Immobilization of Glucoamylase onto Lewis Metal Ion Chelated Magnetic Affinity Sorbent: Kinetic, Isotherm and Thermodynamic Studies

In this study, magnetic metal-chelate beads, m-poly(ethylene glycol dimethacrylate-vinyl imidazole) [m-poly(EGDMA-VIM)] with an average diameter 150-200 m was synthesized by copolymerizing ethylene glycol dimethacrylate (EGDMA) with vinyl imidazole (VIM). The spesific surface area of the m-poly(EGDMA-VIM) beads was found 63.1 m2/g. Cu2+ ions were chelated on the m-poly(EGDMA-VIM) beads and used in immobilization of Aspergillus niger glucoamylase in a batch system. The maximum glucoamylase adsorption capacity of the m-poly(EGDMA-VIM)-Cu2+ beads was observed as 120 mg/g at pH 6.5. The optimum pH for free and m-poly(EGDMA-VIM)-Cu2+ immobilized glucoamylase were found 4.0 and 4.5, respectively. The optimum temperature of glucoamylase was not changed after immobilization and determined as 60oC for free and immobilized enzyme preparations. The glucoamylase adsorption capacity and adsorbed enzyme activity slightly decreased after 10 batch successive reactions, demonstrating the usefulness of the enzyme-loaded beads in biocatalytic applications. Storage stability was found to increase with immobilization. The effect of various experimental parameters such as pH, glucoamylase concentration, contact time and temperature in aqueous solution were also investigated. Adsorption isotherm obtained for m-poly(EGDMA-VIM)-Cu2+ was consistent with Langmuir model. Kinetic studies showed that the adsorption process agreed with both the pseudo-second-order kinetic model and the modified Ritchie's-second-order kinetic model. Various thermodynamic parameters, free energy (G0), enthalpy (H0) and entropy (S0), were also calculated and the results showed that the adsorption process strongly depended on temperature of medium.