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Re: [ccp4bb]: perfect twinning



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> Li Minghui wrote:
> 
> 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


 You can indeed solve a MR search against twinned data - you will find
two overlapping molecules of course..

 SHELX and CNS can both refine your models against the twinned data, and
also refine the twinning factor.

 You will have difficulties with generating maps for rebuilding though.. 
There are ways to detwin the Iobs using the calculated values as a guide
but I have no experience of how much bias it introduces.

 Eleanor