Thermodynamics of antibody-antigen reactions. 1. The binding of simple haptens to two classes of antibodies fractionated according to affinity.

The thermodynamic parameters which characterize the binding of dinitrophenylglycine and dinitrophenylmethoxypoly(ethylene glycol) to selected affinity classes of equine IgG and IgG(T) antibodies were determined by fluorescence quenching and flow calorimetry. The binding enthalpies and entropies were in all cases large and negative, falling in the ranges -14 to -17 kcal/mol and -18 to -25 eu, respectively. The differences in the enthalpies and entropies of binding for different affinity classes and for different haptens are discussed with reference to differences in the structures of the haptens studied and as indications of differences in binding site structure. In addition, the apparent existence of fluorescent side chains which can transfer energy to either hapten binding site in IgG(T) antibodies but not in IgG antibodies is interpreted as indicative of a smaller average interbinding site distance in IgG(T) than in IgG antibodies.