Study of binding between protein A and immunoglobulin G using a surface tension probe.

Molecular interactions and binding are one of the most important and fundamental properties in the study of biochemical and biomedical systems. The understanding of such interactions and binding among biomolecules forms the basis for the design and processing of many biotechnological applications, such as bioseparation and immunoadsorption. In this study, we present a novel method to probe molecular interactions and binding based on surface tension measurement. This method complements conventional techniques, which are largely based on optical, spectroscopic, fluorescence polarization, chromatographic or atomic force microscopy measurements, by being definite in determining molecular binding ratio and flexible in sample preparation. Both dynamic and equilibrium (or quasi-equilibrium) information on molecular binding can be obtained through dynamic and equilibrium surface tension measurements. For an important pair of biological ligand and ligate, Protein A and immunoglobulin G (IgG), the existence of molecular interactions and the binding ratio of 1:2 have been determined unequivocally with the proposed surface tension method. These results are confirmed/supported by a mass balance calculation and spectrophotometry experiment. In addition, adsorption isotherms for Protein A and IgG separately at the air/water interface have been established with the dynamic surface tension measurements. The results show that the Langmuir isotherm equation can describe the adsorption data satisfactorily for both Protein A and IgG solutions.