Hi Alun, 1. What does phenix.xtriage think of the dataset? 2. What kind of redundancy do you have in the dataset? 3. Sometimes with a large crystal and a small beam the twin fraction varies as you rotate the crystal and illuminate different parts of the crystal (which can have different twin fractions). If you have enough data sometimes processing a subset of the frames will help you find a part of the crystal that has a consistent twin fraction. 4. I have had rod like p61 crystals where if you shoot either end of the rod they are not twinned, but if you shot the middle they looked twinned. They were just two crystals with opposite polarity growing together. You could have the same thing in I4.
Ben On 1 Mar 2017, at 22:03, Alun R Coker <alun.co...@ucl.ac.uk> wrote: Hi Everyone, I have a (2.5 - 2.3 Angstroms) data set the process as I4 or I222. Aimless indicates twinning in both space groups and all the lower symmetry space groups consistent with the reduced cell. Molecular replacement works in I4 but not I222 etc. I can see new density for a ligand we are interested in some of the 6 subunits so all seems promising. I've refined in refmac without the twinning turned on in order to avoid model bias and end up with an R-factor/R-free of 0.268/0.305. However, if I repeat the last round of refinement with twinning turned on the R-factor drops but not the R-free (R/R-free 0.237/0.305). Remac reckons the twin fraction is 0.628/0.372. My question is does the drop in R-factor but not R-free suggest I am over-fitting the model by refining twinning? Running the data and model through pdb redo suggest the data isn't twinned. Thanks, Alun -- Dr Alun R. Coker Senior Lecturer Wolfson Institute for Biomedical Research University College London The Cruciform Building London WC1E 6BT Tel: 020 7679 6703 Ext 46703 Web: www.ucl.ac.uk/pxmed Dr Ben Bax Dept. Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK ben.d.v....@gmail.com
