Proteins at liquid interfaces

The adsorption data presented in the preceding paper (Part II of this series) have been used to deduce the molecular structures of β-casein, bovine serum albumin (BSA), and lysozyme films at the air-water and oil-water interfaces. The hydrophobic, disordered β-casein molecules are more surface-active than the globular BSA and lysozyme molecules. β-Casein is in an all-train configuration at low surface pressures (π < 8 mN m−1) at the air-water interface and can be described by Singer's equation of state for linear polymers. At higher π values, loop formation ensues, and the loops increase in density at the expense of the trains until in the close-packed, condensed state the ratio of amino acid residues in loop and train configurations is about 2:1. This conformational change does not occur at the oil-water interface because loops already form at low π; enhanced loop formation in the nonaqueous phase is favored because oil molecules solvate the hydrophobic residues. Multilayer formation occurs at high substrate protein concentrations (> 10−2 wt%) giving films thicker than 100 Å. Although the primary layer is irreversibly adsorbed, molecules in subsequent layers are reversibly adsorbed. Lysozyme molecules are denatured at low π values (π < 8 mN m−1), and at higher values essentially native molecules (which are reversibly adsorbed) coexist in the surface film. Much residual native structure remains even in the denatured film because lysozyme is very resistant to denaturation. At the oil-water interface, the adsorbed lysozyme molecules are denatured to a greater extent than those at the air-water interface; native molecules are not stable at this interface. The adsorbed BSA molecules contain residual native structure, but there is no abrupt conformational change in the film at a particular packing density. The structure at the air-water interface is intermediate to those of lysozyme and β-casein. BSA has the same π-A curve as lysozyme at the oil-water interface.