Unexpected differences in the behavior of ovotransferrin at the air–water interface at pH 6.5 and 8.0

Adsorption of purified apo-ovotransferrin at the air–water interface was studied by ellipsometry, surface tension, polarization–modulation infrared reflection–absorption spectroscopy (PM-IRRAS), and shear elastic constant measurements. No significant difference was observed between pH 6.5 and 8.0 as regards the final value of surface concentration and surface pressure. However at low concentration, a weak barrier to adsorption is evidenced at pH 6.5 and confirmed by PM-IRRAS measurements. At a pH where the protein net charge is negative (pH 8.0), the behavior of ovotransferrin at the air–water interface is more influenced by charge effects rather than bulk concentration effects. At this pH, the interface exhibits a low shear elastic constant and a spectral signature not usual for globular proteins.

Surface pressure versus surface concentration for the lowest subphase concentrations for which the surface pressure is initiated at pH 6.5 (filled circles) and pH 8.0 (open squares): a weak barrier to adsorption is evidenced at pH 6.5.Figure optionsDownload high-quality image (61 K)Download as PowerPoint slideResearch highlights
► Adsorption is delayed at pH 8.0 (negative charge) and low concentration.
► Stronger interactions in the first steps of adsorption are evidenced at pH 8.0.
► Interfacial shear elastic properties are weak.
► Interfacial ovotransferrin conformation depends on the bulk pH (6.5 or 8.0).
► Protein spectral signature is non standard at pH 8.0.