Metallurgy |
The unique relationship between the compressibility factor and the reduced temperature, , and the reduced pressure, , was first recognized by Johannes Diderik van der Waals in 1873 and is known as the two-parameter principle of corresponding states. The principle of corresponding states expresses the generalization that the properties of a gas which are dependent on intermolecular forces are related to the critical properties of the gas in a universal way. That provides a most important basis for developing correlations of molecular properties.
As for the compressibility of gases, the principle of corresponding states indicates that any pure gas at the same reduced temperature, , and reduced pressure, , should have the same compressibility factor.
The reduced temperature and pressure are defined by
: and
Here and are known as the critical temperature and critical pressure of a gas. They are characteristics of each specific gas with being the temperature above which it is not possible to liquify a given gas and is the minimum pressure required to liquify a given gas at its critical temperature. Together they define the critical point of a fluid above which distinct liquid and gas phases of a given fluid do not exist.
The pressure-volume-temperature (PVT) data for real gases varies from one pure gas to another. However, when the compressibility factors of various single-component gases are graphed versus pressure along with temperature isotherms many of the graphs exhibit similar isotherm shapes.
In order to obtain a generalized graph that can be used for many different gases, the reduced pressure and temperature, and , are used to normalize the compressibility factor data. Figure 2 is an example of a generalized compressibility factor graph derived from hundreds of experimental PVT data points of 10 pure gases, namely methane, ethane, ethylene, propane, n-butane, i-pentane, n-hexane, nitrogen, carbon dioxide and steam.
There are more detailed generalized compressibility factor graphs based on as many as 25 or more different pure gases, such as the Nelson-Obert graphs. Such graphs are said to have an accuracy within 1–2 percent for values greater than 0.6 and within 4–6 percent for values of 0.3–0.6.
The generalized compressibility factor graphs may be considerably in error for strongly polar gases which are gases for which the centers of positive and negative charge do not coincide. In such cases the estimate for may be in error by as much as 15–20 percent.
The quantum gases hydrogen, helium, and neon do not conform to the corresponding-states behavior and the reduced pressure and temperature for those three gases should be redefined in the following manner to improve the accuracy of predicting their compressibility factors when using the generalized graphs:
: and
where the temperatures are in kelvins and the pressures are in atmospheres. | 7 | Physical Chemistry |
The oxidase catalyzes the transfer of four electrons from reduced plastoquinone to molecular oxygen to form water . The net reaction is written below:
2 QH + O → 2 Q + 2 HO
Analysis of substrate specificity revealed that the enzyme almost exclusively catalyzes the reduction of plastoquinone over other quinones such as ubiquinone and duroquinone. Additionally, iron is essential for the catalytic function of the enzyme and cannot be substituted by another metal cation like Cu, Zn, or Mn at the catalytic center.
It is unlikely that four electrons could be transferred at once in a single iron cluster, so all of the proposed mechanisms involve two separate two-electron transfers from reduced plastoquinone to the di-iron center. In the first step common to all proposed mechanisms, one plastoquinone is oxidized and both irons are reduced from iron(III) to iron(II). Four different mechanisms are proposed for the next step, oxygen capture. One mechanism proposes a peroxide intermediate, after which one oxygen atom is used to create water and another is left bound in a diferryl configuration. Upon one more plastoquinone oxidation, a second water molecule is formed and the irons return to a +3 oxidation state. The other mechanisms involve the formation of Fe(III)-OH or Fe(IV)-OH and a tyrosine radical. These radical-based mechanisms could explain why over-expression of the PTOX gene causes increased generation of reactive oxygen species. | 5 | Photochemistry |
Fatty acids are broken down to acetyl-CoA by means of beta oxidation inside the mitochondria, whereas fatty acids are synthesized from acetyl-CoA outside the mitochondria, in the cytosol. The two pathways are distinct, not only in where they occur, but also in the reactions that occur, and the substrates that are used. The two pathways are mutually inhibitory, preventing the acetyl-CoA produced by beta-oxidation from entering the synthetic pathway via the acetyl-CoA carboxylase reaction. It can also not be converted to pyruvate as the pyruvate dehydrogenase complex reaction is irreversible. Instead the acetyl-CoA produced by the beta-oxidation of fatty acids condenses with oxaloacetate, to enter the citric acid cycle. During each turn of the cycle, two carbon atoms leave the cycle as CO in the decarboxylation reactions catalyzed by isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. Thus each turn of the citric acid cycle oxidizes an acetyl-CoA unit while regenerating the oxaloacetate molecule with which the acetyl-CoA had originally combined to form citric acid. The decarboxylation reactions occur before malate is formed in the cycle. Only plants possess the enzymes to convert acetyl-CoA into oxaloacetate from which malate can be formed to ultimately be converted to glucose.
However, acetyl-CoA can be converted to acetoacetate, which can decarboxylate to acetone (either spontaneously, or catalyzed by acetoacetate decarboxylase). It can then be further metabolized to isopropanol which is excreted in breath/urine, or by CYP2E1 into hydroxyacetone (acetol). Acetol can be converted to propylene glycol. This converts to pyruvate (by two alternative enzymes), or propionaldehyde, or to -lactaldehyde then -lactate (the common lactate isomer). Another pathway turns acetol to methylglyoxal, then to pyruvate, or to -lactaldehyde (via -lactoyl-glutathione or otherwise) then -lactate. D-lactate metabolism (to glucose) is slow or impaired in humans, so most of the D-lactate is excreted in the urine; thus -lactate derived from acetone can contribute significantly to the metabolic acidosis associated with ketosis or isopropanol intoxication. -Lactate can complete the net conversion of fatty acids into glucose. The first experiment to show conversion of acetone to glucose was carried out in 1951. This, and further experiments used carbon isotopic labelling. Up to 11% of the glucose can be derived from acetone during starvation in humans.
The glycerol released into the blood during the lipolysis of triglycerides in adipose tissue can only be taken up by the liver. Here it is converted into glycerol 3-phosphate by the action of glycerol kinase which hydrolyzes one molecule of ATP per glycerol molecule which is phosphorylated. Glycerol 3-phosphate is then oxidized to dihydroxyacetone phosphate, which is, in turn, converted into glyceraldehyde 3-phosphate by the enzyme triose phosphate isomerase. From here the three carbon atoms of the original glycerol can be oxidized via glycolysis, or converted to glucose via gluconeogenesis. | 1 | Biochemistry |
haemagglutination activity domain - haemolysin expression modulating protein family - hairpin - haploid - haploinsufficiency - HdeA family - helix-loop-helix - helminth protein - hematopoietic stem cell - hemophilia - heteroduplex DNA - heterozygous - highly conserved sequence - Hirschsprungs disease - histone - HLA-Y - hnRNA - holoprosencephaly - homologous recombination - homology - homozygous - host strain (bacterial) - HspQ protein domain - human artificial chromosome - Human Genome Project - human immunodeficiency virus - HumHot - Huntingtons disease - hybridization - hybridoma - hydrophilicity plot - hydroxydechloroatrazine ethylaminohydrolase - | 1 | Biochemistry |
Biological soil uptake is the dominant sink of atmospheric H. Both aerobic and anaerobic microbial metabolisms consume H by oxidizing it in order to reduce other compounds during respiration. Aerobic H oxidation is known as the Knallgas reaction.
Anaerobic H oxidation often occurs during interspecies hydrogen transfer in which H produced during fermentation is transferred to another organism, which uses the H to reduce CO to CH or acetate, to HS, or Fe to Fe. Interspecies hydrogen transfer keeps H concentrations very low in most environments because fermentation becomes less thermodynamically favorable as the partial pressure of H increases. | 1 | Biochemistry |
Igor V. Komarov graduated with distinction from Taras Shevchenko National University of Kyiv, and started to work at the same university in 1986 first as an engineer. He obtained his Candidate of Sciences degree in 1991 in organic chemistry at Taras Shevchenko National University of Kyiv under supervision of Mikhail Yu. Kornilov; the candidate thesis was devoted to the use of lanthanide shift reagents in NMR spectroscopy. Afterwards, he was a postdoctoral fellow at the University Chemical Laboratory in Cambridge (1996–1997, United Kingdom) and at the [https://www.catalysis.de/home/ Institut für Organische Katalyseforschung] in Rostock (2000–2001, Germany). He holds the Supramolecular Chemistry Chair of Institute of High Technologies at Taras Shevchenko National University. Komarov earned his Doctor of Sciences degree in 2003; the title of his thesis is "Design and synthesis of model compounds: study of stereoelectronic, steric effects, reactive intermediates, catalytic enantioselective hydrogenation and dynamic protection of functional groups" He is also a scientific advisor for Enamine Ltd. and Lumobiotics GmbH. Igor V. Komarov was awarded the title of Professor in 2007. | 0 | Organic Chemistry |
The stability of the Keggin structure allows the metals in the anion to be readily reduced. Depending on the solvent, acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple-electron steps. For example, the silicotungstate anion can be reduced a −20 state. Some anions such as silicotungstic acid are strong enough as an acid as sulfuric acid and can be used in its place as an acid catalyst. | 7 | Physical Chemistry |
In chemistry and thermodynamics, the standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their reference state, with all substances in their standard states. The standard pressure value is recommended by IUPAC, although prior to 1982 the value 1.00 atm (101.325 kPa) was used. There is no standard temperature. Its symbol is ΔH. The superscript Plimsoll on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 °C or 298.15 K).
