beta-Casein adsorption at the hydrophobized silicon oxide-aqueous solutioninterface and the effect of added electrolyte

The effect of the presence of NaCl, CaCl2, or MgCl2 at the same ionic stren gth on the structure of beta -casein layers adsorbed on hydrophobic surface s has been investigated by neutron reflectivity measurements. The data were fitted to a four-layer model. The volume fraction versus distance profiles have a similar shape whether beta -casein is adsorbed from NaCl, CaCl2, an d MgCl2 of the same ionic strength or whether the protein concentration is lowered 10 times. In particular at larger distances from the surface, the v olume fraction values are low and similar. However, close to the hydrophobi c surface the volume fraction of protein decreases in the order CaCl2 > MeC l2 > NaCl. We have also used a specific proteolytic enzyme, endoproteinase Asp-N, which cleaves off the hydrophilic part of beta -casein, as a tool to reveal the interfacial structure of the protein. For all the different typ es of added electrolytes, endoproteinase Asp N only affects the outermost b eta -casein layer. Subsequent addition of beta -casein in all cases led to large increases in amounts adsorbed and in the thickness of the outer layer s.