Effect of temperature on the conformation of lysozyme adsorbed to silica particles

Steady-state and lifetime fluorescence spectroscopy was used to study the conformation of hen egg white lysozyme adsorbed to differently charged colloidal silica particles as a function of temperature. While electrostatic interactions appear to be the driving force for adsorption, a decreased charge density of the substrate was found to enhance attractive protein-silica interactions. In the adsorbed state the temperature of unfolding is lowered by about 12-20 degreesC, reflecting a decreased thermal protein stability. Applying a two-state thermodynamic model significantly smaller enthalpy and entropy changes have been found for the temperature-induced unfolding of lysozyme when it is adsorbed to the silica particles. From intrinsic fluorescence lifetime measurements a characteristic change of the lifetime distribution of lysozyme due to adsorption has been observed over a wide temperature range. These results were found to be consistent with an adsorption-induced modification of the lysozyme structure and a spreading of lysozyme on the silica particles in the process of unfolding.