Anomalous dispersion of small-angle scattering of horse-spleen ferritin at the iron K absorption edge

Small-angle scattering experiments of horse-spleen ferritin solutions have been performed at various wavelengths near the K absorption edge of iron, using synchrotron radiation from the storage ring DORIS. The anomalous dispersion of the atomic form factor f as described by f' and if” has been monitored by the dependence of the radius of gyration R and the absorption coefficient μ respectively. There is a 4% increase of R at the absorption edge. The relative full half width of this peak of 0.0016 corresponding to 1 1 eV of the energy scale reflects the drastic variation of f' by 7 electrons. The predictable relation between R and has been verified by the Kramers-Kronig relation. As small-angle scattering is primarily influenced by f', best advantage from anomalous dispersion in small-angle scattering can be taken by measurements very near the absorption edges.