Configuration of the four iron atoms in dissolved human hemoglobin as studied by anomalous dispersion

The anomalous dispersion of iron at its K-absorption edge in small-angle scattering of an aqueous solution of hemoglobin has been used to establish the geometrical arrangement of the four iron atoms in this protein. Although the anomalous contributions are 0.001-0.01 of the total scattering, experiments with synchrotron radiation from the storage ring DORIS have shown that these effects can be measured with an average precision of approximately 10% at each of the 50 points of the scattering curve. The anomalous scattering represents the convolution of the whole structure with the configuration of the four iron atoms of hemoglobin. Analysis in terms of multipoles suggests that tetrahedral symmetry of both the subunit arrangement and the iron structure is a dominant feature. The mean distance between the iron atoms of 26 A derived from this experiment compares well with that derived from crystallographic data.