DEPMPO is a nitrone that is used to spin trap reactive O-, N-, S-, and C-centered radicals and allow their characterization when used in association with electron spin resonance. It is noted for the stability of adducts formed. DEPMPO can be used in vitro or in vivo, as it crosses lipid bilayer membranes and is a good trapping agent in biological systems. DEPMPO-biotin is a biotinylated form of DEPMPO which retains the outstanding persistency of its adducts. The biotin moiety offers an effective means for monitoring biodistribution in cells, tissues, and organs when used with an avidin-conjugated reporter. Importantly, DEPMPO-biotin binds free radicals, such as S-nitroso groups, on proteins, producing adducts that can be analyzed via the biotin tag. This direct labeling of S-nitrosothiols (SNO) thus serves as an effective alternative to the more cumbersome biotin-switch method for monitoring SNO formation.