Re: [ccp4bb]: Detwin and Molecular Replacement

["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         ***

Toshimitsu Tanaka wrote:
> 
> ***  For details on how to be removed from this list visit the  ***
> ***          CCP4 home page http://www.ccp4.ac.uk         ***
> 
> Hello,everyone.
> I hope somebody advise me.
> I have a data set that was processed in R3, a=108A,b=108A,c=155A.


 First you should check for twinning! Look at your truncate plots - 
the cumulative intensity distribution and the 2nd moment plots in
particular will show whether your data is twinned or not.

See the ccp4 documentation: notes on twinning for explanations.


If the crystal is twinned as you say you need the twinning fraction. If
this is 0.46, you really have no hope of detwinning the data
satisfactorily.. Values up to 0.35 can be used, but a value of 0.5 is
indeterminate ( see notes) and 0.465 is very close to 0.5! 
Presumably your data looks like R32? 

The best solution is to go back and test more crystals; in our
experience smaller and more perfect is better, but you will need to try
several..



 It is formally possible to solve the molecular replacement with twinned
data; there is a paper by G. Jamieson amongst others on doing this.
However you dont say how many molecules you expect in the asymm unit, it
gets harder the more you have..

Eleanor