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
