盐水溶液中离子与丙氨酸极性基团间的作用对丙氨酸缔合的影响:密度泛函理论与分子动力学模拟

采用密度泛函理论和经典分子动力学模拟研究了盐水溶液中Na⁺、Cu²⁺、Zn²⁺、Cl^-与丙氨酸分子间的相互作用对丙氨酸分子缔合的影响. 密度泛函理论的计算结果显示丙氨酸分子与Na⁺、Cu²⁺、Zn²⁺、Cl^-之间的相互作用可增强其电荷分离. 经典分子动力学模拟结果显示在水溶液中两性离子形式的丙氨酸存在三种缔合结构.盐水溶液中, 阳离子、阴离子与丙氨酸间的相互作用均能一定程度上减弱丙氨酸分子的缔合. 但是阳离子与丙氨酸间的相互作用明显受离子水合作用的影响. 由于Cu²⁺水合作用较强, 虽在气相中Cu²⁺与丙氨酸分子之间相互作用明显比Na⁺强, 但是在水溶液中则情况刚好相反. 在ZnCl₂稀溶液中, Zn²⁺与丙氨酸间的相互作用被其第一水合壳层隔开. 但这种相互作用仍能明显影响丙氨酸分子的缔合, 这与Zn²⁺的水合壳层特征有关. 另外, 离子与丙氨酸之间的相互作用, 不仅会削弱丙氨酸的缔合, 也可导致丙氨酸分子间的缔合结构发生转变. 离子浓度也会影响其与丙氨酸分子间的缔合形式以及丙氨酸的缔合结构.

Effects of Interactions between Ions and Alanine Polar Groups on Alanine Associations in Saline Solution: Density Functional Theory and Molecular Dynamics Simulation

Density functional theory (DFT) and classical molecular dynamics simulations were used to study the effects of the interactions between zwitterionic alanine and some ions (Na⁺, Cu²⁺, Zn²⁺, and Cl^-) in saline solution on the association of alanine molecules. The DFT calculation results show that the association of alanine with these ions can enhance charge separation of zwitterionic alanine. Classical molecular dynamics simulation results also show that three associated structures of zwitterionic alanine molecules are present in alanine aqueous solution, and the associations can be weakened to a certain extent by the interactions between the cations/anions and alanine polar groups. The interaction between a cation and the carboxyl group of alanine can be greatly affected by hydration of the cation in dilute saline solution. The interaction between Cu²⁺ and alanine is much stronger than that between Na⁺ and alanine in the gas phase, but the situation is reversed in dilute aqueous solution, because the hydration of Cu²⁺ is much stronger than that of Na⁺. In dilute ZnCl₂ aqueous solution, the interaction between Zn²⁺ and the carboxyl group of the alanine molecule is less direct, because of the first hydration shell of Zn²⁺. However, indirect interactions between Zn²⁺ and alanine still lead to a decreased association among alanine molecules. In addition, the interactions of cations/anions with alanine not only weaken the association between alanine molecules, but also result in transformation between two typical conformations of associated alanine molecules. The ion concentration affects the conformations of associated cation/anion-alanine species, and associated alanine molecules.