Dear Stefan, The picture you give of your crystals is indeed quite messy. The first thing I would do is to sort all the information at hand and try to see what could be going on. Based on the (limited) information you give, I have to following comments:
1) Autoindexing suggests a tetragonal space group. This suggest to me that 4 fold symmetry is probably present, which may be crystallographic, non-crystallographic or due to twinning or some other packing defects. How do the statistics look like if you process in say P4, are they somewhat worse, or horrible? If they are only somewhat worse, you could try MR in a tetragonal space group as well. Even if the solution is only to a rough approximation correct, it could give you a feeling what might be going on. 2) Some twinning statistics do not work in the presence of translational symmetry like the 0.46 vector you have, so I would not trust these. 3) With two dimers related by a pseudotranslation, you can only generate 2 fold symmetry, not 4 fold. Since the autoindexing suggests 4 fold symmetry, this may point to twinning. 4) It is not clear how you can build a pseudo-octameric oligomer with only 4 proteins. For me, octameric means that one would need 8 proteins. 5) I think that your crystal may be twinned with a second twin domain 90° rotated with respect to the solution you have right now. Since your data has pseudo 4/mmm symmetry, all 7 possible twin laws may be approximately true and your crystals have pseudo P4x2x2x symmetry. You could rotate your current solution 90, 180 and 270°. If these solutions are non-overlapping e.g. due to the 0.46 vector, you may have to correct for at least 4 fold twinning. The best solution would be to grow crystals in a different space group. Best regards, Herman Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Stefan Gajewski Gesendet: Freitag, 18. Oktober 2013 08:52 An: CCP4BB@JISCMAIL.AC.UK Betreff: [ccp4bb] Triclinic solution for a dataset with pseudo-translational symmetry, possible pseudo-centering and possible pseudo-merohedral twinning. How to proceed? Dear experts, This is a messy question concerning a messy dataset and I try to put it correctly. I have a severely anisotropic dataset up to 2.3A that merges quite fine in all 4/mmm. Automatic indexing is primitive tetragonal but I can push it to a centered tetragonal cell if I want to. The crystals and the diffraction images are highly reproducible. There is one non-origin peak of ~45% origin peak height with vector 0.5 0.5 0.46. Intensity statistics do not indicate twinning. Now I have a good MR solution with a 99% model in P1 that reveals four proteins in the cell. Dimer A,C and Dimer B,D with pseudo-translation A to B and C to D and a twofold between A,C and B,D. The twofold lies almost perfectly within a=b and B,C mimic a center at half c. Each protein (A,B,C,D) is a pseudo-octameric homo-oligomer. It is also still possible that the orientation of the multimers is off by one rotation in my MR solution and there actually exists a higher symmetry, although there are no convincing MR solutions in C2, P2, P2(1), yet. Initial refinement in P1 stalled at R=0.35 and R-free 0.40. There are seven possible pseudo-merohedral twin laws and all of them decrease the R-factors to R~0.30 and R-free ~0.35 in refinement, so I can't tell which one is the correct one or if it is twinned at all. What should I try next? Is there a good advice how to refine such a type of structure? How can I determine if it is twinned or not ? Should I omit anisotropic scaling in this case and cut back the resolution to preserve the weak reflections on the original scale? feeling a little lost here with all the "pseudo" symmetries , Stefan Gajewski
