水液相下羟基负离子(水分子簇)催化脯氨酸旋光异构的理论研究

Theoretical study on the optical isomerization of proline catalyzed by hydroxyl anion(water clusters)in aqueous-liquid environment

采用密度泛函理论的M06-2X和MN15方法与自洽反应场理论的SMD模型方法,研究水液相下羟基负离子水分子簇催化S-型两性脯氨酸分子(S-Pro)的旋光异构。首先考察了S-Pro异构成中性亚氨基酸后,羟基负离子(水分子簇)抽取α-H,脯氨酸-碳负离子抽取水分子(簇)的质子,S-Pro旋光异构的反应通道;然后考察了羟基负离子(水分子簇)直接抽取S-Pro的α-H,脯氨酸-碳负离子抽取水分子(簇)的质子,以及S-Pro旋光异构的反应通道。势能面计算表明:S-Pro异构成中性亚氨基酸后,羟基负离子(水分子簇)抽α-H的通道最具优势,决速步的自由能垒为46.9~59.1 kJ·mol^(−1)。羟基负离子(水分子簇)直接抽两性S-Pro的α-H的反应通道是劣势通道,决速步的自由能垒为70.8 kJ·mol^(−1)。结果表明,碱性环境下脯氨酸的消旋反应会温和地进行。

The optical isomerization of S-type amphoteric proline(S-Pro)is investigated by using the M06-2X and MN15 methods of density functional theory and the SMD model method of self-consistent reaction field theory.Firstly,the reaction channels of S-Pro optically isomerism are investigated after S-Pro isomerized the neutral imino acids,α-H is extracted by hydroxyl anion(water clusters),and the proton of water molecule(clusters)is extracted by proline-carbon anion.Then,the reaction channels of S-Pro optically isomerism are investigated,in which α-H of S-Pro is directly extracted by hydroxyl anion(water clusters)and the proton of water molecule(clusters)is extracted by proline-carbon anion.The calculation of the potential energy surface shows that the channel ofα-H extraction by hydroxyl anion(water clusters)is the most advantageous after S-Pro isomerized the neutral imino acids,and the free energy barrier of the rate-determining step is between 46.9 and 59.1 kJ·mol^(−1).The reaction channel of the hydroxyl anion water molecule clusters directly extracting theα-H of amphoteric S-Pro is the inferior channel,and the free energy barrier of the rate-determining step is 70.8 kJ·mol^(−1).The results show that the racemization of proline would proceed gently in alkaline environment.