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

Studies of the adsorption of beta-casein to hydrophobic and hydrophilic sil ica surfaces by time-resolved ellipsometry and direct measurements of the f orces 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 a s the surfaces were brought into closer contact, this was overcome by an at tractive force. Adsorption of beta-casein to hydrophilic surfaces was much slower and the plateau surface excess was higher. Accessibility of the adso rbed protein to endoproteinase Asp-N was also higher on this surface. Inter actions between the coated hydrophilic surfaces were entirely repulsive. Po ssible structures for the adsorbed protein layers are discussed. (C) 1999 E lsevier Science Ltd. All rights reserved.