Expanding the Genetic Code for a Dinitrophenyl Hapten

Haptens, such as dinitrophenyl (DNP) are small molecules that induce strong immune responses when attached to proteins or peptides and, as such, have been exploited for diverse applications. We engineered a Methanosarcina barkeri pyrrolysyl‐tRNA synthetase (mbPylRS) to genetically encode a DNP‐containing unnatural amino acid, N⁶‐(2‐(2,4‐dinitrophenyl)acetyl)lysine (DnpK). Although this moiety was unstable in Escherichia coli, we found that its stability was enhanced in mammalian HEK 293T cells and was able to induce selective interactions with anti‐DNP antibodies. The capability of genetically introducing DNP into proteins is expected to find broad applications in biosensing, immunology, and therapeutics.