Preparation And Papain Immobilization Of Mesocellular Siliceous Foams

Siliceous mesocellular foam (MCF), a new material with continuous 3D pore structure connected by window, the largest pore size compared to other mesoporous materials and high hydrothermal stability, may be one of the most promising supports for enzyme immobilization. As an environmently friendly biocatalyst, immobilized enzyme can catalyze some chemical reactions highly selectively and efficiently under gentle reaction conditions. This dissertation focuses on the preparation and structure characterization of MCF materials, the immobilization of papain on MCF, and the properties and the stabilities of immobilized papain.Siliceous mesocellular foam with pore size ranging from 12.4 to 30.1 nm, and window size from 3.2 to 18.5 nm, was successfully synthesized by the acid catalyzed sol-gel reaction of tetraethoxysilane (TEOS) in the presence of triblock copolymer P123 as a structure-directing agent, trimethylbenzene (TMB) as a swelling agent and ammonium fluoride (NH4F) as a window size regulator. The amino-functionalized MCF (NH2-MCF), prepared via post-grafting method with aminopropyltrimethoxyl -silane (APTMS) as silane additive, had been demonstrated to possess a desirable mesoporous structure by means of XRD, TEM, FT-IR and N2 adsorption.Both MCF and NH2-MCF material were employed as carriers for papain immobilization. The properties of immobilized enzyme were closely related to the window size of MCF. A window size nearly equal to the diameter of enzyme led to the highest adsorbed amount and activities of immobilized enzyme. Compared to unmodified MCF materials, the animo-functionalized MCF was more efficient in the enzyme immobilization. It was shown that the experimental conditions for enzyme immobilization affected the properties and the stabilities of immobilized papin. The optimal conditions of papain immobilization could be summarized as follows: papain concentration, 0.67 mg/ml; pH, 7.0; temperature, 37℃and immobilization time, 6h.The optimal pH and reaction temperature of the immobilized papain prepared under the above optimal conditions were 7.5 and 50℃, compared to 7.0 and 40℃for the free papain, respectively. The Michaelis constant (Km) of immobilized papain was disclosed as 6.99×10-3 mol/L by the Lineweaver-Burk plot at 37℃. After stored at 4℃for 60 days, the immobilized papain retained 75% of its activity, compared to 53.6% for the free papain. About 65.1% of the immobilized papain activity remained after repeating the catalysis of casein for 8 times. Due to the large pore microenvironment and the shield of the mesopores of MCF, the pH, thermal, operational and storage stabilities of the immobilized enzyme had been considerably improved, which was beneficial to the reuse and storage of immobilized enzyme. Siliceous mesocellular foam can be considered as excellent supports for papain immobilization, showing a good prospect for industrial applications.