A structural and computational study of citrulline in biochemical reactions

Structural Chemistry - Citrulline, a non-essential amino acid, is used therapeutically in mitochondrial diseases, especially since it is a well-tolerated medicinal compound. Athletes use citrulline...     

The Kirkwood-Buff theory and the effect of cosolvents on biochemical reactions

Cosolvents added to aqueous solutions of biomolecules profoundly affect protein stability, as well as biochemical equilibria. Some cosolvents, such as urea and guanidine hydrochloride, denature proteins, whereas others, such as osmolytes and crowders, stabilize the native structures of proteins. The way cosolvents interact with biomolecules is crucial information required to understand the cosolvent effect at a molecular level. We present a statistical mechanical framework based upon Kirkwood-Buff theory, which enables one to extract this picture from experimental data. The combination of two experimental results, namely, the cosolvent-induced equilibrium shift and the partial molar volume change upon the reaction, supplimented by the structural change, is shown to yield the number of water and cosolvent molecules bound or released during a reaction. Previously, denaturation experiments (e.g., m-value analysis) were analyzed by empirical and stoichiometric solvent-binding models, while the effects of osmolytes and crowders were analyzed by the approximate molecular crowding approach for low cosolvent concentration. Here we synthesize these previous approaches in a rigorous statistical mechanical treatment, which is applicable at any cosolvent concentration. The usefulness and accuracy of previous approaches was also evaluated.     

Lipids of defective cotton seeds and the influence of microflora on their composition

A comparative study has been made of the lipid composition of defective and first-grade cotton seeds after their prolonged storage under industrial conditions. On the basis of the change in the lipids of the seeds subjected to thermal spoilage it is proposed to consider the process of spontaneous heating as a group of enzymatic processes taking place mainly in not fully ripe seeds when they are damaged by pathogenic microflora.     

Lipids of defective cotton seeds and the influence of microflora on their composition

A comparative study has been made of the lipid composition of defective and first-grade cotton seeds after their prolonged storage under industrial conditions. On the basis of the change in the lipids of the seeds subjected to thermal spoilage it is proposed to consider the process of spontaneous heating as a group of enzymatic processes taking place mainly in not fully ripe seeds when they are damaged by pathogenic microflora.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 677–686, November–December, 1986.     

The role of natural and man-made ice-forming nuclei in the atmosphere

Water-soluble and insoluble, organic and inorganic, natural and man-made aerosol particles participate in vapor-liquid, vapor-solid (ice), and liquid-solid phase transitions in the atmosphere. Hydrosol particles (aerosol particles that have been transferred into water droplets) nucleate ice through freezing. A small without scavenging or being scavenged by another aerosol particle. It is also difficult to imagine that pure mineral particles can be lifted from soil surfaces. In view of this, an ice-nucleating site may be a much smaller particle attached to a larger clay particle. To this category belong, e.g., silver iodide-clay mixed particles. Limited studies indicate that decaying leaves and forest litter under the surface of soils are a potential source of biogenic ice-forming nuclei but that their contribution to the atmosphere is very limited. Research should be directed to study possible relations between cloud condensation nuclei and ice-forming nuclei derived from natural organic compounds (terpenes, leaf-derived nuclei, bacteria, etc.).

A balance must be maintained between large cloud chambers, in which duplication of in-cloud processes is possible, and the special instrumentation which provides information about the modes of ice nucleation on aerosol particles. The two modes of instrumentation should supplement each other.

The greatest difficulty in attempting to make a comparison between the number of ice-forming nuclei estimated in the laboratory and the number in a cloud is the lack of knowledge of the time-temperature-humidity history of the aerosol particles. In nature, the ability of an aerosol particle to nucleate ice may be destroyed or“poisoned“ in the presence of pollutants. An aerosol particle may, on the other hand, become an activated or warmer ice-forming nucleus, e.g.,after the sublimation of ice once formed on it. The temperature of ice nucleation is not a singular property of a particle; the warmest temperatures of ice nucleation of, e.g., particles of a certain soil 10cm in diameter are-15°C,-10°C, and-8°C for nucleation through freezing, condensation followed by freezing and contact, respectively (ref.26). The progress made in instrumentation permits studies of the modes of ice nucleation. Understanding the physical and chemical processes taking place in clouds makes estimates of the rates of ice particle formation more realistic (Young [ref.157]).

