Modeling thymine photodimerizations in DNA: mechanism and correlation diagrams.

The basic mechanistic traits of the main photochemical reactions in DNA, the formation of the cyclobutane and oxetane thymine dimerization adducts, are established with the help of CASSCF and CASPT2 calculations for a gas-phase model of two stacked thymines. Both reactions go through conical intersections between the ground and the excited state that are connected through minimum energy paths to the corresponding products. This explains the ultrafast formation of the cyclobutane adduct detected experimentally, and it suggests that the oxetane formation also occurs on that time scale. Moreover, the states responsible for the photoproduct formation correlate with two high-lying states of the pair in its ideal B-DNA conformation. These states are different from the delocalized states resulting from coupling of the localized ones, which suggests that the origin of the reactive electronic states lies in the pi stacking. Formation of the photoproducts requires population of these states, by direct excitation of favorable conformations, or preceded by a localized excitation.