Tetrazine Marks the Spot

For decades, bioorthogonal reactions have pushed the frontiers of modern science. Using these selective chemistries, it is now possible to tag and retrieve bioactive metabolites directly from cells, generate libraries of complex drug molecules, and access dynamic materials for cell adhesion and patterning. The breadth of such achievements is remarkable, and as the bioorthogonal toolbox continues to grow, so too will the number of innovative applications and discoveries mediated by these reactions. Despite their myriad successes in vitro, most bioorthogonal reactions have been slow to transition in vivo. This is due, in part, to a lack of exceptionally fast reactions. Organisms with big “volumes” demand chemistries that not only are robust and selective, but also proceed with large rate constants. If a reaction is too slow, a sizable bolus of one reagent must be used to drive covalent bond formation. Large quantities of any reagent can promote off-target toxicities or require long washout periods to deplete excess probe. Fast reactions, by contrast, require less material to achieve comparable ligation yields and are thus highly desirable for in vivo work. Perhaps one bioorthogonal chemistry, out of the dozens reported to date, best satisfies this need for speed: the inverse electron-demand Diels−Alder (IED-DA) cycloaddition of trans-cyclooctene (TCO) and tetrazine.