INTERFACIAL FREE-ENERGIES IN PROTEIN SOLUTION DROPLETS ON FEP-TEFLON BY AXISYMMETRICAL DROP SHAPE-ANALYSIS BY PROFILE IGG VERSUS BSA

In this paper, interfacial free energy changes occurring during IgG adsorption to FEP-Teflon from protein solution droplets are measured and compared with those occurring during BSA adsorption. Protein solution droplets (100 μl) were put on FEP-Teflon and the contact angle Θ and surface tension γlv were determined by ADSA-P. Subsequently, these results were combined with the Young equation while assuming a constant surface free energy γsv to yield the interfacial free energy γsl. For both proteins, γsl decreased in time up to at least 1 h in a concentration-dependent way despite the fact that similar experiments with radiolabeled proteins indicated stationary state adsorption within 30 min. IgG showed a greater decrease in interfacial free energy γsl than did BSA, particularly at the higher concentrations. Hypothetically, this was suggested to be due to a lower internal stability of IgG as compared to BSA at the experimental pH, enabling more extensive conformational changes. Analysis of the results using the Gibbs equation yielded unrealistic amounts of proteins adsorbed to the various interfaces, most likely due to the irreversible nature of protein adsorption. Analysis of the results by the Lucassen-Reynders equation gave possibly more realistic results, since this approach is based on the ratio between surface excess quantities and thus possible errors due to the irreversible nature of protein adsorption processes might cancel each other. © 1993 Academic Press, Inc.