(S)-2-(5-(Dimethylamino)naphthalene-1-sulfonamido)-5-guanidinopentanoic acid, commonly known as N^α-Dansyl-L-arginine, is a fluorescently labeled amino acid derivative used in biochemical and biophysical research. It acts as a substrate for proteases, particularly trypsin and other arginine-specific enzymes, enabling the monitoring of enzymatic hydrolysis through changes in fluorescence intensity and wavelength. The dansyl moiety provides strong fluorescence with environmental sensitivity, making it ideal for studying protein-ligand interactions, conformational changes, and binding affinities in fluorescence spectroscopy and polarization assays. It is also applied in drug discovery for screening protease inhibitors and in studies of peptide-protein interactions. Suitable for in vitro assays, with potential for cellular applications when appropriately formulated.