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Before looking for complicated issues involving twining, I would
recommend to
really run ARP/wARP - its straightforward and at 1.5 it must work and
\produce an
excellent map unless something is very wrong.
If indeed with 1.5 A data Rfree still does not go down, get worried
about twinning, which indeed given the cell axes is indeed likely.
Also look space group choices with POINTLESS from Phil Evans ?
I agree that most likely its Cmmm or twinned, but at 1.5 A restrained
refinement alone without automated model rebuilding is a bit like
having a Ferrari and not using
the 4th and 5th gear ...
A.
On May 22, 2006, at 18:10, 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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