Broken symmetries in macromolecular crystallography: phasing from unmerged data
The space-group symmetry of a crystal structure imposes a point-group symmetry on its diffraction pattern, giving rise to so-called symmetry-equivalent reflections. Instances in macromolecular crystallography are discussed in which the symmetry in reciprocal space is broken, i.e. where symmetryrelated reflections are no longer equivalent. Such a situation occurs when the sample suffers from site-specific radiation damage during the X-ray measurements. Another example of broken symmetry arises from the polarization anisotropy of anomalous scattering. In these cases, the genuine intensity differences between symmetry-related reflections can be exploited to yield phase information in the structure-solution process. In this approach, the usual separation of the data merging and phasing steps is abandoned. The data are kept unmerged down to the Harker construction, where the symmetry-breaking effects are explicitly modelled and refined and become a source of supplementary phase information.
Keywords: X-Ray-Absorption ; Isomorphous Replacement Method ; Anomalous Scattering ; Radiation-Damage ; Synchrotron-Radiation ; Crystal-Structure ; Structural-Changes ; Catalytic Pathway ; Single-Wavelength ; Diffraction
Record created on 2010-04-12, modified on 2016-08-08