Structural evolution and stability of sol-gel biocatalysts

Immobilisation strategies for catalytic enzymes are important as they allow recovery and reuse of the biocatalysts. In this work, sol-gel matrices have been used to immobilise Candida antarctica lipase B (CALB), a commonly used industrial enzyme. The sol-gel bioencapsulate is produced through fluoride-catalysed hydrolysis of mixtures of tetramethylorthosilicate (TMOS) and methyltrimethoxysilane (MTMS) in the presence of CALB, yielding materials with controlled pore sizes and surface chemistries. Sol-gel matrices prolong the catalytic life and enhance the activity of CALB, although the molecular basis for this effect has yet to be elucidated due to the limitations of analytical techniques applied to date. Small angle neutron scattering (SANS) allows such multi-component systems to be characterised through contrast matching. In the sol-gel bioencapsulate system at the contrast match point for silica, residual scattering intensity is due to the CALB and density fluctuations in the matrix. A SANS contrast variation series found the match point for the silica matrix, both with and without enzyme present, to be around 35%. The model presented here proposes a mechanism for the interaction between CALB and the surrounding sol-gel matrix, and the observed improvement in enzyme activity and matrix strength. Essentially, the inclusion of CALB modulates silicate speciation during evolution of the inorganic network, leading to associated variations in SANS contrast. The SANS protocol developed here may be applied more generally to other encapsulated enzyme systems