Unexpected lability of cysteine acetamidomethyl thiol protecting group Tyrosine ring alkylation and disulfide bond formation upon acidolysis

Cleavage and deprotection of the peptidyl resin H-Asn-Gly-Gly-Cys(Acm)-Glu(OBu^t)-Gln-Tyr(Bu^t)-Cys(Acm)-Ser(Bu^t)-Asp(OBu^t)-[(p-alkoxy)benzyloxy polystyrene resin] using standard conditions with various trifluoroacetic acid-containing mixtures were found to result in partial removal of ordinarily acid-stable S-Acm groups. Thus, apart from the desired peptide H-Asn-Gly-Gly-Cys(Acm)-Glu-Gln-Tyr-Cys(Acm)-Ser-Asp-OH, a disulfide-cyclic peptide derivative was also isolated. Furthermore, it was found that in another major by-product of the peptide resin cleavage the tyrosine side chain had been alkylated with an Acm group in a position ortho to the phenolic function. The formation of both by-products could be suppressed by carrying out the cleavage/deprotection reaction at higher dilution and by inclusion of scavengers such as phenol. An authentic sample of the disulfide-cyclic peptide was obtained by oxidation of H-Asn-Gly-Gly-Cys-Glu-Gln-Tyr-Cys-Ser-Asp-OH using Ellman's reagent. © Munksgaard 1997.