Covalent immobilization of invertase onto the surface-modified polyaniline from graft copolymerization with acrylic acid

Invertase was covalently immobilized on the emeraldine (EM) base form of polyaniline (PAN) films and powders with surface-grafted acrylic acid (AAc) polymer. The immobilization proceeded via the amide linkage formation between the amino groups of invertase and the carboxyl groups of the grafted AAc polymer chains on EM in the presence of a water-soluble carbodiimide. The surface structure and composition of the grafted–modified and enzyme-functionalized EM base were characterized by X-ray photoelectron spectroscopy (XPS). It was found that the amount of immobilized invertase increased linearly with the concentration of surface-grafted AAc polymer chains. EM powders could be graft-modified and enzyme-functionalized more effectively than EM films. The decrease in activity of the immobilized invertase was considered to be due to, among other factors, the reduced accessibility of substrate molecules to the active sites of the enzyme and the conformational change of the invertase molecules as a result of the covalent immobilization. However, the immobilized enzyme was less sensitive to temperature deactivation below the optimum temperature as compared to that of the free form. The optimum pH value of invertase was not affected by the immobilization reaction, but the pH stability range was broadened. The immobilized invertase also exhibited a significantly improved stability during storage in buffer solution over that of the free enzyme.