Thermodynamics of oligosaccharides binding to a dextran-specific monoclonal IgM

The binding thermodynamics of seven different oligosaccharide haptens to the dextran-specific IgM secreted by the murine plasmacytoma MOPC-104E were studied by direct calorimetric measurements. The enthalpy change values observed for the binding process range between -5 and -16 kcal/mol depending on the hapten and the temp of measurement. The antibody-hapten interactions were characterized by a positive heat capacity change [delta Cp approximately 300 cal/(mol.degree)] and a resultant process of enthalpy-entropy compensation. The calculated change in unitary entropy of the reaction, delta Su, ranged between -20 and -30 eu (4 degrees C), corresponding to an expected entropy loss due to immobilization of the hapten molecules. The entropy of binding increased with rising temp, thus compensating for the decreasing enthalpy contribution to the free energy of binding. The data are consistent with a hapten binding induced conformational transition to a more relaxed state in the immunoglobulin molecule.