General Shape and Hapten‐Induced Conformational Changes of Pig Antibody against Dinitrophenyl: A Small‐Angle Scattering Study

Abstract

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‐oxooctanoic 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. Copyright © 1981, Wiley Blackwell. All rights reserved