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[ccp4bb]: Heavy atom refinement

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Dear Xtallographers :

I am trying to solve a structure of a 45 kDa protein by MAD. We have
three datasets at three wavelenghts of a mercury derivative with
reasonable statistics till 3.1 A. Spacegroup is P61 with one molecule in
the assimetric unit. Anomalous difference Patterson maps show clear
peaks in the Harker sections. We have tried several automatic Patterson
search programs like RSPS and also SHELX. All the programs are able to
find several atoms, with cross-vectors fitting the Patterson peaks in
the map. The problem is that when we try to refine the heavy atom
positions with MLPHARE or VECREF, always the occupancy drops practically
to zero and the Bs go up till 150. 

I suppose that the heavy atom positions are wrong, but anyone has some
advice about how to proceed for the heavy atom search to avoid these

Thanks in advance.



Francisco J. Enguita, Ph.D.
Protein Crystallography laboratory
Instituto de Tecnologia Química e Biológica
Ap. 127
2781-901 Oeiras
Phone : 351-21-4469669
Fax : 351-21-4411277
E-mail : fenguita@itqb.unl.pt