General shape and hapten-induced conformational changes of pig antibody against dinitrophenyl. A small-angle scattering study.

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 Rg (radius of gyration), Rq1 and Rq2 (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 Rg, Rq1 and Rq2 of complexes of antibodies with the hapten, 8-dinitrophenyl-5,8-aza-4-oxo-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.