title: Ninhydrin as a reversible protecting group of amino-terminal cysteine.
abstract: The proximity of the alpha-amine and beta-thiol of alpha-amino terminal-cysteine (NT-Cys) residues in peptides imparts unique chemical properties that have been exploited for inter- and intra-molecular ligation of unprotected peptides obtained through both synthetic and biological means. A reversible protecting group orthogonal to other protection strategies and reversible under mild conditions would be useful in simplifying the synthesis, cleavage, purification and handling of such NT-Cys peptides. It could also be useful for the sequential ligation of peptides. To this end, we explored tri-one chemistry and found that ninhydrin (indane-1,2,3 trione) reacted readily with cysteine or an NT-Cys-containing peptide on- or off-resin at pH 2-5 to form Ninhydrin-protected Cys (Nin-Cys) as a thiazolidine (Thz). The Thz ring, protecting both the amino and thiol groups in Nin-Cys, completely avoids the formylation and Thz side reactions found during hydrofluoric acid (HF) cleavage when N-pi-benzyloxymethyl histidine groups are present. Nin-Cys is stable during coupling reactions and various cleavage conditions with trifluoroacetic acid or HF, but is deprotected under thiolytic or reducing conditions. These properties enable a facile one-step deprotection and end-to-end-cyclization reaction of Nin-Cys peptides containing C-terminal thioesters.
authors.affiliation: Department of Microbiology/Immunology, Vanderbilt University, Nashville, TN, USA.
authors.forename: C T
authors.initials: CT
authors.lastname: Pool
authors.forename: J G
authors.initials: JG
authors.lastname: Boyd
authors.forename: J P
authors.initials: JP
authors.lastname: Tam
mesh_terms: D003545:Cysteine / Q000737:chemistry*; D015394:Molecular Structure; D009555:Ninhydrin / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D010456:Peptides, Cyclic; D009555:Ninhydrin; D003545:Cysteine