Proton transfer reactions of nucleic acid bases: A semiempirical molecular orbital study

The possibilities open for tautomerism, for protonation and for deprotonation in the five nucleic acid bases are examined theoretically using the semiempirical AM1 SCF-MO methodology. The tautomers predicted to be the most stable, other than the usual forms, all involve proton shifts to adjacent sites. The sites predicted to be the most favourable for protonation are the N7-G, N3-A, N3-C, O4-T and O4-U positions of guanine, adenine, cytosine, thymine and uracil respectively. The protons predicted to be the most acidic for each base are the N1-G, N9-A, N1-C, N3-T and N3-U protons. These predictions accord well with the conclusions drawn from experimental work so far as assignments of acidic protons and basic sites for the particular bases are concerned. However, the relative feasibility of these reactions for the different bases is not well predicted by these gas-phase calculations. © 1995 Indian Academy of Sciences.