8-羟基鸟嘌呤自由基的开环反应机理

Ring-Opening Reaction Mechanism of 8-Hydroxyguanine Radical

使用经实验校准的B3LYP/DZP++方法研究了8-羟基鸟嘌呤自由基的开环反应机理.计算结果表明,该反应先后历经C8—N9键的断裂、羟基H原子向N7原子转移两个步骤完成,转移中的H原子具有阳离子的特征.当没有水存在时,羟基H原子的转移反应需经历一个四元环的过渡态,具有较高的活化能,反应较困难.但如果有水分子存在,羟基H原子的转移步骤将经历一个低活化能的六元环过渡态,使整个8-羟基鸟嘌呤自由基的开环反应可以在较温和的条件下顺利完成.在无水催化时,羟基H转移是反应的速率控制步骤;而有水催化时,反应的速率由C8—N9键的断裂步骤控制.

The theoretical studies on the mechanisms of ring-opening reaction of 8-hydroxyguanine radical were performed by the experimentally calibrated theoretical method of B3LYP/DZP＋＋. All of the computational results revealed that the ring-opening reaction occured through a pathway including two elementary reactions, the C8-N9 bond-broken step followed by a transfer step of cation-like hydrogen atom of the hydroxyl moiety. The high activation energy in the transfer reaction of hydrogen atom in the absence of water led this reaction to be somewhat difficult; on the contrary, the hydrogen transfer might occur feasibly with assistance of one water molecule due to the formation of a six-member ring transition state which was beneficial to lower the reaction activation energy. The difference of the activation energies of two elementary reactions demonstrated that the rate-limiting steps of ring-opening reaction in the absence of water and in the presence of water were the hydrogen transfer step and C8-N9 bond-broken process, respectively.