Standard states are defined for various types of substances. For a gas, it is the hypothetical state the gas would assume if it obeyed the ideal gas equation at a pressure of 1 bar. For a gaseous or solid solute present in a diluted ideal solution, the standard state is the hypothetical state of concentration of the solute of exactly one mole per liter (1 M) at a pressure of 1 bar extrapolated from infinite dilution. For a pure substance or a solvent in a condensed state (a liquid or a solid) the standard state is the pure liquid or solid under a pressure of 1 bar.
For elements that have multiple allotropes, the reference state usually is chosen to be the form in which the element is most stable under 1 bar of pressure. One exception is phosphorus, for which the most stable form at 1 bar is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation.
For example, the standard enthalpy of formation of carbon dioxide is the enthalpy of the following reaction under the above conditions:
All elements are written in their standard states, and one mole of product is formed. This is true for all enthalpies of formation.
The standard enthalpy of formation is measured in units of energy per amount of substance, usually stated in kilojoule per mole (kJ mol), but also in kilocalorie per mole, joule per mole or kilocalorie per gram (any combination of these units conforming to the energy per mass or amount guideline).
All elements in their reference states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.
The formation reaction is a constant pressure and constant temperature process. Since the pressure of the standard formation reaction is fixed at 1 bar, the standard formation enthalpy or reaction heat is a function of temperature. For tabulation purposes, standard formation enthalpies are all given at a single temperature: 298 K, represented by the symbol . | 7 | Physical Chemistry |
ATP has recently been proposed to act as a biological hydrotrope and has been shown to affect proteome-wide solubility. | 1 | Biochemistry |
The Central Pollution Control Board of India released the Air (Prevention and Control of Pollution) Act in 1981, amended in 1987, to address concerns about air pollution in India. While the document does not differentiate between VOCs and other air pollutants, the CPCB monitors "oxides of nitrogen (NO), sulphur dioxide (SO), fine particulate matter (PM10) and suspended particulate matter (SPM)". | 0 | Organic Chemistry |
Proteins formed by the translation of the mRNA (messenger RNA, a coded information from DNA for protein synthesis) play a major role in regulating gene expression. To understand how they regulate gene expression it is necessary to identify DNA sequences that they interact with. Techniques have been developed to identify sites of DNA-protein interactions. These include ChIP-sequencing, CUT&RUN sequencing and Calling Cards. | 1 | Biochemistry |
In the US, biological energy is expressed using the energy unit Calorie with a capital C (i.e. a kilocalorie), which equals the energy needed to increase the temperature of 1 kilogram of water by 1 °C (about 4.18 kJ).
