Newsletter 161 December 3, 2007 |
The
NIH X-Ray Diffraction
Interest Group
Newsletter
web site: http://mcl1.ncifcrf.gov/nihxray
Please share your experience in twinning by presenting a case study. To learn or to review the basics of twinning, see CCP4 General and Paul Adams' presentation. Dr. Xinhua Ji (NCI): Perfect Merohedral Twinning - No News Is Good NewsPerfect Merohedral twinning involves a crystal with a twin fraction equal to ½ We knew the possibility of perfect twinning, but we did not bother until we had to. This "no news is good news" approach offered three advantages. First, there was one
Dr. Mark Mayer (NICHD): A Narrow Escape from Merohedral Twinning Merohedral twinning is a special form of disorder that most crystallographers will be forced to deal with at some point in their career. It is different from twinning which arises when crystals fuse during growth, which is easily recognized either in the light microscope, or from diffraction images which reveal the presence of more than one lattice. In the case of merohedral twinning, the crystal contains microdomains in which the same lattice is present but in different orientations related by a twinning operator. As a result, the observed intensities are not accurate, in the sense that they arise from the sum of the unrelated intensities of the twin components. Because the intensities do not correspond to those generated by a single lattice, refinement stalls at unreasonably high R values, and in some cases the structure cannot be solved at all. (Full Article)
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