Azide is a linear, polyatomic anion with the formula N3 and structure N=N+=N. Used as substrate of Strain-promoted azide-alkyne cycloaddition (SPAAC) together with alkyne substrate when both come close in the active site of an enzyme ( 'click reaction') and in 'Staudinger ligation'. lending to combinatorial chemistry. Azide handle are used for copper-catalyzed alkyne-azide cycloaddition (CuAAC) reactions to easily install fluorescent dyes (biotin, TAMRA) or for enrichment or fluorescent dyes for fluorescence based SDS-PAGE experiments and particularly useful in Activity-Based Probes for detecting or purifying serine hydrolases