Journal of Applied Crystallography

Volume 33, Part 3 Number 2 (June 2000)

research papers

J. Appl. Cryst. (2000). 33, 876-881 [ doi:10.1107/S0021889800001175 ]

Use of Cr K radiation to enhance the signal from anomalous scatterers including sulfur

W. Kwiatkowski, J. P. Noel and S. Choe

Abstract: The anomalous signals from scatterers such as sulfur (S) and arsenic (As) were compared in diffraction data sets collected from an X-ray source with three different targets, Au, Cu and Cr, on a multi-target rotating anode. HIV-1 integrase crystals served as the test case for this study. The crystalline specimen of HIV-1 integrase contains in each protein molecule two As atoms, each covalently bound to a cysteine S atom, and two additional S atoms derived from methionine. It was found that the Cr K [alpha] radiation gave the clearest peaks in anomalous difference Fourier maps, although the signal-to-noise ratios of the anomalous signal for the Cu K [alpha] and Cr K [alpha] data were similar but better than that for Au L [alpha] . This result was in spite of the fourfold higher flux from the Cu anode versus the Cr anode. For all three X-ray wavelengths, anomalous difference Fourier maps calculated with bias-removed phases derived from the known atomic model revealed clear peaks at the two As sites. However, only in the map calculated using the Cr K [alpha] data were both peaks of the expected ellipsoidal shape, enveloping the As atom and the adjacent S atom. None of the S sites was apparent in difference maps calculated using the Au L [alpha] data. The ability to enhance the S-derived anomalous signal using Cr K [alpha] radiation has particularly useful applications in the structure determination of proteins, for example in resolving ambiguities in the chain tracing of a protein with numerous disulfide bonds and in assigning amino acid identities. Additionally, anomalous difference Patterson maps calculated from the Cr K [alpha] data were sufficiently clear to identify the As-related peaks. These results form the groundwork for in-house phase determination with the multi-wavelength anomalous diffraction method.