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 <bert.van-den-b...@newcastle.ac.uk> 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
