Easy to check whether it is a metal by looking at an anomalous difference
map..
But there are examples of di-sulphides formed between symmetry related
molecules..
Query the wwwpdb -


On Tue, 21 Jan 2020 at 18:06, Guenter Fritz <
guenter.fritz.phenix.c...@gmail.com> wrote:

> Dear Chris, are there any metal ions in your buffer or in your protein. We
> had a similar looking case. A Zn2+ ion bridged two monomers. Our protein is
> a Zn2+ binding protein. The Zn2+ originated from some denatured protein in
> the drop. No extra Zn2+ was in the crystallization buffer.
>
> http://www.rcsb.org/structure/5CHT
> https://www.nature.com/articles/nsmb.3371
> HTH
> Guenter
>
> Dear CCP4BB Users,
>
> I've recently solved the ~2.2 angstrom structure of a protein. In my
> electron density there are unusual monomer-monomer interfaces involving
> pairs of His and Cys residues (see https://ibb.co/wdWBcdk). Note the
> positive Fo-Fc density between the four side chains. As there is not
> adequate space for a water molecule or metal ion, perhaps the Cys residues
> are partially tied up disulfide bonds? However, the protein looks to be
> fully monomeric based on LC-MS measurements. Has anyone else observed
> crystal-driven formation of disulfide bridges?
>
> Aside from this region, there is no extensive interface between momoners,
> and PDBePISA suggests a monomeric state.
>
> Thanks in advance for any advice!
>
> Best wishes,
> Chris
>
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