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Hello. I am working with a protein that turns a yellowish-brown color when it is concentrated to around 2 mg/ml or higher in a small volume (a few hundred uL). I was wondering if the protein bound a metal or other prosthetic group that would give it this color? The protein's color somewhat resembles iron binding proteins, but there is no peak in the 400 nm range that would suggest heme, and an iron sulfur cluster is not that likely since there are only five cysteines in the protein. Proteins with structures homologous to the one I am studying bind magnesium, but are not know to bind other metals. Any information about what this color might suggest about the protein or how I could analyze possible bound metals or prosthetic groups using only a small amount of protein would be helpful. Matt