Determination of the surface tension of protein coated materials by means of the advancing solidification front technique.

We investigated whether substrates with different surface tensions would induce a different degree of conformational change in adsorbed protein molecules, which would be reflected by differences in the surface tension of the adsorbed layers. The solidification front technique allowed this study without requiring to expose the protein coated-particle to an air interface which would induce conformational changes in the adsorbed protein layer. With a low bulk albumin concentration (0.1%) decreasing surface tension of the adsorbed protein layer with increasing hydrophobicity of the substrate, suggested more extensive conformational changes on the more hydrophobic surfaces. At high bulk concentrations (0.5% and above) the surface tension of the adsorbed albumin layer was independent of the substrate material and increased to a value of approximately 70.2 ergs/cm2. This was consistent with the surface tension of albumin derived from other independent techniques such as contact angle measurements on thick layers of the protein material or from adsorption data. Freezing front measurements with albumin (BSA or HSA), immunoglobulin G (IgG), and fibrinogen adsorbed onto one and the same substrate material, octyl-sepharose beads, indicate that the hydrophobicity of the protein coated sepharose increased in the following order: BSA less than HSA less than IgG less than Fibrinogen. This was in good agreement with the relative hydrophobicity of these proteins determined by other independent methods.