TCO (tetrazine – trans-cyclooctene) ligation is one of the fastest reactions used for bioconjugation. It is an inverse electron demand [4+2] cycloaddition that takes place between tetrazine and trans-cyclooctene or other strained olefins.nnsulfo-Cyanine5 tetrazine is a fluorophore derivative bearing a tetrazine group for the TCO-ligation-based labeling.nnThis reagent possesses good aqueous solubility and stability in biological environments.