Pig antibodies against dinitrophenyl were studied by neutron small-angle scattering and X-ray small-angle scattering with particular attention to the analysis of cross-section plots and determination of the radii of gyration of cross-section. The experimentally determined molecular parameters R, (radius of gyration), R, and R,z (two different radii of gyration of cross-section characterizing every antibody sample) show that the shapes of the two antibody types, precipitating and non-precipitating, are similar. The non-precipitating antibody is slightly more compact.The parameters R,, R,1 and Rq2 of complexes of antibodies with the hapten, 8-dinitrophenyI-5,8-aza-4-0~0-octanoic acid, are smaller than those of the free antibody. This indicates that a conformational change is induced by the binding of the hapten. The character of the change of parameters is consistent with a view that the observed contraction of the molecule proceeds via similarity transformation.In order to design a model of a pig antibody molecule, isolated building blocks of the molecule, the Fab and Fc fragments, were first studied. A comparison of the scattering curves with various models of fragments showed, however, that the isolated fragments acquire in solution elongated rod-like shapes.Over 300 tentative models of the intact antibody molecule, built of small identical spheres, were constructed before a good fit with the experimental data was achieved. The most probable models have a cavity in the Fc part and the Fab parts are either fully extended or slightly bent downwards to the Fc part.Although detailed information on the three-dimensional structure of immunoglobulin domains is available from X-ray analysis of crystalline immunoglobulin fragments [I], much uncertainty still exists concerning immunoglobulin size, shape and dynamic behaviour in solution. In particular, the character of conformational changes of antibody molecules following the binding of specific ligands, antigens or haptens, is far from being perfectly understood [2,3].
