Influence of the polarity of the microenvironment of pyrene on the intensity of its solid-phase luminescence

The sorption of pyrene by a filter paper modified with lead acetate is studied by luminescence methods. Based on changes in the vibrational structure of fluorescence spectra of pyrene, it was found that the polarity of the microenvironment of pyrene adsorbed from solutions in ethanol increases with the concentration of acetic or hydrochloric acid in these solutions. It was revealed that, when an acid is added, pyrene fluoresces and phosphoresces more intensely at room temperature. It is shown that this is associated with a more efficient sorption of pyrene by the filter paper from acid solutions in ethanol.     

Tuning the interaction energy of hydrogen bonds: the effect of the substituent

The effect of the substituent R in the hydrogen bonding properties of FH···FR (R = H, Al, Li, Cl and CCH) complexes has been studied by theoretical calculations. The dependency of the interaction energy with the hydrogen bond distance and R is explained in terms of the topologies of the electron density and the electrostatic potential. A simple model of the hydrogen bond interaction energy, which can be assimilated to an interaction potential, is defined in terms of a stabilizing mutual polarization of the monomers and an overall destabilizing contribution associated with the electron density reorganization when the overlap of the closed shells is large enough. This model shows an excellent agreement with the ab initio interaction energies and is common for all the analyzed complexes. The substituent effect is represented in the model by a single parameter that can be calculated from the electron distribution in the acceptor atom region. The perturbation in the hydrogen bonding interaction induced by the change of R presents a close similarity with that produced by an external electric field of the same order of magnitude than those found in crystalline solids, indicating that both perturbations should play a significant and similar role on the properties of hydrogen bonds in condensed matter.     

On the role of the conserved aspartate in the hydrolysis of the phosphocysteine intermediate of the low molecular weight tyrosine phosphatase

The usual rate-determining step in the catalytic mechanism of the low molecular weight tyrosine phosphatases involves the hydrolysis of a phosphocysteine intermediate. To explain this hydrolysis, general base-catalyzed attack of water by the anion of a conserved aspartic acid has sometimes been invoked. However, experimental measurements of solvent deuterium kinetic isotope effects for this enzyme do not reveal a rate-limiting proton transfer accompanying dephosphorylation. Moreover, base activation of water is difficult to reconcile with the known gas-phase proton affinities and solution phase pK(a)'s of aspartic acid and water. Alternatively, hydrolysis could proceed by a direct nucleophilic attack by a water molecule. To understand the hydrolysis mechanism, we have used high-level density functional methods of quantum chemistry combined with continuum electrostatics models of the protein and the solvent. Our calculations do not support a catalytic activation of water by the aspartate. Instead, they indicate that the water oxygen directly attacks the phosphorus, with the aspartate residue acting as a H-bond acceptor. In the transition state, the water protons are still bound to the oxygen. Beyond the transition state, the barrier to proton transfer to the base is greatly diminished; the aspartate can abstract a proton only after the transition state, a result consistent with experimental solvent isotope effects for this enzyme and with established precedents for phosphomonoester hydrolysis.     

On evaluate of the integral methods for the determination of the activation energy

Recently, Órfão obtained two simple equations for the estimation of the relative error in the activation energy calculated by the integral methods [2]. In this short communication, the validity of the equations has been evaluated by comparing the results calculated by the equations with the results calculated by the equation from theoretical derivation without introducing any assumption.     

Determination of hydroxyproline in tissues and the evaluation of the collagen content of the tissues

The concentrations of hydroxyproline (an amino acid specific of collagen) in a number of connective tissues were determined. Two procedures were compared. In one of them, amino acids were preseparated by chromatography and then determined on a standard amino acid analyzer. In the other procedure, hydroxyproline was selectively oxidized without amino acid separation and determined by a spectrophotometric reaction with Ehrlich’s reagent. Data obtained for purified collagen preparations in accordance with the two procedures were consistent with each other. The results can be somewhat different in unpurified preparations and tissues because of the presence of polysaccharide components in the tissues.     

