An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Designer C-terminal peptide amides are accessed in an efficient and epimerization-free approach by pairing an electrochemical oxidative decarboxylation with a tandem hydrolysis/reduction pathway. Resembling Nature's dual enzymatic approach to bioactive primary α-amides, this method delivers secondary and tertiary amides bearing high-value functional motifs, including isotope labels and handles for bioconjugation. The protocol leverages the inherent reactivity of C-terminal carboxylates, is compatible with the vast majority of proteinogenic functional groups, and proceeds in the absence of epimerization, thus addressing major limitations associated with conventional coupling-based approaches. The utility of the method is exemplified through the synthesis of natural product acidiphilamide A via a key diastereoselective reduction, as well as bioactive peptides and associated analogues, including an anti-HIV lead peptide and blockbuster cancer therapeutic leuprolide.This work was supported by the Australian Research Council (DE180100092; fellowship to L.R.M.)

Single addition of an allyl amine monomer enables access to end-functionalized RAFT polymers via native chemical ligation

A novel method for the introduction of a single protected amine-functional monomer at the chain end of RAFT polymers has been developed. This monomer addition, in concert with native chemical ligation, facilitated the development of a simple and versatile method for the end-functionalisation of polymers with peptides

Peptide Ligation–Desulfurization Chemistry at Arginine

The utility of a new β‐thiol arginine building block in ligation–desulfurization chemistry has been demonstrated through reactions and kinetic studies with a range of peptide thioesters. Application of the method is highlighted by a one‐pot, kinetically controlled, rapid ligation to generate a 7 kDa MUC1 glycopeptide

Rapid Additive-Free Selenocystine–Selenoester Peptide Ligation

We describe an unprecedented reaction between peptide selenoesters and peptide dimers bearing N-terminal selenocystine that proceeds in aqueous buffer to afford native amide bonds without the use of additives. The selenocystine-selenoester ligations are complete in minutes, even at sterically hindered junctions, and can be used in concert with one-pot deselenization chemistry. Various pathways for the transformation are proposed and probed through a combination of experimental and computational studies. Our new reaction manifold is also showcased in the total synthesis of two proteins

Structurally Diverse Acyl Bicyclobutanes: Valuable Strained Electrophiles

Bicyclo[1.1.0]butanes (BCBs) are highly strained carbocycles that have emerged as versatile synthetic tools, particularly for the construction of functionalized small molecules. This work reports two efficient pathways for the rapid preparation of over 20 structurally diverse BCB ketones, encompassing simple alkyl and aryl derivatives, as well as unprecedented amino acid, dipeptide, bioisostere, and bifunctional linchpin reagents currently inaccessible using literature methods. Analogues are readily forged in two steps and in high yields from simple carboxylic acids or through unsymmetrical ketone synthesis beginning with a convenient carbonyl dication equivalent. The utility of this novel toolbox of strained electrophiles for the selective modification of proteinogenic nucleophiles is highlighted.This work was supported by the Australian Research Council (DE180100092; fellowship to L.R.M.

Site-Selective Solid-Phase Synthesis of a CCR5 Sulfopeptide Library To Interrogate HIV Binding and Entry

Tyrosine (Tyr) sulfation is a common post-translational modification that is implicated in a variety of important biological processes, including the fusion and entry of human immunodeficiency virus type-1 (HIV-1). A number of sulfated Tyr (sTyr) residues on the N-terminus of the CCR5 chemokine receptor are involved in a crucial binding interaction with the gp120 HIV-1 envelope glycoprotein. Despite the established importance of these sTyr residues, the exact structural and functional role of this post-translational modification in HIV-1 infection is not fully understood. Detailed biological studies are hindered in part by the difficulty in accessing homogeneous sulfopeptides and sulfoproteins through biological expression and established synthetic techniques. Herein we describe an efficient approach to the synthesis of sulfopeptides bearing discrete sulfation patterns through the divergent, site-selective incorporation of sTyr residues on solid support. By employing three orthogonally protected Tyr building blocks and a solid-phase sulfation protocol, we demonstrate the synthesis of a library of target N-terminal CCR5(2-22) sulfoforms bearing discrete and differential sulfation at Tyr10, Tyr14, and Tyr15, from a single resin-bound intermediate. We demonstrate the importance of distinct sites of Tyr sulfation in binding gp120 through a competitive binding assay between the synthetic CCR5 sulfopeptides and an anti-gp120 monoclonal antibody. These studies revealed a critical role of sulfation at Tyr14 for binding and a possible additional role for sulfation at Tyr10. N-terminal CCR5 variants bearing a sTyr residue at position 14 were also found to complement viral entry into cells expressing an N-terminally truncated CCR5 receptor

Peptide modification and cyclization via transition-metal catalysis

Transition-metal catalysis has unlocked new paradigms for the late-stage modification and cyclization of peptides by harnessing the innate reactivity of proteinogenic amino acids. The field is rapidly evolving, with recent advances encompassing three fundamental areas - heteroatom couplings, decarboxylative cross-couplings, and C-H functionalizations - which have markedly extended the scope of conventional peptide modification and bioconjugation strategies. The advances outlined herein facilitate access to high-value peptide targets with promising applications in materials science and drug discovery

Development and utility of novel peptide ligation methodologies

The chemical synthesis of homogeneous peptides and proteins is crucial for application in detailed biological studies and the development of novel therapeutics. This thesis describes important contributions to synthetic peptide chemistry, highlighting a number of advances in chemical ligation methodologies, with the aim of facilitating the efficient construction of biologically relevant peptide and protein targets. This work specifically focuses on expanding the scope of native chemical ligation (NCL), the most widely used ligation technique for the construction of peptides, which involves the reaction of a C terminal peptide thioester with a peptide bearing an N-terminal cysteine (Cys) residue to afford new amide bonds. The low abundance of Cys in naturally occurring proteins (1.9%), however, substantially limits the number of accessible targets using this methodology. The thesis will highlight efforts to expand the scope of NCL to include non-Cys ligation sites. Chapters 2 and 3 describe the synthesis of unnatural amino acids bearing β-thiol auxiliaries for use as Cys surrogates in peptide ligation chemistry, including in the construction of a glycopeptide fragment of the human epithelial protein, mucin 1, via an iterative ligation strategy. Chapter 4 highlights advances in selenocysteine (Sec)-mediated ligation, through the application of a β-selenol phenylalanine (Phe) derivative in ligation chemistry, together with the post-ligation, chemoselective removal of the selenol auxiliary in the presence of unprotected thiols. Chapter 5 discusses a novel approach to tryptophan (Trp) ligation junctions that utilizes a chemoselective sulfenylation reaction to install a thiol ligation auxiliary onto the 2 position of the Trp indole ring. By facilitating access to a number of non-Cys ligation junctions for the construction of diverse peptide and protein targets, the work described in this thesis represents an important contribution to synthetic peptide chemistry