title: 'Broken symmetries' in macromolecular crystallography: phasing from unmerged data.
abstract: 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 symmetry-related 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.
authors.affiliation: Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Cristallographie, CH-1015 Lausanne, Switzerland. marc.schiltz@epfl.ch
authors.forename: Marc
authors.initials: M
authors.lastname: Schiltz
authors.forename: Gérard
authors.initials: G
authors.lastname: Bricogne
mesh_terms: D016880:Anisotropy; D018360:Crystallography, X-Ray / Q000379:methods*; D046911:Macromolecular Substances / Q000032:analysis* / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D012313:RNA / Q000032:analysis / Q000737:chemistry
chemical_list: D046911:Macromolecular Substances; D012313:RNA
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