The use of noncrystallographic symmetry averaging to solve structures from data affected by perfect hemihedral twinning


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SABIN Charles David PLEVKA Pavel

Year of publication 2016
Type Article in Periodical
MU Faculty or unit

Central European Institute of Technology

Field Biochemistry
Keywords hemihedral perfect twinning; noncrystallographic symmetry averaging; molecular replacement; virus structure; symmetry; detwinning; twin domain; merohedral twinning; mask envelope
Description Hemihedral twinning is a crystal-growth anomaly in which a specimen is composed of two crystal domains that coincide with each other in three dimensions. However, the orientations of the crystal lattices in the two domains differ in a specific way. In diffraction data collected from hemihedrally twinned crystals, each observed intensity contains contributions from both of the domains. With perfect hemihedral twinning, the two domains have the same volumes and the observed intensities do not contain sufficient information to detwin the data. Here, the use of molecular replacement and of noncrystallo-graphic symmetry (NCS) averaging to detwin a 2.1 angstrom resolution data set for Aichi virus 1 affected by perfect hemihedral twinning is described. The NCS averaging enabled the correction of errors in the detwinning introduced by the differences between the molecular-replacement model and the crystallized structure. The procedure permitted the structure to be determined from a molecular-replacement model that had 16% sequence identity and a 1.6 angstrom r.m.s.d. for C-alpha atoms in comparison to the crystallized structure. The same approach could be used to solve other data sets affected by perfect hemihedral twinning from crystals with NCS.
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