Unraveling the N-Co coordination mechanism in chitosan-cobalt complexes: Structural elucidation and in vivo cobalt supplementation effect

The interaction energy, enthalpy change, and Gibbs free energy of four coordination models (pendant, bridge 1, bridge 2, and central) were calculated to screen the most thermodynamically favorable mode. The direction of electron transfer was elucidated using FMO analysis (by examining the electron cloud distribution of HOMO/LUMO) and charge decomposition analysis (CDA). The intrinsic nature of the chemical bond was revealed through ESP (to localize regions of positive electrostatic potential), interaction region indicator (IRI, to distinguish coordination bonds from van der Waals interactions), electron localization function (ELF, to evaluate electron delocalization), and atoms in molecules (AIM, to calculate topological parameters of bond critical points).

Notably, ALIE and ESP analyses confirmed that the -NH₂ groups of CS are the preferred coordination sites for Co(II). The central model exhibited the lowest energy. FMO and CDA results demonstrated that CS acts as an electron donor while Co(II) serves as an electron acceptor, with electrons transferring from the -NH₂ groups of CS to Co(II).

The N-Co(II) bond is predominantly ionic with a weak covalent character.