DEFINITION OF REQUIREMENTS

The instrumentation requirements for macromolecular crystallography at the synchrotron are quite diverse and technically exacting. The diversity arises because of the different classes of experiment, namely:

1. (a) routine data collection usually at a fixed wavelength;

2. (b) variable single or multiple wavelength anomalous dispersion measurements;

3. (c) time resolved crystallography;

4. (d) small crystals.

There are certainly needs common to each category.

In order to use the synchrotron X-radiation effectively, the ‘white beam’ of photons diverging from the source must be collected by beam line optical element (s), and brought to a focus with a size approximately equal to the protein crystal size. The sample must be centred in the beam and oriented on a goniostat of some kind. The diffraction pattern has to be measured accurately and efficiently with a detector. In the case of monochromatic experiments the beam line optical scheme will include a monochromator of which there are several common types. There are some special needs for each experimental class.

‘Routine’ data collection often involves the measurement of relatively weak, high resolution diffraction data. Critical design goals here include a very high intensity at the specimen and a short wavelength (often ≤0.9 Å) monochromatised beam to reduce radiation damage, which affects these high angle data first.