Beta-casein adsorption at the silicon oxide-aqueous solution interface: calcium ion effects

Neutron reflectometry was used to investigate effects of calcium ions on the interfacial behavior of beta-casein at the silicon oxide-aqueous solution interface. The structural characteristics of the adsorbed layer were determined from reflectivity curves fitted to three- and two-layer optical models. The results showed that the presence of divalent calcium ions decreased the specific electrostatic adsorption affinity of the protein to silica compared with the calcium-free buffer system studied in an earlier work. In addition, it speeded up the adsorption suggesting that the slow kinetics seen in the calcium-free system are related to conformational adjustments of the beta-casein structure driven by the maximization of the number of positive charges on the polypeptide interacting with negative surface charges. In the calcium-free system, a dense inner layer resulted from this process, with cationic segments firmly bound to the negative surface, whereas in the presence of calcium, a less dense inner layer was formed. The difference in binding is also mirrored by the effects on the interfacial layer of a specific proteolytic enzyme, i.e., endoproteinase Asp-N. In the calcium-free case, an inner dense layer remained at the surface after the proteolytic cleavage of the polypeptide, whereas virtually nothing was left after enzymatic action in the presence of calcium ions.