Driving Forces for the Adsorption of Enzymes at the Water/Silica Interface Studied by Total Internal Reflection Fluorescence Spectroscopy and Optical Reflectometry

Abstract

Total internal reflection fluorescence (TIRF) spectroscopy has been used to study conformational changes of hen egg white lysozyme induced by interaction with the water/quartz interface. TIRF spectra have been measured over a large temperature range and are compared to the corresponding solution spectra. It has been found that the wavelength of maximum fluorescence intensity of adsorbed lysozyme is red-shifted by about 7 nm relative to that of dissolved lysozyme in the temperature range of about 20–60°C. This observation indicates that lysozyme is partially unfolding when it is adsorbing on quartz. Using optical reflectometry a drastic temperature-induced increase of the degree of adsorption of lysozyme and staphylococcal nuclease (SNase) on silicon wafers has been measured which suggests that the corresponding adsorption processes are endothermic and thus entropically driven. The major contribution to this entropy gain will probably originate from conformational changes at lower temperatures. The experimental results indicate that proteins with a smaller Gibbs energy of unfolding have a higher tendency for adsorption. Above the temperatures of unfolding of the proteins in solution, the dehydration of hydrophobic residues, which are exposed to water in the thermally unfolded state, are the most likely driving force for the adsorption of lysozyme and SNase on silicon oxide.