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Updated: Aug 23, 2026

Microcrystallography of Protein Crystals and In Cellulo Diffraction
Published on: July 21, 2017
Orientation of non-crystallographic symmetry axes in protein crystals
1Laboratoire de Biologie Structurale, UMR 9920 CNRS, Université Paris-Sud, Orsay, France.
Abstract:
A survey of 129 protein crystal structures with more than one molecule per asymmetric unit shows that local (non-crystallographic) symmetry axes are not randomly oriented. When compared to the crystal cell edges, face diagonals, body diagonal and reciprocal cell edges, 65% of the local symmetry axes are found to be parallel to one of the reference directions to within 15 degrees; another 18% are orthogonal to within 3 degrees; only 17% are in general orientations. In monoclinic, trigonal and hexagonal crystals, a majority of the local symmetry axes are orthogonal to the unique axis, while preferred orientations are parallel to the cell edges in orthorhombic crystals.
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