黄嘌呤及其互变异构体的密度泛函理论研究

嘌呤碱及其衍生物在生物系统中起重要作用。对人具有兴奋和利尿作用的茶碱和咖啡碱就是黄嘌呤的甲基衍生物。黄嘌呤存在多种互变异构体，从理论计算的角度研究这些互变异构体的几何结构、电子结构及相对稳定性，进一步研究溶剂对其结构和性质的影响是有意义旧。密度泛函理论方法既考虑了电子相关，又较其它CI（组态相互作用）或MPn（n级微扰）方法节省机时。其计算结果较好，被广泛应用于研究各种化合物。本文采用密度泛函B3LYP／6—311G方法对14种黄嘌呤可能的互变异构体（见图1），分别在气相和水相中进行几何构型全自由度优化和能量计算，讨论了异构体的相对稳定性，水的溶剂化作用对异构体的能量、几何构型、电荷分布和偶极矩的影响，探讨了溶剂极性对异构体的能量和偶极矩的影响。

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.