Dansyl chloride

Dansyl chloride or 5-(Dimethylamino)naphthalene-1-sulfonyl chloride is a reagent that reacts with primary amino groups in both aliphatic and aromatic amines to produce stable blue- or blue-green–fluorescent sulfonamide adducts. Dansyl chloride is widely used in protein sequencing and amino acid analysis.

In addition, these adducts are generally good acceptors for fluorescence resonance energy transfer from the amino acid tryptophan, allowing this labelling technique to be used in investigating protein flexibility. The fluorescence of these sulfonamide adducts can be enhanced by adding cycloheptaamylose. Importantly, dansyl chloride is unstable in dimethyl sulfoxide and this solvent should never be used to prepare solutions of the reagent.