A novel vicinal lesion obtained from the oxidative photosensitization of TpdG: characterization and mechanistic aspects.

A new type of vicinal base lesion was isolated from the photosensitization of TpdG in aerated aqueous solution. One- and two-dimensional NMR measurements were used together with mass spectrometry to accurately characterize the new adduct. Chemical detection of guanidine provided additional structural information on the base moiety at the 3'-OH terminal end. Altogether the experiments results were indicative of the occurrence of a covalent bonding between the pyrimidine ring on the 5'-OH terminal end and the imidazole ring on the 3'-OH terminal end through a methylene bridge. Photosensitization studies of TpdG, thymidine, and 2'-deoxyguanosine in the presence of either benzophenone, menadione, or riboflavin associated with isotopic labeling experiments using enriched oxygen and water provided relevant information on the mechanism of formation of the adduct. The results of these experiments clearly demonstrated that the initial event leading to the formation of the lesion is the abstraction of a hydrogen atom from the methyl group of the thymine base moiety of TpdG. This is followed by the addition of the methyl-centered radical to the C-4 atom of the guanine ring which gives rise to the vicinal lesion after reaction with molecular oxygen and subsequent rearrangement.

Conformational properties of a cycle oligosaccharide: cyclotrikis-(1-->6)-[alphaD-glucopyranosyl-(1-->4)-beta-D-glucopyranosy l].

The title compound is a cyclic oligosaccharide having six glucopyranose residues linked alternatively by alpha-(1-->4) and beta-(1-->6) glycosidic linkages. Like cyclodextrin analogues it is expected to exhibit an internal cavity and to form inclusion complexes with other species. In order to investigate its conformational preferences, an extensive conformational search was carried out using a combination of Metropolis Monte-Carlo (MMC) procedure in the glycosidic torsion angle space and molecular mechanics procedures. To this end a specific program (METROCYCLIX) was developed. To reduce the MMC search, conformational maps of parent disaccharides were considered as starting entries. Fully minimized conformations were gathered into families using a clustering technique based on RMS fitting over the glycosidic torsion angle values. A wide range of local energy minima were identified in spite of ring closure conditions that constrained the structure of the oligosaccharide. Low energy conformers were stabilized by intramolecular interactions between distant residues. From the Bolzmann population of the best structures derived from the clustering results, various average properties were calculated and compared with experimental data obtained by high resolution NMR. Interpretation of these experimental values (heteronuclear coupling constants, rotating frame nuclear Overhauser effects, relaxation times) relies on the use of Karplus like equations (coupling constants) and analysis of the full relaxation rate matrix treatment (ROE). The quality of the molecular modelling strategy used is assessed by the agreement obtained between calculated and measured observables.