Abstract The mechanisms for the deamination reactions of 6-thioguanine (6-TG) with H2O, OH−, and OH−/H2O and for protonated 6-thioguanine (6-TGH+) with H2O have been studied by theoretical calculations. Six pathways, paths A–F, have been explored and relative energies, enthalpies, and Gibbs energies were calculated for each reaction path. Compared with the deamination reaction of 6-TG with H2O or 6-TGH+ with H2O, the deamination reaction of 6-TG with OH− shows a lower overall Gibbs energy of activation, indicating that the 6-TG deamination reaction is favorably to occur for the deprotonated form 6-TG− with H2O. With the assistance of an additional water molecule, the deamination reaction of 6-TG with OH−/H2O (path F) is found to be the most feasible pathway in gas phase and also in aqueous solution. The overall Gibbs energy of activation for path F in gas phase and in aqueous solution is 169.8 and 140.6 kJ mol−1 (SMD model), respectively, at the B3LYP/6–311++G(d,p) level of theory.
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