Kinetics of adsorption of lysozyme at the air-water interface and the role of protein charge

The adsorption kinetics of hen egg white lysozyme at the air-water interface has been studied using specular neutron reflectometry. Experiments were performed at a number of pH values to examine the effect of charge on the rate of protein adsorption. Solutions of hen egg white lysozyme in air Contrast matched water at 1 mg/mL were made. These allow direct determination of the surface excess of protein. High repetition experiments, with short collection times, were used to accurately determine only the surface excess-derived from the product of the film thickness and the scattering length density of the layer. The kinetic traces at pH values where the protein is charged are well fitted by a first-order rate equation with two linear regions, where the change in the gradient occurs as the surface concentration reaches a steady state. This behaviour is characteristic of the transport and distortion of protein molecules, followed by rearrangement in the surface layer. The equilibrium concentration is a function of protein charge with steady state surface concentrations reaching 1.4 mg m^-2 at pH 4 and 3 mg m^-2 at pH 11. Protein charge is inversely related to the rate of adsorption. This dependency has been explored through thermodynamic analysis.