Theoretical Investigation on Tautomerization Mechanism of 6-Thioguanine

Abstract

The tautomerization reaction mechanisms between three stable 6-thioguanine tautomers were investigated theoretically using B3LYP/6-311+G(d,p) method. The results show that the pathway P(1) is to isomerize from TG(9,10,10,11) to TG(1,9,10,10) and the needed activation Gibbs free energy barrier is 112.7 kJ/mol with the rate constant of 1.12×10-7 s-1. Another two pathways P(2) and P(3) are to isomerize from TG(1,9,10,10) to TG(1,7,10,10) and the activation Gibbs free energy barriers of the rate-determining steps are 227.4 and 281.6 kJ/mol, respectively, with the related rate constants of 8.96×10-28 s-1 and 2.86×10-37 s-1, these results implying the intramolecular proton transfer reactions are infeasible in the gas phase.