Silica-borax pearl samples impregnated with 0.17 and 0.64% Cr3+ were characterized by specific surface area measurements, UV-Vis spectroscopy, energy-dispersive X-ray fluorescence and laser-scanning confocal microscopy. Pearl stability against oxidizing conditions was tested by adding samples to an aqueous hydrogen peroxide solution. The reaction was examined by UV-Vis spectroscopic measurements of the supernatant and laser-scanning confocal microscopy images of the substrate. Overall, hydrogen peroxide-induced Cr3+ to Cr6+ oxidation across the solid-liquid interface promoted solid matrix cleavage pearl degradation and concomitant formation of multiple scattering centers was observed. A dual-detection scheme was employed in the confocal microscopy measurements allowing us to separate scattering and absorptive contributions to the observed signals. The confocal microscopy images indicate that Cr3+ oxidation induced by hydrogen peroxide solutions occurs throughout the entire pearl sample and indicate that oxidation reactions induce leakage of chromate ion into aqueous solutions.

Palladium nanoparticles (NPs) stabilized by ImS3-14, a zwitterionic surfactant structurally related to ionic liquids, are revealed here to be good catalysts for the reduction of a large number of substituted aromatic nitro compounds. Our mass spectrometry results are consistent with the formation of amino products in a direct route, where the aromatic nitro compounds are initially reduced to nitroso compounds, which are then reduced to the hydroxylamine derivatives and finally to the anilines. Activation parameters showed that for most Pd catalysts reported in the literature, the mechanism seems to be similar, with lower enthalpy of activation (ΔH‡) being compensated by more negative entropy of activation (ΔS‡). As a result, the reaction is thermally compensated and the rate constants for most reactions rather similar. Furthermore, Pd NPs stabilized by ImS3-14 showed efficient catalytic activities for the reduction of aromatic nitro compounds, with high conversion and good selectivity even using very low loadings of metal.

A reação entre um novo derivado de imidazol, 3,5-bis((1H-imidazol-1-il)metil)anilina (BIm), e o triéster de fosfato dietil 2,4-dinitrofenil fosfato (DEDNPP), que procede por um mecanismo SN2(P), é substancialmente acelerada em presença do surfactante catiônico brometo de cetiltrimetrilamônio (CTAB) e do surfactante aniônico dodecil sulfato de sódio (SDS). Tal efeito micelar atípico ilustra o emprego de efeitos hidrofóbicos dos reagentes para otimizar o efeito catalítico de micelas aquosas. Tal observação é importante e útil no planejamento de modelos miméticos de reações biológicas, já que a melhoria da incorporação dos reagentes aumentará efetivamente a qualidade dos sistemas biomiméticos, sendo que ambos os surfactantes catiônico e aniônico poderão atuar como nanoreatores homogêneos, concentrando os reagentes neutros hidrofóbicos.

The reaction of a new imidazole derivative, 3,5-bis((1H-imidazol-1-yl)methyl)aniline (BIm), and the model triester diethyl 2,4-dinitrophenyl phosphate (DEDNPP), which proceeds via a SN2(P) mechanism, is substantially enhanced in the presence of either cationic cetyltrimethylammonium bromide (CTAB) or anionic sodium dodecyl sulfate (SDS). This unusual micellar effect illustrates the use of the hydrophobic character of the reactants to enhance micellar catalysis. This is an important and useful guideline in planning mimics for biological reactions, because improving incorporation will increase the quality of the biomimetic systems, and both cationic or anionic surfactants will act as homogeneous nanoreactors concentrating the neutral, hydrophobic reactants.

This article provides an overview on the recent achievements to combat Gram-positive bacteria and the mechanisms related to antimicrobial activity and bacterial resistance. Selected synthetic methodologies to access structurally diverse bioactive compounds are presented in order to emphasize the most important substances currently developed to overcome multiresistant strains. The main properties of vancomycin and related glycopeptide antibiotics are also discussed as a background to understanding the design of new chemotherapeutic agents.

