THE SIZES AND CONFORMATIONS OF THE PROTEINS IN ADSORBED LAYERS OF INDIVIDUAL CASEINS ON LATICES AND IN OIL-IN-WATER EMULSIONS

The dimensions of individual and mixed caseins adsorbed to the polystyrene latex-water interface, were measured. Each casein gave a different hydrodynamic thickness of the adsorbed layer, from 5.4 nm for alpha(s1)-casein to 11.1 nm for beta-casein, and the dimensions of adsorbed layers of kappa-casein depended on prior treatment of the protein. Digestion of the adsorbed protein with trypsin caused the layer thicknesses to decrease, but in all cases residual layers of peptides remained on the surfaces of the particles. Mixtures of alpha(s1)-casein and either alpha(s1)- or beta-caseins gave adsorbed layers in which the behaviour of the mixed interface depended on the nature of the second protein. Different results were obtained for alpha(s2)- and beta-caseins. Casein/soya oil emulsions gave very similar results to the latex system. The build up of the adsorbed layers cannot be studied in emulsions, but trypsinolysis caused almost identical changes in emulsions and latex based systems. Addition of a second casein to an already formed emulsion increased the thicknesses of the adsorbed layers exactly as in the corresponding latex;protein combinations. Attempts were made to consider the relationship between the hydrophobicity maps and possible conformations of the adsorbed proteins. It was concluded that there is no criterion by which the conformation of the adsorbed protein can be estimated simply by considering the hydrophobicity of regions of the polypeptide chain, calculated on the basis of the primary structure of the protein.