I second Kay's observation. Mine were lovely single looking crystals in
P3 but had a twin fraction that varied from 0-0.5 from crystal to crystal.
If anyone is still following this thread, I have a question for the
lattice jocks out there:
I once had a crystal in P3121 which routinely diffracted to >2.8A on a
rotating anode source. There was always a significant diffuse scatter
which I atttributed to a large disordered loop, but after screening many
crystals I realized that for some data this "diffuse scatter" was very
much looking like a second lattice. I was able to index it using hand
picked spots (it was weak and only extended to about 3.5A) The processed
data had unpleasant-looking statistcis but was clearly of the same
spacegroup yet with different cell dimensions in a seemingly (to me)
random orientation with respect to the main lattice. I was able to find
a convincing MR solution in the second lattice with epmr (found the
biological dimer that exists in the large lattice).
Since these crystals were visually perfect (sharp edges, no sign of
twinning and displayed an intense and even birefringence throughout the
crystal) how is the above data possible? Has anyone else seen this?
cheers.
-jason
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
--
Jason Greenwald
UPR9050 CNRS
Pole API - ESBS
Bd Sebastien Brandt
BP 10413
67412 Illkirch Cedex
France
Tel 33 (0)3 90 24 47 31
Fax 33 (0)3 90 24 48 29
[EMAIL PROTECTED]
http://recepteurs.u-strasbg.fr
