Shape and volume of fragments Fab' and (Fab')2 of anti-poly(D-alanyl) antibodies in the presence and absence of tetra-D-alanine as determined by small-angle x-ray scattering.

The conformation of two fragments derived from anti-poly(D-alanyl) antibodies, the divalent fragment (Fab')2 and the monovalent fragment Fab', was studied by small-angle X-ray scattering before and after interaction with the tetra-D-alanine amide hapten. More than 90% of the combining sites were occupied by the hapten. No significant changes were observed in the volume or in the radius of gyration, with either of the fragments. This contrasts with the significant decrease in these two parameters found upon reacting the hapten with intact anti-poly(D-alanyl) antibodies (I. Pilz, O. Kratky, A. Licht, and M. Sela (1973), Biochemistry 12, 4998). For Fab', the radius of the whole particle was found to be 3.48 nm in the absence of the hapten and 3.46 nm in its presence, the radius of gyration of the cross-section was 1.37 nm without hapten and 1.38 nm in its presence, and the volume of the particle was 98 nm3 in the absence of the hapten and 91 nm3 in its presence. For (Fab')2 the respective values were 5.06 and 5.05, 1.38 and 1.37, and 182 and 182. These results suggest that a conformational change occurs within the antibody molecule, but not within its Fab fragment, upon reaction with the tetraalanine hapten.