[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [ccp4bb]: Summary of Replies: Data Reduction for mosaic crystals...



***  For details on how to be removed from this list visit the  ***
***          CCP4 home page http://www.ccp4.ac.uk         ***

On Mon, 10 Jun 2002, Anastassis Perrakis wrote:

> > Here are all the responses to my questions! Thank you very much.
> > Raji
> >
> > Questions:
> > I am working on a dataset where the crystal has a very high mosaicity - my
> > estimate is ~2.0 or more. Hence, I have had to take 0.1deg frames
> > during data
> 
> Since I am a bit dissapointed that I failed to provoke a good discussion
> with my last answer on that one, but I still find it interesting,
> I will rephrase my last mail as direct questions and wait for some lively
> discussion:
> 
> If you DO have crystals with 2.0deg mosaicity:
> 1. Am I alone thinking Raji did *not 'have had'* to take 0.1deg frames ?
>     Besides the good ideas offered for really processing the data,
>     does fine phi slicing improve anything for *very mosaic* crystals ?

You're not alone. I _assumed_ that he had a large cell and was doing his
best to avoid overlaps by having a very small oscillation angle. However,
as the problem was stated, i.e. he has a mosaicity of ~2 and _hence_ had
to have a small oscillation angle, I can't back up this assumption (in
the absence of other information)! 

I think it's unlikely that overlaps would be avoided in any case with a
protein dataset and a mosaicity this large; it may be worthwhile
investigating Dominique Bourgeois's program PrOW (Acta Cryst. (1999). D55,
1733-1741), but I have no experience of this.

> 2. Do people get good maps out of such datasets ?
> 3. Have people solved non MR structures out of >1.5 def mosaicity data ?

I've seen a couple of _MR_ structures from extremely high mosaicity
crystals reported in the last few years (powder structures of myoglobin
and insulin), but this doesn't answer your question!! 

> 4. Whats at the end the minimal meangfull phi slice to get as fucntion of
> mosaicity ?

I'd be tempted to provoke someone from the 3D integration world to answer
this! I don't have Jim Pflugrath's 1999 paper to hand, but I have a vague
recollection that he addresses the point in that.

My thought is that once the phi-slice is smaller than the width of the
rocking curve (i.e. all reflections are partials) there would be no
improvement in signal/background which could be optimized by 3D profile
fitting; (see also Pierre's reply on this thread).

I was a little disappointed (from my partisan point-of-view!!) that Raji
didn't include the Mosflm/Scala option in his summary, especially as this
is the CCP4 BB...

Harry 
-- 
Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre, Hills
Road, Cambridge, CB2 2QH