Synthetic Amino Acids for Applications in Peptide Ligation–Desulfurization Chemistry

Native chemical ligation is a powerful tool for the convergent assembly of homogeneous peptide and protein targets from unprotected peptide fragments. The method involves the chemoselective coupling of a peptide thioester with a peptide bearing an N-terminal cysteine (Cys) residue and is mediated by the nucleophilic Cys thiol functionality. A widely adopted extension of the technique for the disconnection of protein targets at alanine (Ala) ligation junctions has been the application of post-ligation desulfurization protocols for the mild removal of the Cys thiol moiety. Recently, attention has turned to the construction of synthetic amino acid building blocks bearing suitably positioned β-, γ-, or δ-thiol ligation auxiliaries with a view to expanding the scope of the ligation–desulfurization manifold. To date, several thiol-derived amino acids have been prepared, greatly increasing the generality and flexibility of chemoselective ligation technologies for the chemical synthesis of diverse protein targets. This review will highlight the current synthetic approaches to these important amino acid building blocks.

Trifluoroethanethiol: an additive for efficient one-pot peptide ligation-desulfurization chemistry

Native chemical ligation followed by desulfurization is a powerful strategy for the assembly of proteins. Here we describe the development of a high-yielding, one-pot ligation-desulfurization protocol that uses trifluoroethanethiol (TFET) as a novel thiol additive. The synthetic utility of this TFET-enabled methodology is demonstrated by the efficient multi-step one-pot syntheses of two tick-derived proteins, chimadanin and madanin-1, without the need for any intermediary purification.

One-pot peptide ligation-desulfurization at glutamate

An efficient methodology for ligation at glutamate (Glu) is described. A γ-thiol-Glu building block was accessed in only three steps from protected glutamic acid and could be incorporated at the N-terminus of peptides. The application of these peptides in one-pot ligation-desulfurization chemistry is demonstrated with a range of peptide thioesters, and the utility of this methodology is highlighted through the synthesis of the osteoporosis peptide drug teriparatide (Forteo).