基于氨基做质子迁移桥梁的苯丙氨酸分子旋光异构反应机理

Optical Isomerization Mechanism of Phenylalanine Molecule Based on Amino Group as Proton Transfer Bridge

采用密度泛函理论的B3LYP方法和微扰理论的MP2方法,对苯丙氨酸分子的3种最稳定构型基于氨基做质子迁移桥梁的旋光异构进行研究.反应通道研究发现:标题反应有3条通道a、b和c.对于构型1和3,a是羧基异构后手性碳上的质子再以氨基为桥迁移,b是手性碳上的质子直接以氨基为桥迁移,c是手性碳上的质子以羧基和氨基联合为桥迁移.对于构型2,3条通道分别是质子只以氨基、顺次以羰基与氨基和顺次以羧基和氨基为桥迁移;势能面计算表明:构型1,3的主反应通道都是a,决速步是第2基元反应,活化吉布斯自由能垒分别为256.7kJ·mol^(-1)和263.4kJ·mol^(-1),由羧基异构后质子从手性碳向氨基氮迁移的过渡态产生.构型2的主反应通道也是a,决速步是第1基元反应,活化吉布斯自由能垒为256.5kJ·mol^(-1),由质子从手性碳向氨基氮迁移的过渡态产生;3种构型的苯丙氨酸分子旋光异构速控步骤的反应速率常数分别为6.27×10-33 s^(-1),6.79×10-33s^(-1)和4.20×10-34s^(-1).

The optical isomerization of three kinds of the most stable configurations of phenylalanine molecules with amino group as proton transfer bridge was researched by using the B3LYP method of density functional theory and the MP2 method of perturbation theory. The study of reaction channels showed that that there were three channels a, b and c in the title reaction. For the configuration 1, 3, firstly carboxyl isomerism, then the proton was transferred with amino group as the bridge in the channel a, in the channel b it was transferred directly with amino group as the bridge, and in the channel c it was transferred with both carboxyl and amino group as the bridge. For the configuration 2, in the three channels, the proton in the chiral carbon was transferred with only amino group, carbonyl and amino group, carboxyl and amino group successively as the bridge. Calculations of potential energy surface showed that in the configuration 1, 3, channel a was the dominant reaction path and the step- determining was the second elementary reaction and the free energy barriers were 256. 7 kJ·mol^-1 and 263.4 kJ·mol^-1 respectively that was generated by the transition state of proton transfer from the chiral carbon to the amino N after carboxyl isomerism. In addition, in the configuration 2, channel a was also the dominant reaction path and the stepdetermining was the first elementary reaction and the free energy barrier was 256.5 kJ ·mol^-1 that was generated by the transition state of proton transfer from the chiral carbon to the amino N. The reaction rate constants of the optical isomerization rate-determining step of three configurations of phenylalanine molecules were 6. 27 ×10^-33 , 6.79×10^-33s^-1 and 4.20×10^-34s^- 1 , respectively.