基于氨基做质子迁移桥梁亮氨酸的手性转变机理及水溶剂化效应

摘要

The title reaction mechanism was explored by using the B3LYP density functional theory,the MP2 method of perturbation theory and smd model method of self consistent reaction field theory.The study of reaction channels showed that there are two channels a,b in the chiral transition reaction,and in the one the proton is transferred from one side to the other of chiral carbon with only the amino as a bridge,in the other with carbonyl and amino N as the bridge successively.The molecular structure analysis showed that the seven-ring structure in the transition state aTS2·2H2O is almost coplanar and each hydrogen bond angles of the seven-ring structure approach straight angle.Calculations of potential energy surface pointed out that the channel a is the dominant reaction channel and the free energy barrier of the rate-determining step is 249.5 kJ·mol-1,which is caused by the transition state with proton migration from chiral carbon to amino nitrogen.Moreover,with consideration of two water molecules as H transfer media and with inclusion of the water solvation effect of the continuous medium model,,the free energy barrier of the rate-determining step is reduced to 113.2 kJ·mol-1 and the reaction rate constant is 5.85×10-8 s-1,and the water solvation effect enables the energy barrier of the amino heterogeneous reaction increased from 13.4 kJ·mol-1 to 19.3 kJ·mol-1.Therefore,water solvation effect can hinder the heterogeneous reaction of non proton transfer,and the interaction of the catalytic action of water molecules and water solvation effect can cooperatively reduce the proton transfer reaction energy barrier to a large degree.%采用密度泛函理论的B3LYP方法、微扰理论的MP2方法和自洽反应场理论的smd模型方法,对标题反应进行了研究.反应通道研究发现:标题反应有2条通道a和b,分别是质子H只以氨基和以羰基与氨基顺次为桥,从手性碳的一侧迁移到另一侧.结构分析表明:过渡态aTS2·2H2O分子内的7元环结构基本共面,7元环结构的各个氢键角接近平角.势能面计算表明:a是主反应通道,决速步自由能垒为249.5 kJ·mol-1,由质子从手性碳向氨基氮迁移的过渡态产生;2个水分子作H迁移媒介,同时考虑连续介质模型的水溶剂化效应时,决速步自由能垒降到113.2 Kj·mol-1,反应速率常数为5.85×10-8 s-1;水溶剂化效应使氨基异构反应的能垒从裸反应的13.4 Kj·mol-1升高到19.3 Kj·mol-1.结果表明:水溶剂化效应对非质子迁移的异构反应具有阻碍作用;水分子的催化和水溶剂化效应的共同作用,使质子从手性碳向氨基迁移反应的能垒大幅度降低.