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Are the imidazole rings of the histidines distorted? If they are, it could be water/hydroxide. If not, it is probably a cobalt ion side show. Cheers, Rob Meijers EMBL Hamburg --- On Thu, 7/7/11, Artem Evdokimov <[log in to unmask]> wrote: From: Artem Evdokimov <[log in to unmask]> Subject: Re: [ccp4bb] Unexplained density near cobalt To: [log in to unmask] Date: Thursday, July 7, 2011, 9:39 PM Could be a hexacoordinated cobalt with a water molecule (or a hydroxyl ion) depending on the chemical environment... Artem On Thu, Jul 7, 2011 at 10:07 AM, Machius, Mischa Christian <[log in to unmask]> wrote: Y'all, I was wondering if anyone had any thoughts about a feature we observe with a metal-binding site: we have a cobalt that is bound by four histidines and one carboxyl group. There is extra density near the cobalt. See pictures below. The extra density spans the NE2 atoms from two histidines. The Fo-Fc peak (green) has a height of up to 10 sigma (eight molecules in the asymmetric unit, all showing the same feature). I placed a water molecule into the density to get some distances: the distances between the peak and the neighboring histidine NE2 atoms is ~1.8Å and ~2.0Å, resp. The distance between the peak and the cobalt is ~1.7Å. The resolution is 1.24Å. Any input would be greatly appreciated. Many thanks in advance! Cheers! MM