EFFECT OF SURFACE-PROPERTIES AND ADDED ELECTROLYTE ON THE STRUCTURE OF BETA-CASEIN LAYERS ADSORBED AT THE SOLID AQUEOUS INTERFACE/

The adsorption of beta-casein at hydrophobic and hydrophilic silica su rfaces has been studied by time-resolved ellipsometry. Marked differen ces in, e.g., adsorption kinetics and plateau adsorption coverage, wer e observed on the two types of surfaces. The miscellaneous adsorption mechanisms at the two surfaces resulted in different structures of the adsorbed layers as evident from the thicknesses and protein densities measured on the two substrates as well as the effect on the adsorbed layer properties of a subsequently added specific proteolytic enzyme, endoproteinase Asp-N. At the hydrophobic surface, the adsorption is fa st and the surface is saturated within a relatively short period. The addition of endoproteinase Asp-N reduces the surface excess and the th ickness by 24 and 45%, respectively. This corresponds to cleavage at a mino acid residues 43 and/or 47 in the hydrophilic portion of the prot ein. Adsorption from solutions containing added electrolyte leads to s ignificant increase of the surface excess. However, no significant cha nge was observed in the ellipsometric layer thickness. At constant ion ic strength, the surface excess increased in the order NaCl < MgCl2 < CaCl2. From the experimental evidence, it was concluded that the adsor bed layer structure at the hydrophobic surface can be described as a m onolayer with an inner dense region comprising the relatively large hy drophobic portions of the protein molecules and an outer region of the highly charged N-terminal portions protruding into the aqueous phase. The adsorption kinetics at the hydrophilic silica surface, although i nitially the same as on the hydrophobic surface, was found to be much slower and plateau surface excess values were not reached even after 2 h of adsorption. This suggests that substantial rearrangements of the protein molecules take place within the adsorbed layer during the ads orption process. Although the maximum surface excess at the hydrophili c surface of 4.3 mg m(-2) is higher than the value of 2.8 mg m(-2) mea sured at the hydrophobic surface, the thickness is slightly smaller, i .e., 60 Angstrom and 66 Angstrom respectively. Hence, the protein adop t a more compact structure at the hydrophilic surface, at least in the inner part of the adsorbed layer. The different structure at the hydr ophilic surface was confirmed by the larger reduction of the surface e xcess and layer thickness associated with the addition of endoproteina se Asp-N, leaving a very thin compact layer at the surface.