Re: [ccp4bb]: Can P43212 and P41212 be distinguished by anomaloussignals?

["Eleanor J. Dodson" <ccp4@ysbl.york.ac.uk>]

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Q. Steven Xu wrote:
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> Hi,
> 
> I have one sel-met cystal in tetragonal form, which turns out to be in
> either P43212 or P41212 space group. It is supposed to have two
> selenium atoms in the asymmetric unit. The MAD data set collected
> form the crystal was subjected to SOLVE for MAD phasing. When I defined
> the space group as P41212, the SOLVE gives a solution with only one
> selenium site. All parameters (like overall Z score, f.o.m) are OK, except
> that the peak height of the site is zero. In the case of P43212, two sites
> show up as expected with very high peak heights (around 30). My question
> is whether these two space groups can be distinguished by anomalous
> signals, as shown by SOLVE?    
> 
> My another questions is, once I determine the space group, do I need to
> flip the sites and try both possibilites? I remember somebody asked the
> similar question before, but just can not find a clue in the archive.
> sorry about that. Thanks in advance.
> 
> regards,
> Q. Steven Xu
> Dept. of Physiology and Biophysics
> Boston Univ. School of Medicine
> Tel: (617)638-4083
> Fax: (617)638-4041
> E-mail: qian@med-biophg.bu.edu
> 
> 
> 


  It sounds like P43212.

  Patterson for two spacegroups are identical, so you can generate a 
solution xyz in P41212, or -x,-y,-z in P43212.

  But the phases generated are not the same, so if you phase on the 
first site and use difference maps  to find the next site, one hand will 
give a better answer than the other..

  Thus by the end of a SOLVE run you have effectively chosen the hand

  Eleanor#

>