Influence of uracil photohydrate formation on the conformational properties of heterodinucleoside monophosphates

Abstract

AbstractThe structural properties of uracil photohydrates at the monomer and dimer level in aqueous solution have been examined in detail by nmr spectroscopy. Based on such evidence, the absolute configurations of the two possible diastereomers have been assigned, and the conformational perturbations induced by photohydration have been evaluated. In all instances, photohydration shifts the 2E ⇌ 3E puckering equilibrium of the sugar ring of the uridylyl fragment towards 2E (from 12–18%). In addition, for both dimers examined in detail, ho6UpA and Apho6hU, the effect of dimerization on sugar pucker is such that the 3′‐terminal unit shows a clear increase in the percentage of 3E (relative to the appropriate 5′‐mononucleotide), whereas the percentage 3E of the 5′‐terminal unit shows no change. This is contrary to the findings in the normal dinucleoside monophosphates, where an increased preference for 3E pucker occurs in both residues on dimerization and increased base stacking. Significant base–base interactions were observed in both hydrated dimers despite the loss of the planar π‐system in the uracil fragment. In addition, the rate of photohydration for a particular dimer pair (e.g., ApU and UpA or GpU and UpG) is shown to be inversely dependent on the amount of base stacking in the parent dimer. This latter parameter has also been correlated with the ratio of the two possible diastereomers formed in the reaction and is associated with a preferential attack at one face of the pyrimidine base ring. The shift of the sugar puckering equilibrium towards 2E has been compared with similar shifts observed when adenosine and guanosine are methylated at N(1) and N(7), respectively. The possible biological significance of the above‐mentioned conformational aspects is discussed.