Macromolecular Crystallography, Part C
Robert B.Von Dreele, in Methods in Enzymology, 2003
Powder diffraction is the workhorse of X-ray crystallographic methods. It is employed by a multitude of sciences and industries where the goal is characterization of a material to see if it has been seen before, or to determine atomic structures of new materials. This is because often the most readily available form for solid materials is that of a polycrystalline powder. Frequently these can be formed over a wide range of conditions and time scales quite unlike the restricted circumstances required for producing large single crystals. In many cases polycrystalline powders are readily made but large single crystals prove impossible to grow. Therefore, we reasoned that the method might be applied to macromolecular crystals. Our results, gleaned from data acquired at two different synchrotrons, Brookhaven Lab’s NSLS and Argonne’s APS, are promising enough that we envision the method could have general usefulness in such applications as screening for the formation of protein⧸drug complexes and even protein structure determinations in favorable cases.
The diffraction pattern that results from powdered crystals consists of a series of rings (Fig. 1) that are the superposition of the individual single-crystal diffraction patterns from the entire ensemble of a very large number (e.g., 109⧸mm3 for 1-μm crystallites) of randomly oriented crystallites. This pattern can display considerable sensitivity to subtle structural changes, typified by shifts in the diffraction peak positions and changes in intensity resulting in readily discernible changes in the powder-diffraction profile (Fig. 2).