Tetrazine-TCO click chemistry

The tetrazine ligation is the reaction of a trans-cyclooctene and an s-tetrazine in an inverse-demand Diels Alder reaction followed by a retro-Diels Alder reaction to eliminate nitrogen gas. The reaction is extremely rapid with a second order rate constant of 2000 M−1–s−1 (in 9:1 methanol/water) allowing modifications of biomolecules at extremely low concentrations.

Not only is the reaction tolerant of water, but it has been found that the rate increases in aqueous media. Reactions have also been performed using TCO or norbornenes as dienophiles at second order rates on the order of 1 M−1•s−1 in aqueous media. The reaction has been applied in labeling live cells and polymer coupling such as tetrazine hydrogels for chemotherapeutic development.