Evaluation of the structure of adsorbed layers of β-casein from ellipsometry and surface force measurements

Studies of the adsorption of beta-casein to hydrophobic and hydrophilic silica surfaces by time-resolved ellipsometry and direct measurements of the forces between adsorbed beta-casein layers using an interferometric surface force apparatus are reviewed. Adsorption to hydrophobic surfaces was rapid and plateau surface excess and mean thickness values of 2.8 mg m(-2) and 66 Angstrom were measured. Coating the surfaces with protein caused the long-range attractive force to be replaced by a long-range repulsive force and as the surfaces were brought into closer contact, this was overcome by an attractive force. Adsorption of beta-casein to hydrophilic surfaces was much slower and the plateau surface excess was higher. Accessibility of the adsorbed protein to endoproteinase Asp-N was also higher on this surface. Interactions between the coated hydrophilic surfaces were entirely repulsive. Possible structures for the adsorbed protein layers are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.