Theoretical Quantum Chemical Study of Tautomerism and Proton Transfer in 6,8-Dithioguanine

Ab initio quantum chemical calculations were performed to study the tautomeric rearrangements in the 6,8-dithioguanine. Molecular geometries of all the possible 35 tautomeric structures have been fully optimized without imposing any constraints at the HF/3-21G level. For the seven most favorable structures, full geometry optimizations were performed at the higher level of theory using the 6-31G** basis set. The effects of electron correlation were further accounted for at the second-order Moller-Plesset perturbation theory level with the frozen-core approximation. The proton-transfer reactions were considered between the normal and selected rare tautomeric forms in the gas phase and also for the water assisted proton transfer. It was shown that at all applied levels of theory the standard 6,8-dithioguanine (with protons at the N1, N7, and N9 sites and on its amino group) is the global minimum on the potential energy surface in the gas phase. For the monohydrated complexes of dithioguanine, the complex 1·H₂O(N1H) is the most stable one and is characterized by the highest interaction energy. The relative stability of monohydrated complexes of normal dithioguanine which interacts with the water through N9H and N7H groups follows stability of 1·H₂O(N1H). Water-assisted proton-transfer reactions considerably decreases the energy barrier as compared to the ability of gas phase processes to do the same.