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RE: [ccp4bb]: Twinning/Merging



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Hi there,

There are many reasons other than twinning to explain the high R-merge:
1- The space group reported is enantiomeric. When you have 3 different
xtals,
you must make sure they all have the same hand. How do you find out? In the
merging stats you will see R-syms (R-merge for the equivalents in one image)
for the images from one xtal worse than the others. This is a clear
giveaway,
which you must check.
2- Another reason is that one of these xtals is genuinely much worse than
the
others which messes up the stats for all 3.
3- A third reason is that the resolution limit is somewhat optimistic. You
do
not report the I/sigma numbers. If these are too small, then you are
obviously
trying too hard to include data that is not there. But if they are
reasonable, then
the first scenario becomes much more probable. As a rule of thumb, if you
can
discern by eye clear diffraction spots without too much change of contrast
in
the image, then the processing program should be able to give you reasonable
stats, keeping in mind the provisos above.
4- One more reason is the approximate nature of the 2-fold. The space group
might be only P32, but there is almost 2-fold symmetry normal to the 3-fold
that you could be fooled into believing the higher symmetry, while getting
somewhat worse stats.

I hope these thoughts will keep you busy before you leave for York. Then
Maybe we might hear how you solved the structure instead. Good Luck.

Pierre Rizkallah

	-----Original Message-----
	From:	Daniel Schlieper [SMTP:Daniel.Schlieper@Uni-Koeln.DE]
	Sent:	15 December 2000 14:23
	To:	ccp4bb@dl.ac.uk
	Subject:	[ccp4bb]: Twinning/Merging

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	***    CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html    ***

	Dear all,

	Of one protein, I used 3 crystals to collect a data set at room
	temperature. The data set is to 96 % complete, diffracts to 2.35 A,
	and has a very high Rmerge value after scalepack: 16 % (77 % in the
	2.38-2.35 A shell). 

	The twinning test (from Yeates,
www.doe-mbi.ucla.edu/Services/Twinning/) 
	gave no twinning at all.

	I used the very same protein to grow crystals of the very same
crystal
	form. (The crystals differ in the substrate co-crystallized). These
	crystals were measured at 100 K, and I collected full data sets of
	every crystal. Tested for twinning, it came true, that all of them
	were twins of 5-15% (P3221, (-h,-k,l)).

	Now I wonder if the 3 crystals measured at room temperature have
been
	twins as well, and if I may explain the high R-value with that. Is
	this true?

	See you in York!

	Best regards, Daniel

	-- 
	Daniel Schlieper                      Institut fuer Biochemie
	                                      Zuelpicher Strasse 47
	Daniel.Schlieper@Uni-Koeln.De         Universitaet zu Koeln 
	Tel.: +49 221 470-6443, Fax: -5092    50674 Koeln, Germany


	-- 
	Daniel Schlieper                      Institut fuer Biochemie
	                                      Zuelpicher Strasse 47
	Daniel.Schlieper@Uni-Koeln.De         Universitaet zu Koeln 
	Tel.: +49 221 470-6443, Fax: -5092    50674 Koeln, Germany