Excited state deactivation mechanisms of protonated adenine: a theoretical study.

The quantum chemical computational method and Born-Oppenheimer (BO) dynamics simulation were employed to investigate the non-radiative relaxation mechanism of protonated 9H- and 7H-adenine (AH + ). We located three conical intersections (CIs) between the first 1 ππ* excite state and the S 0 ground state potential energy surfaces for the two most stable protonated isomers of adenine. It was predicted that the barrier-free potential energy profile along the out-of-plane deformation coordinates of the six-member ring plays the most prominent role in the deactivation of the excited AH + from 1 ππ* to the ground state via ultrafast internal conversions. This ring deformation was predicted to provide a common deactivation pathway in protonated DNA/RNA bases, describing their high level of photostability, and corresponding neutral homologues.