OPTIMISING SYNCHROTRON FACILITIES FOR DATA COLLECTION IN PROTEIN CRYSTALLOGRAPHY. Colin Nave, CLRC Daresbury Laboratory, Warrington WA4 4AD, UK

A method is presented for specifying the optimum x-ray source, optics and detectors for protein crystallographic data collection. There is intense effort in obtaining facilities matched to the large volume demand and most exacting requirements in protein crystallography. Simply asking for the highest flux or highest brightness is misleading. A better definition of the requirements would be to have the maximum flux in to the phase space volume required by the specimen. A proper evaluation of the requirements should therefore start from the characteristics of the range of specimens which are to be studied.

A graphical representation is derived to illustrate how various sources and optics are matched to parameters such as crystal size and unit cell dimensions. The perfection of the crystal should also be considered, the ultimate (and impractical) extreme being to match the incident beam to the diffraction characteristics of each diffraction spot using Du Mond (or similar) diagrams. Recent investigations have found that some protein crystals have a very low mosaicity. Less attention has been devoted to the divergence of beams diffracted from protein crystals. Simple measurements of mosaicities and diffracted beam divergences have therefore been made from crystals of various degrees of perfection. It was found that the procedure of converting crystal rocking widths to arcs in the detector plane does not apply. However, a simple 3 parameter model can be used to describe crystal perfection. The procedures described have consequences for the design of beamlines and detectors for protein crystallography. Typical issues are whether there is a limit to the source emittance required or the size of an area detector system. The aim is to stimulate discussion of these issues and encourage further similar measurement from a much wider range of specimens, including those with the higher mosaicities typically obtained at cryo-temperature.