Ab initio studies on the photophysics of the guanine–cytosine base pair

The low-lying excited singlet states of the Watson–Crick form of the guanine–cytosine base pair have been investigated with multi-reference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). The reaction paths and energy profiles for single proton transfer from guanine to cytosine in the 1ππ* guanine-to-cytosine charge-transfer state and for twisting of the CC double bond of the cytosine ring in the locally excited 1ππ* state of cytosine have been explored by excited-state geometry optimization using the configuration-interaction-with-singles (CIS) method and single-point energy calculations at the CASPT2 level. Avoided crossings of the 1ππ* potential-energy functions with the electronic ground-state potential-energy function have been identified along both reaction paths. The results suggest the existence of low-lying conical intersections of the 1ππ* potential-energy surface with the S0 surface which become accessible by possibly barrierless single proton transfer as well as out-of-plane deformation of cytosine and may trigger an ultrafast radiationless decay to the ground state. The relevance of these results for the rationalization of the photostability of the genetic code is briefly discussed.