COMPETITIVE AND SEQUENTIAL ADSORPTION OF BETA-CASEIN AND BETA-LACTOGLOBULIN ON HYDROPHOBIC SURFACES AND THE INTERFACIAL STRUCTURE OF BETA-CASEIN

The competitive and sequential adsorption of γ-casein and γ-lactoglobulin on hydrophobized silica surfaces is studied by in situ ellipsometry. Native γ-casein, as well as the segments 1-93 and 1-42, constituting the hydrophilic part of γ-casein, are used in this study. Both the adsorption kinetics and the total amount adsorbed are quite different for γ-lactoglobulin as compared to γ-casein. Although the amount adsorbed is larger for γ-casein, the number of molecules adsorbed per surface area unit following a rinse with pure buffer is nearly the same, since almost none of the adsorbed γ-lactoglobulin desorbs. A preadsorbed layer of γ-casein, but not γ-CN (1-93) (γ-casein (1-93)), prevents a sequential adsorption of γ-lactoglobulin. Addition of endoproteinase Asp-N to an adsorbed layer of γ-casein reduces the adsorbed amount by approximately 20%, suggesting a cleavage at residues Asp 43 and/or Asp 47. Furthermore, it is possible to adsorb γ-lactoglobulin on this enzyme-treated surface. The proteinase has only a minor effect on adsorbed layers of γ-CN (1-93). Our results indicate that the large C-terminal hydrophobic domain, absent in γ-CN (1-93), is essential for the orientation at the interface of the N-terminal hydrophilic moiety of γ-casein. The N-terminal part of an adsorbed γ-casein molecule is then likely to protrude into the solution and thus hinder the sequential adsorption of γ-lactoglobulin. © 1994 by Academic Press, Inc.