Molecular properties of the PCO radical: heat of formation and the isomerization pathways

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Monomolecular films of cholesterol oxidase and S-Layer proteins

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

DFT study of the water-assisted tautomerization process between hydrated oxide, MO(H2O)(+), and dihydroxide, M(OH)(2)(+) cations (M = V, Nb and Ta)

Density functional theory (DFT) calculations point out that the participation of water can effectively lower the barrier height for the isomerization process between hydrated oxide cation, MO(H2O)(+), and dihydroxide cation, M(OH)(2)(+), (M = V, Nb and Ta). The catalytic effect is achieved by a water-assisted mechanism in which water acts as proton donor and acceptor, via a transition structure corresponding to a six-membered ring. In the case of vanadium atom, the presence of two water molecules has been taken into account and the tautomerization becomes nearly barrierless, decreasing both the stability of the transition structures relative to intermediates and the depths of wells associated with the intermediates. (C) 2003 Elsevier B.V. All rights reserved.

Thermo-analyses of polyaniline and its derivatives

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Sources of errors in the quantitative analysis of food carotenoids by HPLC

Several factors render carotenoid determination inherently difficult. Thus, in spite of advances in analytical instrumentation, discrepancies in quantitative results on carotenoids can be encountered in the international literature. A good part of the errors comes from the pre-chromatographic steps such as sampling scheme that does not yield samples representative of the food lots under investigation; sample preparation which does not maintain representativity and guarantee homogeneity of the analytical sample; incomplete extraction; physical losses of carotenoids during the various steps, especially during partition or washing and by adsorption to glass walls of containers; isomerization and oxidation of carotenoids during analysis. on the otherhand, although currently considered the method of choice for carotenoids, high performance liquid chromatography (HPLC) is subject to various sources of errors, such as: incompatibility of the injection solvent and the mobile phase, resulting in distorted or split peaks; erroneous identification; unavailability, impurity and instability of carotenoid standards; quantification of highly overlapping peaks; low recovery from the HPLC column; errors in the preparation of standard solutions and in the calibration procedure; calculation errors. Illustrations of the possible errors in the quantification of carotenoids by HPLC are presented.

The generator coordinate method for a reaction coordinate coupled to a harmonic oscillator bath

This paper investigates the usefulness of the generator coordinate method (GCM) for treating the dynamics of a reaction coordinate coupled to a bath of harmonic degrees of freedom. Models for the unimolecular dissociation and isomerization process (proton transfer) are analyzed. The GCM results, presented in analytical form, provide a very good description and are compared to other methods Like the basis set method and multiconfiguration time dependent self-consistent field. (C) 1998 American Institute of Physics. [S0021-9606(98)50934-8].

Caracterização e propriedades catalíticas da zeolita HZSM5 modificada com nióbio

HZSM5 zeolite was modified by exchanging proton by niobium (V). Several samples were obtained with various degrees of exchange. Pore volumes and acidity were measured to characterize these exchanged zeolites. Catalytic properties were evaluated with two reaction tests: m-xylene transformation and n-heptane cracking. The introduction of niobium on HZSM5 zeolite decreases the diffusion coefficient of 2-methyl-pentane and increases the zeolite acidity. The sample containing niobium are initially more active in cracking of n-heptane and m-xylene isomerization than HZSM5 alone.

Isomerismo cis-trans: de Werner aos nossos dias

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Temperature dependence of photoinduced birefringence in polystyrene doped with Disperse Red-1

A theoretical model on the basis of the free-volume concept is presented explaining the temperature dependence of photoinduced birefringence in polystyrene films that contain Disperse Red-1. Birefringence increases with temperature up to 180 K as the free volume for isomerization increases, and then decreases as thermally activated processes dominate. The fast process of birefringence decay has a time constant that increases with temperature at low temperatures, due to the change kin photoisomerization.

Kinetics of photoinduced birefringence in the guest-host system of poly(methyl methacrylate) doped with azobenzene-containing crown ethers

The kinetics of the buildup and decay of photoinduced birefringence was examined in a series of host-guest systems: azobenzene-containing crown ethers, differing in the size of the crowns, dissolved in a poly (methyl methacrylate) matrix. In all samples, the kinetics of the buildup of the birefringence was reasonably described by a sum of two exponential functions, the time constants being inversely proportional to the intensity of the pumping light and the magnitudes of the signals at the saturation level depending on the pumping light intensity and sample thickness. The dark decays were best described by the stretched exponential function, with the characteristic parameters (time constant and stretch coefficient) being practically independent of the type of crown ether. The time constants of the signal decay were orders of magnitude shorter than the respective constants of the dark isomerization of the azo crown ethers, thus indicating that the process controlling the decay was a relaxation of the polymer matrix and/or a rearrangement of the flexible parts of the crowns. (C) 2007 Wiley Periodicals, Inc.

