Dear Bert, The first thing I would do is to calculate the Matthews number: Does at least one monomer fit in the P622 asymmetric unit? If not, your crystals are definitively twinned. As mentioned below, I would also check the <I^2>/<I>^2 ratio, but I would do it with the data processed in P6, since processing true P6 data in P622 will produce a twinned ratio even when the P6 data was not twinned. If it turns out, that some crystals are twinned and others not, I would look at the diffraction patterns to see if something funny is going on (ice rings, high background, strange spot shape etc.). In this case, I would try to solve the structure with untwinned crystals. Maybe less fun, but also less hassle, frustration and cleaner maps.
Best, Herman Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Dirk Kostrewa Gesendet: Dienstag, 28. Januar 2014 22:01 An: CCP4BB@JISCMAIL.AC.UK Betreff: Re: [ccp4bb] twinning fun Dear Bert Van-Den-Berg, as far as I understand this, if you have true P622, process the data in P6 and then test for twinning, both the Britton-test and H-test will indicate perfect merohedral twinning. This is because the Britton-test checks for a sudden increase of negative intensities after de-twinning, which happens only at twin fractions close to 0.5 if the intensities used for de-twinning are the same. But this is true if they are related by crystallographic symmetry. The H-test relates the absolute difference to the sum of the presumably twinned intensities, which gives "0" for intensities related by crystallographic symmetry, again resulting in twin fractions close to 0.5. In other words, intensities related by crystallographic symmetry would indicate "perfect" twinning in both of these tests. A better test for perfect merohedral twinning would be the ratio of <I^2>/<I>^2 which should be 2 for untwinned and 1.5 for perfectly twinned data, tested in the higher space group. These values are reported by data processing programs like XDS. Please, be aware that these ratios have rather strange values if you have an unusually high background (loop fiber diffraction, ice rings, etc.) or extremely weak data. For a really good discussion of twin tests, see Yeates, Methods. Enzymol. 276, 344-358, 1997. Best regards, Dirk. Am 28.01.14 18:26, schrieb Bert Van-Den-Berg: 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 -- ******************************************************* Dirk Kostrewa Gene Center Munich, A5.07 Department of Biochemistry Ludwig-Maximilians-Universität München Feodor-Lynen-Str. 25 D-81377 Munich Germany Phone: +49-89-2180-76845 Fax: +49-89-2180-76999 E-mail: kostr...@genzentrum.lmu.de<mailto:kostr...@genzentrum.lmu.de> WWW: www.genzentrum.lmu.de<http://www.genzentrum.lmu.de> *******************************************************
