Ab initio study of guanine tautomers in the S0 and D0 states

The electronic and stabilization energies of the guanine six most stable tautomers were calculated in the neutral ground electronic, S-0 and the cationic ground, D-0 states. Calculations using ab initio MP2 method shows that guanine 9-H keto is the most stable tautomer in the S-0 state. It was found that upon ionization to the D-0 state guanine 9-H-enol-trans is the most stable form. The ionization energy of the six tautomers of guanine was calculated and found to agree well with the values reported from experiments and with theoretically predicted ionization energies. The molecular structures in the S-0 and D-0 states were predicted and the results from geometry optimization show that all tautomers are nonplanar in the So state. It was predicted that the ionization of all guanine forms to the D-0 state may originate from the removal of one of the electron pairs on the terminal NH2, group of the pyrimidine ring, and the other electron enhances the interaction between nitrogen atoms of the NH2, and the immediate carbon atom neighbors via hyperconjugation. These interactions force the molecule to become planar in the D-0 state. The planar geometry is justified by the electron delocalization in and outside both rings. The calculations of atomic charges and dipole moments support the findings of electrons delocalization around the ring causing the geometrical changes in the S-0 and D-0 states of all tautomers. The calculations of the rotational constants show that the geometries are different for all tautomers, in both the S-0 and D-0 states, verifying the findings from atomic charge and dipole moment analysis that these tautomers suffer large geometrical changes upon ionization. (c) 2005 Wiley Periodicals, Inc.