Anomalous X-ray diffraction with soft X-ray synchrotron radiation

Anomalous diffraction with soft X-ray synchrotron radiation opens new possi bilities In protein crystallography and materials science. Low-Z elements l ike silicon, :phosphorus, sulfur and chlorine become accessible as new labe ls in structural studies. Some of the heavy elements like uranium exhibit a n unusually strong dispersion at their M-V absorption edge (lambda(MV)= 3.4 97 Angstrom, E-MV=3545 eV) and so does thorium. Two different test experime nts are reported here showing the feasibility of anomalous X-ray diffractio n at long wavelengths with a protein containing uranium and with a salt con taining chlorine atoms. With 110 electrons the anomalous scattering amplitu de of uranium exceeds by a factor of 4 the resonance scattering of other st rong anomalous scatterers like that of the lanthanides at their Lm edge. Th e resulting exceptional phasing power of uranium is most attractive in prot ein crystallography using the multi-wavelength anomalous diffraction (MAD)) method. The anomalous dispersion of an uranium derivative of asparaginyl-t RNA synthetase (hexagonal unit cell; a = 123.4 Angstrom, c = 124.4 Angstrom has been measured for the first time at 4 wavelengths near the My edge usi ng the beamline ID 1 of ESRF (Grenoble, France). The present set up allowed to measure only 30% of the possible reflections at a resolution of 4 Angst rom, mainly because of the low sensitivity of the CCD detector. In the seco nd experiment, the dispersion of the intensity of 5 X-ray diffraction peaks from pentakismethylammonium undecachlorodibismuthate (PMACB, orthorhombic unit cell; a = 13.003 Angstrom, b = 14.038 Angstrom, c = 15.450 Angstrom) h as been measured at 30 wavelengths near the K absorption edge of chlorine ( lambda(K) = 4.397 Angstrom, E-K= 2819.6 eV). All reflections within the res olution range from 6.4 Angstrom to 3.4 Angstrom expected in the 20 degrees scan were observed. The chemical state varies between different chlorine at oms of PMACB, and so does the dispersion of different Bragg peaks near the K-edge of chlorine. The results reflect the performance of the beamline ID1 of ESRF at wavelengths beyond 3 Angstrom at the end of 1998. A gain by a f actor 100 for diffraction experiments with 4.4 Angstrom photons was achieve d in Autumn 1999 when two focusing mirrors had been added to the X-ray opti cs. Further progress is expected from area detectors more sensitive to soft X-rays. Both CCD detectors and image plates would provide a gain of two or ders of measured intensity. Image plates would have the additional advantag e that they can be bent cylindrically and thus cover a larger solid angle i n reciprocal space. In many cases, samples need to be cooled: closed and op en systems are presented. A comparison with the state of art of soft X-ray diffraction, as it had been reached at HASYLAB (Hamburg, Germany), and as i t is developing at the Brookhaven National Laboratory (USA), is given.