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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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