Shape of the diffraction spots changes in the statistical disorder <--> > twinning continuum. At both ends spots shape is like in diffraction from crystals without such disorder. However, in the intermediate case, electron density autocorrelation function has additional component to one resulting from ordered crystal. This additional component of autocorrelation creates characteristic non-Bragg diffraction, e.g. streaks aligned with particular unit cell axis.
In the absence of such diffraction pattern, the ambiguity is binary. The description of the problem indicates statistical disorder. Zbyszek Otwinowski > Hi, > > If there's an NCS translation, recent versions of Phaser can account for > it and give moment tests that can detect twinning even in the presence of > tNCS. But I agree with Eleanor that the L test is generally a good choice > in these cases. > > However, the fact that you see density suggests that your crystal might be > more on the statistical disorder side of the statistical disorder <--> > twinning continuum, i.e. the crystal doesn't have mosaic blocks > corresponding to one twin fraction that are large compared to the > coherence length of the X-rays. So you might want to try refinement with > the whole structure duplicated as alternate conformers. > > Best wishes, > > Randy Read > > ----- > Randy J. Read > Department of Haematology, University of Cambridge > Cambridge Institute for Medical Research Tel: +44 1223 336500 > Wellcome Trust/MRC Building Fax: +44 1223 336827 > Hills Road > E-mail: rj...@cam.ac.uk > Cambridge CB2 0XY, U.K. > www-structmed.cimr.cam.ac.uk > > On 11 Mar 2014, at 14:10, Eleanor Dodson <eleanor.dod...@york.ac.uk> > wrote: > >> Sorry - hadnt finished.. >> The twinning tests are distorted by NC translation - usually the L test >> is safe, but the others are all suspect.. >> >> >> >> On 11 March 2014 14:09, Eleanor Dodson <eleanor.dod...@york.ac.uk> >> wrote: >> What is the NC translation? If there is a factor of 0.5 that makes SG >> determination complicated.. >> Eleanor >> >> >> On 11 March 2014 14:04, Stephen Cusack <cus...@embl.fr> wrote: >> Dear All, >> I have 2.6 A data and unambiguous molecular replacement solution for >> two copies/asymmetric unit of a 80 K protein for a crystal >> integrated >> in P212121 (R-merge around 9%) with a=101.8, b=132.2, c=138.9. >> Refinement allowed rebuilding/completion of the model in the noraml way >> but the R-free does not go below 30%. The map in the model regions looks >> generally fine but there is a lot >> of extra positive density in the solvent regions (some of it looking >> like weak density for helices and strands) and unexpected positive >> peaks within the model region. >> Careful inspection allowed manual positioning of a completely different, >> overlapping solution for the dimer which fits the extra density >> perfectly. >> The two incompatible solutions are related by a 2-fold axis parallel to >> a. >> This clearly suggests some kind of twinning. However twinning analysis >> programmes (e.g. Phenix-Xtriage), while suggesting the potentiality >> of pseudo-merohedral twinning (-h, l, k) do not reveal >> any significant twinning fraction and proclaim the data likely to be >> untwinned. (NB. The programmes do however highlight a >> non-crystallographic translation and there are systematic intensity >> differences in the data). Refinement, including this twinning law made >> no difference >> since the estimated twinning fraction was 0.02. Yet the extra density is >> clearly there and I know exactly the real-space transformation between >> the two packing solutions. >> How can I best take into account this alternative solution (occupancy >> seems to be around 20-30%) in the refinement ? >> thanks for your suggestions >> Stephen >> >> -- >> >> ********************************************************************** >> Dr. Stephen Cusack, >> Head of Grenoble Outstation of EMBL >> Group leader in structural biology of protein-RNA complexes and viral >> proteins >> Joint appointment in EMBL Genome Biology Programme >> Director of CNRS-UJF-EMBL International Unit (UMI 3265) for Virus Host >> Cell Interactions (UVHCI) >> ********************************************************************** >> >> Email: cus...@embl.fr >> Website: http://www.embl.fr >> Tel: (33) 4 76 20 7238 Secretary (33) 4 76 20 7123 >> Fax: (33) 4 76 20 7199 >> Postal address: EMBL Grenoble Outstation, 6 Rue Jules Horowitz, BP181, >> 38042 Grenoble Cedex 9, France >> Delivery address: EMBL Grenoble Outstation, Polygone Scientifique, >> 6 Rue Jules Horowitz, 38042 Grenoble, France >> ********************************************************************** >> >> > Zbyszek Otwinowski UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd. Dallas, TX 75390-8816 Tel. 214-645-6385 Fax. 214-645-6353
