CNDO/2 calculations of the electronic structure of the guanine-cytosine base pair: Proton transfer in excited states

A self-consistent field, all-valence electron, molecular orbital theory has been applied to the guanine-cytosine base pair. Consideration of the total energy of the system predicts that, in its ground and low-lying excited states, the base pair is stable with respect to the separate bases and that the protons in the hydrogen bonds linking the bases will each transfer from one base to the other in at least two excited states. The proton transfers appear to result from the transfer of charge from one base to the other upon excitation, rather than from the charge distribution on the atoms directly involved in the hydrogen bonds. No direct evidence was found that simultaneous transfer of two protons occurs in excited states. In the ground state, the potential barriers to single proton transfers were found to be lower than those for double proton transfers.