A MAD experiment performed at the white line of the iridium LIII absorption edge in lysozyme

A multiple-wavelength anomalous diffraction (MAD) experiment was performed on an iridium derivative of hen egg-white lysozyme. Diffraction data were measured at five wavelengths on the X31 EMBL beamline on the DORIS III storage ring and at two further wavelengths on the X11 beamline. Four iridium-binding sites were located from the dispersive anomalous differences between two wavelengths at the rising and falling inflection points of the Ir LIII-edge white line using direct methods. All other attempts to determine the heavy-atom positions failed. The results demonstrate an experimental method whereby systematic error in MAD data due to sample absorption can be reduced where a white line is present in the absorption spectrum of a heavy atom.