Re: [ccp4bb]: perfect twinning

["Peter Mueller" <peterm@mbi.ucla.edu>]

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Dear Li,

It is not difficult to untwin data of a perfect twin (k = 0.5), it is simply
mathematically impossible. However, you should never REFINE against twinned data
anyway. The detwinning is used mostly to find a solution for the phse problem,
and since you alreaddy solved that you don't need to detwin the data. You
should, however, take the twinning (the twin law in your case is probably 
0 1 0 1 0 0 0 0 -1) into account when you refine the data.

Best wishes

Peter


Li Minghui <lmh@moon.ibp.ac.cn> said:

> Dear all,
> I have a perfectly twinned dataset.
> At first, I determined that the spacegroup was P43212 and could find MR
solution using Molrep. There were two molecules in the ASU, and the initial
R-factor was about 0.5. After a cycle of rebuilding and refinement the R-factor
dropped to 0.36. The density map calculated from the new model was good, and
some omitted parts of the initial model also have clear densities. I found there
were also many other densities outside the molecules. I could dock part of an
other model in them, but the remaining parts would overlap on the existing
molecules.
> I processed the data again in P4 spacegroup and found the data was perfectly
twinned with a twinning fraction of 0.48. It is said that perfect twinned data
is difficult to detwin.
> Does anybody have successful experience with perfect twinned data? Any
suggestion will be apperciated.
> Minghui Li
> National Laboratory of Biomacromolecules 
> Institute of Biophysics, Academia Sinica 
> Tel: 86-10-64888507 
> E-mail: lmh@moon.ibp.ac.cn 
> 



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