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Thio- versus oxo-derivatives of DNA bases: theoretical study on possible mutagenic effect of sulfur atom

The molecular structures, energetics and mechanisms of proton transfer in 9-methyl-2N-substituted guanines and thioguanines have been studied at the gas phase (QCISD/cc-pVDZ//B3lyp/6-31G(d,p)) and polar solvent (IEFPCM//B3LYP/6-31G(d,p)) environments. Calculations predict the height of the proton transfer barrier for the thione → thiol reactions to be approximately 2–3 times lower than that for the keto → enol process. Similarly, presence of sufficiently long 2N substituent with the terminal OH group reduces the barrier. The polar media (water) significantly stabilizes thione and keto forms. Tautomeric concentration ratio and rate constants have also been calculated for all the considered processes.