Charakterisierung und rationale Immobilisierung von Lipasen in biphasischen Reaktionssystemen
Despite the broad use of lipases in ester synthesis procedures, crucial aspects on optimal pH and enzyme behaviour in biphasic reaction media are still not well-known. Such information is highly desired for the rational process design. Therefore, the influence of the pH in lipase-based esterifications in biphasic media was studied. For Candida rugosa and Thermomyces lanuginosa lipases the pH-optima of lipase-catalysed esterifications were pH 3.5 and 4.25, respectively, what significantly differ from the optima of the hydrolysis reaction. Enzyme activity increased with increasing concentrations of protonated acid, allegeable by the reaction mechanism: Different from the protonated, the deprotonated acid cannot react with the nucleophilic serin residue due to its delocalized negative charge. Hence the protonated acid is the substrate for esterification. Conclusively, high synthetic activity is only accessible at lower pH. The importance and generality of this conclusion was verified by testing of other lipases under the same reaction conditions. The pH drop in biphasic systems – caused by enrichment of the acid in the aqueous phase – is not a severe problem as assumed by many researchers and efforts for pH-control are dispensable. Moreover, a strong linear correlation between the enzyme activity and the specific interface was shown for esterifications. An indepth analysis of the experiments in consideration of the thermodynamic facts resulted in the development of a detailed hypothesis about the nature of lipase-catalysed esterification in biphasic systems. The ester synthesis predominantly takes places at the interface, thus in interest of high productivity the specific interface needs to be maximized. Based on this hypothesis, the application of lipase containing hydrogels is useless and an alternative concept for the immobilisation of lipases in biphasic systems was developed. The innovative concept – the static emulsion – consists in the emulsification of an aqueous lipase solution within a hydrophobic silicone elastomer. Spherical immobilisates were obtained with a gum-like consistency. The catalytic and mechanic properties of the static emulsion were closely characterised. According to their mechanical properties, the static emulsion is superior to hydrogels and sol-gels. For all investigated lipases it was shown that the immobilisation comes along with a strong enhancement of the catalytic activity, which was so far only accessible in a similar extent by immobilization in alkylsubstituted sol-gels. The practical suitability was demonstrated by the solvent-free synthesis of fatty acid ethyl hexyl ester, propyllaurate and the kinetic resolution of racemic benzoine in tetrahydrofuran and makes clear that the static emulsion is a method with high future potential, since it enables an improved performance of immobilised lipases.
info:eu-repo/classification/ddc/570, Biowissenschaften, Biologie, Enzymtechnologie, Biokonversion, Biotechnologie, Bioverfahrenstechnik, Immobilisierung, Lipasen, enzyme technology, immobilisation, lipase, silicone, static emulsion
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