Effect of temperature on the adsorption of lysozyme at the silica/water interface studied by optical and neutron reflectometry

The effect of temperature on the adsorption of hen egg white lysozyme at the silica/water interface has been studied. By use of optical reflectometry, the adsorbed mass per surface area has been determined over a large temperature range with protein solution concentrations ranging from 0.03 to 0.09 mg mL(-1). It has been found that the amount of adsorbed protein is strongly increased by an increase in temperature, which indicates an endothermic and thus an entropy-driven adsorption process. This can be explained by an adsorption-induced modification of the protein conformation. At high temperatures, where dissolved lysozyme is mainly unfolded, hydrophobic dehydration effects will play a role as well. The results support the concept that thermodynamically unstable proteins adsorb more strongly at interfaces than stable proteins The positive enthalpy of adsorption suggests that significant repulsive electrostatic interactions between the protein molecules of the adsorbate are present. Concentration profiles of lysozyme adsorbates have been determined at 23, 63, and 80 degreesC using neutron reflectometry. At each temperature, three reflectivity curves have been measured applying the contrast variation method. The reflectivities have been fitted globally on the basis of a four-layer model, Si/SiO2/adsorbate/solution. The obtained concentration profiles clearly show that in the range of 23-63 degreesC the temperature-induced increase of the degree of lysozyme adsorption at the silica/water interface is caused in part by a denser packing of the lysozyme,in the adsorbate. Whereas the neutron reflectivity data are consistent with a monolayer adsorption at 23 and 63 degreesC, at least two layers of protein molecules are adsorbed at 80 degreesC.