Photoreactive derivatives of corticotropin. 2. Preparation and characterization of 2-nitro-4(5)-azidophenylsulfenyl derivatives of corticotropin

Two new photoreactive arylsulfenyl chlorides, 2-nitro-4-azidophenylsulfenyl chloride (2,4-NAPS-Cl) and 2-nitro-5-azidophenylsulfenyl chloride (2,5-NAPS-Cl), have been synthesized and used for the selective modification of corticotropin (ACTH). Both reagents reacted rapidly with N-acetyltryptophanamide and ACTH under acidic conditions. The NAPS derivatives of ACTH were purified by partition chromatography and characterized by absorption spectra, amino acid analysis, and peptide mapping. The spectral changes caused by photolysis as well as the kinetics of photolysis are described. Tritiated 2,5-NAPS-ACTH was attached covalently to a pituitary protein fraction FI by photolysis. The photolabeling of FI was blocked in the presence of excess ACTH.