There are some useful notes on the CCP4 program documentation.
http://www.ccp4.ac.uk/dist/html/twinning.html
The roblem of twinning is more or less independent of space group, but
for point groups like P6 there are some twinning operators which are
frequently seen ( in that case k h -l) . They are listed in the above
documentation.
Eleanor
Ramanathan NATESH wrote:
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Dear All,
Could anyone please point me to similar references for P61/P6122
twinning issues, on similar lines suggested for P21/C2221 by James ?
Thanks in advance.
natesh
R. Natesh
IGBMC
On Tue, 23 May 2006, James Whisstock wrote:
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Hi David - Check out
> Yang F, Dauter Z, Wlodawer A. Effects of crystal twinning on the
ability to solve a macromolecular structure using multiwavelength
anomalous diffraction.
> Acta Crystallogr D Biol Crystallogr. 2000 Aug;56(Pt 8):959-64.
> PMID: 10944332 [PubMed - indexed for MEDLINE]
and
Ban N, Nissen P, Hansen J, Capel M, Moore PB, Steitz TA. 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. and related papers
> Which provides a nice description of the P21 / C2221 twinning issue and
also some ways of resolving it.
J
Eleanor Dodson <[EMAIL PROTECTED]> wrote:
In this space group with |a| ~ equal |c| you could have a twinning
matrix l.-k,h
You should have been offered the possibility of processing the data with
point group Cmmm
did that give a reasonable Rsym?
And you would expect to see some 2 fold symmetry in the h0l h1l etc
sections - look at these with hklview.
The newest SFCHECK will also do a twinning test with the above operator
Eleanor
David Schwefel wrote:
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Hi there!
I have a problem during refinement of apparently good data...
the protein crystallised in space group P21 with following cell parameters:
44.5 86.5 44.6 90 112.5 90
the crystal diffracted up to 1.5 A and the diffraction pattern looked very
good
some statistics from the CORRECT.LP:
Res. Range I/sigma(I) Chi² R obs R exp refl. accepted rej.
43.162 1.498 8.56 1.00 4.91 5.35 139902 138325 750
after Molrep, rigid body and restrained refinement with ccp4, the R factor
is still around 40 % and won't go further down, and there are some regions
where the density looks really bad...
I read that twinning could be an explanation so i checked <I²>/<I>² which
ranges from 1.75 to 2 depending on the resolution, which means that the
crystal is twinned.
As far as I understood is it possible to detwin the data if you know the
symmetry between and the relative fractions of the twin domains, but I only
found possible twin laws for higher symmetry crystals.
Has anyone a clue or some reference how to proceed with this data (which
looked so fine in the beginning...)?
thanks in advance,
David
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