Tables for
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 25.2, p. 737   | 1 | 2 |

Section Similar geometry and NCS restraints

G. M. Sheldricku* Similar geometry and NCS restraints

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When there are several identical chemical moieties in the asymmetric unit, a very effective restraint is to assume that the chemically equivalent 1,2 and 1,3 distances are the same, but unknown. This technique is easy to apply using SHELXL and is often employed for small-molecule structures and, in particular, for oligosaccharides. Similarly, the terminal P—O bond lengths in DNA structures can be assumed to be the same (but without a target value), i.e. it is assumed that the whole crystal is at the same pH. For proteins, the method is less suitable because of the different abundance of the different amino acids, and, in any case, good target distances are available (Engh & Huber, 1991[link]).

Local noncrystallographic symmetry (NCS) restraints (Usón et al., 1999[link]) may be applied to restrain corresponding 1,4 distances and isotropic displacement parameters to be the same when there are several identical macromolecular domains in the asymmetric unit; usually, the 1,2 and 1,3 distances are restrained to standard values in such cases and so do not require NCS restraints. Such local NCS restraints are more flexible than global NCS constraints and – unlike the latter – do not require the specification of a transformation matrix and mask.


Engh, R. A. & Huber, R. (1991). Accurate bond and angle parameters for X-ray protein structure refinement. Acta Cryst. A47, 392–400.Google Scholar
Usón, I., Pohl, E., Schneider, T. R., Dauter, Z., Schmidt, A., Fritz, H.-J. & Sheldrick, G. M. (1999). 1.7 Å structure of the stabilised RE!v mutant T39K. Application of local NCS restraints. Acta Cryst. D55, 1158–1167.Google Scholar

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