Raji,

Assuming that the real space group is P212121, in P21 you will still get
a solution except for the extra NCS which will closely resemble the
extra 2-fold screw.  Then you can probably tell if there are any
significant differences between NCS-related copies that justify lower
symmetry space group.  It's not impossible, as Fred points out, that P21
is the true space group, but I think most people here would agree it may
be less likely.  Did your colleague actually try going orthorhombic?

One possibility is that symmetry breakdown results from lattice
distortion upon cryocooling.  I would expect that shifts/rotations are
small and naturally the effect on the refinement is
resolution-dependent.  I am sure there are more examples in the
literature, but this one should hit close to the hills of Waltham:

http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040099

which is PDB ID 2PZV - check out that P1 with beta=89.98.  Colonel Pybus
could tell you more (if the brass lets him, of course :), but what I
recall is that most of the time and at lower resolution KSI can be
processed in C2221 without problems.  With that particular dataset (and
the goal was to get as high resolution as possible to discern minute
changes in hydrogen bonds) it processed fine too, except that R-values
stayed a little too high (lower 20s?) for authors comfort.  But they do
go down significantly in P1.

However, you are at much lower resolution.  My own unpublished example
was at 2.4A, processed fine in C2221, gave a clear MR solution, but then
stayed in R~40% zone and had every loop missing in electron density.
Turned out to be P21 with NCS resembling higher symmetry just enough to
confuse denzo (and me).

I guess the message is that everything is P1 and higher symmetry is just
a dream within a dream so you can experience a drop at any time :)

Cheers,

Ed.

On Thu, 2011-07-07 at 23:30 -0400, Raji Edayathumangalam wrote:
> Hello Everyone,
> 
> I have a 3.1 Ang dataset for which I'd like to get to the bottom of
> what the correct space group is.
> 
> The current unit cell in p212121 is 98.123   101.095   211.201
> 90.000    90.000    90.000
> I fed the reflection data into Xtriage to look for twinning and
> pseudotranslational NCS and there is no indication for either issue in
> the Xtriage output. Also, all odd 00h, 00k, 00l reflections are
> systematically absent as they should be for p212121.
> 
> However, my colleague who is also working on the same dataset recently
> reprocessed the data in P21. Here's the cell in p21:
> 98.010  100.940  210.470  90.00  90.04  90.00 p21
> 
> I am not sure if BETA=90.04 is significant enough to treat as p21
> (0.04% deviation of beta angle from ideal lattice for p212121). I
> don't think so but I could be wrong. Could someone please clarify? 
> 
> Also, what kind of twinning and twinning operators can relate a
> p212121 cell to a p21 cell with almost identical unit cell parameters
> as that of the p212121 cell and leave all systematic absences intact?
> 
> Thanks much.
> Raji
> 
> 
> -----------
> Raji Edayathumangalam
> Instructor in Neurology, Harvard Medical School
> Research Associate, Brigham and Women's Hospital
> Visiting Research Scholar, Brandeis University
> 

-- 
Edwin Pozharski, PhD, Assistant Professor
University of Maryland, Baltimore
----------------------------------------------
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Then knowledge and wisdom are born along with hypocrisy.
When harmonious relationships dissolve then respect and devotion arise;
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------------------------------   / Lao Tse /