Photochemistry of DNA fragments via semiclassical nonadiabatic dynamics.

Forming upon absorption of a UV photon, excited states of DNA are subject to nonadiabatic evolution, via either internal conversion (IC) back to the ground state or mutagenesis. Nonadiabatic processes following the formation of the first singlet excited states, S1, in 10 different small DNA fragments--4 single 4'H-nucleosides, 2 Watson-Crick base pairs, and 4 nucleotide quartets--have been investigated. Simulations were done via the nonadiabatic direct trajectory surface hopping semiclassical dynamics. The electronic wave function was obtained with configuration interaction, based on the semiempirical AM1 and PM3 Hamiltonians with fractional orbital occupation numbers. The evolution of the electronic wave function was governed by the time-dependent Schrödinger equation with a locally diabatic representation, intrinsically stable near surface crossings. The nuclei evolved on adiabatic potential energy surfaces, as prescribed by classical Newtonian dynamics, with sudden hops between potential energy surfaces to account for nonadiabatic transitions. The "fewest switches" surface hopping algorithm coupled the quantum and classical parts of the system. The dynamics simulations revealed several routes of nonadiabatic relaxation in these systems, which were not reported previously, and also recovered known routes of IC.