Multiconfiguration Quantum Chemistry Calculations for Describing Photoinduced DNA Repair

Electronic excitation energies and dipole moments have been computed using multiconfiguration CASSCF method further corrected to the second order perturbation theory XMCQDPT2 for a molecular-cluster modeling the active site of (6–4) photolyase containing а thymine–thymine (6–4) photoproduct. A set of models is considered in which the excitation energies change in a wide range since anionic phosphate groups of the photoproduct and cofactor flavin adenine dinucleotide (FADH^–) are included in the models. The results demonstrate that an account for solvent effects using a PCM model reproduces the screening of the negative charges of the phosphate groups by the environment of the active site. The limits of the two-state model for estimating the electronic coupling matrix elements of the electron transfer reactions are investigated. The electronic coupling matrix elements may become overestimated due to the presence of a bright excited state in the flavin excitation spectrum.