Labeling Proteins at Site-Specifically Incorporated 5-Hydroxytryptophan Residues Using a Chemoselective Rapid Azo-Coupling Reaction.

Chemoselective protein labeling is a valuable tool in the arsenal of modern chemical biology. The unnatural amino acid mutagenesis technology provides a powerful way to site-specifically introduce nonnatural chemical functionalities into recombinant proteins, which can be subsequently functionalized in a chemoselective manner. Even though several strategies currently exist to selectively label recombinant proteins in this manner, there is considerable interest for the development of additional chemoselective reactions that are fast, catalyst-free, use readily available reagents, and are compatible with existing conjugation chemistries. Here we describe a method to express recombinant proteins in E. coli site-specifically incorporating 5-hydroxytryptophan, followed by the chemoselective labeling of this residue using a chemoselective rapid azo-coupling reaction.