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

Abstract

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 Zn Cl2 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.