Credit: © 2010 ACS

Many biochemical techniques rely on the ability to form covalent bonds to proteins. For example, coupling proteins to fluorescent probes allows them to be tracked within a cell, and can allow mechanistic studies of biological transformations. Such bioconjugation techniques usually rely on selective coupling reactions to the side chains of specific amino acid residues. The most common amino acid residues that are targeted are lysine and cysteine, but neither residue is ideal: lysines are too abundant and cysteines are rare and often linked in disulfide pairs.

Now, Carlos Barbas and co-workers from the Scripps Research Institute in La Jolla, California, have developed1 a method of selectively functionalizing tyrosine residues under very mild aqueous conditions. In their method, the aromatic (phenolic) side chain of a tyrosine reacts with a cyclic diazodicarboxamide — a highly reactive electrophile that is also stable to the aqueous conditions desired. Tyrosine residues reacted selectively over other amino acids with aromatic side chains to form new C–N bonds that are stable to both strongly acidic and basic conditions even at high temperature.

A wide variety of molecules can be functionalized with the necessary cyclic diazocarboxamide group, including fluorescent dye reagents and other peptides. These include a bifunctional linker with both the diazo functionality and an azide, so the system is primed for the now widely used copper-catalysed [3+2] cycloaddition or 'click' reaction.