Interaction of glycine with Li⁺ in the (H₂O)_n (n = 0-8) clusters

We investigated the interaction between glycine and Li⁺ in water environment based on the Gly·Li⁺(H₂O)_n (n = 0-8) cluster. Our study shows that for Gly·Li⁺, Li⁺ binds to both carbonyl oxygen and amino nitrogen to form a bidentate structure, and the first three water molecules preferentially interact with Li⁺. For n = 0-5, the complexes of Gly·Li⁺(H₂O)_n exist in neutral form, and when the water number reached 6, the complex can coexist in neutral and zwitterionic form, then zwitterionic structures are dominant for n = 7, 8. The analyses by RDG, AIM, and ESP in conjunction with the calculated interaction energies show that the interaction between Li⁺ and Gly decreases gradually with the water molecules involved successively from n = 1 to 6 and then increases for n = 7-8. Additionally, the infrared spectra of Gly·Li⁺(H₂O)_n (n = 0-8) are also calculated.

The initial structures were optimized using Gaussian 09 program package in B3LYP-D3 (BJ)/6-311G(d, p) method, and the frequency was calculated with 6-311 + G(2d, p) basis set. GaussView5.0.9 was used to view simulation infrared spectra. The noncovalent interaction method (NCl), energy decomposition (EDA), atoms in molecules (AIM) analysis, and electrostatic potential (ESP) analyses were conducted using Multiwfn software to gain a deeper understanding of the interaction properties of Gly, Li⁺, and water.