Efficient immobilization of lipases by entrapment in hydrophobic sol-gel materials

The commercial application of lipases as biocatalysts for organic synthesis requires simple but efficient methods to immobilize the enzyme, yielding highly stable and active biocatalysts which are easy to recover. In this study, we present a novel method to achieve lipase immobilization by entrapment in chemically inert hydrophobic silica gels which are prepared by hydrolysis of alkyl-substituted silanes in the presence of the enzyme. A typical immobilization procedure uses: an aqueous solution of lipase; sodium fluoride as a catalyst; and additives like polyvinyl alcohol or proteins and alkoxysilane derivatives like RSi-(OMe)(3) with R = alkyl, aryl, or alkoxy as gel precursors. The effect of various immobilization parameters like stoichiometric ratio of water, silane, type and amount of additive, type and amount of catalyst, and type of silane has been carefully studied. The new method is applicable for a wide variety of lipases, yielding immobilized lipases with esterification activities enhanced by a factor of up to 88, compared to the commercial enzyme powders under identical conditions. Studies on the stability of sol-gel immobilized lipases under reaction conditions or storage (dry, in aqueous or organic medium) revealed an excellent retention of enzymatic activity. The possible reasons for the increased enzyme activities are discussed.