An ab initio SCF study on the tautomerisation of fapy-guanine

The quantum chemistry calculations were performed on the fapy-guanine tautomers. The semiempirical PM3 as well as ab initio (STO-3G, 6-31G, 6-31G*) methods were used for geometry optimisations. The gradient minimisation (6-31G*) was followed by single point correlation energy estimation in the MP2 (6-31G*) terms. The fapy-guanine may have a complicated structure. Among a variety of potential tautomers two seem to be dominant. The most stable in the isolated state is the di-keto-di-amino isomer. The next stable structure corresponding to 4-enol-8-keto-2,6-diamino tautomer is about 2 kcal mol(-1) (6-31G* MP2) above the most stable one. The third tautomer (6,8-diketo-2-imino-9-amino) is destabilised by more than 10 kcal mol(-1). The rest of tautomers are even less stable. Thus the fapy-guanine exists in the isolated state mainly as a mixture of two tautomers with predominantly the first structure. As a result in the significant differences in the electrostatic potential properties these tautomers are supposed to have different miscoding potential.