– Cy5.5-PEG-N3 (MW 3400) is a fluorescent dye azide used for labeling amino groups in peptides, proteins, and oligonucleotides. As a click chemistry reagent, it contains an azide group that participates in a copper-catalyzed azide-alkyne cycloaddition (CuAAC) with alkyne-containing molecules. Additionally, it can engage in a strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules that include DBCO or BCN groups.