Hydrogen-Bonded Double-Proton Transfer in Five Guanine–Cytosine Base Pairs after Hydrogen Atom Addition

The double-proton transfer reactions in Watson-Crick guanine-cytosine (GC) base pairs after hydrogen atom addition are studied theoretically. The structural changes and energy differences among the structures are compared to explore the double-proton transfer mechanisms, concerted and stepwise. The concerted mechanism is found in all five radicals (GC+H)(•) considered, while the stepwise mechanism is predicted only for structures G-H(•)C(C6) and H(•)G(N7)-C. The geometrical features have been found to change regularly in the concerted double-proton transfer. This is different from the single-proton transfer, for which the structural perturbations are dispersed throughout the GC base pair. The energy analyses demonstrate that the concerted double-proton transfer mechanism is more favorable in the gas phase, while the stepwise mechanism dominates in water. The structures of proton transfer products become less favored energetically.