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. Theoretical^2–5 and spectroscopic^6–8 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.