Interactions Study between the Copper II Ion and Constitutive Elements of Chitosan Structure by DFT Calculation

Molecular modeling is particularly useful to understand interactions between various kinds of molecules and ions. This study is aimed at studying the interactions between one Cu²⁺ ion and one or several glucosamine residues. The geometries and the interaction energies of all of the complexes involving all of the dimers obtained from glucosamine and N-acetylglucosamine were computed by means of density functional theory (DFT) methods. In a first step, for the two dimers A−A and A−B (A for glucosamine and B for N-acetyl glucosamine), a starting geometry was built, and the energies were calculated using a rigid rotation of 30° intervals for each of the dihedral angles (Φ and Ψ) of the glycosidic bond, spanning the whole angular range. These calculations allowed us to retrieve the minimal energy conformation and investigate all possible conformations. The results were compared to some experimental data. In a second step, we investigated the interactions of Cu²⁺ with the different possible coordination sites of A. For all complexes considered, the Cu²⁺ site was completed with H₂O and/or OH^- ligands to have a global neutral charge. The calculations confirmed that the most stable interactions involved the free amino site in a “pending complex”. Another pending form was possible considering the participation of the heterocyclic O site, but the latter was less favored. On the other hand, we also showed that glucosamine could not act as a bidentate ligand and that N-acetyl glucosamine was not coordinating with Cu²⁺. Finally, our results evidenced a cooperative fixation of Cu²⁺ ions when considering the complexation of two successive metal ions on the two consecutive glucosamine residues of the dimer A−A.