Sulfo-CY3 tetrazine potassium is a sulfo-Cyanine3 derivative that contains methyltetrazine moiety (Ex=548 nm, Em=563 nm). Sulfo-CY3 tetrazine potassium can be used for the labeling of metabolically engineered cell-surface glycoconjugates. Sulfo-CY3 tetrazine (potassium) is a click chemistry reagent, it contains a Tetrazine group that can undergo an inverse electron demand Diels-Alder reaction (iEDDA) with molecules containing TCO groups.