Dont forget that with twinning in apparent point group PG6/mmm the
true SG may be P6i or P3i21
See the twinning notes: http://www.ccp4.ac.uk/dist/html/twinning.html
Detecting twinning can be problematic -
My rule of thumb, following the procedure od ctruncate::
0) Check the matthews coefficient for likely number of molecules.
Half a molecule must mean you are assigning too high a symmetry count.
Lots of molecules means you need to check for non-crystallographic
translation etc.
1) Look at the <I^2>/<I>^2 plot after correction for anisotropy
If it isnt reasonably straight with resolution you probably have some
data problems, and these can make all the tests pretty useless.
2) Is there a NC translation - truncate tells you that.
If not, and the data is OK, you are unlikely to have twinning if
<I^2>/<I>^2 for acentrics is ~ 2, and the L test looks OK.
H test and Britten tests a bit more influenced by other NC symmetry
considerations
3) If there IS NC translation <I^2>/<I>^2 for acentrics will probably
be > 2 but the L test is still pretty reliable.
Good luck Eleanor
experimental phasing is tricky with perfect twinning but it has been
done. Sorry I have forgotten reference though..
Eleanor
On 29 January 2014 09:17, Kay Diederichs <[log in to unmask]> wrote:
> Dear Bert,
>
> as Dirk has pointed out, if P622 is the correct space group, then the twinning statistics printed out if you process in P6 are meaningless.
>
> Intensity statistics, like the ratio of <I^2>/<I>^2 , can be misleading if there is (e.g. pseudo-translational) NCS in the crystal; however, the effect of NCS on the value of the ratio of <I^2>/<I>^2 is opposite to that of twinning. Thus if a crystal is twinned and has NCS, you might not notice any problem in the ratio of <I^2>/<I>^2 .
>
> The other statistics, like Britton and H-test, present the intensity statistics in a different way, but from my understanding do not give substantially different information.
>
> The L-test does look at a different kind of information and therefore gives additional insight.
>
> If your measurements suffer from high background, diffuse scatter, ice rings, smeared reflections, additional crystals in the beam, or any other pathology, then all these tests may give distorted answers. In other words, even if twinning is not really present, any test designed to convert the deviation of data from ideality into an estimate of the twinning fraction will give you an alpha > 0. So my experience is: if your data are very good, then the tests give good answers; if the data are mediocre or bad, don't necessarily believe the numbers.
>
> Finally, it's not only twinning of P6 that would give you P622, it's also twinning of P3x21, P3x12 that gives P6y22.
>
> Hope this helps,
>
> Kay
>
>
>
>
> On Tue, 28 Jan 2014 17:26:23 +0000, Bert Van-Den-Berg <[log in to unmask]> wrote:
>
>>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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