SYNCHROTRON RADIATION FOR HIGH RESOLUTION DATA COLLECTION Z. Dauter, EMBL c/o DESY, Notkestrasse 85, 22603 Hamburg, Germany

Recent advances in the macromolecular data collection technology has made very high resolution data collection on macromolecular crystals much more tractable. Several points have contributed to this.

The construction of very bright beam lines at different synchrotron sites based on wigglers or undulators provides X-radiation of unprecedented intensity. This is required to obtain a meaningful signal from protein crystals giving thousands of inherently weak reflections.

Macromolecular crystals are susceptible to radiation damage, especially when exposed to strong synchrotron beam. Shock freezing of the sample alleviates this problem and allows to collection of data to maximum resolution from a single crystal.

The possibility of using fast and accurate 2-dimensional detectors, such as imaging plates or CCD's, plays in practice a very important role. The number of reflections increases with the cube of the resolution so does the contrast in intensity between the low and high resolution reflections. This requires the use of fast detectors with high dynamic range and spatial resolution. IP and CCD fulfill this condition.

Equally important is the availability of data processing programms, which are fast, efficient and easy to use. Significant progress in this field has taken place in the last few years. It is possible to obtain the complete data set, perhaps consisting of several hundreds of thousands of merged reflections, just minutes after finishing the synchrotron session.

Advances in macromolecular data collection do not make the human contribution to this process redundant. Progress in macromolecular crystallography and especially in computing in the recent years means that the data collection plays even more important role, since it is much easier to repeat all the other stages, if necessary. It pays off to collect the data as optimally as possible, since it makes all later steps of the structure analysis considerably easier.