The high-anti conformation of 7-halogenated 8-aza-7-deaza-2 '-deoxyguanosines: A study of the influence of modified bases on the sugar structure of nucleosides

The conformation of the 7-bromo- and 7-iodo-substituted 8-aza-7-deazapurine nucleosides 1 and 2 in the solid state and in aqueous solution was studied by single-crystal X-ray analyses and by H-1-NMR spectroscopy. In the solid state, both compounds display a high-anti conformation around the glycosyl ic bond, and their 2'-deoxy-beta-D-ribofuranose moieties adopt an N-type su gar puckering. The orientation of the exocyclic C(4')-C(5') bond was found to be crp in both cases. In D2O solution, both compounds display i) an 8-10 % higher N-conformer population than 2'-deoxyguanosine and ii) a preference of the -sc conformation about the C(4')-C(5') bond. A comparative study on the influence of modified bases on the sugar structure of nucleosides is m ade.