R. Kahn et al., Feasibility and review of anomalous X-ray diffraction at long wavelengths in materials research and protein crystallography, J SYNCHROTR, 7, 2000, pp. 131-138
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 t
heir analyses are described. The first study reports on the use of uranium
which exhibits an unusually strong anomalous dispersion at its M-V absorpti
on edge (lambda(MV) = 3.5 Angstrom). The anomalous scattering amplitude of
uranium with 110 anomalous electrons exceeds the resonance scattering of ot
her strong anomalous scatterers like that of the rare earth ions by a facto
r of four. The resulting exceptional phasing power of uranium is most attra
ctive in protein crystallography using the MAD method. The anomalous disper
sion of a uranium derivative of asparaginyl-tRNA synthetase (hexagonal, a =
124.4 Angstrom, c = 123.4 Angstrom) has been measured at three wavelengths
near the M-V edge using beamline ID1 of the ESRF. The present set-up allow
ed the measurement of 10% of the possible reflections at a resolution of 8
Angstrom. This is mainly due to the low sensitivity of the CCD camera. The
second study, involving DAFS experiments at wavelengths near the K-absorpti
on edge of chlorine (lambda(K) = 4.4 Angstrom), reports the use of salt cry
stals which give rise to much stronger intensities of diffraction peaks tha
n those of protein crystals. In the case of a crystal of pentamethylammoniu
m undecachlorodibismuthate (PMACB, orthorhombic, a = 13.00 Angstrom, b = 14
.038 Angstrom, c = 15.45 Angstrom), all reflections within the resolution r
ange from 6.4 Angstrom to 3.5 Angstrom and the total scan width of 24 degre
es were collected. The crystalline structure of PMACB implies two chemicall
y distinct states of the Cl atom. Consequently, different dispersions near
the K-edge of chlorine are expected. The dispersion of the intensity of fiv
e Bragg peaks of the PMACB crystal has been measured at 30 wavelengths. The
relative success of these preliminary experiments with X-rays of long wave
length shows that the measurement of anomalous X-ray diffraction at wavelen
gths beyond 3 Angstrom 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 comp
arison with different techniques of anomalous diffraction which rely on the
use of argon/ethane-filled multiwire chambers and image plates as detector
s for wavelengths near the K-edge of sulfur and phosphorus is also given.