RE: [ccp4bb]: perfect twinning

[raji <raji@lamar.colostate.edu>]

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

Yes, CNS does have a routine to detwin perfect hemihedral twins 
(detwin_perfect.inp).
It detwins data based on a twinning operator, twinning fraction and model 
amplitudes. I quite dont understand how that works though but I think it works 
better when you have a starting model.
Raji

>===== Original Message From Bjoern Mamat 
<Bjoern.Mamat@mpibp-frankfurt.mpg.de> =====
>***  For details on how to be removed from this list visit the  ***
>***          CCP4 home page http://www.ccp4.ac.uk         ***
>
>>From my very limited experience, I got the impression that
>CNS can handle partial, but not perfect merohedral twinning.
>
>To my knowledge, all structures solved using perfect twins so far
>have been refined with SHELXL --
>
>any comments on that?
>
>-- bjoern
>
>>
>> ***  For details on how to be removed from this list visit the  ***
>> ***          CCP4 home page http://www.ccp4.ac.uk         ***
>>
>> > 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
>
>--
>----------------------------------------------------------------------------
>Bjoern Mamat					mamat@mpibp-frankfurt.mpg.de
>----------------------------------------------------------------------------
>Max-Planck-Institute of Biophysics 		http://www.biophys.mpg.de
>Heinrich-Hoffmann-Str. 7			phone  +49 69 96769 384/393
>60528 Frankfurt/Main, Germany			fax    +49 69 96769 423
>----------------------------------------------------------------------------

Rajeswari Edayathumangalam
Dept. of Biochemistry & Molecular Biology
Colorado State University
Fort Collins, CO 80523 USA