Marked variations of proton release energy of the G-M+ (MÂ =Â Li, Na) in the water circumstance

The variations of N1-H proton release energy of G-M (M = Li, Na) cation have been investigated employing density functional theory using B3LYP/6-31++G∗∗//B3LYP/6-31+G∗ method. There are three modes (NO mode, N mode and O mode) when the hydrated-M+ bonds to guanine. The bonding energy of the hydrated M+ to the guanine reduces following the increase in the number of water molecules. The proton release energies of the G-M+ complexes are calculated at the condition of the different numbers of water molecules and the different modes of water molecules bonded on the G-M+. The results show that the difference of proton release energy on three modes is very small, and the proton release energies of the Na+ complexes are slightly larger than those of the Li+ complexes. The effect on the N1-H proton release is very small when the water molecules bond on the M+ cation, but the effect is very large when the water molecule bonds on the N1-H proton and the proton releases as the hydrated proton. The IR vibrational frequencies of the hydrated G-M+ complexes are calculated using analytic second derivative methods at the B3LYP/6-31+G∗ level. The vibrational frequency analyses show that the changes of the vibrational frequency are consistent with the changes of geometry and the changes of the N1-H proton release energy. The N1-H proton release (N1-H proton release energy: 45-60 kcal/mol) of the guanine occurs easily under the biological environment.