On Mon, Feb 4, 2013 at 12:24 PM, Roger Rowlett <rrowl...@colgate.edu> wrote: > It's possibly a transition metal ion. Zinc is a common adventitious > contaminant of solutions. Typical Zn-O distances (tetrahedral or > pseudo-tetrahedral coordination) are 2.0 A. ICP-OES or ICP-MS of the protein > solution might offer a clue to the possible identity of the metal ion, since > it appears to be nearly stoichiometric with your protein.
Zinc tends not to bind carbonyl oxygens, however, but calcium does quite frequently. Also, the presence of calcium acetate in the crystallization solution strongly suggests that this is what is actually bound (especially if it's at a concentration around 200mM, as is common in many crystallization screens). Jared: what do you mean by "proper coordination"? Surface ions bound non-specifically frequently don't have recognizable coordination, and they become even more vague as resolution decreases. As always, it would be worth looking at the anomalous difference map, although whether you'll actually see anything depends on the element and on wavelength, data quality, anomalous completeness, etc. -Nat
