The binding of zinc(II) ions onto gamma-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(13)C NMR spectra of gamma-pga and Zn(gamma-pga) complex revealed binding of zinc(II) with carboxylate and amide groups on gamma-pga. The strong interaction between zinc(II) ions and gamma-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 gamma-pga was estimated to be 13.97 mmol/g at 30 degrees C and 20.58 mmol/g at 45 degrees C. The values of activation energy (E(a)18.23 kJ/mol), changes in free energy ( increment G degrees), enthalpy ( increment H degrees), and entropy ( increment S degrees) indicate that the formation of Zn(gamma-pga) complex is an endothermic spontaneous physisorption process. The antimicrobial activity of Zn(gamma-pga) complex was examined against a gram-negative bacteria (i.e.,Escherichia coli) and a gram-positive bacteria (i.e.,Bacillus subtilis). The IC(50)values of Zn(gamma-pga) complex were estimated to be (0.76 +/- 0.03) mmol/L forE. coliand (0.94 +/- 0.02) mmol/L forB. subtilis, respectively. Therefore, Zn(gamma-pga) complex can be used as an antimicrobial agent against gram-positive and gram-negative microorganisms.