[Study of the structure of immunoglobins by small-angle x-ray diffraction. I. The structure of IgMCep in solution]

The data of small-angle X-ray scattering from monoclonal immunoglobulin MCep (IgM) enabled the shape and geometrical parameters of the molecule in solution at 23 degrees C to be established. The molecule is a flat, strongly anisometric particle with radius of gyration 115 A, volume 1,8 X 10(6) A3, maximum size 380 A, thickness 35-40 A. The most probable molecular model in the approximation of homogeneous electron density in the molecule was suggested, its geometry fitting the experimental parameters. The five IgM subunits are located in the equatorial plane, low-electronic-density regions are located in the centre and at the periphery of the macromolecule. In addition, the absence of fixed angle values between Fab-regions in each subunit is indicative of rather high structural mobility at the periphery of the IgM molecule.