Generation of an S-Peptide via an N–S Acyl Shift Reaction in a TFA Solution

An N–S acyl shift reaction of thiol-containing peptides in a trifluoroacetic acid (TFA) solution was confirmed by a combination of 13C NMR spectroscopy, reversed-phase (RP) HPLC, and MS analyses. A model peptide containing a cysteine residue was transformed into an S-peptide in a TFA solution. The S-peptide was quickly transformed to the original peptide during RP-HPLC analysis even when an eluent that contained 0.1% TFA was used. A peptide that had an N-2-mercapto-4,5-dimethoxybenzyl (Dmmb) group was also transformed into an S-peptide, forming a thioester bond with the thiol of the Dmmb group by TFA treatment. The generated S-peptide was isolated by directly injecting the reaction mixture into a RP-HPLC and was readily converted to the corresponding 2-sulfoethyl thioester via an intermolecular thiol exchange reaction with sodium 2-mercaptoethanesulfonate. The 2-sulfoethyl peptide thioester is widely used as a building block in polypeptide synthesis. The N–S acyl shift reaction observed in peptides with or without an auxiliary group provides a new route for the preparation of peptide thioesters.