Abstract

Peptides and proteins may be tagged with metallic elements in order to use them as imaging reporters or for other applications. The polypeptide of interest is first conjugated to a suitable chelating agent that forms stable complexes with the element of interest. This conjugation step is undertaken either in aqueous or in non-aqueous conditions depending on the solubility of the substrate. For polypeptides of greater than |[sim]|10 kDa in size, this is normally done in aqueous medium. Most commonly the chelators are reacted with lysine amino groups. The protein is first desalted into a suitable buffer at pH 8–9 and a molar excess of a bifunctional chelating agent is added. After a suitable period of incubation, excess, unreacted or hydrolyzed chelator is removed and the protein conjugate is desalted into an acidic buffer. The conjugate can then be tagged by addition of a suitable metal salt followed, if necessary, by removal of unchelated metal. As described in the protocol that follows, the entire conjugation, purification and labeling procedure takes about 2 d.