Orientation of non-crystallographic symmetry axes in protein crystals.
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.
Laboratoire de Biologie Structurale, UMR 9920 CNRS, Université Paris-Sud, Orsay, France.
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Janin