Assignment of Individual Metal Redox States in a Metalloprotein by Crystallographic Refinement at Multiple X-ray Wavelengths

A method is presented to derive anomalous scattering contributions for individual atoms within a protein crystal by collecting several sets of diffraction data at energies spread along an X-ray absorption edge of the element in question. The method has been applied to a [2Fe:2S] ferredoxin model system with localized charges in the reduced state of the iron−sulfur cluster. The analysis shows that upon reduction the electron resides at the iron atom closer to the protein surface. The technique should be sufficiently sensitive for more complex clusters with noninteger redox states and is generally applicable given that crystals are available.