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.

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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.
Eleanor
On 8 August 2017 at 14:15, Giorgio Giardina <giorgio.giard...@uniroma1.it>
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
>
> and
>
> 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,
> Giorgio
>