On Tuesday 06 January 2009 08:35:19 Stephen Hare wrote: > Dear All, > > We are currently working on a structure of apparent P21 symmetry which > has been solved by molecular replacement. The data are to 2.7Å but the > Rfree will not drop below 30%. The density is clear for the model we > have, however there is extra density that suggests a shift of the > structure by 16Å in either direction - resulting in three possible > overlapping positions for the structure. We assume this is the result > of twinning.
It sounds like an interesting case. You might want to go back and closely inspect your diffraction images after reading this paper: J. Appl. Cryst. (2008). 41, 600-605 Protein crystals can be incommensurately modulated JJ Lovelace, CR Murphy, L Daniels, K Narayan, CE Schutt, U Lindberg, C Svensson and GEO Borgstahl I am not certain that your description matches this case, but it might. Ethan > The unit cell dimensions are 102.7Å, 83.0Å, 115.3Å, 90°, 101.8°, 90°. > Examining the data with phenix.xtriage also suggests pseudo > translational symmetry with a separation of 16Å. A Patterson peak at > 0.097, 0.000, -0.096 is approximately 30% of the origin peak, while a > second peak of double the translation at 0.192, 0.000, 0.195 is 7% of > the origin peak. The structure contains a dimer of dimers with an NCS > 2 fold axis almost perpendicular to the crystallographic 2 fold. This > NCS axis almost coincides with the diagonal between the A and C axes. > A twin axis along the A C diagonal (l,-k,h) could explain the observed > extra density, however this is not possible because A and C are > different lengths. > > As a result of the NCS axis running almost perpendicular to the > observed P21 axis, it is possible to merge the reflections in a larger > orthorhombic unit cell - dimensions 137.1Å, 83.3Å, 169.8Å although > here the Rmerge is higher and it is not possible to get a molecular > replacement solution. > > Is it possible to define the (l,-k,h) twinning operator in our > original unit cell? or have we missed the actual unit cell? > Or....something else? > > Steve > > > > Stephen Hare PhD > Post doctoral research associate > Jefferiss Research Laboratories > Wright-Fleming Institute > Division of Medicine > Imperial College London > Norfolk Place > London W2 1PG > UK > > Phone: +44 (0) 20 7594 3908 > Fax: +44 (0) 20 7594 3906 > -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742