The immobilization of proteins by entrapment in optically clear, porous glasses prepared by sol-gel techniques appears to be a promising approach to optical biosensor development. However, little is known about the physical environment of the immobilized protein or the mechanism(s) of entrapment. In this study, absorbance and fluorescence spectroscopies have been used to characterize the properties of two model proteins, bovine serum albumin (BSA) and horse heart myoglobin (Mb), entrapped in wet sol-gel glass bulks. The fluorescence behavior of dissolved and entrapped BSA in the presence of acid, a chemical denaturant, and a collisional quencher was examined. The results show that a large fraction of the BSA added to the sol is entrapped within the gelled glass in a native conformation. However, the reversible conformational transitions that BSA undergoes in solution are sterically restricted in the gel. In contrast, the native properties of Mb are largely lost upon entrapment, as judged by the changes in the visible absorbance spectra of dissolved and entrapped Mb in acidic solutions. Fluorescence studies of dissolved and entrapped apomyoglobin support this conclusion. (C) 1994 Academic Press, Inc.