It is highly desirable to study the kinetics and spectroscopy of enzymes in a crowded and controllable microenvironment. In this work, we employ mesoporous silica of matching pore sizes to confine lysozyme in order to mimic enzyme in a crowded environment. The stability and activity of lysozyme immobilized in mesoporous silica nanoparticle (MSN) of various pore sizes were studied and correlated to spectroscopic data of the immobilized enzyme. By site-selective surface functionalization, we were able to avoid protein adsorbing on the external surfaces of MSNs and study specifically the protein immobilized in the nanochannels. Solution spectroscopic methods, CD and fluorescence, were used to study the secondary and tertiary structures of the immobilized enzyme because MSNs could be suspended very well in solution. To study the catalytic activity of lysozyme, we employed 4-methylumbelliferyl β-d-N,N,N′- triacetylchitotrioside as a substrate that was hydrolyzed and detected by fluorescence spectroscopy. 8-Anilino-1-naphthalenesulfonic acid was utilized as a fluorescence probe to characterize the protein-binding site. The conformation, thermal stability, and catalytic activity of lysozyme were sensitive to the curvature of the silica materials. The activity of the lysozyme immobilized in the 5.6 nm mesopores of MSNs was higher than those of native enzymes. The enhanced activity was attributed to subtle change in tertiary structure of lysozyme in the crowded microenvironment in the mesopores.
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