Certainly you may have a lower symmetry spacegroup, but with NC translation the twinning tests are a bit uncertain.
Your moments seem too high, whereas twinning usually makes them too low. But we need to see the plots v resolutio which should be pretty linear.. Sometimes these go haywire at higher resolution.. And also the spacegroup determination - absences along a* and C*could be caused by the translation (1/2,0.4,1/2) so you need to test P2 21 2 and P 21 21 2 and P 2 21 21 etc.. This is messy but you can test the data h'=h/2 k l'=l/2 ( Ie for a cell 2a,b,2c and see if that seems twinned (Use reindex and requset h/2,k,l/2 as the reindexing operator then input it to ctruncate ..) And of course if th integration is a bit ropy you can get strange effects.. Do you see the same results for all crystals? Eleanor On 3 December 2017 at 18:42, Kay Diederichs <kay.diederi...@uni-konstanz.de> wrote: > Hi Carmela, > > as Jacob suggests, the real space group may be P2 or P2(1). You need to > prepare 3 MTZ files, each corresponding to a different unique b axis. > So just run XDS 3 times, each time with JOB=CORRECT and > SPACE_GROUP_NUMBER=3, and afterwards XDSCONV to create a MTZ file (or > pointless/aimless). > > First time: > UNIT_CELL_CONSTANTS= 58 103 220 90 90 90 > Second time: > UNIT_CELL_CONSTANTS= 103 220 58 90 90 90 > Third time: > UNIT_CELL_CONSTANTS=220 58 103 90 90 90 > > Then, if you have a good model, do MR with each MTZ file, taking care > that both P2 and P2(1) are tested. Or experimental phasing, but that > requires good data. > > HTH, > Kay > > On Sun, 3 Dec 2017 18:29:57 +0100, Carmela Garcia <c.gar...@bioc.uzh.ch> > wrote: > > >Hi, > > > >The dimensions for a native are 58 103 220, with small differences for > the derivatives. > > > >Best, > > > >Carmela. > > > >> On 3 Dec 2017, at 18:18, Sridhar Prasad <spra...@calasiapharma.com> > wrote: > >> > >> Hello, > >> Can you please share the unit cell dimensions. > >> > >> Cheers, > >> Sridhar > >> > >> On Dec 3, 2017 9:13 AM, "Carmela Garcia" <c.gar...@bioc.uzh.ch <mailto: > c.gar...@bioc.uzh.ch>> wrote: > >> Dear all, > >> > >> I know that some years ago a similar situation was discussed here, and > I wonder if someone has new insights about these problems. > >> > >> My protein is a dimer in solution. I tried several derivatives for SAD, > and all my datasets seem to have the same problem, including the native > crystals. I processed the data with XDS and the space group determination > was done with Pointless, being a P212121. > >> > >> Checking the quality of the data, I found several problematic results: > >> > >> - Translational NCS is detected. There is a peak at (0.50, 0.40, 0.50) > >> - The L test suggests twinning (L statistic = 0.41) > >> - The mean acentric moments I from input data have the following values: > >> <I^2>/<I>^2 : 4.396 > >> <I^3>/<I>^3: 34.478 > >> <I^4>/<I>^4: 361.084 > >> All these values are way higher than the expected ones for > non-twinned data. > >> - The twinning fraction from L-test is 0.22 > >> > >> This would all suggest that my space group is wrong, and that I should > proceed in a lower symmetry group, but I don’t know how should I continue. > I know about cases where a P43212 or a P41212 were suggested but in fact > the correct space group was P212121, but I would not know how to continue > going down. Checking my images, I can see some streaky spots, and the > crystals grow first as needles that eventually become plates and rarely > crystals. All these would be a clear indicative of twinning, but I would > not expect to have the same results with different derivatives. We thought > about some kind of reticular twinning, and I wonder if there is a way to > solve it. > >> > >> Any idea about how to proceed or to identify the problem would be > welcome. > >> > >> Thanks, > >> > >> Carmela. > >> > >> > >> > >> Carmela Garcia-Doval > >> University of Zurich > >> Department of Biochemistry > >> Winterthurerstrasse 190 <https://maps.google.com/?q= > Winterthurerstrasse+190&entry=gmail&source=g> > >> CH-8057 Zurich > >> Switzerland > >> E-mail: c.gar...@bioc.uzh.ch <mailto:c.gar...@bioc.uzh.ch> > > > > >
