Dear Eleanor,
This was all in the reply (below) that I sent to you earlier but
omitted to Cc to the Bulletin.
Best wishes,
Gerard.
----- Forwarded message from Gerard Bricogne <gb10> -----
Date: Tue, 8 Oct 2024 16:47:14 +0100
From: Gerard Bricogne <gb10>
Subject: Re: [ccp4bb] a twinning question..
To: Eleanor Dodson <eleanor.dod...@york.ac.uk>
Dear Eleanor,
It would be a good idea to make sure that you are not using data that
have been produced by the BUILT=20240723 of XDS Version 20240630, as this
gives an abnormally high percentage of twinning indications. See the section
about "Warning about twinning (POINTLESS)" of the Comparison Summary on
https://www.globalphasing.com/autoproc/wiki/index.cgi?ComparisonProcessing202409
Parkhurst & al. pointed out (https://doi.org/10.1107/S1600576716013595)
that
"[...] traditional methods [of background estimation] can result in the
systematic underestimation of background values. This then results in the
reflection intensities being overestimated and gives rise to a change in
the overall distribution of reflection intensities in a dataset such that
too few weak reflections appear to be recorded. Statistical tests
performed during data reduction may mistakenly attribute this to
merohedral twinning in the crystal."
which may be relevant to your case.
With best wishes,
Gerard.
--
On Tue, Oct 08, 2024 at 04:01:24PM +0100, Eleanor Dodson wrote:
> I have always had great faith in the L test and other statistics to detect
> twinning..
> But now I have a puzzling data set.
> Seems to be orthorhombic with a, b and c all different
> One molecule/asymm unit and no NC translation..
> BUT clear indication of twinning given by second moments, etc..
>
> I have seen this before when the spacegroup was actually monoclinic and the
> pointless analysis of the two fold axes along a b or c showed one was
> better than the other two and in fact the crystal proved not to be twinned
> when the data was processed as monoclinic..
>
> But in this case no amount of reprocessing gets rid of the twin indicator ..
> Does anyone have any suggestions of why this might be?
> ( The data quality isnt great - 2.5A but the Rmerges are reasonable..)
> Eleanor
>
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