黄嘌呤及其互变异构体的密度泛函理论研究

Density functional theory calculations on xanthine tautomers

Molecular structures of fourteen xanthine tautomers were calculated by using the B3LYP/6-311G**method both in the gas and aqueous phases with full geometry optimization.The Onsager solvation model was employed for aqueous solution calculations.The structures,total energy,standard enthalpy,standard entropy and standard free energy were obtained.The calculations showed that xanthine existing as the diketo form is the predominant isomer in the gas and aqueous phases.The dioxo-N（7）（H）isomer is more stable than the dioxo-N（9）（H）isomer in the gas and aqueous phases.The results are in good agreement with available experimental results.The entropy effect on the Gibbs free energy of xanthine base is very small and there is little significance for the tautomeric equilibria of the base.The enthalpic term is dominant in the determinatio of tautomeric equilibria.The free energy of solvation is well correlated with the dipole moments of xanthine tautomers.Additionally,origins of the solvent effects were examined by analyzing the changes of the dipole moments due to the change of a solvent polarity.

Molecular structures of fourteen xanthine tautomers were calculated by using the B3LYP/6 -311G method both in the gas and aqueous phases with full geometry optimization. The Onsager solvation modal was employed for aqueous solution calculations. The structures, total energy, standard enthalpy, standard entropy and standard free energy were obtained. The calculations showed that xanthine existing as the diketo form is the predominant isomer in the gas and aqueous phases. The dioxo - N （7） （H） isomer is more stable than the dioxo - N （9） （H） isomer in the gas and aqueous phases. The results are in good agreement with avail- able experimental results. The entropy effect on the Gibbs free energy of xanthine base is very small and there is little significance for the tautomeric equilibria of the base. The enthalpic term is dominant in the determinatio of tautomeric equilibria. The free energy of solvation is well correlated with the dipole moments of xanthine tautomers. Additionally, origins of the solvent effects were examined by analyzing the changes of the dipole moments due to the change of a solvent polarity.