C8-amino purine nucleosides. A well-defined steric determinant of glycosyl conformational preferences

100 MHz proton magnetic resonance measurements were performed on dilute solutions of adenosine and guanosine and their 8MH2, 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 indicated 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 shifs 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.