The effects of the pause to pulse ratio in the regime of pulsating overpotential on the formation of honeycomb-like structures

The formation of honeycomb-like structures in the pulsating overpotential (PO) regime was examined by scanning electron microscopy (SEM). The honeycomb-like structures obtained with a square-wave PO of different pause to pulse ratios were compared to the one obtained in the constant potentiostatic regime. Increasing the pause to pulse ratio led to a decrease of the diameter of the holes formed by attached hydrogen bubbles, as well as to an increase in the number of holes formed at the surface area of copper electrodes. The size of the agglomerates of copper grains, of which the walls of the holes were constructed, was reduced with increasing duration of the pause. Also, the uniformity of the honeycomb-like structures was increased by application of the PO regime. It was shown that the effects attained by application of this regime were comparable with those obtained by electrodeposition in a constant potential regime with the addition of specific additives.     

On the volume-dependence of the index of refraction from the viewpoint of the complex dielectric function and the Kramers-Kronig relation

How indices of refraction n(omega) of insulating solids are affected by the volume dilution of an optical entity and the mixing of different, noninteracting simple solid components was examined on the basis of the dielectric function epsilon(1)(omega) + iepsilon(2)(omega). For closely related insulating solids with an identical composition and the formula unit volume V, the relation [epsilon(1)(omega) - 1]V = constant was found by combining the relation epsilon(2)(omega)V = constant with the Kramers-Kronig relation. This relation becomes [n(2)(omega) - 1]V = constant for the index of refraction n(omega) determined for the incident light with energy less than the band gap (i.e., h omega < E(g)). For a narrow range of change in the formula unit volume, the latter relation is well approximated by a linear relation between n and 1/V.     

The influence of the addition of electrolytes on the degree of dispersion of silver iodide hydrosol

The changes in the degree of dispersion of AgI hydrosols produced by reaction of AgNO₃ and KI with the addition of other electrolytes (1–60 min after addition of AgNO₃ to KI) were investigated by photometric methods. The results obtained suggest that in the process of rapid condensation (caused by a sudden increase in concentration to a value considerably higher than the critical one) colloidal particles do not form by continuous growth of individual nuclei but through disaggregation of loose and amorphous, i. e. thermodynamically unstable clusters of previously formed embryos. This disaggregation process, termed protopeptization by the authors in order to underline its analogy with the peptization process, increases in magnitude with the strengthening adsorption of potential-determining ions (protopeptizing agents) by the particles formed in the precipitation process.     

On the influence of ion exchange on the local structure of the titanosilicate ETS-10

The effect of ion exchange with different monovalent cations (NH(4)(+), K(+), Na(+) and Cs(+)) on the local structure of the titanosilicate ETS-10 has been studied by (29)Si MAS NMR and Raman spectroscopy. Although X-ray diffraction shows no significant influence of ion exchange on the long range order, ammonium exchange is found to result in substantial damage to the local structure. Ion exchange experiments with alkali cations under significantly more acidic conditions clearly show that the structural damage brought about by ammonium exchange is not caused by the low pH of the exchange solution. The exchange with potassium and caesium ions also leads to significant changes in the (29)Si NMR and Raman spectra. However, these changes can largely be reversed by sodium back-exchange.     

Role of the conformation in the reactivity of 1,3-diphosphapropenes

The symmetrical and unsymmetrical 1,3-diphospha-propenes 3 and 4 were obtained from the corresponding diposphiranes 1 and 2 . The chemical behavior of these compounds has been studied. Phosphonium-phosphaalkenes 7a and 10a have been obtained in the reactions with aluminium trichloride. Whereas the symmetrical diphosphaallene 13 can be obtained by reaction of 1 or 3 with lithio compounds, the unsymmetrical diphosphaalene 14 cannot be prepared by a similar route. Reduction of 3a and 4a (obtained with a different conformation) by lithium aluminum hydride afforded phosphino-phosphaalkenes 17a and 18a (with a similar conformation); further dehydrochlorination with amines led to the symmetrical and unsymmetrical diphosphaalenes 13 and 14 , respectively. The formation of allenes strongly depends on the conformation of the starting diphosphapropenes.     

