Shape and volume of anti-poly(D-alanyl) antibodies in the presence and absence of tetra-D-alanine as followed by small-angle x-ray scattering.

The conformation of two anti-poly-D-alanyl antibodies was studied before and after interaction with hapten by small-angle X-ray scattering. With both samples a volume contraction could be observed upon interaction with the tetra-D-alanine hapten. The anti-poly-D-alanyl antibodies obtained by immunization with poly-D-alanyl diphtheria toxoid showed a decrease of the volume by 10% and a decrease of the radius of gyration by 7.7 % when 90% of the binding sites were occupied by hapten. With anti-poly-D-alanyl HSA antibodies, a smaller decrease of the volume, of 3.2 %, and of the radius of gyration of 1.4 % were found, when 62 % of the binding sites were occupied. Since the other data determined for the antibodies, such as molecular weight, radii of gyration of the cross section, and form of the scattering curve, were unchanged upon interaction with hapten within the errors of measurement, it must be assumed that the change in conformation upon interaction consists in a volume contraction without significant changes in the shape. The shape of all antibodies (free and occupied by hapten) is best described by a T-shaped model. The data found for the two antibody fractions, in the free and bound states, respectively, are the following: radius of gyration of the whole particle, 6-08 to 6-00 nm ( 1-4 %) and 6"50 to 6-00 nm ( 7"7 %); volume, 371 to 359 nm 3 ( 3-2 %) and 399 to 358 nm 3 ( 10 %). 187