Labelling proteins and peptides with phosphorescent d6 transition metal complexes

The phosphorescent properties of Ru(II), Re(I) and Ir(III) transition metal complexes, each with a nominal d 6 electron configuration, are well suited to label peptides and proteins to allow biological imaging by luminescent microscopy. This review focuses on the development of chemistry that enables the ready incorporation of a wide range of phosphorescent metal complexes into peptides or proteins. Different strategies for labelling proteins are presented including the direct coordination of amino acid functional groups to the metal ion, metal complexes where one or more of the ligands coordinated to the metal ion contain reactive functional groups for conjugation to peptides or proteins, and metal complexes designed for site selective bioorthogonal conjugation. Phosphorescent metal complexes that bind and interact with proteins in a non-covalent fashion are also discussed as is the use of metal complexes as single-electron transfer photo-catalysts for the fluorescent labelling of proteins.