The reader should examine two previous reviews written by Mossop (1963) and Montefinale . (1971) for a more complete list of references.     

On the influence of solvents on the rate of ion-transfer reactions

The literature data on the kinetics of cation electrodeposition on mercury in different solvents were analysed. For all cations considered in different solvents there was a linear decrease of the logarithm of the standard charge-transfer rate constant with increasing basicity of solvent and with more negative formal potential of the electrode reaction expressed in the scale of a solvent-independent electrode, as well as a linear dependence of the activation energy on the Gibbs energy of cation transfer. No dependence of the logarithm of the heterogeneous rate constant on the rate of exchange of solvent molecules from the first solvation sphere was observed. For the different electrode systems studied in one solvent, the dependences of the activation energy on (i) the cation solvation energy, (ii) the Gibbs energy of metal amalgamation, and (iii) metal solubility in mercury were analysed.     

Porosity control of Pd@SiO2 yolk-shell nanocatalysts by the formation of nickel phyllosilicate and its influence on Suzuki coupling reactions

The surface of Pd@SiO(2) core-shell nanoparticles (1) was simply modified by the formation of nickel phyllosilicate. The addition of nickel salts formed branched nickel phyllosilicates and generated pores in the silica shells, yielding Pd@SiO(2)-Niphy nanoparticles (Niphy = nickel phyllosilicate; 2, 3). By removal of the silica residue, Pd@Niphy yolk-shell nanoparticles (4) was uniformly obtained. The four distinct nanostructures (1-4) were employed as catalysts for Suzuki coupling reactions with aryl bromide and phenylboronic acid, and the conversion yields were in the order of 1 < 2 < 3 < 4 as the pore volume and surface area of the catalysts increased. The reaction rates were strongly correlated with shell porosity and surface exposure of the metal cores. The chemical inertness of nickel phyllosilicate under the basic conditions rendered the catalysts reusable for more than five times without loss of activity.     

Quantitative rate constants for radical reactions in the nanopores of cotton

The understanding of radical reactions in nanostructured materials is important for developing new synthetic procedures and controlling degradation reactions. To develop this area, an easy method for measuring quantitative rate constants of some radical reactions in nanostructures is required. A simple method for measuring the rate constant of dye bleaching, kdye, by organic radicals in such materials is introduced, involving the measurement of microsecond bleaching kinetics by diffuse reflectance spectroscopy, following laser flash creation of the radicals. Using wet and dry cotton as model substrates, we obtained kdye of 2-hydroxy-2-propyl and 1-hydroxy-1-cyclohexyl radicals with reactive red 3 and reactive orange 4 and compared them to solution-phase values. Surprisingly, the reactions in cotton follow simple liquid-phase kinetics and are diffusion-controlled. A cage effect in cotton is also found.     

The influence of basic calcium chloride nuclei on the hydration of calcium chloridesilicate

The influence of CaCl₂ · Ca(OH)₂ · H₂O nuclei on the hydration of Ca₃SiO₄Cl₂ was studied by means of calorimetry, X-ray diffraction, simultaneous DTA-TG and chemical analysis of the liquid phase.

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Zusammenfassung Der Einfluss von CaCl₂ · Ca(OH)₂ · H₂O-Zusätzen auf die Hydratation von Calciumchlorid-silicat Ca₃SiO₄Cl₂ wurde durch Kalorimetrie, Röntgenbeugung, simultane TG-DTA und chemische Analyse der flüssigen Phase untersucht.

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l₃ · ()₂ Ca₃SiO₄Cl₂ , .

