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RE: [ccp4bb]: Tetragonal Twinning and Detwin - solution?

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Hi There,

While I cannot contribute to the twinning debate, I can offer a quick word
about the choice of space group. Somewhere in the recesses of my mind, I
recall a piece of wisdom from somebody (sorry, can't remember who) which
says that one should always go for the highest symmetry that gives
consistent results:

If the true symmetry is P4, you might be looking at twice as many molecules
in the asymmetric unit, with an 'accidental' packing that looks like P422.
To distinguish between them, you might want to do rigid body refinement of
the P422 derived model in P4 (using the appropriate 422 symmetry operator to
complete the contents of the P4 asymmetric unit), and then observe how far
apart the two are. If there are genuine differences, go for the lower sp.
gr. However, Rigid Body refinement only tells you about gross errors in
positioning the molecules. This might not be significant. So you might have
to go further and do a full refinement in both sp. gr. and observe
particularly the side chains near interfaces that make lattice contacts. A
few of these differences would force a lower symmetry (P4), but if you
assume the higher symmetry (P422) you would not notice in the statistics,
always taking into account the degree of difference (the resolution
obviously has a great deal of impact on the significance of the
differences). 'Accidental' packing that looks like a higher sp.gr. usually
gives a slightly odd N(z) plot in TRUNCATE, where the observed graphs are to
the left of the theoretical ones. If they are to the right of the
theoretical graphs, especially in the bottom left corner, then you should
suspect twinning.

I hope this helps.


Pierre

-----Original Message-----
From: David Briggs [mailto:d.briggs@mail.cryst.bbk.ac.uk]
Sent: 20 September 2001 15:26
To: ccp4bb@dl.ac.uk
Cc: perrakis@nki.nl; roman.hillig@schering.de; bhazes@ualberta.ca;
jacqueline.gulbis@hsn.csiro.au; Nick Keep; Ajit
Subject: [ccp4bb]: Tetragonal Twinning and Detwin - solution?


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***          CCP4 home page http://www.ccp4.ac.uk         ***

Dear all....

	Following on from my problems regarding tetragonal twinning and
some ambiguity between P4 (twinned) and P422 (non-twinned), we took an
un-scaled mtz file from a solved/published structure from our college that
was solved in P422 (4/m mm). This integrated mtz file was in p4.

	We then re-indexed this in p422 and repeated scala/truncate/detwin
on both P4 and P422 datasets.

	Both my data and the solved data scale equally well in P4 and P422
(sensible stats, very few rejections...)

	The P4 centric intensity distribution was also a little odd, where
as the p422 looks fine. All the various moments in P4 and P422 indicated
that the data was not twinned.
	Detwin also indicated that, in P4, this data was an almost perfect
twin. The UCLA perfect twinning test for P422 indicated "no twin", but the
partial test in P4 indicated almost perfect twin.

At some point I will put all the new graphs on the webpage...

http://student.cryst.bbk.ac.uk/~ebrig02/twin/

	As this structure has been solved to about 2.8A, it is fairly
safe to assume that it is not twinned...
(CLANG!!, famous last words etc...)

When data scales equally well in both higher and lower space groups,
provided that there are NO indications of twinning in both intensity
distribution AND moments, then is it safe to assume that it isn't twinned,
and it IS the higher symmetry, despite the fact that Twinning tests indicate
that the lower symmetry is almost perfectly twinned? (making us believe that
the higher symmetry is an artifact of the merohedral twinning).... (!)

	 Therefore, for near perfect twinning, should one pay more
attention to the UCLA "perfect Twinning Test" than other tests designed
for partial twinning?

	Cheers

		Dave

	P.S. So maybe my crystal isn't twinned...?

	P.P.S (CLANG!)
____________________________________________________________________________
___
Dave Briggs
School of Crystallography
Birkbeck College
Malet Street
Bloomsbury
London
WC1E 7HX