SYNCHROTRON RADIATION IN PROTEIN CRYSTALLOGRAPHY

Zbigniew Dauter

SAIC/NCI Frederick, Brookhaven National Laboratory, Bldg. 725A-X9, Upton, NY 11973, USA;
e-mail: dauter@bnl.gov

Keywords : synchrotron radiation, macromolecular crystallography

Synchrotron radiation has been used by macromolecular crystallographers since the 1970's, but in recent years it has become indispensable. This is due to several advantages of the X-ray beam generated at synchrotrons over the conventional home-laboratory sources of radiation. The importance of structural results on biological macromolecules, mainly for human health-care research, is appreciated by science financing agencies and in response to increasing demand for radiation several new synchrotron facilities with beam lines dedicated for diffraction experiments on macromolecular crystals have been built recently or are being constructed.

Synchrotron radiation has certain properties which cannot be achieved by X-ray tubes or rotating anodes. The first, very important, property is the intensity of the synchrotron beam, particularly that generated by insertion devices, such as wigglers or undulators. Coupled with fast 2-D detecors and crystal freezing, this allows the collection of data from crystals of even very large macromolecules where an enormous number of generally very weak reflections has to be measured. Synchrotron beams can be very efficiently collimated and focused. That is important for very small samples and crystals with large cell dimensions, where the spatial resolution of reflections on the detector is crucial.

The radiation generated by bending magnets or wigglers is "white", i.e. non-monochromatic. The Laue technique, using the full potential of white radiation, can be used to investigate short-lived intermediates of enzymatic reactions.

The broad spectrum of synchrotron radiation allows tuning the wavelength of X-rays to the absorption edge of selected elements to optimise the effect of anomalous scattering.

This is the principle of the Multiwavelength Anomalous Dispersion (MAD) method, which has recently become the most popular method of solving novel crystal structures.

X-ray data collection with synchrotron radiation becomes easier and faster. It should be remembered that this process is not a purely technical achievement, but is rather a vital scientific step in crystal structure analysis which is often very difficult to repeat.