Interaction of Cysteine with Li⁺ and LiF in the Presence of (H₂O)_n (n = 0–6) Clusters

The interaction between cysteine with Li⁺ and LiF in the microcosmic water environment was investigated to elucidate how ions interact with amino acids and the cation–anion correlation effect involved. The structures of Cys·Li⁺(H₂O)_n and Cys·LiF­(H₂O)_n (n = 0–6) were characterized using ab initio calculations. Our studies show that the water preferentially interacts with Li⁺/LiF. In Cys·Li⁺(H₂O)_0–6, Li⁺ interacts with amino nitrogen, carbonyl oxygen, and hydrophobic sulfur of Cys to form a tridentate mode, whereas in Cys·LiF­(H₂O)_n, Li⁺ and F^– work in cooperation and interact with carbonyl oxygen and hydroxyl hydrogen of Cys to form a bidentate type. The neutral and zwitterionic forms are essentially isoenergetic when the water number reaches three in the presence of Li⁺, whereas this occurs at four water molecules in the presence of LiF. Further research revealed that the interaction between Li⁺/LiF and Cys was mainly electrostatic, followed by dispersion, and the weakest interaction occurs at the transition from the neutral form to zwitterionic form. Natural population analysis charge analyses show that for Cys·Li⁺(H₂O)_n, the positive charge is mostly concentrated on Li⁺ except for the system containing three water molecules. For Cys·LiF­(H₂O)_n, the positive charge is centered on the LiF unit in the range n = 0–6, and at n = 5, electron transfer from Cys to water occurs. Our study shows that the contribution of anions in zwitterionic state stabilization should be addressed more generally along with cations.