Re: [ccp4bb] strange density

[Roger Rowlett]

This looks suspiciously like a metal ion with 3 or more resolved water ligands. It's hard to tell from the images provided, but it looks like the metal could be close to octahedral, with 2 His and 3 water ligands well-defined. The estimated metal-nitrogen bond distances might help narrow down the metal ion. For example, tetrahedral Zn(II) has a Zn-N bond distance of around 2.0 A. I suppose this could be a Mg(II) ion, since that is in your crystallization mix, but I think that Mg(II) typically prefers harder protein  ligands, e.g., carboxylates. Zn(II) is a very common contaminant in solution, however, and should not necessarily be discounted; indeed Zn(II) and other first row transition metal ions would be expected to have a reasonable affinity for medium hard/soft ligands such as vicinial His residues. Cheers. On 2/23/2011 7:34 PM, Alex Singer wrote: cocrystallized in 2.5mM Glycero-3Phosphocholine and cryoprotected by dipping in -- Roger S. Rowlett Professor Department of Chemistry Colgate University 13 Oak Drive Hamilton, NY 13346 tel: (315)-228-7245 ofc: (315)-228-7395 fax: (315)-228-7935 email: rrowl...@colgate.edu