Cy5.5-PEG-N3 (MW 2000) is a Cy5.5 azide fluorescent dye that targets amino groups in peptides, proteins, and oligonucleotides. As a click chemistry reagent, it features an azide group that undergoes copper-catalyzed azide-alkyne cycloaddition (CuAAC) with alkyne-containing molecules. Additionally, it participates in strain-promoted azide-alkyne cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.