QM/MM study of the role of the solvent in the formation of the charge separated excited state in 9,9'-bianthryl

In this paper the role of the solvent in the formation of the charge-separated excited state of 9,9'-bianthryl (BA) is examined by means of mixed molecular mechanical/quantum mechanical (QM/MM) calculations. It is shown that in weakly polar solvents a relaxed excited state is formed with an interunit angle that is significantly smaller than 90 degrees . This relaxed excited state has a considerable dipole moment even in weakly polar solvents; for benzene and dioxane dipole moments of ca. 6 D were calculated, which is close to experimental data. These dipoles are induced by the solvent in the highly polarizable relaxed excited state of BA, and the dipole relaxation time is governed by solvent reorganizations. In polar solvent the charge separation is driven to completion by the stronger dipoles in the solvent and a fully charged separated excited state is formed with an interunit angle of 90 degrees.     

Modeling the stereoselectivity of the Johnson-Claisen rearrangements in the Danishefsky synthesis of gelsemine

[reaction: see text] The stereochemistries of [3,3] sigmatropic Johnson-Claisen (J-C) rearrangements of six intermediates studied in the synthesis of gelsemine were modeled using DFT methodology. The possible origins of the rearrangement stereoselectivity are explored and compared with the experimentally suggested rationalizations by Danishefsky et al. (J. Am. Chem. Soc. 2002, 124, 9812-9824). In the intermediate used for the J-C rearrangement in the Danishefsky synthesis (3), the closure is inhibited by the repulsive interactions between the enolate terminus and the carbon atoms of the double bond as well as with the hydrogen on C7. The closure is favored by stabilizing interactions between the enolate terminus and the H's of the oxetane ring.     

Annelation of the Pyrrole to the Steroid Skeleton Using the Trofimov Reaction

The product of annelation of the N-vinyl pyrrole ring with steroid skeleton of 5-cholestene was obtained in a single regioselective step by the reaction of 5-cholesten-3-one oxime with acetylene in KOH-DMSO by the Trofimov reaction.     

Identification of the Enzymes Mediating the Maturation of the Seryl-tRNA Synthetase Inhibitor SB-217452 during the Biosynthesis of Albomycins

Albomycin δ₂ is a sulfur-containing sideromycin natural product that shows potent antibacterial activity against clinically important pathogens. The l -serine-thioheptose dipeptide partial structure, known as SB-217452, has been found to be the active seryl-tRNA synthetase inhibitor component of albomycin δ₂. Herein, it is demonstrated that AbmF catalyzes condensation between the 6′-amino-4′-thionucleoside with the d -ribo configuration and seryl-adenylate supplied by the serine adenylation activity of AbmK. Formation of the dipeptide is followed by C3′-epimerization to produce SB-217452 with the d -xylo configuration, which is catalyzed by the radical S-adenosyl-l -methionine enzyme AbmJ. Gene deletion suggests that AbmC is involved in peptide assembly linking SB-217452 with the siderophore moiety. This study establishes how the albomycin biosynthetic machinery generates its antimicrobial component SB-217452.     

Composition of the triacylglycerols of the seeds of some representatives of the family labiatae

The fatty acid compositions of five species and the compositions of the triacylglycerols of 22 species of the family labiatae have been studied for the first time. Octadeca-ε12,13-dienoic acid has been detected in five species. The typical compositions of the triacylglycerols differs from those of known plant oils with a similar set of fatty acids by the absence of triacylglycerols of the S₃ type and the presence of the S₂U type (0.1–1.6%). The main types are SU₂ (5–24%) and U₃ (74–95%). In a comparison of the position-species composition of the oils studied it was found that the oils of the plants of this family are distinguished by a greater diversity of species of triacylglycerols and also by the nature of the distribution of the unsaturated acyl residues between the 1,3- and 2-positions. In the majority of oils studied, the 2- position is enriched with the 18:1 acid, while the 18:2 acid is distributed predominantly in the 1,3- positions, and the nature of the distribution of the 18:3 acid is determined by its proportion in the total.     

