The high-mosaicity illusion: revealing the true physical characteristics of macromolecular crystals

Typical measurements of macromolecular crystal mosaicity are dominated by the characteristics of the X-ray beam and as a result the mosaicity value given during data processing can be an artifact of the instrumentation rather than the sample. For physical characterization of crystals, an experimental system and software have been developed to simultaneously measure the diffraction resolution and mosaic spread of macromolecular crystals. The contributions of the X-ray beam to the reflection angular widths were minimized by using a highly parallel, highly monochromatic synchrotron source. Hundreds of reflection profiles over a wide resolution range were rapidly measured using a charge-coupled device (CCD) area detector in combination with superfine phi-slicing data collection. The Lorentz effect and beam contributions were evaluated and deconvoluted from the recorded data. Data collection and processing is described. From 1 degrees of superfine phi-slice data collected on a crystal of manganese superoxide dismutase, the mosaicities of 260 reflections were measured. The average mosaicity was 0.0101 degrees (s.d. 0.0035 degrees) measured as the full-width at half-maximum (FWHM) and ranged from 0.0011 to 0.0188 degrees. Each reflection profile was individually fitted with two Gaussian profiles, with the first Gaussian contributing 55% (s.d. 9%) and the second contributing 35% (s.d. 9%) of the reflection. On average, the deconvoluted width of the first Gaussian was 0.0054 degrees (s.d. 0.0015 degrees) and the second was 0.0061 degrees (s.d. 0.0023 degrees). The mosaicity of the crystal was anisotropic, with FWHM values of 0.0068, 0.0140 and 0.0046 degrees along the a, b and c axes, respectively. The anisotropic mosaicity analysis indicates that the crystal is most perfect in the direction that corresponds to the favored growth direction of the crystal.