Re: [ccp4bb]: Questions about translational NCS

[Jianghai Zhu <zhu5@purdue.edu>]

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Hi, 

Thanks for all your responses. 

Based on Rizkallah's example, there is pure translational NCS 
without any rotational NCS associated with it. But in his case, 
since the translational vector has such a special direction and 
length, he can shrink the unit cell to half volume and make the 
translational symmetry as a crystallgraphic symmetry. But if 
the translational vector doesn't have a special direction and 
length, he probably can not just shrink the unit cell and get 
rid of the translational NCS (e.g 2 molecules related by a 
general translation vector in a triclinic unit cell). Am I 
right?

Based on the paper Randy read pointed out (Wang & Janin,Acta 
Cryst. D49:505-512, 1993), most of the NCSs are either 
parrallel to one of the crystallographic axis or orthogonal to 
it. Here comes another question. If a 2-fold NCS is orthogonal 
to the crystallographic rotation axis, it is possible that you 
will get a higher apparent symmetry in you diffration pattern 
(e.g. if I have a 2-fold NCS along a* and b* in a P3 space 
group, I could get a 622 symmetry). This is quite like a 
perfect twinning crystal data. But will the intensity 
distribution also behave like a twinned data? Why?

Thanks.

Jianghai


-- 
===========================
Jianghai Zhu
Biochemistry & Molecular Biology
Purdue University
Tel: 765-4949249 (O)
      765-4633336 (H)
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