MOR分子筛与水复合环境对赖氨酸手性转变反应的共催化机理

Theoretical study on the co-catalysis of MOR zeolite and water combined environment on the chiral transition of lysine molecules

采用量子力学与分子力学组合的ONIOM方法,研究了赖氨酸限域在水与MOR分子筛复合环境下的手性转变.结构分析表明:2个水分子比1个水分子助氢迁移反应的过渡态分子氢键键角显著增大。反应通道研究发现:标题反应有a、b和c三个通道,是赖氨酸在MOR分子筛限域环境下,水助质子以氨基、羰基和羟基为桥从手性碳的一侧迁移到另一侧,实现手性转变。势能面计算表明,a是主反应通道,质子从手性碳向氨基的迁移是决速步骤,在2个水分子助决速步时,吉布斯自由能垒被降到最低值101.9 k J/mol,与裸反应、限域在MOR分子筛和限域在水环境的此通道决速步能垒252.6、229.7和123.9 k J/mol相比较,均有明显降低。结果表明:水与MOR分子筛复合环境对赖氨酸手性转变具有较好的共催化作用,左旋赖氨酸在生命体内可以缓慢地旋光异构。

The chiral transition of lysine molecules confined in water and MOR zeolite combined environ-ment was studied in the paper by introducing the ONIOM methods using combination of quantum mechan-ics and molecular mechanics.The molecular structure researches show that hydrogen bond angle of the transition state molecules in the hydrogen transfer reactions with the help of two water molecules is signifi-cantly larger than that with the help of one water molecule.The study of reaction channels shows there are three channels a,b and c in the title reaction where lysine is confined in MOR zeolite,protons transfer with the help of water molecules from one side to the other of the chiral C with amino,carbonyl and hy-droxyl as a bridge,and at last the chiral transition is achieved.Calculations of potential energy surface show that channel a is the dominant reaction path and protons transfer the chiral C to amino is the step-determining where gibbs free energy barrier of two water-assisted proton transfers reaction is reduced to the minimum value 1 01.9 kJ · mol-1 ,that is significantly lower than the gibbs free energy barrier 252.6,229.7 and 1 23.9 kJ·mol-1 ,respectively corresponding to the bare reaction,confined in MOR zeolite and confined in water environment.The results show that water and MOR zeolite combined envi-ronment has a good co-catalysis on the chiral transition of lysine molecules,and S-Lys in vivo can be slowly optical isomerism.