Selective separation of tryptophan derivatives using sulfenyl halides

Upon reaction of tryptophan containing proteins with 2-nitrophenylsulfenyl chloride (NPS-Cl) the nitroaryl chromphore (lambda max 365 nm, epsilon 4000) is covalently linked at the 2-position of the indole ring of tryptophan. From the mixture of peptides obtained by enzymatic or chemical fragmentation of the labelled protein, the yellow tryptophan peptides are much more easily separated by column as well as paper chromatographic techniques, since their easier detection by spectrophotometry. Tryptophan peptides have been also isolated by diagonal electrophoresis, upon introduction of a carboxylate anion in the peptide by reaction with the carboxy-substituted reagent 2-nitro-4-carboxyphenylsulfenyl chloride. Sulfenl halides have been also used for the selective chemical coupling of tryptophan peptides to a solid support, thus facilitating their separation. N-acetyl-tryptophan has been covalently coupled to an amino-resin (aminoethylated Bio-Gel), and the resulting polymer reacted with a mixture of peptides in presence of 4,6-dinitrophenyl-1,3-disulfenyl chloride in glacial acetic acid. Selective covalent binding of tryptophan peptides has been checked.