Synchrotron radiation: micrometer-sized x-ray beams as fine tools for macromolecular crystallography

Structural data play a central role in understanding biological function at the molecular level. At present, the majority of high-resolution structural data about biological macromolecules and their complexes originates from crystallography. In crystal structure determination, the major hurdle to overcome is the production of crystals of sufficient size and quality. High-flux x-ray beams with diameters of a few micrometers or less help to alleviate this problem as small beams allow the use of small crystals or scanning of large crystals for regions of acceptable diffraction. Using sophisticated x-ray optics and mechanics with submicrometer precision, Riekel et al.[Acta Crystallogr., Sect. D: Biol. Crystallogr., 64, 158-166 (2008)], have recently demonstrated that an x-ray beam of 1 mum can be used to determine the crystal structure of a protein to a resolution of 1.5 A. The smallest volume from which usable diffraction data were collected amounted to 20 mum(3), corresponding to not more than 2x10(8) unit cells. In a diffraction volume of micrometer dimensions, radiation damage is expected to be reduced with respect to large volumes as a significant fraction of the photoelectrons produced by the incident radiation escapes from the diffracting volume before dissipating their energy. The possibility to make use of small andor inhomogeneous crystals in combination with a possible reduction in radiation damage due to size effects has the potential to make many more systems amenable to crystal structure analysis.