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
9700 South Cass Avenue
Argonne, IL 60439
email: [log in to unmask]
Tel: (630) 252-0682
Fax: (630) 252-0687
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