An oscillation data collection system for high-resolution protein crystallography

Abstract

An oscillation data collection system for protein crystallography is described. The system consists of a modified Enraf-Nonius precession camera with cylindrical cassette, and stepping motor driven by a flexible microprocessor control system. The X-ray source is graphite-monochromatized radiation from an Elliott GX-21 rotating-anode generator run at 5.5 kW on a focal spot measuring 0.5 × 5.0 mm. The potential advantages of using a relatively large focal spot in conjunction with a graphite monochromator are discussed. Conditions for optimum collimation and X-ray intensity are considered, and it is shown that appropriately designed collimators with adjustable apertures can have substantial advantages over commercially available pinhole collimators. The oscillation films are processed by a procedure based on that of Rossmann [J. Appl. Cryst. (1979), 12, 225-238]. Determination of the initial alignment of the film is facilitated by a pair of reference pins incorporated in the cylindrical cassette. These pins ensure that the position of the film in the cassette is known, and avoid the need for fiducial marks. The crystal alignment and film measurement technique is fully automatic, requiring no prior input other than the approximate starting orientation of the crystal, the approximate unit-cell dimensions, and the angular oscillation range. An alternative method for the determination of crystal orientation is proposed which has been found to be somewhat superior to that of Rossmann, especially for smaller unit cells.