A density functional theory study on the intramolecular proton transfer in 6-thioguanine and alkali metal cations substituted derivatives (M-TG, M = Li+, Na+, K+)

Abstract

The intramolecular proton transfer in cationized thione form and thiol form of 6-thioguanine substituted derivatives with M=Li+, Na+ and K+ has been studied with the level of B3LYP/6-31+G (d) density functional theory. Which can help us to further explore the pharmacological properties of 6-thioguanine. Vibrational analysis has been performed on these two configurations to obtain the vibrational frequencies, by means of which the entropy of the proton transfer of 6-thioguanine has been evaluated. It is found that the coordination of the alkali metal ions to the nitrogen of 6-thioguanine stabilized the thione form. For all cases the intramolecular proton transfer occurs readily with small energy barriers. The necessary energy barriers of the intramolecular proton transfer are within 105.938–173.141kJ/mol. The energy barriers become higher when 6-thioguanine complexes with the alkali metal cations than that without the alkali metal cations. For the alkali metal cations Li+, Na+ and K+ systems, the thione form becomes very stable. The intramolecular proton transfer reaction takes more easily according to the order: Li+, Na+, K+.