On Tuesday 06 January 2009 08:35:19 Stephen Hare wrote:
> Dear All,
> 
> We are currently working on a structure of apparent P21 symmetry which  
> has been solved by molecular replacement. The data are to 2.7Å but the  
> Rfree will not drop below 30%. The density is clear for the model we  
> have, however there is extra density that suggests a shift of the  
> structure by 16Å in either direction - resulting in three possible  
> overlapping positions for the structure. We assume this is the result  
> of twinning.

It sounds like an interesting case. You might want to go back and
closely inspect your diffraction images after reading this paper:

	J. Appl. Cryst. (2008). 41, 600-605
	Protein crystals can be incommensurately modulated
	JJ Lovelace, CR Murphy, L Daniels, K Narayan, CE Schutt, 
	U Lindberg, C Svensson and GEO Borgstahl

I am not certain that your description matches this case, but it might.

	Ethan 
 
> The unit cell dimensions are 102.7Å, 83.0Å, 115.3Å, 90°, 101.8°, 90°.  
> Examining the data with phenix.xtriage also suggests pseudo  
> translational symmetry with a separation of 16Å.  A Patterson peak at  
> 0.097, 0.000, -0.096 is approximately 30% of the origin peak, while a  
> second peak of double the translation at 0.192, 0.000, 0.195 is 7% of  
> the origin peak. The structure contains a dimer of dimers with an NCS  
> 2 fold axis almost perpendicular to the crystallographic 2 fold. This  
> NCS axis almost  coincides with the diagonal between the A and C axes.  
> A twin axis along the A C diagonal (l,-k,h) could explain the observed  
> extra density, however this is not possible because A and C are  
> different lengths.
> 
> As a result of the NCS axis running almost perpendicular to the  
> observed P21 axis, it is possible to merge the reflections in a larger  
> orthorhombic unit cell - dimensions 137.1Å, 83.3Å, 169.8Å although  
> here the Rmerge is higher and it is not possible to get a molecular  
> replacement solution.
> 
> Is it possible to define the (l,-k,h) twinning operator in our  
> original unit cell? or have we missed the actual unit cell?  
> Or....something else?
> 
> Steve
> 
> 
> 
> Stephen Hare PhD
> Post doctoral research associate
> Jefferiss Research Laboratories
> Wright-Fleming Institute
> Division of Medicine
> Imperial College London
> Norfolk Place
> London W2 1PG
> UK
> 
> Phone: +44 (0) 20 7594 3908        	
> Fax: +44 (0) 20 7594 3906 	
> 



-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742