Energy balance, through biosynthetic reactions, can be measured with the following equation:
:Energy intake (from food and fluids) = Energy expended (through work and heat generated) + Change in stored energy (body fat and glycogen storage)
The first law of thermodynamics states that energy can be neither created nor destroyed. But energy can be converted from one form of energy to another. So, when a calorie of food energy is consumed, one of three particular effects occur within the body: a portion of that calorie may be stored as body fat, triglycerides, or glycogen, transferred to cells and converted to chemical energy in the form of adenosine triphosphate (ATP – a coenzyme) or related compounds, or dissipated as heat. | 1 | Biochemistry |
Ge was born in China. She attended Peking University for her undergraduate studies, where she studied chemistry. After graduating in 1997 Ge moved to the United States, where she joined Cornell University as a doctoral student. Here she started to work on mass spectrometry, using electron-capture dissociation to study proteins. She worked under the supervision of Tadhg Begley and Fred McLafferty. After completing her doctorate, Ge worked as a research scientist at Wyeth. | 1 | Biochemistry |
In the book, Lane discusses what he considers to be a major gap in biology: why life operates the way that it does, and how it began. In his view as a biochemist, the core question is about energy, as all cells handle energy in the same way, relying on a steep electrochemical gradient across the very small thickness of a membrane in a cell – to power all the chemical reactions of life. The electrical energy is transformed into forms that the cell can use by a chain of energy-handling structures including ancient proteins such as cytochromes, ion channels, and the enzyme ATP synthase, all built into the membrane. Once evolved, this chain has been conserved by all living things, showing that it is vital to life. He argues that such an electrochemical gradient could not have arisen in ordinary conditions, such as the open ocean or Darwin's "warm little pond". He argues instead (following Günter Wächtershäuser) that life began in deep-sea hydrothermal vents, as these contain chemicals that effectively store energy that cells could use, as long as the cells provided a membrane to generate the needed gradient by maintaining different concentrations of chemicals on either side.
Once cells similar to bacteria (the first prokaryotes, cells without a nucleus) had emerged, he writes, they stayed like that for two and a half billion years. Then, just once, cells jumped in complexity and size, acquiring a nucleus and other organelles, and complex behavioural features including sex, which he notes have become universal in complex (eukaryotic) life forms including plants, animals, and fungi.
The book is illustrated with 37 figures taken by permission from a wide variety of research sources. They include a timeline, photographs, cladograms, electron flow diagrams and diagrams of the life cycle of cells and their chromosomes. | 1 | Biochemistry |
The du Noüy Padday rod consists of a rod usually on the order of a few millimeters square making a small ring. The rod is often made from a composite metal material that may be roughened to ensure complete wetting at the interface. The rod is cleaned with water, alcohol and a flame or with strong acid to ensure complete removal of surfactants. The rod is attached to a scale or balance via a thin metal hook. The Padday method uses the maximum pull force method, i.e. the maximum force due to the surface tension is recorded as the probe is first immersed ca. one mm into the solution and then slowly withdrawn from the interface. The main forces acting on a probe are the buoyancy (due to the volume of liquid displaced by the probe) and the mass of the meniscus adhering to the probe. This is an old, reliable, and well-documented technique.
An important advantage of the maximum pull force technique is that the receding contact angle on the probe is effectively zero. The maximum pull force is obtained when the buoyancy force reaches its minimum,
The surface tension measurement used in the Padday devices based on the du Noüy ring/maximum pull force method is explained further here:
The force acting on the probe can be divided into two components:
: i) Buoyancy stemming from the volume displaced by the probe, and
: ii) the mass of the meniscus of the liquid adhering to the probe.
The latter is in equilibrium with the surface tension force, i.e.
where
:* is the perimeter of the probe,
:* is the surface tension and the weight of the meniscus under the probe. In the situation considered here the volume displaced by the probe is included in the meniscus.
:* is the contact angle between the probe and the solution that is measured, and is negligible for the majority of solutions with Kibron’s probes.
Thus, the force measured by the balance is given by
where
:* is the force acting on the probe and
:* is the force due to buoyancy.
At the point of detachment the volume of the probe immersed in the solution vanishes, and thus, also the buoyancy term. This is observed as a maximum in the force curve, which relates to the surface tension through
The above derivation holds for ideal conditions. Non-idealities, e.g. from defect probe shape, are partly compensated in the calibration routine using a solution with known surface tension. | 7 | Physical Chemistry |
Source:
If a solid body is modeled by a constant hard-core field, then
or
where
Here
For the hard solid potential
where is the position of the potential discontinuity. So, in this case | 7 | Physical Chemistry |
The rising level of the Caspian Sea between 1995 and 1996 reduced the number of habitats for rare species of aquatic vegetation. This has been attributed to a general lack of seeding material in newly formed coastal lagoons and water bodies.
