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I'm guessing this is translational pseudosymmetry, where spots that should
be fully absent in I222 are faintly present in the P212121 data? In that
case, I think the indices of the individual spots should be the same either
way, and you should just keep the old Rfree indices and expand the set from
there.
Thought for the community as a whole - in cases where some fraction of the
spots is systematically weak, should the Rfree set be chosen only from the
strong ones, since the translational symmetry and not the structural
details is dictating the overall magnitude of the weak ones?
Phoebe Rice
At 01:57 AM 11/8/2006, you wrote:
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Hello,
We have solved the structure of a protein that crystallised in space group
I222 with 1 molecule per asymmetric unit. After refining the structure to
3.5 A, we obtained a different crystal form that diffracts to higher
resolution; in this case, the space group appears to be P21212, with the
same unit cell as the first type of crystals and 2 molecules per
asymmetric unit. The packing between the two molecules in the P21212
asymmetric unit is identical to that of symmetry-related molecules in the
I222 crystals, suggesting that the space group of crystal form 1 could
also be P21212, with non-crystallographic symmetry giving rise to strong
pseudo-I222 symmetry at low resolution.
Since we have already refined our model against the low resolution data
processed in I222, what would be the best way to define a new free R set
in order to continue refinement against the higher resolution P21212 data?
If our interpretation is correct, I would think that using a completely
new set would introduce some bias; on the other hand, is there any way to
somehow take into account the pseudo-symmetry we observe?
Thank you for any advice,
Zorg
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Phoebe A. Rice
Assoc. Prof., Dept. of Biochemistry & Molecular Biology
The University of Chicago
phone 773 834 1723
fax 773 702 0439
http://bmb.bsd.uchicago.edu/index.html
http://www.nasa.gov/mission_pages/cassini/multimedia/pia06064.html
