气相S-异亮氨酸向R-别异亮氨酸的旋光异构机理及水溶剂化效应

Optical Isomerism Mechanism and Water Solvation Effect from Gaseous Phase S-Isoleucine to R-Allo-isoleucine

采用密度泛函理论的B3LYP方法,微扰理论的MP2方法及自洽反应场(SCRF)理论的SMD模型方法,研究气相S-异亮氨酸向R-别异亮氨酸的旋光异构机理及水溶剂化效应.结果表明:该反应有a,b,c 3个通道,在通道a和c实现旋光异构反应需经过3个基元反应,在通道b实现旋光异构反应需经过4个基元反应;a为主反应通道,决速步骤Gibbs自由能垒为255.0kJ/mol,由质子从α手性C向氨基N迁移的过渡态产生,决速步骤的反应速率常数为1.25×10-32 s^(-1);水溶剂效应使决速步骤能垒降至114.1kJ/mol,反应速率常数增至2.73×10-7 s^(-1),即水环境对S-异亮氨酸旋光异构具有较好的催化作用.

We studied optical isomerism mechanism and water solvation effect from gaseous phase S-isoleucine to R-allo-isoleucine by using the B3LYP method of density functional theory, the MP2 method of perturbation theory, and SMD model method of self-consistent reaction field （SCRF） theory. The results show that there are three channels a, b and c in the reaction. The optical isomerism reaction requires three elementary reactions in the channel a and c, four elementary reactions in the channel b. Channel a is the main reaction channel and the step-determining Gibbs free energy barrier is 255.0 kJ/mol, which is generated by the transition state of proton transfer from the chiral carbon to the amino N. The step-determining reaction rate constant is 1.25×10-7 s^-1. The water solvation effect enables the step-determining energy barrier to reduce to 114.1 kJ/mol and the reaction rate constant to increase to 2. 73×10-7 s^-1. It shows that water environment has better catalytic effect on S-isoleucine optical isomerization.