The protein net electric charge determines the surface rheological properties of ovalbumin adsorbed at the air–water interface

Adsorption of purified diphosphorylated Al-ovalbumin at the air–water interface was studied by ellipsometry, surface tension, and shear elastic constant measurements. The value of pH did not significantly affect the final value of surface concentration. It affected slightly the kinetics of surface pressure increase and the final value of surface pressure. The interfacial rheology was affected strongly by pH. The interface exhibited a maximum of the shear elastic constant at a pH close to the isoelectric pH of ovalbumin. The bulk protein concentration also had a more pronounced effect on the surface rheology when the protein net charge was low. At a pH where the protein net charge is negative, an increase of the ionic strength increased the final value of the shear elastic constant. The results suggest that interactions between adsorbed ovalbumin molecules, which form slowly in the adsorbed layer upon conformational rearrangements, impart rigidity to the interface, and that these intermolecular associations are hindered at high protein net charge.