[40] Sulfenyl halides as modifying reagents for polypeptides and proteins.

The conversion of 2-thio-aryl-tryptophan and related derivatives to the corresponding 2-hydroxycompounds by acidic hydrolysis was investigated. The thioether function is introduced into the tryptophan residue by the selective reaction of sulfenyl halides with the indole nucleus. The sub­ stitution occurs under mild conditions only when the amino group of the tryptophan residue is free. The assistance of the amino group was confirmed using model compounds. Thus 2-thio-(2nitrophenyl)-tryptamine is smoothly converted to 2-hydroxy-tryptamine whereas 2-thio(2-nitrophenyl)-/?-indolyl-propionic acid is completely stable under the same conditions. The conversion of 2-thio-aryl-tryptophan units linked in a polypeptide chain to the corresponding 2-hydroxy-tryptophan occurs only under conditions during which hydrolysis of the peptide bond occurs. It was also observed that the 2-nitro-thiophenols which are released during the hydrolysis of 2-thio-(2-nitrophenyl)-tryptophan and related compounds are converted under the conditions of the reaction to 2-aminobenzene sulfonic acids.