O estudo da sorção e dessorção de Cd(II) na interface de vários sorventes e soluções aquosas mostrou que a capacidade de troca depende do tipo de sorvente, do pH e do sistema tampão. Tampões como succinato e citrato, os quais são bons complexantes de Cd(II) suprimem completamente a incorporação dentro dos diferentes sorventes. Na presença do tampão Tris, a capacidade de troca observada decresce na seguinte ordem, gama-alumina > montmorilonita > caolinita <FONT FACE=Symbol>»</FONT> sílica. A precipitação de Cd(II) torna-se importante na presença de bicarbonato aquoso na faixa de pH 5,5-7. As isotermas observadas para as diferentes argilas mostram saturação em concentração bastante altas de Cd(II). Isto sugere que em águas naturais, a sorção e distribuição de Cd(II) entre as fases geoquímicas dos sedimentos deve seguir uma relação linear. A cinética de dessorção de Cd(II) da superfície da montmorilonita, na presença do íon citrato, mostra que este é um ligante orgânico eficiente para remover o Cd(II) ligado em sítios facilmente acessíveis. O processo ocorre em duas etapas, com liberação inicial de aproximadamente 70% do Cd(II), seguido pela liberação lenta do metal de sítios com acesso controlado pela difusão.

The sorption and desorption of Cd(II) that occurs at the interface between several sorbents and aqueous solutions has been investigated and it is shown that the observed exchange capacity depends on the type of sorbent, the pH and the buffering system. Buffers such as succinate and citrate, which are good complexants of Cd(II) completely supress its incorporation into the different sorbents. In the presence of Tris buffer the observed exchange capacity decreases in the order gamma-alumina > montmorillonite > kaolinite <FONT FACE=Symbol>»</FONT> silica, while precipitation is important in the presence of aqueous bicarbonate in the pH range 5.5-7. The sorption data fit with general purpose isotherms well, and the sorption isotherm shows saturation at rather high concentrations of Cd(II) in both clays. This suggests that under many experimental conditions in natural waters, the sorption and distribution of Cd(II) between geochemical phases of sediments should follow a linear relationship. The kinetics of desorption from montmorillonite, in the presence of citrate ion, is particularly important, showing that this efficient organic ligand removes all the Cd(II) bound to readily accessible sites in a fast step, while removal of Cd(II) from sites with diffusion-restricted access is much slower.

This work summarizes important aspects of the chemistry of phosphate esters, with special emphasis on: i) advances in the design of enzymatic models for phosphatases; and ii) nerve agents and currently used strategies for detoxification. The physical organic chemical aspects that are responsible by the reactivity of phosphate esters are also discussed.

As constantes relacionadas ao equilíbrio entre calix[4]areno e aminas alifáticas em acetonitrila foram medidas por métodos condutivimétrico e espectrométrico e as estruturas dos sais estudadas por espectroscopia de RMN. Em concentrações próximas a 10-4 mol L-1 predominam os íons livres, enquanto que a 10-2 mol L-1 aumenta a proporção de pares iônicos. Os valores das constantes para as reações ácido-base permitem avaliar o valor de pKa do calix[4]areno em acetonitrila, igual a 16,6 unidades de pKa. A análise do conjunto de dados está de acordo com a proposta de complexos com o íon amônio em posição exo-calix.

The constants related to the equilibria between calix[4]arene and aliphatic amines in acetonitrile were measured by conductimetric and spectrophotometric methods, and the structures of the salts studied by NMR spectroscopy. In concentrations about 10-4 mol L-1, free ions are at higher proportion, whereas at 10-2 mol L-1 there is an increase of ion-pairs proportion. The values of the constants found for acid-base reactions allow an evaluation of the value of pKa for calix[4]arene in acetonitrile as 16.6 pKa units. The analysis of data agrees with a proposal of exo-calix ammonium ion.

Ésteres de N-ftaloil amino ácidos e p-terc-butilcalix[4]areno foram sintetizados, e somente os produtos bis-substituídos em conformação 1,3 alternada foram isolados. Os ésteres de aminoácidos quirais sofreram racemização completa ou parcial no meio reacional (Et3N/CH2Cl2).

1-3-Diesters of p-tert-butylcalix[4]arene were obtained upon reaction with N-phtaloyl amino acid chlorides. Reactions with chiral amino acids were accompanied by complete or partial racemization in the reaction media (Et3N/CH2Cl2).

A broad picture of the graduate programs in chemistry in Brazil is analyzed from the data collected during the period 1996/1997 for evaluation by CAPES. The results are compared with those from previous evaluations, indicating significant quantitative and qualitative progresses.

The chemical and biological properties of energy-rich phosphate compounds, e.g. ATP and acetyl phosphate, were revised. The role of water in the formation of this class of energy-rich compounds in biological systems is also discussed.

A on-line thermostatization system that use simples materials, for flow injection and continuous flow analysis is described. The proposed system showed good performance between 10 to 40ºC.

This review gives a critical idea on the importance of intramolecular reactions as models for enzymatic catalysis. Intramolecular lactonizations, ester and amide hydrolysis studies result in theories which try to explain the difference between intermolecular, intramolecular and enzyme reactions and rationalize the enhancement promoted by these biological catalyst.