Electron correlation effects in the ab initio study on tautomerism of guanine

Abstract

The molecular structures of three quanine tautomers are examined using ab initio method at the MP2/6–31G**//HF/6–31G level. The study predicts amino—oxo Gua N(7)H tautomer (1) to be the global minimum. The second amino—oxo Gua N(9)H (2) and amino—hydroxy Gua* N(9)H (3) tautomers have relative energies of 0.5 and 4.6 kJ mol−1 (MP2/6–31G**+ZPE), respectively. These results explain well recent experimental IR matrix-isolation and UV photoelectron studies on guanine. The shift of tautomeric equilibria towards 2 on going from the isolated molecules towards polar solvents is predicted. The influence of the basis set and electron correlation contributions on the relative energies of tautomers is discussed.