Thermodynamics of antibody-antigen reactions. 2. The binding of bivalent synthetic random coil antigens to antibodies having different antigen precipitating properties

Abstract

The objects of this study were the equine Ig[immunoglobulin]G and IgG(T) classes of antibodies with immunologic specificity for the dinitrophenyl group and bivalent antigens consisting of linear poly(ethylene glycol) polymers which terminated at both ends in dinitrophenyl groups. Complex formation between antibodies of both classes and 1 of several sharp fractions of antigen having average MW in the range 25,000-75,000 were studied by measuring the light scattered from solutions containing equimolar amounts (.apprx. 5 .times. 10-6 mol/l) of 1 of the antibodies and 1 size fraction of antigen, and variable amounts of monovalent hapten. The data were analyzed in the context of a model that accounted for the formation of linear and cyclic complexes of all extents of aggregation. Two parameters in addition to the intrinsic antibody-dinitrophenyl group association constant were necessary in the assumed equilibrium model to account for the behavior of the system. One of these accounted for the losses in configurational entropy that resulted when a random-coil polymer became bound at 1 end to a space-occupying antibody. The other was a ring closure factor for the formation of cyclic complexes. Ring closure factors for the formation of larger cyclic complexes (present in only small amounts under the conditions studied) were related to the ring closure factor for the formation of the smallest cyclic complexes, which increased as antigen size decreased, and for each antigen size the closure factor was consistently higher for IgG(T) antibody than for IgG antibody. Comparisons of the theoretically estimated values of the 2 parameters with their measured values indicated that the average conformation of IgG antibodies in solution is open (T shaped), but the average inter-Fab arm angle in IgG(T) antibodies is approximately 60.degree. or less.