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Dear Bert,
In my own review:-
http://www.tandfonline.com/doi/abs/10.1080/08893110802360925?journalCode=gcry20#.UulGyGtYCSM
molecular replacement emerged in my mind as the most robust option for structure determination in such a case, apart from finding an untwinned crystal form of course.
Best wishes,
John

Prof John R Helliwell DSc FInstP CPhys FRSC CChem F Soc Biol.
Chair School of Chemistry, University of Manchester, Athena Swan Team.
http://www.chemistry.manchester.ac.uk/aboutus/athena/index.html
 
 

On 28 Jan 2014, at 17:26, Bert Van-Den-Berg <[log in to unmask]> wrote:

Dear all,

I recently collected several datasets for a protein that needs experimental phasing.
The crystals are hexagonal plates, and (automatic) data processing suggests with high confidence that the space group is P622. This is where the fun begins.
For some datasets (processed in P622), the intensity distributions are normal, and the L-test (aimless, xtriage) and Z-scores (xtriage) suggest that there is no twinning (twinning fractions < 0.05). However, for other datasets (same cell dimensions), the intensity distributions are not normal (eg Z-scores > 10). Given that twinning is not possible in P622, this suggests to me that the real space group could be P6 with (near) perfect twinning.

If I now process the "normal L-test P622" datasets in P6, the twin-law based tests (britton and H-test in xtriage) give high twin fractions (0.45- 0.5), suggesting all my data is twinned.
Does this make sense (ie can one have twinning with "normal" intensity distributions)?
If it does, would the "normal L-test" datasets have a higher probability of being solvable?

Is there any strategy for experimental phasing of (near) perfect twins? SAD would be more suitable than SIR/MIR? (I also have potential heavy atom derivatives).

Thanks for any insights!

Bert