Two more papers on twinning I found informative: ===========================================
Acta Cryst. (2003). D59, 2004-2016 [ doi:10.1107/S0907444903021085 ] Twinned crystals and anomalous phasing Z. Dauter Abstract: Merohedral or pseudomerohedral twinning of crystals cannot be identified from inspection of the diffraction patterns. Several methods for the identification of twinning and the estimation of the twin fraction are suitable for macromolecular crystals and all are based on the statistical properties of the measured diffraction intensities. If the crystal twin fraction is estimated and is not too close to 0.5, the diffraction data can be detwinned; that is, related to the individual crystal specimen. However, the detwinning procedure invariably introduces additional inaccuracies to the estimated intensities, which substantially increase when the twin fraction approaches 0.5. In some cases, a crystal structure can be solved with the original twinned data by standard techniques such as molecular replacement, multiple isomorphous replacement or multiwavelength anomalous diffraction. Test calculations on data collected from a twinned crystal of gpD, the bacteriophage [lambda] capsid protein, show that the single-wavelength anomalous diffraction (SAD) method can be used to solve its structure even if the data set corresponds to a perfectly twinned crystal with a twin fraction of 0.5. Keywords: twinning; merohedral; pseudomerohedral; anomalous scattering; SAD. =========================================== Acta Crystallogr D Biol Crystallogr. 2008 Jan;64(Pt 1):99-107. Epub 2007 Dec 5. Surprises and pitfalls arising from (pseudo)symmetry. Zwart PH, Grosse-Kunstleve RW, Lebedev AA, Murshudov GN, Adams PD. Author information Abstract It is not uncommon for protein crystals to crystallize with more than a single molecule per asymmetric unit. When more than a single molecule is present in the asymmetric unit, various pathological situations such as twinning, modulated crystals and pseudo translational or rotational symmetry can arise. The presence of pseudosymmetry can lead to uncertainties about the correct space group, especially in the presence of twinning. The background to certain common pathologies is presented and a new notation for space groups in unusual settings is introduced. The main concepts are illustrated with several examples from the literature and the Protein Data Bank. PMID: 18094473 [PubMed - indexed for MEDLINE] PMCID: PMC2394827 Free PMC Article
