Preparation of photoreactive derivatives of α-melanotropin by selective modification of the lysine or tryptophan residue

Photoreactive derivatives of α-melanotropin were prepared by selective modification of the single tryptophan residue in position 9 or the amino group of the lysine residue in position 11 by reaction with 2-nitro-4-azidophenylsulfenyl chloride. [2-Nitro-4-azidophenylsulfenyl-Trp9]-α-melanotropin was found to be five times as potent as α-melanotropin in stimulating lipolysis in isolated rabbit adipocytes. [Ne-2-nitro-4-azidophenylsulfenyl-Lys11]-α-melanotropin had only 11% of the lipolytic potency of α-melanotropin.