Intermolecular hydrogen bond interactions in water clusters of zwitterionic glycine: DFT, MESP, AIM, RDG, and molecular dynamics analysis

Abstract

The putative global minima geometries of zwitterionic glycine-water complexes [zGly.Wn (1 = 1–10)] are controlled by the cyclic network of water molecules bonded to the zwitterionic glycine. Their stability is governed by the number of water-water interactions along with the existence of the intramolecular glycine-water bonds. The computed intramolecular distances suggest that water molecules tend to coordinate within themselves. The computed association energy attains saturation at 6, implying that up to 6 water molecules are coordinated to the zwitterionic glycine. Further addition of water molecules happens to be an intramolecular addition to the existing water. The computed incremental association energy decreases with the increase in the number of water molecules, except for the six and nine water molecules. This implies that compared to the water clusters, the zwitterionic clusters are less stable. The higher stability of n = 6 and 9 can be marked to the resemblance of the cyclic network of water molecules to the bare global minima water clusters. Benchmarking calculations suggest that M06-2X-D3 functional provides binding energy close to the PW6B97D3(BJ) method. The PGM clusters' thermodynamic parameters show that the reaction's enthalpy remains constant and is negative, indicating that the hydration process is exothermic and independent of the number of water molecules. The electron density difference (EDD) diagram shows the build-up of charge is more pronounced in the intramolecular hydrogen bonds (HBs) between water molecules than between zwitterionic glycine and water. QTAIM analysis shows that the water-water HBs are comparatively weaker than zwitterionic glycine-water interactions.