Theoretical study of the ground and excited states of 7-methyl guanine and 9-methyl guanine: comparison with experiment

The keto-enol tautomerization of 7-methyl-guanine and 9-methyl-guanine in the excited state was investigated using the time-dependent DFT (TDDFT) method. For both species, the potential energy surfaces of the ground state and two lowest singlet excited states (due to pi-->pi* and n-->pi* transitions) have been investigated and their features discussed in terms of consequences on the excited state dynamics. The findings suggest that, for both species, the state due to the n-->pi* transition, suspected to be an intermediate in the excited state deactivation, exhibits two minima with the second minimum characterized by an elongated N1-H distance. This structure, intermediate between enol and keto tautomers, might play a role in the excited state relaxation. The existence of this second well, however, is observed in both 7- and 9-methyl-guanine, which suggests that it cannot account alone for the different photophysical behavior of these species.