[ccp4bb]: Phasing with FeS cluster

[gb10@globalphasing.com]

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Dear Paul,

     The problem is that an uranium atom is a very bad model for the FeS 
cluster at the resolution at which you are working, as it concentrates density 
at the centre of the cluster, which is actually empty.

     There is an undocumented feature in SHARP, which was used on the 30S 
ribosome subunit work and performed well, which uses clusters assuming that 
they are rotationally disordered (with all orientations equally probable), 
but keeping track of the chemical identity of all atoms within it. The idea 
is that you can first refine and phase with this rotationally disordered 
model - which, unlike an U-atom model, has the merit of having the correct 
radial distribution of scattering matter - and then examine the anomalous 
residual map to look for indications of broken rotational symmetry and locate 
the individual atoms if applicable.

     This feature could be documented if absolutely crucial to your project, 
but even at 3.0 Angstrom resolution it should be possible to resolve the Fe 
atoms and start with a sensible model of the cluster. Please keep us posted 
on your progress.


     With best wishes,

         Gerard.

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     * Gerard Bricogne                     gb10@GlobalPhasing.com  *
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