Selective Derivatization of Hexahistidine-Tagged Recombinant Proteins

Covalent modification of proteins is extensively used in research and industry for biosensing, medical diagnostics, targeted drug delivery, and many other practical applications. The conventional method for production of protein conjugates has changed little in the last 20 years mostly relying on reactions of side chains of cysteine and lysine residues. Due to the presence of large numbers of similar reactive amino acid residues in proteins, common synthetic methods generally produce complex mixtures of products, which are difficult to separate. An emerging alternative technology for covalent modification of proteins involves formation of a covalent bond with a hexahistidine affinity tag present in a majority of recombinant proteins without interfering with other amino acid residues. The approach is based on formation of a ternary complex of the hexahistidine sequence with a bivalent metal cation chelated by ligand bearing an electrophilic Baylis-Hillman ester group capable of subsequent formation of a covalent bond with one of the histidine residues of the tag. The reaction proceeds under mild reaction conditions in neutral aqueous solutions under high dilutions (10^-5 to 10^-4 M) providing a stable covalent bond between the label and an imidazole residue in a hexahistidine tag at either C- or N-terminus. Because hexahistidine affinity tag methodology is a de-facto standard for preparation of recombinant proteins our approach can be easily implemented for selective derivatization of these proteins with fluorescent groups, alkynyl groups for "click" reactions, or biotinylation.