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+)

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 105.938–173.141 kJ/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+.