Assessing the extent of protein intermolecular interactions at air-water interfaces using spectroscopic techniques

There is an ongoing debate about whether a protein surface film at an air-water interface can be regarded as a gelled layer. There is literature reporting that such films show macroscopic fracture behavior and a rheology comparable to three-dimensional protein bulk-networks. If this is the case, a complete description of the formation of adsorbed layers should include a transition from single, freely moving proteins to a gelled layer. This report presents studies using spectroscopic techniques, such as infrared, fluorescence and neutron spectroscopy, or ellipsometry, to derive molecular insight in situ to substantiate the intermolecular networking in surface films of chicken egg ovalbumin. It is concluded that protein films, generated by equilibrium adsorption from the bulk, behave as a densely packed colloidal repulsive particle system, where the proteins still have a significant rotational mobility, have a predominantly retained globular fold, and show distinct (lateral) diffusion. Applied stresses on the surface film (by compressions of the interface) may result in protein denaturation and aggregation. This process renders a surface film from a colloidal particle into that of a gelled system.