Thioester

Abstract: The straightforward C-terminal modification of peptides assembled on a solid support remains a significant challenge in peptide and protein chemistry. In particular, C-terminal thioester peptides are important intermediates for the generation of active esters, amides and hydrazides[1,2] and are an essential component of many synthetic strategies for protein synthesis.[3] Currently, the most effective approach for the synthesis of peptidyl thioesters is the in situ neutralization protocol for Boc solid phase peptide synthesis (Boc-SPPS)[4] using thioester linkers.[2,5] However, many laboratories use Fmoc-SPPS exclusively and such protocols are favored when synthesizing glyco- and phosphopeptides. The thioester linkers used for Boc-SPPS have limited utility for Fmoc-SPPS due to the requirement for repeated Fmoc removal under basic conditions. Considerable effort has been applied to address this challenge[6] including optimized Fmoc deprotection cocktails,[7] thiol labile safety catch linkers,[8] activation of protected peptides in solution,[9] and recently thioesters have been generated using O to S[10] or N to S[11] acyl transfer. Despite these notable advances, the synthesis of thioester peptides by Fmoc-SPPS remains significantly more challenging than the synthesis of the corresponding acid or amide peptide.

Abstract: The native chemical ligation reaction (NCL) involves reacting a C-terminal peptide thioester with an N-terminal cysteinyl peptide to produce a native peptide bond between the two fragments. This reaction has considerably extended the size of polypeptides and proteins that can be produced by total synthesis and has also numerous applications in bioconjugation, polymer synthesis, material science, and micro- and nanotechnology research. The aim of the present review is to provide a thorough mechanistic overview of NCL and extended methods. The most relevant properties of peptide thioesters, Cys peptides, and common solvents, reagents, additives, and catalysts used for these ligations are presented. Mechanisms, selectivity and reactivity are, whenever possible, discussed through the insights of computational and physical chemistry studies. The inherent limitations of NCL are discussed with insights from the mechanistic standpoint. This review also presents a palette of O, S-, N, S-, or N, Se-acyl shift systems as thioester or selenoester surrogates and discusses the special molecular features that govern reactivity in each case. Finally, the various thiol-based auxiliaries and thiol or selenol amino acid surrogates that have been developed so far are discussed with a special focus on the mechanism of long-range N, S-acyl migrations and selective dechalcogenation reactions.