4-Azido-L-phenylalanine hydrochloride is an unnatural amino acid, which is used as an effective vibrational reporter of local protein environments. 4-Azido-L-phenylalanine (hydrochloride) is a click chemistry reagent, it contains an Azide group and can undergo copper-catalyzed azide-alkyne cycloaddition reaction (CuAAc) with molecules containing Alkyne groups. It can also undergo strain-promoted alkyne-azide cycloaddition (SPAAC) reactions with molecules containing DBCO or BCN groups.