Space-group and origin ambiguity in macromolecular structures with pseudo-symmetry and its treatment with the programZanuda

Abstract

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 andvice versa. A mistake only becomes apparent when theRfreeceases 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 programZanudapresented in this article was developed for the automation of SG validation.Zanudaruns 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.