Feasibility and review of anomalous X-ray diffraction at long wavelengths in materials research and protein crystallography

The feasibility and a review of progress in the long-wavelengths anomalous dispersion technique is given in the context of the development of beamline ID1 of the ESRF for such studies. First experiments on this beamline and their analyses are described. The first study reports on the use of uranium which exhibits an unusually strong anomalous dispersion at its M_V absorption edge (λ_MV = 3.5 Å). The anomalous scattering amplitude of uranium with 110 anomalous electrons exceeds the resonance scattering of other strong anomalous scatterers like that of the rare earth ions by a factor of four. The resulting exceptional phasing power of uranium is most attractive in protein crystallography using the MAD method. The anomalous dispersion of a uranium derivative of asparaginyl-tRNA synthetase (hexagonal, a = 124.4 Å, c = 123.4 Å) has been measured at three wavelengths near the M_V edge using beamline ID1 of the ESRF. The present set-up allowed the measurement of 10% of the possible reflections at a resolution of 8 Å. This is mainly due to the low sensitivity of the CCD camera. The second study, involving DAFS experiments at wavelengths near the K-absorption edge of chlorine (λ_K = 4.4 Å), reports the use of salt crystals which give rise to much stronger intensities of diffraction peaks than those of protein crystals. In the case of a crystal of pentamethylammonium undecachlorodibismuthate (PMACB, orthorhombic, a = 13.00 Å, b = 14.038 Å, c = 15.45 Å), all reflections within the resolution range from 6.4 Å to 3.5 Å and the total scan width of 24° were collected. The crystalline structure of PMACB implies two chemically distinct states of the Cl atom. Consequently, different dispersions near the K-edge of chlorine are expected. The dispersion of the intensity of five Bragg peaks of the PMACB crystal has been measured at 30 wavelengths. The relative success of these preliminary experiments with X-rays of long wavelength shows that the measurement of anomalous X-ray diffraction at wavelengths beyond 3 Å is feasible. Starting from the experience gained in these experiments, an increased efficiency of the instrument ID1 by two to three orders of magnitude will be achieved in this wavelength range. A comparison with different techniques of anomalous diffraction which rely on the use of argon/ethane-filled multiwire chambers and image plates as detectors for wavelengths near the K-edge of sulfur and phosphorus is also given.