气相环境下丙氨酸Ca2+配合物的手性转变机理及水分子的催化作用

采用基于密度泛函理论的M06方法，研究了气相环境下2种稳定构型的丙氨酸（Ala）与Ca²⁺配合物的手性转变及水分子的催化。研究发现，Ala_1·Ca²⁺的手性转变有a和b 2个通道，a通道是α-氢只以羰基氧为桥迁移；b通道是α-氢迁移到羰基氧后，氨基上的质子在纸面内侧向α-碳迁移。Ala_2·Ca²⁺的手性转变有a、b、c、d 4个通道，a和b通道分别是羧基内质子迁移后，α-氢只以羰基氧为桥迁移和α-氢迁移到羰基氧接质子从氨基氮向α-碳迁移；c通道是钙与氮的配位键断裂后，α-氢向氨基氮迁移；d通道是钙与氮的配位键断裂后，Ala_2·Ca²⁺向Ala_1·Ca²⁺异构，再接Ala_1·Ca²⁺的手性转变。势能面计算表明，Ala_1·Ca²⁺手性转变的a通道具有优势，总包能垒为134.8 kJ·mol^－1，Ala_2·Ca²⁺手性转变的d通道具有优势，总包能垒为235.3 kJ·mol^－1；水分子的催化使能垒分别降至40.8和141.3 kJ·mol^－1。结果表明，Ca²⁺对Ala的手性转变具有催化作用，水分子对丙氨酸Ca²⁺配合物的手性转变具有极好的催化作用。

Chiral transition mechanism of Ala and Ca2+ complexes in gas phase and catalysis of water molecules

The catalysis of water molecules and the chiral transition of two stable configurations of alanine (Ala) and Ca²⁺ complexes in gas phase have been studied using the M06 method based on density functional theory. The study shows that the Ala_1·Ca²⁺ chiral transition has two pathways a and b. In pathway a, α-H is transferred merely using carbonyl O atom as a bridge. In pathway b, the proton on the amino group transfers to α-C from inside paper after α-H is transferred to carbonyl O atom. The Ala_2·Ca²⁺ chiral transition has four pathways a, b, c and d. In pathway a, α-H is transferred merely using carbonyl O atom as a bridge after the transition of proton on the carboxyl group. In pathway b, α-H is transferred to carbonyl O atom and then proton transfers from the amino group N atom to α-C after the transition of proton on the carboxyl group. In pathway c, α-H is transferred to the amino group N atom after the coordination bond between calcium and nitrogen is broken. In pathway d, the coordination bond between calcium and nitrogen is broken and Ala_2·Ca²⁺ is isomerized to Ala_1·Ca²⁺, then Ala_1·Ca²⁺ achieves the chiral transition. The potential energy surface calculation shows that the dominant pathway of the chiral transition of Ala_1·Ca²⁺ is pathway a, and the total energy barrier is 134.8 kJ·mol^－1. The dominant pathway of the chiral transition of Ala_2·Ca²⁺ is pathway d, and the total energy barrier is 235.3 kJ·mol^－1. The catalysis of water molecules then reduces them to 40.8 kJ·mol^－1 and 141.3 kJ·mol^－1 respectively. The results show that Ca²⁺ has a catalytic effect on the chiral transition of Ala, and water molecule has an excellent catalytic effect on the chiral transition of Ala and Ca²⁺ complexes.