[ccp4bb] Fwd: Twinning

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

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From: Derek Logan derek {- dot -} logan {- at -} MBFYS {- dot -} LU {- dot -} SE
Date: 2009-01-06

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From: Phil Jeffrey pjeffrey {- at -} PRINCETON {- dot -} EDU
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Subject: Fwd: Twinning
From: Stephen Hare s {- dot -} hare {- at -} MAIL {- dot -} CRYST {- dot -} BBK {- dot -} AC {- dot -} UK
Date: 2009-01-06

Dear All,

We are currently working on a structure of apparent P21 symmetry which
has been solved by molecular replacement. The data are to 2.7Å but the
Rfree will not drop below 30%. The density is clear for the model we
have, however there is extra density that suggests a shift of the
structure by 16Å in either direction - resulting in three possible
overlapping positions for the structure. We assume this is the result
of twinning.

The unit cell dimensions are 102.7Å, 83.0Å, 115.3Å, 90°, 101.8°, 90°.
Examining the data with phenix.xtriage also suggests pseudo
translational symmetry with a separation of 16Å. A Patterson peak at
0.097, 0.000, -0.096 is approximately 30% of the origin peak, while a
second peak of double the translation at 0.192, 0.000, 0.195 is 7% of
the origin peak. The structure contains a dimer of dimers with an NCS
2 fold axis almost perpendicular to the crystallographic 2 fold. This
NCS axis almost coincides with the diagonal between the A and C axes.
A twin axis along the A C diagonal (l,-k,h) could explain the observed
extra density, however this is not possible because A and C are
different lengths.

As a result of the NCS axis running almost perpendicular to the
observed P21 axis, it is possible to merge the reflections in a larger
orthorhombic unit cell - dimensions 137.1Å, 83.3Å, 169.8Å although
here the Rmerge is higher and it is not possible to get a molecular
replacement solution.

Is it possible to define the (l,-k,h) twinning operator in our
original unit cell? or have we missed the actual unit cell?
Or....something else?

Steve

Stephen Hare PhD
Post doctoral research associate
Jefferiss Research Laboratories
Wright-Fleming Institute
Division of Medicine
Imperial College London
Norfolk Place
London W2 1PG
UK

Phone: +44 (0) 20 7594 3908
Fax: +44 (0) 20 7594 3906

CCP4bb navigation

CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

Previous message:
Subject: Re: Mac pro
From: Derek Logan derek {- dot -} logan {- at -} MBFYS {- dot -} LU {- dot -} SE
Date: 2009-01-06

Next message:
Subject: Re: Mac pro
From: Phil Jeffrey pjeffrey {- at -} PRINCETON {- dot -} EDU
Date: 2009-01-06