COADSORPTION AND EXCHANGE OF LYSOZYME BETA-CASEIN MIXTURES AT THE AIR-WATER-INTERFACE

The coadsorption and exchange of lysozyme and β-casein at the air/water interface have been studied using the radiotracer technique. The amount of protein adsorbed from mixtures containing 1.1-1.5 μg/ml of each protein in 0.1M NaCl, pH 7.0 phosphate buffer exceeded the amount expected if the proteins adsorbed in a simple competitive fashion. The β-casein surface concentration was approximately twice the value expected for simple competitive adsorption while the lysozyme coverage was 50-60% of the expected value. Adsorption experiments performed with sequential addition of the two proteins show that β-casein cannot displace adsorbed lysozyme and lysozyme cannot displace adsorbed β-casein. β-casein that is coadsorbed with lysozyme can exchange with β-casein in solution, but lysozyme coadsorbed with β-casein cannot exchange with lysozyme in solution. Also, coadsorption of lysozyme and β-casein does not enable β-casein to displace lysozyme or lysozyme to displace β-casein. While β-casein was able to exchange at 0.1 and 1.0M NaCl, lysozyme was not able to exchange at either salt concentration. The experiments performed at 1.0M NaCl suggest that electrostatic interactions do not play a major role in determining the exchange behavior. Apparently, the flexible, rod-like struture of β-casein allows it to exchange with itself, while the rigid, globular structure of lysozyme prevents it from exchanging. These physical traits of the two proteins also seem to allow β-casein to adsorb in intermolecular spaces which lysozyme cannot access when both proteins are present. A kinetic analysis reflecting this proposed mechanism was able to model the final surface coverages for coadsorption experiments quantitatively and the kinetic data for coadsorption qualitatively. © 1991 Academic Press, Inc. All rights reserved.