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

International Tables for Crystallography (2006). Vol. F, ch. 5.1, p. 112   | 1 | 2 |

Section Twinning

H. L. Carrella* and J. P. Gluskera

aThe Institute for Cancer Research, The Fox Chase Cancer Center, Philadelphia, PA 19111, USA
Correspondence e-mail: Twinning

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Twinning is a phenomenon that can cause much grief in X-ray diffraction data measurements. It has been described as `a crystal growth anomaly in which the specimen is composed of separate crystal domains whose orientations differ in a specific way … some or all of the lattice directions in the separate domains are parallel' (Yeates, 1997[link]). Thus, a twin consists of two (or more) distinct but coalescent crystals. This effect has been described in terms of their diffraction patterns as follows: `Some crystals show splitting of diffraction spots owing to the different tilts of the two lattices. Others pretend to be single crystals with no split spots, and their symmetries of intensity distribution vary with every data set. These latter have been called hemihedral, and in them the unique axes of the two crystals are exactly reversed parallel with each other.' (Igarashi et al., 1997[link]). Perfect hemihedral twinning (when there are equal proportions of each twin member) can be detected from the value of [\langle I^{2} \rangle/\langle I \rangle^{2}] for the acentric data; it is near 2 for untwinned data and 1.5 for twinned data (Yeates, 1997[link]).

Twinning may sometimes be prevented by introducing variations into the crystallization setup. Such changes could involve the pH or the nature of the buffer. Variations in the seeding technique used, or the introduction of additional agents, such as metal ions or salts, detergents, or certain amino acids, can also be tried. One method for estimating the degree of twinning is based on the fact that each measured X-ray diffraction intensity is the sum of the intensities from the two (or more) crystal lattices (suitably weighted according to the proportion in which each lattice alignment occurs in the crystal). The relative proportion of each component can therefore be estimated. Detwinned intensities obtained by this method should only be positive or zero (not negative) within experimental error (Stanley, 1972[link]; Britton, 1972[link]; Rees, 1980[link]). Crystal structures of hemihedrally twinned crystals are now being determined (Gomis-Rüth et al., 1995[link]; Breyer et al., 1999[link]).


Breyer, W. A., Kingston, R. L., Anderson, B. F. & Baker, E. N. (1999). On the molecular-replacement problem in the presence of merohedral twinning: structure of the N-terminal half-molecule of human lactoferrin. Acta Cryst. D55, 129–138.Google Scholar
Britton, D. (1972). Estimation of twinning parameter for twins with exactly superimposed reciprocal lattices. Acta Cryst. A28, 296–297.Google Scholar
Gomis-Rüth, F. X., Fita, I., Kiefersauer, R., Huber, R., Avilés, F. X. & Navaza, J. (1995). Determination of hemihedral twinning and initial structural analysis of crystals of the procarboxypeptidase A ternary complex. Acta Cryst. D51, 819–823.Google Scholar
Igarashi, N., Moriyama, H., Mikami, T. & Tanaka, N. (1997). Detwinning of hemihedrally twinned crystals by the least-squares method and its application to a crystal of hydroxylamine oxidoreductase from Nitrosomonas europaea. J. Appl. Cryst. 30, 362–367.Google Scholar
Rees, D. C. (1980). The influence of twinning by merohedry on intensity statistics. Acta Cryst. A36, 578–581.Google Scholar
Stanley, E. (1972). The identification of twins from intensity statistics. J. Appl. Cryst. 5, 191–194.Google Scholar
Yeates, T. O. (1997). Detecting and overcoming crystal twinning. Methods Enzymol. 276, 344–358.Google Scholar

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