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 inter face was studied by ellipsometry, surface tension, and shear elastic consta nt measurements. The value of pH did not significantly affect the final val ue of surface concentration. It affected slightly the kinetics of surface p ressure increase and the final value of surface pressure. The interfacial r heology was affected strongly by pH. The interface exhibited a maximum of t he 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 su rface rheology when the protein net charge was low. At a pH where the prote in net charge is negative, an increase of the ionic strength increased the final value of the shear elastic constant. The results suggest that interac tions between adsorbed ovalbumin molecules, which form slowly in the adsorb ed layer upon conformational rearrangements, impart rigidity to the interfa ce, and that these intermolecular associations are hindered at high protein net charge. (C) 2000 Elsevier Science Ltd. All rights reserved.