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

The computational chemistry section employed quantum chemical tools (density functional theory DFT) to investigate the coordination mechanism between chitosan (CS) and Co(II). The core research hypothesis is that Co(II) tends to accept electrons from the -NH₂ groups of CS rather than the -OH groups to form CS-Co(II) complexes. The data analysis focused on four key aspects: confirmation of coordination sites, screening of coordination modes, electron transfer mechanism, and intrinsic nature of chemical bonding. Chitobiose was used as a simplified model of CS. Three chitobiose configurations were optimized via DFT calculations at the B3LYP/6-311G(d,p) basis set level (with the LanL2DZ basis set for Co(II)), followed by comparisons of their energies, frontier molecular orbital (FMO) parameters. The coordination sites were confirmed by combining electrostatic potential (ESP) and average local ionization energy (ALIE) analyses. 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.