International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. F. ch. 9.1, p. 193   | 1 | 2 |

Section 9.1.13.2. Anomalous scattering, MAD and SAD

Z. Dautera* and K. S. Wilsonb

a National Cancer Institute, Brookhaven National Laboratory, NSLS, Building 725A-X9, Upton, NY 11973, USA, and bStructural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, England
Correspondence e-mail:  dauter@bnl.gov

9.1.13.2. Anomalous scattering, MAD and SAD

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The requirements for collecting data with an intrinsically weak anomalous signal are several. As with the isomorphous measurements in the previous section, the highest possible resolution may not be the primary consideration. Here the emphasis lies in data quality, as the measurement of very small differences in macromolecular amplitudes, which are already in themselves relatively weak, is required. Important considerations include the following.

  • (1) Optimization of the wavelength, particularly for MAD experiments.

  • (2) Ensuring that the anomalous data are complete in terms of all possible Bijvoet pairs. This is not always addressed by the currently available data-processing software.

  • (3) High redundancy of measurements significantly enhances the quality of the signal, as this provides effective averaging of errors and allows the rejection of statistical outliers. The latter is especially important for direct-methods solution of the anomalous-scattering constellation.

For MAD experiments (Hendrickson, 1991[link]; Smith, 1991[link]), which can only be carried out at SR sites, the optimum number of wavelengths at which data should be recorded remains unclear. The minimum is one (SAD) and the conventional wisdom is that four are optimal. Given finite beam time, the trade-off is between measuring with limited redundancy at several wavelengths as against higher redundancy at a smaller number of wavelengths. The jury is still out on this one.

Single-wavelength anomalous dispersion (SAD) represents the limiting case. All data are recorded at one wavelength, reducing the requirement for fine monochromatization and for fine tunability and stability. Now quality, especially in the form of redundancy, is the dominating factor since all phasing is based purely on a single anomalous difference for each reflection.

References

First citation Hendrickson, W. A. (1991). Determination of macromolecular structures from anomalous diffraction of synchrotron radiation. Science, 254, 51–58.Google Scholar
 First citation Smith, J. L. (1991). Determination of three-dimensional structure by multiwavelength anomalous diffraction. Curr. Opin. Struct. Biol. 1, 1002–1011.Google Scholar








































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