Wetting of β-casein layers adsorbed at the solid-aqueous interface

The wetting by water of the adsorbed layer of beta-casein on hydrophobised silica and pure (hydrophilic) silica surface was investigated by dynamic contact angle measurements based on the Wilhelmy plate principle. The results are discussed in relation to adsorption data obtained for the protein on similar surfaces by in situ ellipsometry, beta-casein adsorption on a hydrophobic surface leads to a significant decrease of the contact angle, in particular in terms of the receding contact angle, which decreased by about 70 degrees. This indicates a strong shielding of the hydrophobic surface by the hydrophilic domain of beta-casein. Adding a specific enzyme, endoproteinase Asp-N, which previously has been proposed to remove a large fraction of the hydrophilic segments, results in a significantly decreased wettability of the solid surface. The layer is now more hydrophobic and the hysterises is much smaller. The receding contact angle after the proteolysis is roughly 70 degrees. The results are consistent with the hypothesis that beta-casein adsorbs at the hydrophobic surface to form a monolayer with the hydrophobic part of the protein anchored at the surface, leaving the hydrophilic segments dangling into the solution. Less dramatic effects are observed in terms of changes of the wettability on the hydrophilic surface. The surface is still quite hydrophilic both after adsorbing beta-casein and exposing the layer to endoproteinase Asp-N. These results confirm the differences in the structure of beta-casein layers on the hydrophobic and hydrophilic surface. (C) 1999 Elsevier Science B.V. All rights reserved.