Microfine magnetic particles of magnetite with a mean diameter of 10-15 nm, which is in the range of superparamagnetism, were prepared by coprecipitation from ferrous and ferric electrolyte solution and applied to the enzyme immobilization support. The surface of the particles was treated by a series of amino-functional silane coupling agents capable of covalently bonding with enzyme. A protease, thermolysin, was immobilized on the particles and transferred to ethyl acetate solution containing N-Cbz-l-aspartic acid and l-phenylalanine methyl ester. The dipeptide, aspartame precursor synthetic reaction was then carried out. Higher enzyme activity was found when the silane coupling agents with long spacer arms between the silane functionality and amino functionality were used such as N-(6-aminohexyl)-3-aminopropyltrimethyoxysilane, compared to conventional 3-aminopropyltrichoxysilane. Magnetic recovery of the immobilized enzyme was also demonstrated upon the application of an external magnetic field.
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