3-Deazauridine: Crystal Structure and Conformation

THE nucleoside analogue 3-deazauridine^1,2 (Fig. 1), 4-hydroxy-1-(β-D-ribo-pentafuranosyl)-2-pyridone, has powerful cytostatic properties; at a concentration of 5 × 10^?6 M it inhibits the growth of Ehrlich ascites cells by 50% (ref. 3). In the codon-specific binding of tRNA^phe to ribosomes, 3-deazauridine can substitute for uridine in the 5′ - and middle position of the codon but not in the wobble position⁴. NMR evidence^1,2 suggests that the major form in solution has a 4-hydroxyl group but a dynamic enol-keto tautomerism is taking place. The present X-ray diffraction study was undertaken to determine the tautomeric form present in the crystalline state and to investigate the hydrogen bonding possibilities of 3-deazauridine. We find the enol form with an unusually short hydrogen bond between the 4-hydroxyl group and the 2-keto oxygen of a neighbouring molecule.