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.

Cysteine protecting groups: applications in peptide and protein science

“…242 Nin has since been used as an Nterminal Cys protecting group used in Boc SPPS. 141 The Nin group can be converted back to the Cys in the presence of an excess of Cys (pH 7.7, 30 min, 23 1C). Cysteine O-methylester in DMF/DIEA will remove the group while the peptide is still attached to the resin.…”

“…Deprotection can be achieved in this case using excess 3-mercaptopropiosulfonic acid (MPS) at pH 7.7 (which is nearly as effective as Cys-mediated deprotection) or under reducing conditions (10% TFA/H 2 O/Zn dust, 1 h). 141 Protection using the Nin group avoids the formylation side reactions normally found during TFA and HF cleavage when His(Bom) groups are present, without the need for additional scavengers. Additionally, in thioester-containing peptides, the products will cyclise under deprotection conditions, enabling a one-step process.…”

Amino Acid-Protecting Groups

“…376 It has been proposed as a protecting group for N-terminal Cys. It In the Fmoc/ t Bu strategy, Met is used unprotected in most of the cases.…”

Enantioresolution of dl‐penicillamine

“…In particular, b-thiol adds to the Schiff 's base intermediate (Fig. 12), trapping both molecules as a spirothiazolidine (Pool et al, 2004;Prota and Ponsiglione, 1973). Thus penicillamine and other aminothiols do not give the expected color reaction with ninhydrin (Friedman, 2004).…”