Many rare and endemic plant species of Russia are associated with the tidal areas of the Volga delta and riparian forests of the Samur River delta. The shoreline is also a unique refuge for plants adapted to the loose sands of the Central Asian Deserts. The principal limiting factors to successful establishment of plant species are hydrological imbalances within the surrounding deltas, water pollution, and various land reclamation activities. The water level change within the Caspian Sea is an indirect reason for which plants may not get established.
These affect aquatic plants of the Volga Delta, such as Aldrovanda vesiculosa and the native Nelumbo caspica. About 11 plant species are found in the Samur River delta, including the unique liana forests that date back to the Tertiary period.
Since 2019 UNESCO has admitted the lush Hyrcanian forests of Mazandaran, Iran as World Heritage Site under category (ix). | 2 | Environmental Chemistry |
Electrolytic refining of copper was first patented in England by James Elkington in 1865 and the first electrolytic copper refinery was built by Elkington in Burry Port, South Wales in 1869.
There were teething problems with the new technology. For example, the early refineries had trouble producing firm deposits on the cathodes. As a result, there was much secrecy between refinery operators as each strove to maintain a competitive edge.
The nature of the cathode used to collect the copper is a critical part of the technology. The properties of copper are highly susceptible to impurities. For example, an arsenic content of 0.1% can reduce the conductivity of copper by 23% and a bismuth content of just 0.001% makes copper brittle. The material used in the cathode must not contaminate the copper being deposited, or it will not meet the required specifications.
The current efficiency of the refining process depends, in part, on how close the anodes and cathodes can be placed in the electrolytic cell. This, in turn, depends on the straightness of both the anode and the cathode. Bumps and bends in either can lead to short-circuiting or otherwise affect the current distribution and also the quality of the cathode copper.
Prior to the development of the Isa Process technology, the standard approach was to use a starter sheet of high-purity copper as the initial cathode. These starter sheets are produced in special electrolytic cells by electrodeposition of copper for 24 hours onto a plate made of copper coated with oil (or treated with other similar face-separation materials) or of titanium. Thousands of sheets could be needed every day, and the original method of separating the starter sheet from the “mother plate” (referred to as “stripping”) was entirely manual.
Starter sheets are usually quite light. For example, the starter sheets used in the CRL refinery weighed 10 pounds (4.53 kilograms). Thus, they are thin and need to be handled carefully to avoid bending.
Over time, the formation of starter sheets was improved by mechanisation, but there was still a high labour input.
Once the starter sheets were formed, they had to be flattened, to reduce the likelihood of short circuits, and then cut, formed and punched to make loops from which the starter sheets are hung from conductive copper hanger bars in the electrolytic cells (see Figure 1).
The starter sheets are inserted in the refining cells and dissolved copper electrodeposits on them to produce the cathode copper product (see Figure 2). Because of the manufacturing cost of the starter sheets, refineries using them tend to keep them in the cells as long as possible, usually 12–14 days. On the other hand, the anodes normally reside in the cells for 24–28 days, meaning that there are two cathodes produced from each anode.
The starter sheets have a tendency to warp, due to the mechanical stresses they encounter and often need to be removed from the refining cells after about two days to be straightened in presses before being returned to the cells. The tendency to warp leads to frequent short-circuiting.
Due to their limitations, it is difficult for copper produced on starter sheets to meet modern specifications for the highest-purity copper. | 8 | Metallurgy |
In mathematics and solid state physics, the first Brillouin zone (named after Léon Brillouin) is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice is broken up into Brillouin zones. The boundaries of this cell are given by planes related to points on the reciprocal lattice. The importance of the Brillouin zone stems from the description of waves in a periodic medium given by Bloch's theorem, in which it is found that the solutions can be completely characterized by their behavior in a single Brillouin zone.
The first Brillouin zone is the locus of points in reciprocal space that are closer to the origin of the reciprocal lattice than they are to any other reciprocal lattice points (see the derivation of the Wigner–Seitz cell). Another definition is as the set of points in k-space that can be reached from the origin without crossing any Bragg plane. Equivalently, this is the Voronoi cell around the origin of the reciprocal lattice.
There are also second, third, etc., Brillouin zones, corresponding to a sequence of disjoint regions (all with the same volume) at increasing distances from the origin, but these are used less frequently. As a result, the first Brillouin zone is often called simply the Brillouin zone. In general, the n-th Brillouin zone consists of the set of points that can be reached from the origin by crossing exactly n − 1 distinct Bragg planes. A related concept is that of the irreducible Brillouin zone, which is the first Brillouin zone reduced by all of the symmetries in the point group of the lattice (point group of the crystal).
