Surface Rheological Properties of Native and S-Ovalbumin Are Correlated with the Development of an Intermolecular β-Sheet Network at the Air−Water Interface

Polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS), ellipsometry, and shear elastic constant measurements were used to study the adsorption and the behavior of ovalbumin and S-ovalbumin at the air−water interface at different values of the subphase pH. Native and S-ovalbumin exhibited similar behaviors, with a maximum plateau value of the shear elastic constant near the isoelectric pH of the protein. However, higher surface concentration values were reached with S-ovalbumin in low net charge conditions, which suggest adsorption of aggregates or multilayer adsorption. For both proteins, the statistical analysis of PM-IRRAS spectra demonstrated that the aging of the interfacial film and the increase of the shear elastic constant were correlated with a significant increase in the relative contribution of intermolecular β-sheets in the amide I band with time. This increase was significantly faster at low pH values. At the same pH value and age of the interface, the relative contribution of intermolecular β-sheets was significantly higher for S-ovalbumin.