Lysozyme adsorption at the air/water interface

Abstract

The adsorption of reductively methylated chicken egg white lysozyme (EC 3.2.1.17) to the air/water interface has been measured by the radiotracer technique. This method enables direct determination of surface excess concentration as well as relative rates of adsorption and desorption. Improvements in calibration and radiolabeling techniques led to differences between the present results and previous lysozyme isotherms reported in the literature. The isotherm indicates monolayer saturation at low concentrations of bulk protein (below 2 × 10 −5 wt%) and multilayer adsorption at high concentrations (above 10 −3 wt%). At intermediate concentrations, an abrupt increase in surface concentration with increasing bulk concentration indicates a change in orientation of the adsorbed protein molecules. Adsorption experiments performed by sequential addition of protein to the bulk solution provide evidence that lysozyme molecules adsorbed at concentrations below the multilayer region do not exchange significantly with lysozyme molecules in the bulk solution. A kinetic model which incorporates the monolayer plateau, the multilayer adsorption, and the change of orientation of adsorbed lysozyme is presented.