The concept of a Brillouin zone was developed by Léon Brillouin (1889–1969), a French physicist.
Within the Brillouin zone, a constant-energy surface represents the loci of all the -points (that is, all the electron momentum values) that have the same energy. Fermi surface is a special constant-energy surface that separates the unfilled orbitals from the filled ones at zero kelvin. | 3 | Analytical Chemistry |
Wild Fermentation: The Flavor, Nutrition, and Craft of Live-Culture Foods is a 2003 book by Sandor Katz that discusses the ancient practice of fermentation. While most of the conventional literature assumes the use of modern technology, Wild Fermentation focuses more on the practice and culture of fermenting food.
The term "wild fermentation" refers to the reliance on naturally occurring bacteria and yeast to ferment food. For example, conventional bread making requires the use of a commercial, highly specialized yeast, while wild-fermented bread relies on naturally occurring cultures that are found on the flour, in the air, and so on. Similarly, the book's instructions on sauerkraut require only cabbage and salt, relying on the cultures that naturally exist on the vegetable to perform the fermentation.
The book also discusses some foods that are not, strictly speaking, wild ferments such as miso, yogurt, kefir, and nattō.
Beyond food, the book includes some discussion of social, personal, and political issues, such as the legality of raw milk cheeses in the United States.
Newsweek has referred to Wild Fermentation as the "fermentation bible". | 1 | Biochemistry |
Since the dawn of modern chemistry, humic substances are among the most studied among the natural materials. Despite long study, their molecular structure remains elusive. The traditional view is that humic substances are heteropolycondensates, in varying associations with clay. A more recent view is that relatively small molecules also play a role. Humic substances account for 50 – 90% of cation exchange capacity. Similar to clay, char and colloidal humus hold cation nutrients. | 9 | Geochemistry |
SINEs are classified as non-LTR retrotransposons because they do not contain long terminal repeats (LTRs). There are three types of SINEs common to vertebrates and invertebrates: CORE-SINEs, V-SINEs, and AmnSINEs. SINEs have 50-500 base pair internal regions which contain a tRNA-derived segment with A and B boxes that serve as an internal promoter for RNA polymerase III. | 1 | Biochemistry |
The traditional synthetic route uses Raney nickel and has been further improved over time, for example by the use of ibuprofen and AlCl.
Overall, it is a cost-effective method with moderate reaction conditions that is easy to handle and suitable for industrial production. | 4 | Stereochemistry |
At neutral pH, thiocarboxylic acids are fully ionized. Thiocarboxylic acids are about 100 times more acidic than the analogous carboxylic acids. For PhC(O)SH pK = 2.48 vs 4.20 for PhC(O)OH. For thioacetic acid the pK is near 3.4 vs 4.72 for acetic acid.
The conjugate base of thioacetic acid, thioacetate is reagents for installing thiol groups via the displacement of alkyl halides to give the thioester, which in turn are susceptible to hydrolysis:
Thiocarboxylic acids react with various nitrogen functional groups, such as organic azide, nitro, and isocyanate compounds, to give amides under mild conditions. This method avoids needing a highly nucleophilic aniline or other amine to initiate an amide-forming acyl substitution, but requires synthesis and handling of the unstable thiocarboxylic acid. Unlike the Schmidt reaction or other nucleophilic-attack pathways, the reaction with an aryl or alkyl azide begins with a [3+2] cycloaddition; the resulting heterocycle expels N and the sulfur atom to give the monosubstituted amide. | 0 | Organic Chemistry |
The large array of tests can be categorised into sub-specialities of:
*General or routine chemistry – commonly ordered blood chemistries (e.g., liver and kidney function tests).
*Special chemistry – elaborate techniques such as electrophoresis, and manual testing methods.
*Clinical endocrinology – the study of hormones, and diagnosis of endocrine disorders.
*Toxicology – the study of drugs of abuse and other chemicals.
*Therapeutic Drug Monitoring – measurement of therapeutic medication levels to optimize dosage.
