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
