Re: [ccp4bb]: Twinning problems (again....).

["Eleanor J. Dodson" <ccp4@ysbl.york.ac.uk>]

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

Mark Roe wrote:
> 
> ***  For details on how to be removed from this list visit the  ***
> ***          CCP4 home page http://www.ccp4.ac.uk         ***
> 
> Dear All,
> 
> Not CCP4 I know, but ccp4bb seems to be the best forum to get an intelligent
> response.
> 
> The problem is that we appear to be getting twinned crystals, but that
> neither truncate nor the twinning server shows this up.  We have tetragonal
> crystals, apparent space group P41212 or P43212. The crystals show 100%
> incorporation of Se by mass-spec and the fluorescence scan shows a Se edge.
> We collected Se-SAD data sets at the peak wavelength for five crystals, all
> diffracting to 2.8 - 3.0A.  The data was processed with mosflm. Parts of the
> scala and truncate logfiles for one are reproduced below. As you can see, the
> anomalous R merge is lower than the normal R merge, indicating (as I
> understand it) that there is little or no anomalous signal. This (as I also
> understand it) indicates twinning and the twinning cancels out any anomalous
> signal. The truncate output, though, clearly indicates an
> untwinned crystal.



 That isnt necessarily true - Rmerge will increase with the number of
observations whilst Ranom will probably get smaller as your data
improves.

And in fact Ranom and Rmerge are defined quite differently - I think the
definitions are listed in the log file, but alas, NOT in the
documentation! Sorry abt that!!

The cumulative intensity more or less proves it is not a twinned crystal
- you dont include the 2nd moment plots which are usually the clearest
indicator.

> 
> The questions are these:
> 
> 1) Are the crystals twinned, or is there another explanation?


 Almost certainly not..

> 2) If so, why doesn't truncate or the twinning server show this?

See above..

> 3) Can any useful info about the twinning be gained from the above two
> questions?

see above..

> 4) Is there any other way of showing the twinning, without the need to
> collect anomalous data (because otherwise it is going to be a hard slog
> screening to find untwinned crystals)?


 What evidence do you have of twinning?

> 5) Back to finding another crystal form?


 This one is prob OK.. persistence is a virtue..