title: Crystal pathologies in macromolecular crystallography.
abstract: Macromolecules, such as proteins or nucleic acids, form crystals with a large volume fraction of water, ~50% on average. Apart from typical physical defects and rather trivial poor quality problems, macromolecular crystals, as essentially any crystals, can also suffer from several kinds of pathologies, in which everything seems to be perfect, except that from the structural point of view the interpretation may be very difficult, sometimes even impossible. A frequent nuisance is pseudosymmetry, or non-crystallographic symmetry (NCS), which is particularly nasty when it has translational character. Lattice-translocation defects, also called order-disorder twinning (OD-twinning), occur when molecules are packed regularly in layers but the layers are stacked (without rotation) in two (or more) discrete modes, with a unique translocation vector. Crystal twinning arises when twin domains have different orientations, incompatible with the symmetry of the crystal structure. There are also crystals in which the periodic (lattice) order is broken or absent altogether. When the strict short-range translational order from one unit cell to the next is lost but the long-range order is restored by a periodic modulation, we have a modulated crystal structure. In quasicrystals (not observed for macromolecules yet), the periodic order (in 3D space) is lost completely and the diffraction pattern (which is still discrete) cannot be even indexed using three hkl indices. In addition, there are other physical defects and phenomena (such as high mosaicity, diffraction anisotropy, diffuse scattering, etc.) which make diffraction data processing and structure solution difficult or even impossible.
authors.affiliation: Macromolecular Crystallography Laboratory, National Cancer Institute, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Zbigniew
authors.initials: Z
authors.lastname: Dauter
authors.affiliation: Zakład Krystalografii, Wydział Chemii, Uniwersytet im. A. Mickiewicza w Poznaniu, ul. Grunwaldzka 6, 60-780 Poznań i Centrum Badań Biokrystalograficznych, Instytut Chemii Bioorganicznej PAN, ul. Z. Noskowskiego 12/14, 61-704 Poznań, Polska.
authors.forename: Mariusz
authors.initials: M
authors.lastname: Jaskólski
mesh_terms: D003460:Crystallization; D003461:Crystallography / Q000379:methods*
keywords: aperiodic crystals; commensurate modulation; crystal twinning; diffraction anisotropy; diffuse scattering; incommensurate modulation; lattice-translocation defects; merohedry; modulated structure; mosaicity; non-crystallographic symmetry (NCS); order-disorder (OD); pseudosymmetry; quasicrystals; superstructure; translational NCS