Involvement of proton transfer in the reductive repair of DNA guanyl radicals by aniline derivatives

The most easily oxidized sites in DNA are the guanine bases, and major intermediates produced by the direct effect of ionizing radiation (ionization of the DNA itself) are electron deficient guanine species. By means of a radiation chemical method (gamma-irradiation of aqueous thiocyanate), we are able to produce these guanyl radicals in dilute aqueous solutions of plasmid DNA where the direct effect would otherwise be negligible. Stable modified guanine products are formed from these radicals. They can be detected in the plasmid conversion to strand breaks after a post-irradiation incubation with a DNA base excision endonuclease enzyme. If aniline compounds are also present, the yield of modified guanines is strongly attenuated. The mechanism responsible for this effect is electron donation from the aniline compound to the guanyl radical, and it is possible to derive rate constants for this reaction. Aniline compounds bearing electron withdrawing groups (e.g., 4-CF3) were found to be less reactive than those bearing electron donating groups (e.g., 4-CH3). At physiological pH values, the reduction of a guanyl radical involves the transfer of a proton as well as of an electron. The mild dependence of the rate constant on the driving force suggests that the electron is not transferred before the proton. Although the source of the proton is unclear, our observations emphasize the importance of an accompanying proton transfer in the reductive repair of oxidative damage to guanine bases which are located in a biologically active double stranded plasmid DNA substrate.     

Ecoulements pulsés de fluides inelastiques en conduites déformables

Résumé Le présent travail concerne l'étude des propagations d'ondes et des champs de vitesses en écoulements pulsés newtoniens et pseudoplastiques en conduites cylindriques souples. Trois méthodes ont été utilisées: une méthode analytique pour les écoulements newtoniens; une méthode numérique aux différences finies dans tous les cas Ostwaldiens; une méthode expérimentale utilisant la vélocimétrie à laser. Les résultats obtenus à l'aide de ces méthodes sont confrontés entre eux et les influences des paramètres rhéologiques et hydrodynamiques sur ces écoulements sont analysées.

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The purpose of this work consists in the study of the pulsating flows of inelastic fluids in non-rigid tubes. The evolution of velocity profiles and the propagation of the pressure wave have been obtained by three methods of investigation: an analytical method, a numerical method and an experimental method based on Laser-Doppler anemometry applied to aqueous solutions of high polymers. The influences of rheological and hydrodynamical parameters have been analysed and compared.

     

Purified oxygenand nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation

Oxygen-and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygenand nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis.