具有氨基和羧基间单氢键的α-Ala分子旋光异构机理及水和羟自由基的作用

Mechanism of Optical Isomerism of α-Ala Molecules with Hydrogen Bonds between Amino and Carboxyl Groups and Roles of Water and Hydroxyl Radicals

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,研究了标题反应.反应通道研究发现:标题反应有两个通道a和b,a是羧基顺反异构后质子以氨基为桥从手性碳的一侧迁移到另一侧;b是质子从手性碳向氨基的迁移与羧基顺反异构协同进行.势能面计算表明:a为优势通道,质子从手性碳向氨基迁移过程是决速步骤.2个水分子簇作氢迁移媒介,使决速步内禀能垒从裸反应的267.41kJ·mol-1降到131.77kJ·mol-1,水溶剂效应又使该能垒进一步降到107.83kJ·mol-1.羟自由基水分子链联合作用可使α-丙氨酸损伤,水分子拔氢和羟自由基拔氢的能垒分别是124.76和21.56kJ·mol-1,水溶剂效应使两个能垒进一步降到6.59和-186.99kJ·mol-1.结果表明:水溶剂环境下,α-丙氨酸分子可以缓慢地旋光异构;在浓度较大的水汽环境下,α-丙氨酸分子的旋光异构更容易进行.水汽环境下,羟自由基的存在可使α-丙氨酸分子损伤;水溶剂环境下,羟自由基的存在可使α-丙氨酸分子迅速损伤.

The title reaction was researched by using the B3 LYP 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 title reaction.In the channel a,the proton was transferred from the side of the chiral carbon to the other with the amino as the bridge after the carboxyl was antiisomeric.In the channel b,the transfer of proton from chiral carbon to amino groups was coordinated with the antiisomerism of carboxyl groups.Calculations of potential energy surface show that the channel a is the dominant one,and the transfer of protons from chiral carbon to the amino group is the step-determining.The intrinsic energy barrier of step-determining is decreased from 267.41 kJ·mol-1 in the bare reaction to 131.77 kJ·mol-1 with two water molecule clusters as hydrogen transfer agents,and the water solvent effect further reduces the barrier to 107.83 kJ·mol-1.The combined effect of water molecular chain hydroxyl radical makesα-Ala damaged,and the energies of water molecules and hydroxyl radical dehydrogenation are 124.76 and 21.56 kJ·mol-1,respectively,and the solvent effect makes the two barrier further be reduced to 6.59 and-186.99 kJ·mol-1.The results show that in water solvent environment,α-Ala molecules can be slowly optically isomerized,and the optical isomerism ofα-Ala is more easily carried out in high concentration of water vapor.In the water vapor environment,the presence of hydroxyl radicals can damage the alanine molecule;in the water solvent environment,the presence of hydroxyl radicals can cause rapid damage to the alanine molecule.