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

International Tables for Crystallography (2012). Vol. F, ch. 9.1, pp. 211-212   | 1 | 2 |

Section 9.1.4. X-ray sources

Z. Dautera* and K. S. Wilsonb

aNCI Frederick & Argonne National Laboratory, Building 202, Argonne, IL 60439, USA, and bYork Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5YW, England
Correspondence e-mail:  dauter@anl.gov

9.1.4. X-ray sources

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There are two principal sources of X-rays appropriate for macromolecular data collection: rotating anodes and synchrotron storage rings. These are not discussed in detail here as they are described in Chapters 6.1[link] and 8.1[link] .

9.1.4.1. Conventional sources

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Rotating anodes were initially developed for biological scattering experiments on muscle samples and have the advantage of higher intensity compared with sealed-tube generators. They usually have a copper target providing radiation at a fixed wavelength of 1.542 Å. Alternative targets, such as silver or molybdenum, provide lower intensities at short wavelengths, but have not found general applications to macromolecules. It is also possible to use a chromium target, giving a longer wavelength of 2.29 Å. Historically, rotating anodes were first used with nickel filters to give monochromatic Cu Kα radiation. Current systems are equipped with either graphite monochromators, a focusing mirror or multilayer optics. The latter provide substantially enhanced intensity. Rotating anodes remain the source of choice in most structural biology laboratories. An important choice for the user is in the selection of optimal collimator aperture: this should roughly match the crystal sample dimensions. For large crystals, especially if the cell dimensions are also large, it may be preferable to use collimator settings smaller than the crystal in order to resolve the diffraction spots on the detector. Considerable progress has been made in the technologies for rotating anodes in recent years, as well as in the development of high-intensity sealed-tube sources for home laboratories (Bloomer & Arndt, 1999[link]).

9.1.4.2. Synchrotron storage rings

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The radiation intensity available from rotating anodes is limited by the heat load per unit area on the target. In the early 1970s, it was realized that synchrotron storage rings produced X-radiation in the necessary spectral range for studies in structural molecular biology (Rosenbaum et al., 1971[link]), and the last four decades have seen great advances in their application to macromolecular crystallography (Helliwell, 2004[link]; Hendrickson, 2000[link]). Synchrotron radiation (SR) is now used for the great majority of newly determined protein-crystal structures.

The general advantages of SR are:

  • (1) High intensity: third-generation sources provide more than 1000 times the intensity of a conventional source.

  • (2) A highly parallel beam allowing the resolution of closely spaced spots from large unit cells.

  • (3) Short wavelengths, less than 1 Å, essentially eliminating the problems of correcting for absorption.

  • (4) Tunability of the wavelength, allowing its optimization for single- or multiple-wavelength applications; this is simply not possible with a conventional source.

  • (5) The ability to use a white, non-monochromated beam, the so-called Laue technique discussed in Chapter 8.2.[link]

SR beamlines take a number of forms. The source may be a bending magnet or an insertion device, such as a wiggler or an undulator. The properties of different beamlines thus vary considerably and it is vital to choose an appropriate beamline for any particular application. The beamline capabilities are, of course, affected by the detector as well as the source itself. As far as the user is concerned, the primary questions regard the intensity, the size of the focal spot, the wavelength tunability and the detector system.

The present consensus for new synchrotron beamlines for macromolecular crystallography is that they should be on sources with an energy of at least 3 GeV and should receive radiation from tunable undulators. Together, these provide high and tunable intensity over the range required for most crystallographic experiments, including multiwavelength anomalous dispersion (MAD). The impact of free-electron lasers, which are currently under construction at a number of sites, is not yet possible to assess.

Present beamlines produce radiation of extremely high quality for macromolecular data collection. At third-generation sources complete data sets can be collected from cryogenically vitrified single crystals in minutes.

References

Bloomer, A. C. & Arndt, U. W. (1999). Experiences and expectations of a novel X-ray microsource with focusing mirror. I. Acta Cryst. D55, 1672–1680.
Helliwell, J. R. (2004). Macromolecular Crystallography with Synchrotron Radiation. Cambridge University Press.
Hendrickson, W. A. (2000). Synchrotron crystallography. Trends Biochem. Sci. 25, 637–643.
Rosenbaum, G., Holmes, K. C. & Witz, J. (1971). Synchrotron radiation as a source for X-ray diffraction. Nature (London), 230, 434–437.








































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