Dear Steve, Presence of several Patterson peaks at impossible (as far as crystal packing is concerned) distances and an NCS two-fold normal to the crystallographic one suggests it could be an order-disorder (OD) structure, like the one we had in Exeter.
An order-disorder twin crystal of L-2-haloacid dehalogenase from Sulfolobus tokodaii. Rye CA, Isupov MN, Lebedev AA, Littlechild JA. Acta Crystallogr D Biol Crystallogr. 2007 Aug;63(Pt 8):926-30. For that case at 1.6 A Andrey Lebedev from York had written a dedicated detwinning (demodulation) program and FreeR went down from 0.27 to 0.22. Original FreeR was nearly acceptable as only several percent of reflections were overlapping. If you feel your case appears similar to our OD-structure I would suggest you to contact Andrey, as he is collecting cases of OD-structures and probably could modify his program for your case. Best wishes Misha Isupov University of Exeter ________________________________________ From: CCP4 bulletin board [[email protected]] On Behalf Of Stephen Hare [[email protected]] Sent: 06 January 2009 16:35 To: [email protected] Subject: [ccp4bb] Fwd: Twinning 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. 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
