PROTEIN CRYSTALLOGRAPHY USING CAPILLARY-FOCUSED X-RAYS Z. Barneaa, D. X. Balaica,b, K. A. Nugenta, R. F. Garrettc, J. N. Varghesed & S. W. Wilkinsb, a School of Physics, University of Melbourne, Parkville VIC 3052 Australia; b CSIRO Division of Materials Science and Technology, Locked Bag 33, Rosebank MDS, Clayton, VIC 3169, Australia; c Australian Nuclear and Scientific Technology Organisation, Private Bag 1, Menai, NSW 2234, Australia; d Biomolecular Research Institute, 343 Royal Parade, Parkville, VIC 3052, Australia.
X-ray concentration using tapered glass capillaries is achieved by exploiting the total external reflection property of glass surfaces for glancing angles of incidence. Our group has recently demonstrated a paraboloidally-tapered glass capillary optic which produced a focused X-ray beam using a monochromatised synchrotron source. The optic produces a focal region for singly-reflected X-rays at a point 40 mm from the end of the capillary. This focus has a FWHM diameter of 40 microns and an intensity gain of two orders of magnitude over the incident X-ray intensity from the channel-cut monochromator for X-ray energies from 5 to 20 keV. We subsequently used a similar optic to obtain X-ray diffraction patterns from a crystal of hen egg-white lysozyme protein on image plates. The use of the capillary-focused beam yielded diffraction patterns 70 to 100 times faster than using an unfocused beam from the channel-cut monochromator alone. Placement of the crystal at different positions in the capillary-focused beam demonstrated the focusing of Bragg reflections and diffraction from a small volume of crystal.
The use of capillary optics with laboratory sources also holds great promise. Currently, the lower X-ray intensity available from these sources dictates long exposure times to obtain sufficient data to perform successful macromolecular crystallography. We expect that tapered capillaries can be used to reduce the exposure times required for macromolecular crystallography by a factor of more than 10, while still maintaining high resolution. The use of focused beams may also permit the study of very small crystals with laboratory X-ray sources.