Influence of the sol–gel chemistry on the activity of a lipase encapsulated in a silica aerogel

Silica aerogels have been shown to be efficient encapsulation media for the lipase of Burkholderia cepacia. The present study has focused on the encapsulation of this lipase in an aerogel made from 80% tetramethoxysilane (TMOS) and 20% methyltrimethoxylsilane (MTMS), dried by the supercritical CO2 method. By varying different parameters in the synthesis chemistry of such materials, the structure and texture of the resulting gels can be significantly affected. The aim of the present study was to examine the possible existence of correlations between modifications of the gel’s synthesis procedure, and the catalytic activity of the gel-encapsulated lipase in the esterification reaction of lauric acid with 1-octanol. The synthesis parameters studied included aging of the wet gels in different solvents, variation of the solvent used during gel synthesis, variation in the molar ratio of hydrolysis water to silicon precursor and replacement of MTMS by another alkoxide. The biocatalytic activity was found to depend significantly on these different treatments. The results were analyzed in the light of the gel texture and structure characterization, respectively analyzed by nitrogen adsorption isotherms and 29Si, 1H and 13C NMR. These results suggest that the main role of the aerogel is to maintain the enzyme dispersed as it would be in an aqueous solution, even though it is used in an organic solvent where the lyophilized enzyme is insoluble. The nature of the gel surface groups, in particular their capability to modify the substrates concentration inside the gel, close to the active site of the enzyme, by comparison with the concentration in the solvent outside the gel, seem to have a more secondary effect.