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CCP4BB Home

CCP4BB  January 2019

CCP4BB January 2019

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Subject:

Re: AW: [ccp4bb] AW: [EXTERNAL] [ccp4bb] translational NCS & twinning

From:

DUMAS Philippe (IGBMC) <[log in to unmask]>

Reply-To:

DUMAS Philippe (IGBMC) <[log in to unmask]>

Date:

Fri, 11 Jan 2019 11:51:47 +0100

Content-Type:

text/plain

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text/plain (135 lines)

 
Le Vendredi 11 Janvier 2019 09:07 CET, [log in to unmask] a écrit: 

Dear Herman,
As far as I understood the twinning problem, what you say is only true in some occasions, and not in others.
If the "macroscopic" domains are so small that they are smaller than the X-rays coherence length, then you may do what you say because the X-rays emitted by the different domains can interfere.
But, if the domains become large, the X-rays emitted by the different domains do not interfere anymore and you have to add the weighted intensities, not the amplitudes, of each domain.
I hope I understood well your comment and did not "interfere negatively" with the thread...
Best
Philippe Dumas
 
> Dear Lan,
> 
> Thank you for your compliment. I do not use Xtriage, so I did not bother looking at the log files.
> 
> What I meant to say is that with twinning, the crystal has different macroscopic domains where the molecules have different orientations, say one domain with orientation A and one domain with orientation B. Since these domains grow on top of each other, they are usually related by a twin operator similar to a crystallographic operator such as a twofold axis. 
> The fourier transform of the electron density of the crystal is the convolution of the fourier transform of the individual molecules with the crystal lattice, with the fourier transform of the individual molecules usually giving the stronger contribution. So to get a solution with a decent R-factor, one must include all orientations (A, B etc.) in the model, with the position of the molecules in the crystal lattice contributing less to the diffraction pattern. So one can put the orientations on top of each other in a small unit cell using twinning, or put them in a larger unit cell at different positions using crystallographic or non-crystallographic symmetry. That is what I meant be "twinning" (N)CS.
> 
> Hope this makes my remark a little clearer.
> 
> Best,
> Herman
> 
> PS: While other BB readers may have had the same question, I have posted the reply to the BB. I hope you don't mind.
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: Guan, Lan [mailto:[log in to unmask]] 
> Gesendet: Donnerstag, 10. Januar 2019 20:53
> An: Schreuder, Herman /DE
> Betreff: Re: [ccp4bb] AW: [EXTERNAL] [ccp4bb] translational NCS & twinning
> 
> Dear Herman,
> I have read your insightful comments on twining and tNCS for years now, which is very useful and helpful!  Thanks,
> 
> For Donghyuk’s case, do you think that he really has a twinning issue?  With a lower symmetry, all possible twining operators is always reported in the Xtriage and did not mean a real twin existed.  His L test shows a twin fraction of 0.00 in his log file.  The intensity statistics does not really indicate an actually twinning.  Base on the refinement, twin law is needed to get refinement going.  It looks like a twin.  I am confused...
> 
> > the molecules are related by "twinning" (N)CS?
> 
> 
> What does this mean “ twinning (N)CS"?  Would you please kindly explain it further?
> 
> Thanks,
> 
> 
> Lan
> 
> 
> ********************************************************************************************
> Lan Guan, MD PhD
> Associate Professor | Department of Cell Physiology and Molecular Biophysics
> Director | Center for Membrane Protein Research
> 
> 3601 4th St. MS 6551 | Lubbock, TX 79430
> 5A148A (Office) | (1) 806 743-3102 (Phone) | [log in to unmask] (E-Mail)
> 
> http://www.ttuhsc.edu/medicine/cell-physiology-molecular-biophysics/faculty/guan/
> https://www.ttuhsc.edu/centers-institutes/membrane-protein-research/

> ********************************************************************************************
> 
> > On Jan 10, 2019, at 11:10 AM, [log in to unmask] wrote:
> > 
> > CAUTION: This email originated from outside of TTUHSC. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > 
> > Dear Donghyuk,
> > 
> > Unfortunately, everything is possible when NCS, twinning etc. get into the game. I do not have answers, but some questions for you to think about:
> > - Do you really have 6 twinning operators, or only one operator and are the other operators generated by (non)crystallographic symmetry?
> > - Both your P6322 cell and your C2 cell have angles of 90 90 90. For P6322, the last angle should be 120 and for C2, only the second angle is constrained to be 90. Maybe you should check that not somewhere something went wrong with the cell angles.
> > - How weak are the reflections that got discarded by halving the a- and b-axes? Do they have significant intensity, or is it only noise?
> > - By shrinking the unit cell, you may have created artificial twinning when in the large unit cell the molecules are related by "twinning" (N)CS.
> > - Since you seem to have found a solution with the small unit cell, you could see if you could fit this solution in the large unit cell: Process in P1 in the large unit cell and use the ensemble (the complete! unit cell of your C2 solution, as a search model.
> > - Your current solution maybe correct after all, but I would analyze it very critically.
> > 
> > Best,
> > Herman
> > 
> > 
> > -----Ursprüngliche Nachricht-----
> > Von: CCP4 bulletin board [mailto:[log in to unmask]] Im Auftrag von Donghyuk Shin
> > Gesendet: Donnerstag, 10. Januar 2019 11:12
> > An: [log in to unmask]
> > Betreff: [EXTERNAL] [ccp4bb] translational NCS & twinning
> > 
> > Dear all,
> > 
> > I am having tough time with my Xtal data sets those seem to be twinned or have translational NCS, and it will be greatly appreciated if you can give me some advices or comments!
> > 
> > Data was initially processed with XDS and scaled with aimless without specifying certain space group (SG).
> > Aimless picked the P 63 2 2 for the best SG, but the xtriage indicates there is non-origin peak after patterson analysis. (attached log) And, I could not get the proper MR-solution from this data sets.
> > 
> > Because I read that twinning and tNCS cannot be properly distinguished at high SG, I went down to subgroup either P32 or P6 assuming that there is twinning which make data set seems to have apparently high SG. (procedure was same XDS->aimless but I specified the SG to keep them) Then, xtriage still indicates there is non-origin peak as before, but found twin laws for the data sets (attached log).
> > However, I still could not get the right MR-solution.
> > Then, I went even further down to P3 or C2, and xtriage found more twin laws which is make sense because of the lower SG. (attached log) Again, I could not get the MR-solution.
> > For all the MR running above, I assumed that phaser(ccp4 module) automatically applied tNCS if they present. or should I have to tick on button in the expert parameters?
> > 
> > Then, I went back to the image and processed the datasets with mosflm by checking the indexed spots.
> > During this step, I played with the threshold for indexing to follow the strong spot for get correct SG.
> > I am not sure whether this is correct or not, but by putting high threshold for indexing (e.g. ~15) I could index the data with C2 which has half dimension for a,b axes (116.348,  67.218, 182.861,  90, 90, 90) to the original unit cell (232.533, 134.202, 182.67, 90, 90, 90).
> > With this, I could put 3 molecules in ASU by MR. During refinement, I felt that the R values were not dropping, and I applied twin refinement.
> > without twin refinement the R values were (0.39/0.44, work/free), and applying twin refinement gave me significantly better values (0.23/0.26).
> > Because there were 6 twin operators which may cause this huge R value drop, I speculate whether this is true or not.
> > 
> > Your comments will be greatly helpful!
> > 
> > With you all the best,
> > Donghyuk
> > 
> > 
> > 
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