Re: [ccp4bb]: Tetragonal Twinning & Detwin - Summary #1 and storyso far!!

[Bart Hazes <bhazes@ualberta.ca>]

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On Thu, 20 Sep 2001, David Briggs wrote:

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>
> Dear all....
>
> 	First I must thank those of you who have replied so far for you
> help.. but I still have doubts... I will try and clear this up...
>
> Roman
> > However, if you are lucky and the NCS axes do not run parallel to the
> > crystallographic axes, you should then be able to differentiate between
> > NCS and pseudo-crystallographic two-fold axes (caused by perfect
> > twinning) by examination of the self rotation function.
> Unfortunately, I am not that lucky!!!
> I have 2mols per asu in p422 (therefore 4 in p4). (everything SHOULD fit)
> My NCS two fold does run parallel to my crystallographic axes, as I have
> rather nice looking pseudo-translations peaks on my native patterson...

The 2-fold NCS parallel to your 4 or 2-fold crystallographic NCS can cause
systematic weakening of some sets of reflections while strengthening others
(depending on whether the pseudo-translationally related molecules scatter in
phase or the other way around). This would result in more weak and more strong
reflections with fewer "average intensity" reflections. Just the opposite of
twinning where you see fewer weak or strong reflections. Your cummulative
intensity distribution plot (the first one) shows such a pattern for the
centrics (black line) which rise quickly (many weak reflections), flattens
off, and then (with a bit of fantasy) rises again at the end. However, in all
cases the line remains below the theoretical line (green) which doesn't make
sense. You also don't see an effect in the acentrics or the P422 curve.
Perhaps it is just bad statistics since you won't have that many centric
reflections in P4 (only the HK0 plane).

> Bart
> > Take a look at your TRUNCATE (there it is again) output to see if your
> > intensity (or better E-value) distributions are normal or not. If they
> > indicate twinning then I fear you do have a problem.
>
> I have put both Intensitiy distributions and 1st and 3rd moments graphs
> for p4 and p422 data on the web at...
>
> http://student.cryst.bbk.ac.uk/~ebrig02/twin/
>
> see what you think....
> (I think I'm getting conflicting answers again!!)

Apart from the P4 centrics deviation mentioned above, I would say the curves
look very good. Perhaps no problem after all.

> Jacqui
> The Detwin program indicated a near perfect twin for the p4 scaled data.
> As there are no twinning operators for p422, I could not use detwin on
> this data.
> The UCLA twinning server, allows you to detect presence of a perfect twin
> using your higher space group (for me, P422) The perfect twin test gave a
> resounding "NO, you are not twinned!". However, the partial twin test
> using p4 data gave a "yes, you are greater that 45% twinned" answer.
>
> Which is right!!!

Correct me if I'm wrong, but I thought P422 couldn't form merohedral twins as
the unit cell morphology has the same P422 symmetry as its content (unless
your c axis happens to be the same as a and b). For P4 you can have twinning.
Is is possible that the UCLA server with the "higher space group option" is
comparing twin-related reflections in this situation rather than intensity
distributions. If so then of course your P4 processed data suggests 50%
twinning.

Based on your truncate data I would suggest to go ahead and assume that things
are ok unless you run into a brick wall somewhere. Your parallel NCS and
crystallographic symmetry may turn out to be a greater problem than the
perceived twinning.

Bart

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