Mechanisms and energetics of benzophenone photosensitized thymine damage and repair from Paternò–Büchi cycloaddition

Abstract

Here, we report the detailed mechanisms of benzophenone (BZP) photosensitized thymine damage and repair by Paternò–Büchi (PB) cycloaddition. It was found that the head-to-head and head-to-tail PB cycloadditions lead to the formation of the C–O bonds in the ³(nπ*) state and the ³(ππ*) state, respectively. The conical intersection occurs before the head-to-tail C–O bonding. Then, the C–C bonds are formed via intersystem crossing (ISC). The C–O bonding is the rate-determining step of PB cycloaddition. For the cycloreversion reactions, the ring-opening processes completely occur in the singlet excited states of oxetanes. The head-to-head oxetane goes through a conical intersection before cycloreversion with a little energy barrier of 1.8 kcal mol⁻¹. The head-to-tail oxetane splits without a barrier. Then, the ISC processes take place to restore thymine. Throughout the ring-closing and ring-opening processes, ISC plays an important role. These findings are in good agreement with the available experimental findings. We hope that this comprehensive work can provide a deeper understanding of photosensitive DNA damage and repair.