Determination of thermodynamic parameters of antigen–antibody interaction from real-time kinetic studies

Real-time kinetic parameters of the interaction of iodinated human chorionic gonodotropin (IhCG) with one of its monoclonal antibodies are sought and are determined after an analysis of the dissociation profile of IhCG–MAb (monoclonal antibody) complexes in the presence of excess unlabelled hCG at various temperatures. The equilibrium association constant (KA) of the epitope–paratope reaction (the first interaction) was determined from this data. KA values obtained at different temperatures were fitted to the nonlinear form of the vant Hoff equation. Values of ln KA0, ∆H0 and ∆CP were obtained from the fit. ∆G0 and ∆S0 were calculated using standard formulae. A change in heat capacity at constant pressure, ∆CP of – 3300 cal/ degree/mol indicated that a large surface area is buried upon complex formation. This method can be an alternative to the micro-calorimetric determination of thermodynamic parameters of ligand–ligate interaction. Our results also raise new questions about the interpretations of micro-calorimetry results from large-molecular weight ligand–ligate interactions.