Hi Sue,
This reference has very useful info
Lebedev AA, Vagin AA, Murshudov GN.
Intensity statistics in twinned crystals with examples from the PDB.
Acta Crystallogr D Biol Crystallogr. 2006 Jan;62(Pt 1):83-95. Epub
2005 Dec 14.
PMID: 16369097 [PubMed - indexed for MEDLINE]
Roberto
On 21 Feb 2007, at 15:22, Sue Roberts wrote:
> Hello
>
> A partially philosophical, partially pragmatic question.
>
> I've noticed a trend, both on ccp4bb and locally, to jump to
> twinning as an explanation for data sets which do not refine well -
> that is data sets with R and Rfree stuck above whatever the
> person's pre-conceived idea of an acceptable R and Rfree are.
> This usually leads to a mad chase through all possible space
> groups, twinning refinements, etc. and, in my experience, often
> results in a lot of time being spent for no significant improvements.
>
> Just out of curiosity, does anyone have a feel for what fraction
> of stuck data sets are actually twinned? (I presume this will vary
> somewhat with the type of problem being worked on).
>
> And a sorta-hypothetical question, given nice-looking crystals;
> images with no visible split spots, extra reflections, or streaks;
> good predictions; nice integration profiles; good scaling with
> reasonable systematic absences; a normal solvent content; and a
> plausible structure solution, and R/Rf somewhat highish (lets say .
> 25/.3 for 1.8 A data), how often would you expect the Stuck R/Rf
> to be caused by twinning (or would you not consider this a failed
> refinement). (My bias is that such data sets are almost never
> twinned and one should look elsewhere for the problem, but perhaps
> others know better.)
>
> Sue
> Sue Roberts
> Biochemistry & Biopphysics
> University of Arizona
>
> [log in to unmask]
---
Dr. Roberto Steiner
Randall Division of Cell and Molecular Biophysics
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King's College London
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London, SE1 1UL
Phone +44 (0)20-7848-8216
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