Twinned crystals and anomalous phasing.

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.