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Re: [ccp4bb]: dodgy mosaicity (nee indexing)



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On Tue, 27 Feb 2001, Martyn Simmons wrote:

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> 
> Dear Jan
>         - I have seen patterns with similar problems at low resolution. I
> wondered if it was due to the outer layers of a crystal becoming more
> disordered than the bulk of the lattice during the cryoprotection
> treatment. These outer layers might contribute more 'mosaic' reflections
> and of course only diffract to lower resolution. The diffraction pattern
> includes everything in the beam of course: good, bad, and ugly. I have
> seen a similar effect with sample inhomogeneity in fibre diffraction. If
> this is right then maybe you can cure the problem by improving the
> treatment for cryoprotection.
>          - regards
>                  Martyn  
> 
> 
> 
> On Tue, 27 Feb 2001, Jan Abendroth wrote:
> 
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> > 
> > Hi,
> > this reminds me my (large) crystal when I measured a high resolution
> > data set at the synchrotron when we did not have enough time for the
> > low-resolution scan. Although the final mosaicity from scalepack
> > postrefinement was lower than the input in denzo: in the low resolution
> > range not all spots were detected - the mosaicity seemed to be higher.
> > The rest (autoindexing, chi^2, ... ) of the scaleing was very smooth.
> > I made a compromize, taking a mosaicity slightly higher than the
> > postrefined from scalepack for a second denzo-scalepack run. The
> > data-set was fine anyway up to high resolution!
> > I don't know exactly the resons for this behaviour. Maybe the crystal
> > cracked a bit during freezing, or a small part of twinning (although
> > merohedral twinning is not possible in this space group),...
> > 
> > Maybe someone has made similar observations and knows a reason.
> > 
> > Regards
> > Jan
> > 
> > 
> > Lauris Kemp wrote:
> > > 
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> > > 
> > > Hello
> > > I'm trying to index a data set with denzo. The problem is with
> > > mosaicity-it looks about 1 but at this value it misses out some low
> > > resolution spots and seems to overfit for the number of spots at high
> > > resolution. Thought it might be misindexed but did a direct beam shot
> > > and the beam values look about right. It manages to autoindex it fine
> > > and the chi values are also fine.
> > > Lauris
> > 
> > -- 
> > ------------------------------
> > Jan Abendroth
> > Institut fuer Biochemie
> > Universitaet Koeln
> > Zuelpicher Strasse 47
> > D-50674 Koeln
> > 
> > Tel: +49-(0)221 470 6455 (Labor)
> >                     6445 (Buero)
> > Fax: +49-(0)221 470 5092
> > 
> 
>