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Abstract

Molecular-orbital calculations are used to compare the hydrogen-bonding characteristics of the two carbonyl groups of uridine nucleosides. Various numbers of water molecules are allowed to interact with uracil. The hydrogen-bond 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 anti-gg global minimum is quite similar to the x-ray 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 17O 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 C2 carbonyl to solvation allows a smaller number of hydrogen bonds with water and hence a reduced shift when the solvent is changed.