Highly selective nickel ethylene oligomerization catalysts based on sterically hindered tris(pyrazolyl)borate ligands

The reaction of TlTp' (Tp' = HB(3-mesitylpyrazolyl)(3)(-) (Tp(Ms)), HB(3-mesitylpyrazolyl)(2)(5-mesitylpyrazolyl)(-) (Tp(Ms)*)) with NiCl(2).6H(2)O affords Tp(Ms)NiCl (1) and Tp(Ms)*NiCl (2) in good yield. The compound 2 undergoes an isomerization process to form [{Tp(Ms)**}NiCl](2) (3) (Tp(Ms)** = HB(5-mesitylpyrazolyl)(2)(3-mesitylpyrazolyl)(-)) in 68% yield. Treatment of the tris(pyrazolyl)-borate nickel compounds 1 and 2 with alkylaluminum cocatalysts such as methylalumoxane (MAO) and trimethylaluminum (TMA) in toluene generates active catalysts for ethylene oligomerization. The compound 1 shows turnover frequencies in the range of (2.2-43.1) x 10(3) h(-1). Oligomerization reaction conditions can be adjusted that lead to selectivities as high as 81% for butene-1.

Molecular engineering strategies to control photo-induced birefringence and surface-relief gratings on layer-by-layer films from an azopolymer

The interactions governing adsorption of layer-by-layer (LBL) films from an azopolymer, PS-119(R), and poly(allylamine hydrochloride) (PAH) have been controlled by modifying the pH of the solutions used to form the films. The thickness per bilayer was varied by one order of magnitude, from 10 to 240 A, as the pH changed from 4 to 10. Thick layers were formed at higher pHs because in this case the PAH is only partially charged adopting a loopy conformation. This change in molecular conformation caused the kinetics of isomerization and mass transport to be much slower than at lower pHs. The writing time defined as the time to achieve 50% of maximum birefringence dropped from 110 to 18 min for films prepared from solutions with pH 10 and pH 4, respectively. This decrease is probably due to the higher free volume for isomerization in the films prepared at lower pHs, in which PAH molecules are less coiled than at higher pHs. For the same reason, the rate of inscription of surface-relief gratings with an interference pattern of p-polarized light was also much slower at higher pHs. (C) 2003 Elsevier B.V. All rights reserved.

Electrochemistry of organometallic compounds. Part IV. Oxidations of benzylic derivatives and p-substituted benzylic derivatives of bis(dimethylglyoximato) and bis(salicylaldehyde)o-phenylenediimine cobalt(III) in dimethylformamide

The electrochemical oxidation of some p-substituted benzylic derivatives of Co(III) dimethylglyoximato and Co(III)bis(salicylaldehydc)o-phenylenediimine in dimethylformamide. 0.2 M in tetraethyammonium perchlorate, on a platinum electrode, at several temperatures, is described as an ECE type, the first electrochemical step being a quasi-reversible one-electron charge transfer at room temperature. At temperatures around -20°C, or lower, the influence of the irreversible chemical decomposition of the oxidized species, via a solvent or other nucleophilic-assisted reaction, is negligible. It is suggested that at low temperatures the oxidation to the formally CoIV-R species is followed by an isomerization reaction in which this complex is partially transformed in a CoIII-(R) species or a s π-complex which undergoes an electroreduction at less positive potentials than those corresponding to the reduction of the CoIV-R species. © 1982.

Structure and catalytic mechanism of glucosamine 6-phosphate deaminase from Escherichia coli at 2.1 Å resolution

Background: Glucosamine 6-phosphate deaminase from Escherichia coli is an allosteric hexameric enzyme which catalyzes the reversible conversion of D-glucosamine 6-phosphate into D-fructose 6-phosphate and ammonium ion and is activated by N-acetyl-D-glucosamine 6-phosphate. Mechanistically, it belongs to the group of aldose-ketose isomerases, but its reaction also accomplishes a simultaneous amination/deamination. The determination of the structure of this protein provides fundamental knowledge for understanding its mode of action and the nature of allosteric conformational changes that regulate its function. Results: The crystal structure of glucosamine 6-phosphate deaminase with bound phosphate ions is presented at 2.1 Å resolution together with the refined structures of the enzyme in complexes with its allosteric activator and with a competitive inhibitor. The protein fold can be described as a modified NAD-binding domain. Conclusions: From the similarities between the three presented structures, it is concluded that these represent the enzymatically active R state conformer. A mechanism for the deaminase reaction is proposed. It comprises steps to open the pyranose ring of the substrate and a sequence of general base-catalyzed reactions to bring about isomerization and deamination, with Asp72 playing a key role as a proton exchanger.

Influência da concentração de antioxidante na estabilidade do ácido retinóico

Retinoic acid, which has been prescribed for some forms of acne since 1962, is subject to isomerization provoked by atmospheric oxygen and ambient light. Therefore it is very important to keep this material intact through the use of antioxidants. We tested two antioxidants: various concentrations of butylhydroxytoluene or BHT, widely used in cosmetic and dermatological preparations, and vitamin E, found in grape oil, an ingredient of skin lotion. Recently prepared lotions were subject to heat stress and the residual concentrations of retinoic acid determined at predetermined intervals through UV spectrophotometry. The shelf life of the various preparations were determined by the Arrhenius equation. The antioxidants helped conserve the retinoic acid, however, there was no synergism in antioxidant activity between the vitamin E in the grape seed oil and BHT.