Abstract
Molecularβorbital calculations are used to compare the hydrogenβbonding characteristics of the two carbonyl groups of uridine nucleosides. Various numbers of water molecules are allowed to interact with uracil. The hydrogenβbond energies and the effects of these bonds on the electronic structure of uracil provide no evidence of a significant difference between the two carbonyls in terms of intrinsic properties. Calculations are also performed upon MMU, a derivative of the uridine nucleoside and a close analog of a system studied previously by nmr measurements. Energy minima are located on the conformational energy surface. The calculated antiβgg global minimum is quite similar to the xβray structure. Electronic parameters examined in the various minima, as well as in hydrated complexes of MMU, again indicate no significant chemical differences between the two carbonyls. The theoretical data offer an explanation of the observed differences in shift of the two carbonyl ^17^O resonance signals when an aprotic solvent is replaced by water. Whereas the two carbonyls are found to be chemically similar, the observed difference in behaviour is attributed to steric factors. The lesser accessibility of the C~2~ carbonyl to solvation allows a smaller number of hydrogen bonds with water and hence a reduced shift when the solvent is changed.