A Proton Magnetic Resonance Study of the Molecular Conformation of a Modified Nucleoside from Transfer RNA. Dihydrouridine

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Abstract
The 220 MHz nuclear magnetic resonance (N.M.R.) spectra of dihydrouridine at 23 °C and 60 °C have been analyzed. From the N.M.R. data a conformational model is derived. The ribose moiety indicates a slight preference for the C3′exo and C2′endo conformations, although rapid interconversion between ring-puckered conformers is taking place. At high temperature the preference for C3′exo and C2′endo is less. The conformation of the exocyclic hydroxymethyl group is a rapid time average of all possible conformers with a preference for the gauche-gauche rotamer. By comparison with β-cyanuric acid riboside the chemical shifts of the ribose moiety in dihydrouridine are not influenced by the diamagnetic anisotropy of the keto group on the base, demonstrating that the base has the anti conformation with respect to the ribose ring. The dihydrouracil base does not exist in any of the classical puckered conformations, but manifests an average torsional angle of approximately 30° apparently due to rapid interconversion between various conformations. This distinguishes dihydrouridine from all other naturally occurring nucleosides, and may be the reason for its occurrence in almost all known tRNA species.

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