| Check this paper below - a C222(1) space group (a=212, b= 300, c=575) frequently appearing as a merohedral twin P2(1) with apparent C222(1) symmetry was exactly a major problem in the H. marismortui 50S structure determination.
Poul
 Placement of protein and RNA structures into a 5 A-resolution map of the 50S ribosomal subunit. Nature. 1999 Aug 26; 400( 6747): 841-7.
On 03/04/2008, at 17.48, Bart Hazes wrote: I just realized that this is an orthorhombic C222(1) space group. I didn't check it up but unless two of the cell-dimensions are nearly identical I think merohedral twinning is not possible for this space group, because the symmetry of the unit cell shape is not higher than the symmetry of the space group.
Bart
Eleanor Dodson wrote:
It is not really possible to detect twinning by the simple moment and cumulative distribution tests for data from a crystal with pseudo translation. As Bart says, twinning decreases the value of the moments, whilst pseudo-translation increases them, so the two effects tend to cancel out. There is a reference to the L test: J. Padilla & T. O. Yeates. A statistic for local intensity differences: robustness to anisotropy and pseudo-centering and utility for detecting twinning. /Acta Crystallogr./ *D59*, 1124-30, 2003. <http://scripts.iucr.org/cgi-bin/paper?S0907444903007947>S They suggest using neighbouring reflections pairs to test . This can often overcome the problem associated with pseudo-translation. However it is quite sensitive to data quality.
See http://nihserver.mbi.ucla.edu/pystats/
Eleanor
Bart Hazes wrote:
Hi Qiang,
A normal data set has a unimodal intensity distribution with a predictable shape. When there is twinning the distribution remains unimodal but becomes sharper and this is picked up in the twinning analysis. When there is pseudo-translational symmetry, as you indicate you have, then the intensity distribution becomes bimodal with one set of reflections systematically strengthened and another systematically weakened. This makes the whole distribution broader, just the opposite of what twinning does, and therefore shows up as "negative twinning" in the analysis.
Bart
Qiang Chen wrote:
Hi all,
The data I am working on has a strong translation vector. The space group
is C2221 and resolution is 2.3 angstrom. There are two molecules per AU
with a pseudo-2-fold axis.
On the cumulative intensity distribution plot, the theor and obser curves
totally do not overlap. I did "detect_twinning" from CNS, and there is the
result:
<|I|^2>/(<|I|>)^2 = 3.2236 (2.0 for untwinned, 1.5 for twinned)
(<|F|>)^2/<|F|^2> = 0.6937 (0.785 for untwinned, 0.865 for twinned)
Does the result mean my data is not twinned?
Any suggestion will be highly appreciated.
Thank you!
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University of Alberta
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