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
[EMAIL PROTECTED]
---
Dr. Roberto Steiner
Randall Division of Cell and Molecular Biophysics
New Hunt's House
King's College London
Guy's Campus
London, SE1 1UL
Phone +44 (0)20-7848-8216
Fax +44 (0)20-7848-6435
e-mail [EMAIL PROTECTED]