Dependence of ion hydration on the sign of the ion's charge

The solvation of simple ions in water is studied using molecular dynamics simulations with a polarizable force field. Previous simulations using this potential demonstrated that anions are more favorably solvated in water than cations. The present work is an attempt to explain this result by examining the effects of ions on the surrounding water structure, with particular focus on the first solvation shell and its interactions with the surrounding water. We conclude that while the first solvation shell surrounding cations is frustrated by competition between ion-water and water-water interactions, solvation of anions is compatible with good water-water interactions.     

Re-examination of the Kinetics of the Thermal Dehydroxylation of Goethite

The kinetics of thermal dehydroxylation of aluminuous goethites [1] synthesised from a ferrous salt has been re-examined using the general reaction order kinetic law. The utilised data processing was based on the procedures employed by dissolution kinetics. Recalculation of the activation energies E_A of the dehydroxylation yielded the values 130, 132, 128, and 123 kJ mol⁻¹ for pure goethite, goethite with 10, 20, and 30 mol% Al substitution, respectively. The values of E_A are in a good agreement with those given for goethite in literature. The E_A values are linearly related with the chemically bound excess H₂O/OH⁻ in the crystal lattice that is apparently influenced by Al substitution. This revised version was published online in August 2006 with corrections to the Cover Date.     

电流型葡萄糖传感器阴极的研究

本文研究了石墨、银、钛基RuO2 /TiO2 涂层材料等几种导电材料取代铂作为葡萄糖传感器阴极的可能性 .结果表明 ,钛基RuO2 /TiO2 涂层电极的化学性质稳定 ,对析氢反应有一定的催化作用 .由石墨_环氧胶粘剂混合物—钛基RuO2 /TiO2 组成的电化学体系可以在 0 .6～ 0 .8V电压范围内检测H2 O2 的稳态氧化电流 ;在 0 .7V电压下 ,以钛基RuO2 /TiO2 涂层材料作为阴极的葡萄糖传感器的性能与以铂片为阴极的传感器性能接近 ,钛基RuO2 /TiO2 涂层材料是取代铂的最佳阴极材料 .     

Influence of the solvent on the quenching of the fluorescence of anthracene by aromatic amines

The influence of the solvent on the quenching of the fluorescence of anthracene by diethylaniline (DEA) and aniline has been investigated. Fluorescence quenching in these systems obeys the equation of Stern and Volmer. A transition from kinetics-controlled to diffusion-controlled processes takes place with the increase in viscosity () in nonpolar solvents. In polar solvents the quenching rate is limited by diffusion over the whole viscosity range investigated. These peculiar effects of the solvent can be explained by the traditional three-stage quenching scheme of the singlet-excited anthracene by electron donors. It is also possible that the increase in the quenching constant () in polar as compared to nonpolar solvents with the same is related to an increase in the rate constant of non-emitting deactivation in polar media, induced by the quenching agent. In the case of the quenching of fluorescence of anthracene by aniline the process is diffusion-controlled, regardless of the nature of the solvent.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 370–374, May–June, 1985.     

Influence of the particle size distribution on the CRTA curves for the solid-state reactions of interface shrinkage type

The kinetic curves at infinite temperature for the solid-state reactions of the interface shrinkage type were drawn theoretically by taking account the particle size distribution in the sample mixture. The CRTA curves for the reactions with the particle size distribution can be drawn by utilizing the universal kinetic curves at infinite temperature. The proper kinetic treatment for the CRTA curves with the particle size distribution is discussed in connection with the property of the kinetic equation with respect to the particle size distribution. The present kinetic consideration is taken as a simulation for the reactions with a certain distribution in among the reactant particles, produced preferably by the mass and heat transfer phenomena during the thermoanalytical measurements. The merit of the rate jump method by a single cyclic CRTA curve is also discussed on the basis of the present results.