Photophysics and photochemistry of DNA molecules : electronic excited states leading to thymine dimerization

Abstract

We combine quantified natural transition orbital (QNTO) analysis with large-scale linear response time-dependent DFT (TDDFT) to investigate the concerted [2 + 2] thymine dimerisation reaction. This reaction is a main cause of UV-light induced damage to DNA, but its mechanism has remained poorly understood. QNTO analysis enables the electronic excitations of a molecule to be identified on the basis of their transition origins across a wide range of molecular geometries, allowing the participating excited states to be identified relatively straightforwardly. We identify a barrierless funnel that is responsible for the ultrafast reaction previously indicated in experiments. The reactive state is found to have crossings with several bright excited states, revealing how the initially populated bright states can decay rapidly to the reactive state. We also examine the contribution of environmental factors such as inclusion of the DNA backbone, which can affect the conformation of the potential energy surfaces of the relevant states