I think you should assign the Free R set randomly, including weak and
stong observations.
If you want to transfer your I222 Free R set to the new P21212 set, when
you use the task import scaled data , or run SCALA you can ask to have
your Free R transfered from another data set. Then the procedure will a)
see what % you had assigned originally b) generate the same % of Free R
assignments for all newly included hkl. This will work providing you
have the same indexing convention for the I222 and the P2i2i2i set .
You may need to reindex one to get the axes aligned..
Eleanor
Becker, Joseph W wrote:
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I222 and P2(1)2(1)2 are closely related. Change one of the
crystallographic two-folds into a non-crystallographic two-fold and
Bob's your uncle.
Demetallized Concanavalin A crystallizes in P2(1)2(1)2. Add cations to
the crystals and they'll change into the same form (I222) that you get
by crystallizing the metallo-protein directly. See Becker et al. (1975)
JBC 250:1513 and Reeke et al. (1978) PNAS 75:2286.
Joe Becker
Merck Research Labs
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of
Zorg Blue
Sent: Wednesday, November 08, 2006 2:58 AM
To: [email protected]
Subject: [ccp4bb]: Pseudosymmetry and free R sets
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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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