Effect of cleaving interchain disulfide bridges on the radius of gyration and maximum length of anti-poly(D-alanyl) antibodies before and after reaction with tetraalanine hapten.

The small-angle x-ray scattering of solutions of rabbits IgG antibodies and their derivatives has been investigated. The reduction and alkylation of the native antibody cause a small increase of the molecular parameters, indicating a limited expansion of the molecule. Binding of native antipoly(D-alanyl) antibodies with hapten (80% saturation) causes a significant change of the quaternary structure, expressed by a decrease in the maximum diameter of about 2 nm, of the radius of gyration by 5.5%, and of the volume. The same antibodies, in which the single inter-heavy-chain disulfide bridge was opened by reduction and carboxamidomethylation, do not show any significant decrease in the overall molecular parameters upon reaction with hapten, except for a local structural change in a part of the molecule. These data lend further support to the notion that binding of hapten induces a conformational transition in its specific antibodies and suggest that the opening of the interchain disulfide bridges affects that transition. The dimensions of the intact antibodies calculated from measurements of small-angle x-ray scattering at low concentrations agree closely with those obtained from crystallographic studies.