Hi
Firstly thanks to all who replied to my original post.
The clear consensus was to look for pseudo-symmetry.
I must admit there is more to the story. Here goes the long version.
Crystals are Hexagonal bi-pyramids (under ideal conditions they are very beautiful nice crisp edges etc. non ideal conditions crystals still grow however they lose the nice edges and look kind of like a football, and do not diffract well). Two different unit cells have been observed for these crystals; 1) a,b=50, c=150, 2) a,b=50, c=300. 90 90 120
The data for both cells is highly anisotropic and has apparent 622 symmetry (self rotation function). Due to the anisotropy data can only be merged to ~3 A even though there is data to ~2 A in the strong reflecting direction.
There is no pseudo-symmetry detected in the small cell however there is pseudo-translation detected in the big cell:
pseudo-translation vector: 0.000 0.000 0.500 (12.3%) from 'SFcheck' (what does the % mean?)
Not surprisingly intensity statistics to detect twinning are kind of all over the place but xtriage does find three twin laws (alpha for all 3 laws ~0.48) and suspects the data to be twinned (in consensus with SFcheck). Using data processed in P3 at the end of xtriage log there is this statement.
[ The results of the L-test indicate that the intensity statistics
Show more centric character than is expected for acentric data.]
I have a MR model with 45% identity. No solutions are found in the big cell. In the small cell solutions can be found in P3212 and P32 ????2 and 4 mol/ASU respectively). Refinement stalls at ~42% and there is missing density for much of the model. I have attempted perfect twin refinement with CNS but I get huge divergence in R - Rfree 30 - 52.
SeMet protein has been crystallized but so far has only exhibited the small cell. Sites appear to have been found with SOLVE/RESOLVE and SHELX, however the maps just look like noise.
Questions:
What can cause acentric data to have centric characteristic?
Is there an option to do perfect twin refinement with phenix?
In the absence of pseudo-symmetry is there any other hypothesis as to why data (lower res data not suffering from anisotropy) displays values near 3 for the perfect twin test?
All comments, questions and suggestions welcome
Sincerely
Ben
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Benjamin Flath
College of Pharmacy and Nutrition
University of Saskatchewan
320 Thorvaldson Building
110 Science Place
Saskatoon, SK
S7N 5C9
