| Abstract: | Oxygen-17 and nitrogen-14 nuclear magnetic resonance techniques have been utilized in studying liquid-state hydrogen bonding interactions between nucleic acid bases and between the bases and solvent. The bases uracil and thymine have been labeled with 17O at positions 2 and 4, cytosine at position 2 and thymine riboside at position 4. The 17O chemical shift was found to be a sensitive structural probe and to provide information concerning the resonance forms that contribute to the total wave function. The chemical shift of the oxygen of thymine and uracil at position 2 was detected at a higher field than that of oxygen at position 4, presumably owing to an increased contribution of single bonded-oxygen valence bond structures at position 2. The 17O and 14N chemical shifts of cytosine were found to be strongly pH dependent, and the results have been interpreted in terms of predominant cytosine resonance structures contributing to the aqueous cation and anion. The 17O line width of cytosine in aqueous solution indicates the presence of hydrogen bonded dimers between neutral and cationic forms. |
