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The presence of pseudo-symmetry in a macromolecular crystal and its interplay with twinning may lead to an incorrect space-group (SG) assignment. Moreover, if the pseudo-symmetry is very close to an exact crystallographic symmetry, the structure can be solved and partially refined in the wrong SG. Typically, in such incorrectly determined structures all or some of the pseudo-symmetry operations are, in effect, taken for crystallographic symmetry operations and vice versa. A mistake only becomes apparent when the Rfree ceases to decrease below 0.39 and further model rebuilding and refinement cannot improve the refinement statistics. If pseudo-symmetry includes pseudo-translation, the uncertainty in SG assignment may be associated with an incorrect choice of origin, as demonstrated by the series of examples provided here. The program Zanuda presented in this article was developed for the automation of SG validation. Zanuda runs a series of refinements in SGs compatible with the observed unit-cell parameters and chooses the model with the highest symmetry SG from a subset of models that have the best refinement statistics.

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