Comparative study on interaction between copper (II) and chitin/chitosan by density functional calculation

The DMol3 calculations, based on density functional theory (DFT), have been employed to investigate the interactions between Cu2+ and chitin/chitosan residues. The possible initial conformations were optimized at the generalized gradient approximation (GGA) level, with spin unrestricted approach, symmetric unrestriction, doublet multiplicity and BLYP/DND methods. For all initial complexes considered, the Cu2+ was completed with H2O and/or OH− groups to neutralize the initial complexes with hexacoordination geometries. The tendency of ligands to coordinate with Cu2+ is NH2 > C3OH > H2O > NHCOCH3, suggesting that amine groups (NH2) on chitosan prefer to bind Cu2+ and acetamide groups (NHCOCH3) on chitin lose their coordination with Cu2+ in aqueous solution. The geometries of bridge and pendant models have been comparatively analyzed. The results show that bridge model is more favorable than pendant model. In terms of the optimized geometries, the initial hexacoordination structures of Cu2+ designed seem more reasonable than initial tetracoordination ones designed.