Macromolecular crystals and twinning
Macromolecular crystal structure analyses can be severely hampered by cases of twinning or of multiple crystals. However, increasingly, twinning can readily be recognized, notably when the X‐ray diffraction data are analyzed using intensity statistics. Remediation of such cases is possible via alteration of crystal‐growth conditions including use, mainly, of chemical additives. The case of multiple crystal growths likewise can hamper crystal‐structure analysis and although not necessarily associated with twinning is a crystal‐growth situation where similar remediation methods can be adopted, or portions of crystals ‘cut out’ or selected through use of X‐ray ‘microbeams’. There is also a nice body of case studies now in the crystallographic literature and several of these are highlighted in this chapter. Practical details of use of MAD, SAD or molecular replacement with different degrees of crystal twinning, as well as software to diagnose such cases, are described.
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