Fig. 1. Model esterification of lauric acid (1) by n-octanol (2) catalyzed by a lipase.
Fig. 2. Encapsulation of a lipase in hydrophobic sol-gel materials.
Usually methyltrimethoxysilane CH3Si(OCH3)3 (MTMS) was used as the precursor and polyvinyl alcohol (PVA) as an additive, the latter possibly acting as a stabilizer of the lipase (17,18).
An important extension of this method pertains to the use of additional porous solid supports during the sol-gel process (19). This type of "double immobilization" involves binding of the lipase-containing gels in the pores of the solid support (e.g., silicates of the type SIRAN® or Celite®) as gelation occurs (see Fig. 3). Such a process results in higher mechanical stability and enhanced enzyme activity (up to a factor of 88).
It should be noted that sol-gel encapsulation is crucial in both variants because conventional adsorption on hydrophobic silicates or on SIRAN alone affords poor catalysts. The structural and morphological properties of the lipase immobilizates were characterized by scanning electron microscopy (SEM), solid state 29Si and 13C NMR spectroscopy, and in studies concerning specific surface area and pore volume (20). Moreover, kinetic studies clearly point to an "alkyl effect" (i.e., enhancement of lipase-activity upon using RSi(OCH3)3 in the series methyl < ethyl < n-propyl < «-butyl) (20). Enhanced hydrophobicity in the silicon oxide matrix correlates with increased enzyme activity. Higher thermal stability and activity appear to result from multipoint interactions through hydrogen bonding as well as ionic and hydrophobic interactions (van der Waals), which can be schematized as shown in Fig. 4. Hydrophobic interactions can result in a type of interfacial activation. The lipase may be conformationally arrested in the matrix in a "lid-opened" and therefore active form. The early work on lipase entrapment in hydrophobic sol-gel materials has been reviewed (21).
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