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CCP4BB  February 2007

CCP4BB February 2007

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Subject:

Re: how many stuck datasets are actually twinned?

From:

James Irving <[log in to unmask]>

Reply-To:

James Irving <[log in to unmask]>

Date:

Thu, 22 Feb 2007 10:38:52 +0000

Content-Type:

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I guess the point is to "know thy data", to avoid problems down the 
track: a statistical test for twinning forms one of a battery of tests 
which can be performed independently of a coordinate set (Matthews 
coefficient, self-RF, etc.), and with relative ease.  Importantly, in 
the case of partial twinning, overlooking the twin relationship can lead 
to errors in the final model.  As noted by Todd Yeates ("Protein 
Crystals and Their Evil Twins" Structure 7:R25-R29), there were "small 
but significant structural differences" between two bacteriorhodopsin 
structures refined with and without correction for twinning.  I agree 
that it should certainly not be assumed that a dataset is twinned merely 
because it is possible, and the Rfactor and Rfree look a bit 
unfashionable - the presence of twinning must be corroborated by other 
evidence.  Twin-related reflections should also be assigned to 'working' 
and 'test' datasets as pairs, to avoid bias in Rfree.

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