A structural study of beta-casein adsorbed layers at the air-water interface using X-ray and neutron reflectivity

New details on the structure of beta-casein adsorbed layers, at the air-water interface, have been obtained using X-ray and neutron reflectivity. The experimental data are fitted well by a power law model and the results discussed in terms of the distribution of amino-acid sequences between trains, loops and tails. This distribution seems to be consistent with statistical theories established for flexible polymers. The trains are present in close proximity to the surface as a dense layer 8-9 A thick. At low surface coverage, the tail effect is negligible and the adsorbed layer is composed of nearly 60% amino-acid sequences in trains and the remaining in loops. When the bulk concentration is increased, a substantial part of the amino-acid residues has to be accommodated in loops and long tails; the adsorbed layer becomes more extended (80-100 A). A striking feature is observed for a high bulk concentration (10(-1) wt.%): trains are forced to eject out of the interface.