Chelation of zinc(II) with poly(γ-glutamic acid) in aqueous solution: kinetics, binding constant, and its antimicrobial activity

The binding of zinc(II) ions onto γ-pga was studied in aqueous solution as a function of contact time, solution pH, initial concentration of metal ion, and solution temperature, respectively. IR and 13C NMR spectra of γ-pga and Zn(γ-pga) complex revealed binding of zinc(II) with carboxylate and amide groups on γ-pga. The strong interaction between zinc(II) ions and γ-pga occurred at pH 5. The binding kinetic data followed a pseudo-second-order kinetic model. The equilibrium binding data were fitted well by Langmuir isotherm model. The maximum amount of zinc(II) ions bound to γ-pga was estimated to be 13.97 mmol/g at 30 °C and 20.58 mmol/g at 45 °C. The values of activation energy (Ea 18.23 kJ/mol), changes in free energy (∆G°), enthalpy (∆H°), and entropy (∆S°) indicate that the formation of Zn(γ-pga) complex is an endothermic spontaneous physisorption process. The antimicrobial activity of Zn(γ-pga) complex was examined against a gram-negative bacteria (i.e., Escherichia coli) and a gram-positive bacteria (i.e., Bacillus subtilis). The IC50 values of Zn(γ-pga) complex were estimated to be (0.76 ± 0.03) mmol/L for E. coli and (0.94 ± 0.02) mmol/L for B. subtilis, respectively. Therefore, Zn(γ-pga) complex can be used as an antimicrobial agent against gram-positive and gram-negative microorganisms.