Theoretical Study of Guanine from Gas Phase to Aqueous Solution:  Role of Tautomerism and Its Implications in Absorption and Emission Spectra

Correlated ab initio methods have been exploited to study geometries and electronic structures of ground and lowest singlet excited states of isolated and solvated keto−amino tautomers of guanine. Nuclear structures have been obtained at the CIS level, whereas calculations of electronic structures have been performed using a multireference perturbed CI method. In all calculations, the solvent (water) has been described within the integral equation formalism continuum model. The results have been successfully compared to experimental absorption and emission spectra. In particular, the analysis of the two computed spectra has shown an opposite role of the different tautomers (being absorption mostly because of 9NH form and the emission induced by 7NH) which has lead us to investigate possible mechanisms giving origin to the keto−9NH and keto−7NH tautomerism in the ground or in the electronically excited states.