Dear Bert Van-Den-Berg,
as far as I understand this, if you have true P622, process the data in
P6 and then test for twinning, both the Britton-test and H-test will
indicate perfect merohedral twinning.
This is because the Britton-test checks for a sudden increase of
negative intensities after de-twinning, which happens only at twin
fractions close to 0.5 if the intensities used for de-twinning are the
same. But this is true if they are related by crystallographic symmetry.
The H-test relates the absolute difference to the sum of the presumably
twinned intensities, which gives "0" for intensities related by
crystallographic symmetry, again resulting in twin fractions close to 0.5.
In other words, intensities related by crystallographic symmetry would
indicate "perfect" twinning in both of these tests.
A better test for perfect merohedral twinning would be the ratio of
<I^2>/<I>^2 which should be 2 for untwinned and 1.5 for perfectly
twinned data, tested in the higher space group. These values are
reported by data processing programs like XDS. Please, be aware that
these ratios have rather strange values if you have an unusually high
background (loop fiber diffraction, ice rings, etc.) or extremely weak data.
For a really good discussion of twin tests, see Yeates, Methods.
Enzymol. 276, 344-358, 1997.
Best regards,
Dirk.
Am 28.01.14 18:26, schrieb Bert Van-Den-Berg:
Dear all,
I recently collected several datasets for a protein that needs
experimental phasing.
The crystals are hexagonal plates, and (automatic) data processing
suggests with high confidence that the space group is P622. This is
where the fun begins.
For some datasets (processed in P622), the intensity distributions are
normal, and the L-test (aimless, xtriage) and Z-scores (xtriage)
suggest that there is no twinning (twinning fractions < 0.05).
However, for other datasets (same cell dimensions), the intensity
distributions are not normal (eg Z-scores > 10). Given that twinning
is not possible in P622, this suggests to me that the real space group
could be P6 with (near) perfect twinning.
If I now process the "normal L-test P622" datasets in P6, the twin-law
based tests (britton and H-test in xtriage) give high twin fractions
(0.45- 0.5), suggesting all my data is twinned.
Does this make sense (ie can one have twinning with "normal" intensity
distributions)?
If it does, would the "normal L-test" datasets have a higher
probability of being solvable?
Is there any strategy for experimental phasing of (near) perfect
twins? SAD would be more suitable than SIR/MIR? (I also have potential
heavy atom derivatives).
Thanks for any insights!
Bert
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Dirk Kostrewa
Gene Center Munich, A5.07
Department of Biochemistry
Ludwig-Maximilians-Universität München
Feodor-Lynen-Str. 25
D-81377 Munich
Germany
Phone: +49-89-2180-76845
Fax: +49-89-2180-76999
E-mail: kostr...@genzentrum.lmu.de
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