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

Abstract

100 MHz proton magnetic resonance measurements were performed on dilute solutions of adenosine and guanosine and their 8-NH 2, 8-NHCH 3, 8-N(CH 3) 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.