C8-AMINO PURINE NUCLEOSIDES - WELL-DEFINED STERIC DETERMINANT OF GLYCOSYL CONFORMATIONAL PREFERENCES

PMR measurements were performed on dilute solutions of adenosine and guanosine and their 8-NH2, 8-NHCH3, 8-N(CH3)2 and 8-bromo derivatives. The chemical shift of the ribose C2''-H and especially the difference in chemical shifts between the C1''-H and C2''-H resonances clearly indicate whether the nucleoside exists in a syn glycosyl conformation (the C8-dimethylamino derivatives) or as a flexible syn-anti mixture (the monomethylamino and amino derivatives). The temperature dependent behavior of these indicators can be employed to define qualitative shifts in syn-anti equilibrium with temperature. An increased C1''-H-C2''-H chemical shift separation implies shift to more anti, a decreased separation a shift to more syn conformers.