组氨酸分子几种稳定构型的手性转变机理及水溶剂化效应

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（SCRF）理论的smd模型方法，研究了组氨酸分子3种最稳定构型的手性转变机理及水溶剂化效应.发现标题反应有a、b、c 3条通道，对于构型1和2，a是手性碳上的质子先以氨基为桥迁移，b是羟基异构后手性碳上的质子再以氨基为桥迁移，c是以羧基和氨基联合作桥实现质子迁移.对于构型3，a是质子只以氨基为桥迁移，b是质子顺次以羰基与氨基为桥迁移，c是质子顺次以羧基和氨基为桥迁移.计算表明：构型1和2的主反应通道都是b，决速步自由能垒分别为250.8和251.7 kJ·mol^-1，来源于羟基异构后的质子从手性碳向氨基氮迁移的过渡态.构型3的主反应通道是a，决速步自由能垒为250.8 kJ·mol^-1，来源于质子从手性碳向氨基氮迁移的过渡态.水溶剂效应使构型1的主反应通道决速步自由能垒降到109.1 kJ·mol^-1.说明水环境对组氨酸的旋光异构有极好的催化作用.

Chiral transition mechanism and water solvation effect of stable configurations of histidine molecules

The chiral transition mechanism and water solvation effect of three kinds of the most stable configurations of histidine molecules were studied by adopting the B3LYP method of density functional theory, the MP2 method of perturbation theory, and smd model method of self consistent reaction field theory. Our study on reaction channels shows that there are three channels a, b and c for the title reaction. For the configuration one and two, the proton of the chiral carbon is transferred directly with amino group as the bridge in the channel a, and in b hydroxyl isomerism firstly, then the proton is transferred with amino group as the bridge, while in c it is transferred with carbonyl/amino groups as the bridge. For the configuration three, the proton is transferred with the following respective groups as the bridge for channels a, b and c:only amino group, carbonyl then amino group, carboxyl then amino group. Calculations of potential energy surface show that channel b is the dominant reaction path in the configuration one and two, step-determining gibbs free energy barriers are 250.8 kJ·mol^-1 and 251.7 kJ·mol^-1, respectively, which are generated by the transition state of proton transfering from the chiral carbon to the amino N after hydroxyl isomerism. In addition, channel a is the dominant reaction path in the configuration three, and step-determining gibbs free energy barrier is 250.8 kJ·mol^-1 that is generated by the transition state of proton transfer from the chiral carbon to the amino N. The water solvation effect reduces the step-determining energy barrier to 109.1 kJ·mol^-1 for the dominant reaction path in the configuration one. It shows that water environment has an excellent catalytic effect on the histidine optical isomerization.