*Urinalysis – chemical analysis of urine for a wide array of diseases, along with other fluids such as CSF and effusions
*Fecal analysis – mostly for detection of gastrointestinal disorders. | 1 | Biochemistry |
The first decade of the 20th century brought the basics of quantum theory (Planck, Einstein) and interpretation of spectral series of hydrogen by Lyman in VUV and by Paschen in infrared. Ritz formulated the combination principle.
John William Nicholson had created an atomic model in 1912, a year before Niels Bohr, that was both nuclear and quantum in which he showed that electron oscillations in his atom matched the solar and nebular spectral lines. Bohr had been working on his atom during this period, but Bohr's model had only a single ground state and no spectra until he incorporated the Nicholson model and referenced the Nicholson papers in his model of the atom.
In 1913, Bohr formulated his quantum mechanical model of atom. This stimulated empirical term analysis. Bohr published a theory of the hydrogen-like atoms that could explain the observed wavelengths of spectral lines due to electrons transitioning from different energy states. In 1937 "E. Lehrer created the first fully-automated spectrometer" to help more accurately measure spectral lines. With the development of more advanced instruments such as photo-detectors scientists were then able to more accurately measure specific wavelength absorption of substances. | 7 | Physical Chemistry |
While RBS is generally used to measure the bulk composition and structure of a sample, it is possible to obtain some information about the structure and composition of the sample surface. When the signal is channeled to remove the bulk signal, careful manipulation of the incident and detection angles can be used to determine the relative positions of the first few layers of atoms, taking advantage of blocking effects.
The surface structure of a sample can be changed from the ideal in a number of ways. The first layer of atoms can change its distance from subsequent layers (relaxation); it can assume a different two-dimensional structure than the bulk (reconstruction); or another material can be adsorbed onto the surface. Each of these cases can be detected by RBS. For example, surface reconstruction can be detected by aligning the beam in such a way that channeling should occur, so that only a surface peak of known intensity should be detected. A higher-than-usual intensity or a wider peak will indicate that the first layers of atoms are failing to block the layers beneath, i.e. that the surface has been reconstructed. Relaxations can be detected by a similar procedure with the sample tilted so the ion beam is incident at an angle selected so that first-layer atoms should block backscattering at a diagonal; that is, from atoms which are below and displaced from the blocking atom. A higher-than-expected backscattered yield will indicate that the first layer has been displaced relative to the second layer, or relaxed. Adsorbate materials will be detected by their different composition, changing the position of the surface peak relative to the expected position.
RBS has also been used to measure processes which affect the surface differently from the bulk by analyzing changes in the channeled surface peak. A well-known example of this is the RBS analysis of the premelting of lead surfaces by Frenken, Maree and van der Veen. In an RBS measurement of the Pb(110) surface, a well-defined surface peak which is stable at low temperatures was found to become wider and more intense as temperature increase past two-thirds of the bulk melting temperature. The peak reached the bulk height and width as temperature reached the melting temperature. This increase in the disorder of the surface, making deeper atoms visible to the incident beam, was interpreted as pre-melting of the surface, and computer simulations of the RBS process produced similar results when compared with theoretical pre-melting predictions.
RBS has also been combined with nuclear microscopy, in which a focused ion beam is scanned across a surface in a manner similar to a scanning electron microscope. The energetic analysis of backscattered signals in this kind of application provides compositional information about the surface, while the microprobe itself can be used to examine features such as periodic surface structures. | 7 | Physical Chemistry |
Mono-BOC-cystamine (mono BOC protected cystamine) is a tert-butyloxycarbonyl (BOC) derivative of cystamine used as crosslinker in biotechnology and molecular biology applications. This compound was originally reported by Hansen et al. | 1 | Biochemistry |
Most of the methyl chloride present in the environment ends up being released to the atmosphere. After being released into the air, the atmospheric lifetime of this substance is about 10 months with multiple natural sinks, such as ocean, transport to the stratosphere, soil, etc.
On the other hand, when the methyl chloride emitted is released to water, it will be rapidly lost by volatilization. The half-life of this substance in terms of volatilization in the river, lagoon and lake is 2.1 h, 25 h and 18 days, respectively.
The amount of methyl chloride in the stratosphere is estimated to be 2 x 10 tonnes per year, representing 20-25% of the total amount of chlorine that is emitted to the stratosphere annually. | 2 | Environmental Chemistry |
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