text: The full text for this article, hosted at <http://journals.iucr.org>, is unavailable due to technical difficulties.
  
  Supporting information
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  PDB reference: <http://scripts.iucr.org/cgi-bin/cr.cgi?rm=pdb&pdbId=4wmd>
  
  PDB reference: <http://scripts.iucr.org/cgi-bin/cr.cgi?rm=pdb&pdbId=4wme>
  
  PDB reference: <http://scripts.iucr.org/cgi-bin/cr.cgi?rm=pdb&pdbId=4wmf>
  