Dear Stefan, Congratulations! It seems that you not only have pseudo-translation and pseudo 4-fold symmetry in your diffraction, but also pseudo 8-fold symmetry in your protein molecules! An excellent case to learn a lot about space groups and NCS symmetry.
3) you are right, you might see pseudo-translation due to the internal pseudo-symmetry of your molecules. Does this fit with the molecular replacement solution you got? 5) Nothing is easier! Here the Refmac people really did an excellent job! Just give your P1 data set and P1 MR solution to Refmac and give the TWIN keyword. Refmac will sort everything out for you. I am currently working on a crystal form which is P43212, but where in most crystals the layers occasionally shift by half an unit cell resulting in a spacegroup of 1/2a, 1/2b, c with the same P43212 symmetry and 1/4 molecule in the unit cell. In a first round, I just processed in P1 and ran MR in P1 and found 1 dimer. Refmac detected and refined 8 twin domains without any problems. Of course the 2-fold symmetry, generated by the dimer, is not twinning, so I still have to reprocess the data in P2 and refine with tetartohedral twinning. I suspect, you might have something similar: perfect 4- or 4x-fold symmetry, but subsequent layers, or twin domains, are randomly shifted somewhat (left-right-up-down) and your NCS 4-folds to not coincide with the positions where crystallographic 4-folds should be. What kind of R-factors do you get when you refine your MR solution without twinning? Herman Von: CCP4 bulletin board [mailto:[email protected]] Im Auftrag von Stefan Gajewski Gesendet: Dienstag, 22. Oktober 2013 06:29 An: [email protected] Betreff: Re: [ccp4bb] Triclinic solution for a dataset with pseudo-translational symmetry, possible pseudo-centering and possible pseudo-merohedral twinning. How to proceed? Dear Herman, 1) R-merge is about 11% in 4/mmm and only slightly lower in lower symmetry. 4/m would be an option since it has only four molecules in the cell but I do not get a solution for any group higher than P1. 2) Agreed. 3) Well, actually the pseudo 4-fold "works" when the pseudo translation also has a rotational component. I attached a diagram for the packing in the cell. My protein has a high internal pseudo-symmetry and overlaps pretty well with itself by rotation. 4) Each of the four proteins has a local 8 fold pseudo-symmetry of monomers. 5) tetartohedral twinning? what refinement software is able to deal with that? Stefan
