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]
Sue,
I seem to be in the other camp: - "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)"
- all of this may happen with merohedrally twinned crystals. I believe
it would be good to teach students to always devote some thought to the
possibility of merohedral twinning in case of a trigonal/ hexagonal/
tetragonal crystal, to avoid a rather common pitfall. I don't have the
percentage at hand, but I believe I saw a paper by George Sheldrick
giving a high percentage (like 20% or so) of merohedral twinned
structures in the above crystal systems for small-molecule structures -
why should that percentage be different for protein crystals?
It is of course true that twinning refinement is painful, and a lot of
additional work! But "man twinning" is always enlightening reading.
Kay
--
Kay Diederichs http://strucbio.biologie.uni-konstanz.de
email: [log in to unmask] Tel +49 7531 88 4049 Fax 3183
Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz
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