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,

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