There are many and various reasons for unpleasantly high R values, and
twinning is undoubtedly one of them.
But you can only have twinning with apparently good diffraction if the
cell dimensions allow it.
It is always a possibility in trigonal, tetragonal and cubic cells.
Certain other combinations of cell dimensions allow it ;- eg monoclinic
with a ~= c or beta ~= 90
But you can usually detect it from the intensity statistics - see the
plots from TRUNCATE for Moments and Cumulative intensities.
Or the output of SFCHECK which suggests possible twinning baed on
interpretation of these tests.
So it should be relatively easy to spot those cases where twinning is a
likely cause of high Rfactor..
Surprisingly some degree of twinning doesnt seem to degrade the map
quality very much..
Just an aside - it is really puzzling why from two sets of apparently
similar data, one set gives Rfacros of 24+ while others refine to R of
18% ..
Eleanor
Kay Diederichs wrote:
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]
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
