Hi Bert, It seems unikely you are experiencing merohedral twinning in your crystal since none of your unit cell dimensions are equal length or integer multiples. For your cell, you would expect to see multiple lattices. Is it possible you have a dimer in the asymmetric unit? Strong NCS parallel to a principle lattice direction can sometimes give "twin-like" statistics especially at lower resolutions.
Hope this helps, Chris On Mon, 16 Feb 2009, Van Den Berg, Bert wrote: >>>Hello all, >>> >>>we have a dataset collected from multiple (2 or 3) parts of the same >>>crystal with a microbeam (20 micron). The merged data scales OK (not great) >>>in monoclinic (1-3% rejections). The resolution is 3.2-3.3 A, so the data is >>>not fantastic. This is the cell (similar for other datasets): >>> >>>Cell: 70.012 126.449 107.988 90.000 89.946 90.000 p21 >>> >>>Processing in orthorhombic makes the scaling a lot worse, so I'm assuming >>>its monoclinic for now. Running xtriage gives the following summary: >>> >>>------------------------------------------------------------------------------- >>>Twinning and intensity statistics summary (acentric data): >>> >>>Statistics independent of twin laws >>> - <I^2>/<I>^2 : 1.877 >>> - <F>^2/<F^2> : 0.834 >>> - <|E^2-1|> : 0.663 >>> - <|L|>, <L^2>: 0.411, 0.235 >>> Multivariate Z score L-test: 6.737 >>> The multivariate Z score is a quality measure of the given >>> spread in intensities. Good to reasonable data are expected >>> to have a Z score lower than 3.5. >>> Large values can indicate twinning, but small values do not >>> necessarily exclude it. >>> >>> >>>Statistics depending on twin laws >>>----------------------------------------------------------------- >>>| Operator | type | R obs. | Britton alpha | H alpha | ML alpha | >>>----------------------------------------------------------------- >>>| h,-k,-l | PM | 0.167 | 0.367 | 0.339 | 0.152 | >>>----------------------------------------------------------------- >>> >>>Patterson analyses >>> - Largest peak height : 5.962 >>> (corresponding p value : 0.72096) >>> >>> >>>The largest off-origin peak in the Patterson function is 5.96% of the >>>height of the origin peak. No significant pseudotranslation is detected. >>> >>>So, I'm assuming that these crystals are monoclinic and that they are >>>pseudo-merohedrally twinned. Is this a reasonable assumption? I get a decent >>>solution for the P21 data from molecular replacement with a 50% identical >>>model (LLG 900, with the rotation Z-scores low (4-5), but the corresponding >>>translation Z-scores high (8-20)). >>> >>>My questions are: what would be the best way to refine? More specifically, >>>what twin fraction should be used as the different tests give different >>>fractions. Is the twin fraction automatically determined in phenix.refine or >>>does this need to be specified? Finally, can twinning be responsible for the >>>fact that the data do not scale well (using data collected on different >>>parts of the same crystal)? >>> >>>Any hints appreciated! >>> >>>Cheers, Bert >>> >>> >>>Bert van den Berg >>>University of Massachusetts Medical School >>>Program in Molecular Medicine >>>Biotech II, 373 Plantation Street, Suite 115 >>>Worcester MA 01605 >>>Phone: 508 856 1201 (office); 508 856 1211 (lab) >>>e-mail: [email protected] >>>http://www.umassmed.edu/pmm/faculty/vandenberg.cfm >>> >>> >>> Christopher L. Colbert, Ph.D. Instructor Phone: (214) 645 5944 University of Texas Southwestern Medical Center FAX: (214) 645 5945 6001 Forest Park Lane Dallas, TX 75390
