Conformation of 6-methyluridine--a pyrimidine nucleoside in the syn conformation.

Abstract

IN a nucleoside the rotation of the nucleobase about the glycosidic bond relative to the sugar moiety is sterically hindered and two conformational ranges, syn and anti, are preferred1. Theoretical2–5 and spectroscopic6–8 investigations suggest that the anti conformation is slightly more energetically favoured than the syn. In the crystalline state, this is also true for purine nucleosides but all pyrimidine nucleosides, except 4-thiouridine9 (a tRNA minor constituent), crystallize in anti conformation. 4-Thiouridine may occur in syn conformation because of the stabilization by the peculiar hydrogen bonding and packing scheme or the particular C(3′)-endo-C(4′)-exo puckering* of the ribose unit. The correlation between conformation about the glycosidic bond and the sugar puckering in nucleosides was further studied by X-ray analysis of 6-methyluridine (Fig. 1). This species, according to nuclear magnetic resonance studies6, exists in the syn conformation in aqueous solution because of the bulky methyl group in position 6 of the pyrimidine ring.