[ccp4bb] perfect twin test

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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