Metal counterion modulated single proton transfer process in guanine base

The influence of metal cations (Mn+ = Na+, K+, Mg2+, Ca2+, Zn2+) on the single proton transfer process in guanine base has been studied at the MP2/6-31G* and B3LYP/6-311++G** levels of theory. Two main basic binding sites, N3 and N7 of guanine, are considered for Mn+ coordination. It is observed that the tautomerism behavior of guanine, induced by intramolecular single proton transfer, can be modulated by surrounding metal counterions. Calculated results show that the single proton transfer process in guanine is favored and even becomes thermodynamically spontaneous due to the presence of Mn+ interacting at the N3 position of guanine. On the contrary, if Mn+ coordinated to N7 of guanine, the single proton transfer process will become unfavorable than that in the neutral system. Moreover, the effects are more pronounced for the divalent cations than for the monovalent ones. All of them can be understood from the electrostatic and oxidative effects of metal cations on the guanine base obtained from further natural bond orbital (NBO) analyses.