基于氨基作氢迁移桥梁天门冬酰胺的旋光异构机理及水的作用

采用密度泛函理论的B3LYP方法、微扰论的MP2方法和自洽反应场(SCRF)理论的smd模型方法,研究了天门冬酰胺分子2个稳定构型的旋光异构裸反应机理、水分子的催化作用及水溶剂化效应.反应通道研究发现:构型1有2条通道a和b,a通道的第一基元反应质子迁移与羧基异构同时进行,是协同机理;b通道羧基先异构而后质子迁移,是分步机理.构型2有1条通道,是质子先以氨基氮为桥从手性碳的一侧迁移到另一侧,然后羧基和氨基再异构.势能面计算表明:构型1的主反应通道是b,决速步自由能垒为252.7kJ·mol~(-1);构型2的决速步自由能垒为254.0kJ·mol~(-1),均来自于质子从手性碳向氨基氮迁移的过渡态.2个水分子作质子迁移媒介时,构型1主反应通道b的决速步能垒降到124.1kJ·mol~(-1),再考虑到水溶剂化效应时,决速步能垒降到104.0kJ·mol~(-1).结果表明:水分子的催化和水溶剂助催化的共同作用,使质子迁移反应能垒大幅度降低.

Optical Isomerism Mechanism of Asparagines and the Role of Water Based on Amino Group as Proton Transfer Bridge

The bare optical isomerism reaction mechanism of two stable configurations of asparagine molecules,catalytic action of water molecules and water solvation effect were researched by using the B3LYP method of density functional theory,the MP2 method of perturbation theory and the SMD model method based on self consistent reaction field(SCRF) theory.The study of reaction channels showed that there were two channels a,b in the configuration one.In the channel a,the proton transfer of the first elementary reaction is in parallel with isomerism of the carboxyl group,that was the concerted mechanism.In b,it was the stepwise mechanism carboxyl first isomerism and then proton transfer.There was one channel in the configuration two where the proton was first transferred to the other side of the chiral carbon and then isomerism of the carboxyl and amino group.Calculations of potential energy surface showed that channel a was the dominant reaction path and the free energy barrier of the rate-determining step was 252.7 kJ · moli in the configuration one,and that was 254.0 kJ · mo1-1 in the configuration two,which were both generated by the transition state of proton transfer from the chiral carbon to the amino N.When two water molecules were used as proton transfer carrier,the free energy barrier of the rate-determining step was reduced to 124.1 kJ · mol-1 in the dominant reaction path b of the configuration one,and that can be reduced to 104.0 kJ · mol-1 considering the water solvation effect.The research results showed the catalytic action of water molecules and the co-catalysis of water solvent can greatly reduce the energy barrier of proton transfer reaction.