水液相下羟基负离子催化半胱氨酸旋光异构的机理

Optical Isomerism Mechanism of Cystenie Catalyzed by Hydroxyl Anion in Water-Liquid Phase Environment

采用密度泛函理论M06-2X和MN15方法,结合自洽反应场理论的SMD模型方法,研究了水液相下羟基负离子(OH^(-))催化半胱氨酸(Cys)分子的旋光异构反应机理。研究发现,两性Cys分子的消旋反应可以通过OH^(-)直接抽取α-H质子和Cys碳负离子抽取水分子(H_(2)O)质子实现,也可以在两性Cys向中性异构后,通过OH^(-)抽取中性Cys的α-H质子和Cys碳负离子抽取H_(2)O质子实现。势能面计算表明:第1种情况下Cys消旋反应的活化能垒是45.8 k J/mol,第2种情况下Cys消旋反应的活化能垒是51.6 k J/mol,均比水液相下Cys消旋反应的活化能垒104.0 k J/mol低很多。结果表明,水液相下OH^(-)对Cys的旋光异构具有很好的催化作用。

The optical isomerism reaction mechanism of cysteine(Cys)molecule catalyzed by hydroxyl anion(OH^(-))in water-liquid phase environment was studied by means of M06-2X and MN15 methods based on density functional theory and SMD model method of self consistent reaction field theory.The study shows that the racemization reaction of amphoteric Cys molecule can be achieved through the process of OH^(-)directly extractingα-H proton and Cys carbanion extracting the proton from water molecule(H_(2)O),it can also be achieved through the process of OH^(-)extractingα-H proton from amphoteric Cys and Cys carbanion extracting the proton from water molecule(H_(2)O)after amphoteric Cys isomerizes to neutral.Investigation on the potential energy surface shows that the activation energy barrier of Cys racemization reaction in the first case is 45.8 k J/mol,the activation energy barrier of Cys racemization reaction in the second case is 51.6 k J/mol,both of barriers are much lower than the 104.0 k J/mol activation barrier of Cys racemization reaction in water-liquid phase environment.The results show that OH^(-)has a good catalytic effect on the reaction of Cys optical isomerism in water-liquid phase environment.