Dear all,
I posted here recently regarding a potential twinning problem I had
and got some very useful feedback.
The summary of what happened is I had a crystal that appeared to be
P42212 from systematic absences after scaling in SCALA (a,b,c;
118,118,192). Interestingly POINTLESS suggested P41212 (or P43212 as
the best solutions), and PHASER could only find a solution in P41212
(2 mol/asu). Cumulative intensity plots suggested twinning, as did
phenix.xtriage, and refinement at 2.6 A resolution was stuck at R/
Rfree of 25.5/32.0. (Rmerge 6.5%). Although people suggested using
refmac with twin refinement, in P41212 no twin laws were identified in
REFMAC or phenix.xtriage.
I tried indexing in lower symmetry P222 (a,b,c; 118,118,192) (Rmerge
5.4 %). PHASER found a solution in p212121 (4 mol/asu). Refinement now
proceeds to R/Rfree of 25.3/27.9.
I tried additional twin refinement in in P212121 using REFMAC, which
found the twin operator k,h,-l with fraction ~43%. However, R/Rfree
only changed marginally (and suspiciously) to 25.6/26.9.
Importantly, in each case the maps seem good and I can't discern any
significant difference between them.
My questions are
(i) which (if any) solution should I trust?
(ii) Is the drop in R factors in p212121 just because parameters/data
ratio is less? i.e Am I just overfitting the data?
(iii) Do I really have twinning?
Cheers,
Brett
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Brett Collins, PhD
Group leader
Institute for Molecular Bioscience
Level 3 North
Queensland Bioscience Precinct
The University of Queensland
St. Lucia, 4072, Qld,
AUSTRALIA
e-mail: [email protected]
phone: 61-7-3346-2043
FAX: 61-7-3346-2101
website: http://www.imb.uq.edu.au/index.html?page=82433
Courier address:
Institute for Molecular Bioscience
Queensland Bioscience Precinct
Building 80, Services Road
University of Queensland
St. Lucia, Brisbane
Queensland, Australia 4072
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