Thymine dimer repair by electron transfer from photo-excited 2′,3′,5′-tri-O-acetyl-8-oxo-7,8-dihydroguanosine or 2′,3′,5′-tri-O-acetyl-ribosyluric acid – a theoretical study

Abstract

Electronic structure calculations are combined with published experimental data from another laboratory to interpret trends in the rates of thymine dimer repair induced by photo-exciting the title molecules or their deprotonated derivatives. Opening of the thymine dimer's cyclobutane ring is believed to be initiated by electron transfer from the photo-excited molecule and to then pass over thermally accessible energy barriers. Therefore, the repair rates are determined by rates of accessing activation barriers connecting the photo-excited state to the electron-transferred state. These barriers are shown to depend on the electronic excitation energy and electron-binding energy of the donor and the electron affinity of the thymine dimer acceptor. For neutral donors, the barriers also depend on the distance between the donor and the thymine dimer through a screened Coulomb interaction between the donor cation and acceptor anion. For the deprotonated (anionic) donors, this Coulomb-derived distance dependence is absent. For both neutral and anionic donors, the range for electron transfer is spatially limited by the strength of the electronic couplings. The model put forth here rationalizes why anionic donors can be expected to perform better than neutrals and offers a framework for designing electron transfer agents optimal for a given electron acceptor.