     

Structure and thermal properties of natural colored cottons and bombax cotton

Chemical compositions, crystalline structures and thermal properties of bombax cotton and natural colored cottons including laurel green, bottle green and brown cotton were investigated by chemical analysis, SEM, IR spectra and X-ray diffraction. The results showed that the crystallinity and crystallite sizes of laurel green cotton were lower than those of bottle green cotton because of the excess content of suberin in the former. The crystallinity of brown cotton was similar to that of white cotton, and bombax cotton had the lowest crystallinity but its crystallite orientation was the highest. Thermal property of bottle green cotton was the most stable, whose decomposition temperature was higher of 30°C than that of common white cotton because of its higher lignin content, and bombax cotton had the lowest thermal degradation temperature. But bottle green cotton reached the highest decomposition speed and made decomposition finish within a very short time, and bombax cotton was just the reverse.     

Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N₂/400-600 °C/40-600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O-CH₃ bond homolysis, which was observed at >400 °C, initiated their decomposition. This homolysis was followed by several temperature-dependent reactions; radical-induced rearrangement to convert the aromatic OCH₃ to aromatic CH₃ structures and condensation into high molecular weight (MW) products were the next steps (≈400 °C); then, coke formation became extensive (≈450 °C); effective gas formation required higher temperature such as >550-600 °C. The syringol- and guaiacol-characteristic GC/MS-detectable low MW products were explained with the above mentioned reactions. As for the difference between guaiacol and syringol, coke and gas (especially CH₄ and CO₂) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH₃ group in syringol, which doubles the opportunity for coke formation. This, in turn, reduces the yields of GC/MS-detectable low MW products. Demethoxylation to form guaiacol was also observed in syringol, even though the reactivity was not so high. These reactions are discussed at the molecular level.     

CIDNP of 13C and 1H nuclei in the reactions of cyclohexadienone carbenes

Thermal decomposition of 4-diazo-2,6-di-tert-butyl-2, 5-cyclohexadien-1-one in perhalogenated solvents has been investigated. The decomposition reaction proceeds via a carbene intermediate first to a singlet and then to an effectively triplet free encounter radical pair formed in a halogen abstraction reaction. The polarisation of the stable reaction product is determined by competing processes in the primary cage, intersystem crossing and escape from the cage.     

Mechanisms of the coupling of one-and two-electron reactions in chemical and biochemical processes

The mechanisms of chemical and biochemical reactions in which one-electron stages ae coupled with two-electron stages are examined. The mechanisms of the coupling of the one- and two-electron stages in the electron-transport systems of the living cell are discussed on the basis of an analysis of chemical models. Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev 253094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, pp. 133–144, May–June, 1999.     

The influence of surface reactions on analytical thermogravimetric results

Two examples are given to show the influence of surface reactions on analytical thermogravimetric results. The first concerns the effect of the crucible material on the reduction of rhenium heptoxide by hydrogen. In the second, the effect of a strong metal-support interaction on the evaluation of the average oxidation number of rhodium present in small percentage in the system RhO_xTiO₂ is discussed.     

Dynamics of the accumulation of the galactomannans ofGleditsia macracantha and their influence on the growth of cotton seeds

The dynamics of the accumulation of galactomannans (GMs) in the seeds ofGleditsia macracantha have been studied. Their maximum content was observed during dormancy. It was found that with the development and growth of the fruit the yield and characteristic viscosity of the GMs and the amount of mannose in them increased.Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 171–173, March–April, 1999.     

Exchange reactions on polyarylate-phenoxy blends: Their influence on the physical and mechanical behaviour

The possibility of interchange reactions in blends of the polyarylate of bisphenol A and the polyhydroxyether of bisphenol A is considered by examining the changes in the melt-viscosity for various mixing times. The immiscibility of this blend is confirmed by measurements of the softening temperature. The DSC technique is used to show the variation produced in the structure of the blends as a consequence of the development of interchange reactions. The influence of the exchange process on the mechanical behaviour is shown by the improvement observed in the elastic modulus of the reacted mixtures compared with that of the corresponding unreacted blends.