Multiwavelength anomalous diffraction analysis at the M absorption edges of uranium

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The National Academy of Sciences

RESUMO

The multiwavelength anomalous diffraction (MAD) method for phase evaluation is now widely used in macromolecular crystallography. Successful MAD structure determinations have been carried out at the K or L absorption edges of a variety of elements. In this study, we investigate the anomalous scattering properties of uranium at its MIV (3.326 Å) and MV (3.490 Å) edge. Fluorescence spectra showed remarkably strong anomalous scattering at these edges (f′ = −70e, f′′ = 80e at the MIV edge and f′ = −90e, f′′ = 105e at the MV edge), many times higher than from any anomalous scatterers used previously for MAD phasing. However, the large scattering angles and high absorption at the low energies of these edges present some difficulties not found in typical crystallographic studies. We conducted test experiments at the MIV edge with crystals of porcine elastase derivatized with uranyl nitrate. A four-wavelength MAD data set complete to 3.2-Å Bragg spacings was collected from a single small frozen crystal. Analysis of the data yielded satisfactory phase information (average difference of 0ϕT − 0ϕA for replicated determinations is 32°) and produced an interpretable electron-density map. Our results demonstrate that it is practical to measure macromolecular diffraction data at these edges with current instrumentation and that phase information of good accuracy can be extracted from such experiments. We show that such experiments have potential for the phasing of very large macromolecular assemblages.

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