title: Unexpected lability of cysteine acetamidomethyl thiol protecting group. Tyrosine ring alkylation and disulfide bond formation upon acidolysis.
abstract: 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.
authors.affiliation: Nycomed Pharma AS, Oslo, Norway.
authors.forename: M
authors.initials: M
authors.lastname: Engebretsen
authors.forename: E
authors.initials: E
authors.lastname: Agner
authors.forename: J
authors.initials: J
authors.lastname: Sandosham
authors.forename: P M
authors.initials: PM
authors.lastname: Fischer
mesh_terms: D000478:Alkylation; D000595:Amino Acid Sequence; D003545:Cysteine; D004220:Disulfides; D004228:Dithionitrobenzoic Acid; D004355:Drug Stability; D009842:Oligopeptides / Q000138:chemical synthesis*; D010456:Peptides, Cyclic / Q000138:chemical synthesis*; D012116:Resins, Plant / Q000737:chemistry*; D013438:Sulfhydryl Compounds; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D004220:Disulfides; D009842:Oligopeptides; D010456:Peptides, Cyclic; D012116:Resins, Plant; D013438:Sulfhydryl Compounds; D014443:Tyrosine; D004228:Dithionitrobenzoic Acid; D003545:Cysteine