Section 9.1.4. X-ray sources

Rotating anodes were initially developed for biological scattering experiments on muscle samples and have the advantage of higher intensity compared to 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. 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. The fine-focus tubes currently being developed may affect the choice of home source over the next years (Arndt, Duncumb et al., 1998; Arndt, Long & Duncumb, 1998).

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), and the last three decades have seen great advances in their application to macromolecular crystallography (Helliwell, 1992). Synchrotron radiation (SR) is now used for more than 70% of newly determined protein-crystal structures.

The general advantages of SR are:

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 likely to be built within the next decade, is not yet possible to assess.

Present beamlines produce radiation of extremely high quality for macromolecular data collection. At third-generation sources, such as the European Synchrotron Radiation Facility (ESRF) or the Advanced Photon Source (APS), complete data sets can be collected from cryogenically frozen single crystals in minutes.