The influence of composition of poly(n-isopropylacrylamide-co-itaconic acid) hydrogel on immobilized Candida rugosa lipase activity

The application of lipases as catalysts in chemical reactions has been deterred by the high cost of isolation and purification of enzymes, the instability of their structure when they are isolated from their natural environment, contamination of products with residual protein, their sensitivity to process conditions, etc. These problems could be overcome using immobilized lipases. Immobilization is achieved by fixing enzymes to or within solid supports and as a result a heterogeneous system is obtained. The present paper reports on the immobilization of Candida rugosa lipase in hydrogels based on N-isopropylacrylamide and itaconic acid. Immobilization of lipase is carried out by two different methods. In the first method, lipase is added to the reaction mixture before polymerization and crosslinking (in situ polymerization), while in the second method the synthetized hydrogels are immersed in lipase solution and left to rich the equilibrium swelling. The specific activities of the immobilized lipase were determined in both cases and compared. The amount of the immobilized lipase is higher if the immobilization is carried out by immersing hydrogel in lipase solution. It was observed that in both cases lipase activity increases with an increase of the itaconic acid content up to 10 wt% and thereafter decreases. From the measurements of shear storage moduli (G') it was concluded that the increase of the itaconic acid content decreases the mechanical properties of the hydrogels. SEM analysis confirmed the highly porous structure of hydrogels. It was found that greater pores were achieved when the enzyme was immobilized by in situ polymerization. When the enzyme was immobilized by in situ polymerization the itaconic acid content had not great effect on the mass of the immobilized enzyme, except for the 100/0 sample. On the contrary, for the samples where the enzyme was immobilized by swelling, the increase of the itaconic acid content increases the mass of the immobilized enzyme. Concerning the activity of the immobilized lipase, the swelling degree and mechanical properties of the investigated hydrogels, the best results were performed by the 95/5 hydrogel sample