Site-Selective Cysteine-Cyclooctyne Conjugation.

We report a site-selective cysteine-cyclooctyne conjugation reaction between a seven-residue peptide tag (DBCO-tag, Leu-Cys-Tyr-Pro-Trp-Val-Tyr) at the N or C terminus of a peptide or protein and various aza-dibenzocyclooctyne (DBCO) reagents. Compared to a cysteine peptide control, the DBCO-tag increases the rate of the thiol-yne reaction 220-fold, thereby enabling selective conjugation of DBCO-tag to DBCO-linked fluorescent probes, affinity tags, and cytotoxic drug molecules. Fusion of DBCO-tag with the protein of interest enables regioselective cysteine modification on proteins that contain multiple endogenous cysteines; these examples include green fluorescent protein and the antibody trastuzumab. This study demonstrates that short peptide tags can aid in accelerating bond-forming reactions that are often slow to non-existent in water.