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Re: [ccp4bb]: Non-merohedral twinning......



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RE: Non-merohedral twinning?...

is this synchrotron data or from a laboratory source?  if the latter, what 
means of monochromatization?  Are the extra reflections always just closer 
to the beam center than the regular reflections?
-- I am wondering if you could be seeing the Cu 
K beta reflections.  some people do not realize that focusing mirrors are 
not adequate for monochramatization, and still require Nickel filters. 
Multi-layer optics do a decent job of both focusing and 
monochromatization. this would not be relevant for synchrotron data.

if this is either Cu K beta or non-merohedral twinning (satellite xtal?  
less likely if this happens for all xtals) i 
would say if the statistics are good the data are probably usable. the 
processing programs know where to find the primary reflections and do not 
look elsewhere (after indexing). the only problem would be if two spots 
coincide, and if the stats are OK this must not be a serious problem.

cheers,
 
=======================================================================
Life is a struggle, not against sin, not against the Money Power, not 
against the malicious animal magnetism, but against hydrogen ions.
                                                - H.L. Mencken
=======================================================================
                        David J. Schuller
                        modern man in a post-modern world
                        MacCHESS, Cornell University
                        djs63@cornell.edu

On Mon, 1 Jul 2002, raji wrote:

> I need help with determining whether my crystal is non-merohedrally twinned or 
> not. And, if my crystal is twinned, can I find a program that can detwin 
> non-merohedrally twinned data?
> Here's what I find so far-
> The crystals by themselves are not microscopically twinned. However, the 
> diffraction pattern has some reflections adjacent to some main reflections. 
> These reflections are NOT being picked up by the indexing routine.
> I am able to index, scale my data fine. The spacegroup is p212121 and the 
> scaled data is very consistent with p212121. I'd like to point out that my a 
> and b axis are almost close to one another (105 109). The R-merges are as low 
> as I have seen with other related data sets and the completeness is excellent. 
> I performed merohedral twinning tests and the crystals are untwinned according 
> to the Yeates test.
> I performed Molecular Replacement on my data and got good solutions for 
> cross-rotation and translation searches. My maps dont look abnormal at all 
> (although I am not experienced at all to determine what one can find out about 
> twinning from a map).
> Is it possible to have completely non-overlapping twins and if so, can one 
> ignore the 2nd lattice? Or, do I have some kind of crystal splitting ?? Is 
> there a way to distinguish between splits and twins ? 
> Since, so far I am unable to decide what's going on, I'd like some suggestions 
> as to how I can either establish or rule out non-merohedral twinning...
> Thank you very much.
> Raji
> 
> Rajeswari Edayathumangalam
> Dept. of Biochemistry & Molecular Biology
> Colorado State University
> Fort Collins, CO 80523 USA
> 
> 

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