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Re: R3 Intensity distribution...

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Good morning Chris,

Your cumulative intensity distribution does look
like you may have a merohedral twinning. However the shape of
this distribution may be not a reliable indicator
(it will also depend on the resolution cut-off, completeness and
quality of your data.) Another thing to look at would be
the <I2>/<I>2 ratio, check out the 'Crystal Twinning Server'
by T. Yeates at al. (http://www.doe-mbi.ucla.edu/Services/Twinning/).

If you do have twinning, next thing to do would be to determine
the twinning operation. Look at your self-rotation function --
are there any strange two-folds? I think that if you have a substantial
twinning fraction then you might even be able to see
some additional symmetry on your pseudo-precession plots.
This is only after that that you go to the DETWIN pgm.

I once had a case of an R3 crystal but with perfect twinning
so that the apparent s/group was R32. I could solve it by molecular
replacement. Interestingly enough I could do some initial refinement
of my model in refmac using the original (prefectly twinned) data without any
'untwinning', and could bring my R and Rfree down to 0.31 and 0.35,

Hope this helps,

Chris Colbert wrote:

> Hi all,
> I am working with some R3 data processed and scaled in the hexagonal
> setting of R3.  I am looking at the cummulative intensity distribution
> output from truncate and am curious about the deviations I see from the
> theoretical.  I realize in R3 there are no centric reflections and
> initially, I thought the deviation was a byproduct of this.  Now, I
> am refining the structure and the R-factor is hung in the forties, so
> I am slightly suspicious of my data.  I have enclosed the xloggraph
> portion of my logfile.  Does anyone see any glaring problems with this
> distribution?

Sergei V. Strelkov, Ph.D.                   Tel. ++41 61 267 2091 (lab)
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