Catalytic and spectroscopic properties of cytochrome-c, horseradish peroxidase, and ascorbate oxidase embedded in a sol-gel silica matrix as a function of gelation time

In this study, we investigated the optical features of the redox metal-dependent proteins cytochrome-c, horseradish peroxidase (HRP), and ascorbate oxidase embedded in a sol-gel-processed silica matrix as a function of gelation time. Circular dichroism, absorbance, and fluorescence spectroscopies revealed that the sol-gel process affects the complex structure of the dimeric ascorbate oxidase (although the prosthetic coppers still remain bound to the enzyme) but not that of monomeric cytochrome-e and HRP. Any modifications in ascorbate oxidase occurred in the initial gelation phase; the drying process induced no further alterations and the enzyme remained stable for months. Unfolding-refolding experiments on cytochrome-e revealed severely restricted motility in the protein moiety in the xerogel, the concentrated matrix that forms after drying. The diffusion time of the solvent within the matrix, which regulated the enzyme-substrate reaction rate, depended on the thickness of the monolith, not on the dryness of the specimen.