C-coordinated O-carboxymethyl chitosan metal complexes: Synthesis, characterization and antifungal efficacy

A novel type of O-carboxymethyl chitosan Schiff bases (O-CSPX) was synthesized via a condensation reaction. After the coordination reaction of cupric ions, zinc ions and nickel ions, metal complexes (O-CSPX-M) were achieved. The theoretical structure of O-CSPX-M calculated by Gaussian 09 reveals that the copper ions and nickel ions underwent dsp2 hybridization, the zinc ions underwent sp3 hybridization, and they all coordinated by the carbon atom in the p-π conjugate group. Then, the structures were confirmed by FT-IR, 1H NMR, CP-MAS 13C NMR, elemental analysis, DSC and XRD. The antifungal properties of O-CSPX-M against Phytophthora capsici (P. capsici), Gibberella zeae (G. zeae), Fusarium oxysporum (F. oxysporum) and Botrytis cinerea (B. cinerea) were evaluated at concentrations ranging from 0.05 mg/mL to 0.40 mg/mL. The experiments indicated that the derivatives have significantly enhanced antifungal activity after metal ions complexation compared with the original chitosan. Moreover, it was shown that 0.20 mg/mL of O-CSPX-Cu can 100% inhibit the growth of P. capsici and 0.20 mg/mL of O-CSPX-Ni can 87.5% inhibit the growth of B. cinerea. In addition, the phytotoxicity assay and cell viability assay were also evaluated. The experimental results may provide a novel direction for the development of metal fungicides.