Maybe, but it may also indicate that the problem isn’t twinning, but simply 
pseudo symmetry.

Zanuda is one way to double-check, and it takes approximately a day to 
re-process the data, run the MR and run Zanuda.


Diana R. Tomchick
Departments of Biophysics and Biochemistry
University of Texas Southwestern Medical Center
5323 Harry Hines Blvd.
Rm. ND10.214A
Dallas, TX 75390-8816<>
(214) 645-6383 (phone)
(214) 645-6353 (fax)

On Aug 8, 2017, at 1:27 PM, Eleanor Dodson 
<<>> wrote:

That seems hard work to me! Data quality suffers when you discard multiplicity..

On 8 August 2017 at 17:15, Sudipta Bhattacharyya 
Hi Giorgio,

I also suspect the presence of twining and as Eleanor has suggested the L test 
is the best test to detect that (if also t-NCS is not present). You can 
reprocess your data in P1, do MR in P1 and then feed your P1 model and mtz file 
to zanuda to see what SG it suggests. That trick once worked for me.


Sudipta Bhattacharyya,
Postdoctoral fellow
Department of Molecular Biosciences
University of Texas at Austin
Texas, USA.

On Tue, Aug 8, 2017 at 8:27 AM, Eleanor Dodson 
<<>> wrote:
First of all - check whether your crystal data shows twinning - thee is an 
L-test plot which usually gives a clear indication and if you are using GUI2 
the report suggests if this is so.

If you have twinning the most likely SG is P31 (or P32 ) - you cant tell the 
difference till the structure is solved.

Check the merging stats in that point group -
Then try MR searches in the two spacegroups.

The C2 and I2 SGs are probably sub-group possibilities if you reindex - there 
are common cell dimensions -pointless output tells you how to do that - but in 
general one always chooses the highest symmetry that gives reasonable merging 
statistics and is sensible.


On 8 August 2017 at 14:15, Giorgio Giardina 
<<>> wrote:
Hello everybody,

I think I have a pseudo-centering problem.

I have a 1.6 ang. dataset of a mutant  protein that is an homodimer in solution.
Data processing gives the following SG:

Space group: P 31 2 1
Average unit cell:  111.36  111.36   28.56   90.00   90.00  120.00
Average mosaicity:   0.24
Rmerge Overall 0.07

However with this SG the unit Cell is too small and monomer doesn't fit the in 
the AU.

I reprocessed the data in other possible SG (including P6) and finally I got 2 
equivalent SG's in which I can get a correct Molecular Replacement solution:

Space group: I 1 2 1
Average unit cell:   57.07  111.11  192.90   90.00   90.07   90.00


Space group: C 1 2 1
Average unit cell:  201.10  111.11   57.07   90.00  106.42   90.00

Both with Average mosaicity:   0.38 and Rmerge Overall 0.06,
but the corresponding MR solutions do not refine, with R-factor stuck to 45-47%.

From what I understand, in the I121 SG I have the NC two-fold axis of the dimer 
at 1/2a and this originates the pseudo centering and the small P3 Cell

Largest Patterson peak with length larger than 15 Angstrom:
 Frac. coord.              :    0.500    0.000    0.000
 Distance to origin        :   28.566
 Height relative to origin :   53.830 %

I'm really not so good with symmetry, so I'll be grateful for any 
suggestion/help/solution from you out there.
Many thanks,


UT Southwestern

Medical Center

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