AM1 CALCULATIONS OF REACTION FIELD EFFECTS ON THE TAUTOMERIC EQUILIBRIA OF NUCLEIC-ACID PYRIMIDINE AND PURINE-BASES AND THEIR 1-METHYL ANALOGS

Semiempirical AM1 quantum-chemical calculations of the principal tautomers of cytosine, thymine, uracil, 1-methylcytosine, 1-methylthymine, 1-methyluracil, adenine, and guanine were carried out for the isolated molecules, and for the same molecules in a polarizable dielectric medium with the relative permittivity of water, using the self-consistent reaction field method. In almost all cases, the same tautomer is predicted as most stable regardless of the method used and environment applied. However, the calculations imply significant quantitative differences in the relative stabilities of the tautomers in different media, which need to be taken account of in the nucleic acid base pairing/mispairing probability estimations based on quantum-chemical data.