Charge-transfer excited states and proton transfer in model guanine-cytosine DNA duplexes in water

Characterization of the excited electronic states and relaxation processes in DNA systems is critical for understanding the physical basis of radiation damage. Spectroscopic studies have shown evidence of coupling between the relaxation dynamics of photoinduced charge-transfer states and interstrand proton transfer in DNA duplexes, where a deuterium isotope effect was observed for duplexes with alternating sequences but not with nonalternating sequences. We performed quantum mechanical/molecular mechanical (QM/MM) calculations of the vertical excitation energies and excited state proton potential energy curves for model DNA duplexes comprised of three guanine-cytosine pairs with alternating and nonalternating sequences in aqueous solution. Our calculations indicate that the intrastrand charge-transfer states are lower in energy for the alternating sequence than for the nonalternating sequence. The more accessible intrastrand charge-transfer states could provide a relaxation pathway coupled to interstrand proton transfer, thereby providing a possible explanation for the experimentally observed deuterium isotope effect in duplexes with alternating sequences.