Sue Roberts wrote:
HelloA 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]
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: [EMAIL PROTECTED] Tel +49 7531 88 4049 Fax 3183 Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz
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