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: [email protected] Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk On 11 Mar 2014, at 14:10, Eleanor Dodson <[email protected]> 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 <[email protected]> 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 <[email protected]> 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: [email protected] > 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 > ********************************************************************** > >
