Molecular confinement influences protein structure and enhances thermal protein stability

The sol-gel method of encapsulating proteins in a silica matrix was investi gated as a potential experimental system for testing the effects of molecul ar confinement on the structure and stability of proteins. We demonstrate t hat silica entrapment (1) is fully compatible with structure analysis by ci rcular dichroism, (2) allows conformational studies in contact with solvent s that would otherwise promote aggregation in solution, and (3) generally e nhances thermal protein stability. Lysozyme, oc-lactalbumin, and metmyoglob in retained native-like solution structures following sol-gel encapsulation , but apomyoglobin was found to be largely unfolded within the silica matri x under control buffer conditions. The secondary structure of encapsulated apomyoglobin was unaltered by changes in pH and ionic strength of KCl. Intr iguingly, the addition of other neutral salts resulted in an increase in th e cr-helical content of encapsulated apomyoglobin in accordance with the Ho fmeister ion series. We hypothesize that protein conformation is influenced directly by the properties of confined water in the pores of the silica. F urther work is needed to differentiate the steric effects of the silica mat rix from the solvent effects of confined water on protein structure and to determine the extent to which this experimental system mimics the effects o f crowding and confinement on the function of macromolecules in vivo.