Try looking into tetartohedral twinning as well--I think I may have such a crystal, and it's tough going. And as Kay pointed out, try the various P3's. Since I have not yet been successful in figuring my similar case out, what do people on the list recommend as an approach to figuring this out--just trying every possible space group with various parameters? I would think there should be some actual advantages at the phasing step to having twins, such as a sort of "NCS" of intensities rather than amplitudes, weighted by twin fraction, but it doesn't seem that any software uses this. Perhaps there is a reason for that?
A paper on tetartohedral twinning I saw: Acta Crystallogr D Biol Crystallogr. 2012 Apr;68(Pt 4):418-24. doi: 10.1107/S0907444912006737. Epub 2012 Mar 16. Tetartohedral twinning could happen to you too. Roversi P, Blanc E, Johnson S, Lea SM. Author information Abstract Tetartohedral crystal twinning is discussed as a particular case of (pseudo)merohedral twinning when the number of twinned domains is four. Tetartohedrally twinned crystals often possess pseudosymmetry, with the rotational part of the pseudosymmetry operators coinciding with the twinning operators. Tetartohedrally twinned structures from the literature are reviewed and the recent structure determination of tetartohedrally twinned triclinic crystals of human complement factor I is discussed. PMID: 22505261 [PubMed - indexed for MEDLINE] PMCID: PMC3322600 Free PMC Article JPK -----Original Message----- From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Kay Diederichs Sent: Wednesday, January 29, 2014 4:17 AM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] twinning fun Dear Bert, as Dirk has pointed out, if P622 is the correct space group, then the twinning statistics printed out if you process in P6 are meaningless. Intensity statistics, like the ratio of <I^2>/<I>^2 , can be misleading if there is (e.g. pseudo-translational) NCS in the crystal; however, the effect of NCS on the value of the ratio of <I^2>/<I>^2 is opposite to that of twinning. Thus if a crystal is twinned and has NCS, you might not notice any problem in the ratio of <I^2>/<I>^2 . The other statistics, like Britton and H-test, present the intensity statistics in a different way, but from my understanding do not give substantially different information. The L-test does look at a different kind of information and therefore gives additional insight. If your measurements suffer from high background, diffuse scatter, ice rings, smeared reflections, additional crystals in the beam, or any other pathology, then all these tests may give distorted answers. In other words, even if twinning is not really present, any test designed to convert the deviation of data from ideality into an estimate of the twinning fraction will give you an alpha > 0. So my experience is: if your data are very good, then the tests give good answers; if the data are mediocre or bad, don't necessarily believe the numbers. Finally, it's not only twinning of P6 that would give you P622, it's also twinning of P3x21, P3x12 that gives P6y22. Hope this helps, Kay On Tue, 28 Jan 2014 17:26:23 +0000, 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 >
