Hari,
What twin tests have you run? Results? If your data really is
P43212 and you drop to P212121 you will still have the additional
two-fold operator in your data. An operator is a mathematical operator,
which could be crystallographic or twin.
I never refine a structure with twin refinement from the beginning
(even if I suspect it is twinned). I do iterative rounds of model
building and conventional refinement until I cannot get the R-factor any
lower. Depending on many factors (resolution, twin fraction, etc.), I
usually get stuck in the mid-30's. At this point I will investigate twin
refinement or other possible SG's.
If you start using twin refinement too early in the model building
process, I have seen programs report much too low R-factors and
exaggerated twin fractions. Probably, a programmer like Garib or Peter
could comment on why.
Jon
On 04/23/2010 04:28 PM, hari jayaram wrote:
I am refining a twinned dataset in possible spacegroup P212121 .
Pointless thinks it is P43212 , but based on reading this posting
(http://www.phenix-online.org/pipermail/phenixbb/2007-September/000501.html)
I think it is P212121.
The starting R/Rfree after molecular replacement ( single site
mutant) was 34/38 to 2.2 A
After an initial round of restrained refinement ( without twin
refinement) and minimal rebuilding I got the R/Rfree to 30/34
Then I did an amplitude based twin refinement - The twin fraction was
0.48 k,h,-l and 0.52 h,k,l and The r/rfree became 24/29
After a little more rebuilding ( a few residues out of 800 residues in
ASU) and another twin refinement I got an r/rfree of 22/27 . Now the
twin fraction was 0.87 (h,k,l) and 0.13 (k,h,-1)
The maps looked a little better allowing me to fix a few more residues
Finally the same twin refinement gives me no twin operators and the
R/Rfree is 22/26
All the twinning tests indicate a serious twinning in my crystal. Any
ideas why I am seeing this
Hari
--
Jonathan P. Schuermann, Ph. D.
Beamline Scientist
NE-CAT, Building 436E
Advanced Photon Source (APS)
Argonne National Laboratory
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