Synthesis of C-coordinated O-carboxymethyl chitosan metal complexes and evaluation of their antifungal activity

Based on a condensation reaction, a chitosan-derivative-bearing amino pyridine group was prepared and subsequently followed by coordination with cupric ions, zinc ions and nickel ions to synthesize chitosan metal complexes. The calculations using the density functional theory (DFT) show that the copper ions and nickel ions underwent dsp(2) hybridization, the zinc ions underwent sp(3) hybridization, and they all formed a coordination bond with the carbon atom in the p-pi conjugate group. The antifungal properties of O-CSPX-M against Phytophthora capsici (P. capsici), Verticillium alboatrum (V. alboatrum), Botrytis cinerea (B. cinerea) and Rhizoctonia solani (R. solani) were also assayed. Apparently, chitosan metal complexes showed enhanced antifungal activity against four fungi at the tested concentrations compared to that of chitosan. It was shown that Cu complexes can inhibit the growth of P. capsici 100%, and Ni complexes can inhibit the growth of B. cinerea 77.1% at a concentration of 0.4 mg/mL and 0.2 mg/mL, respectively. The pot experiment also verified the result. In addition, the phytotoxicity experiment showed that O-CSPX-M had no obvious toxicity on wheat leaves. This kind of complexes may represent as an attractive direction for chemical modifications of metal fungicides.