Boc-SPPS: Compatible Linker for the Synthesis of Peptide o-Aminoanilides

Review on the o-Aminoaniline Moiety in Peptide and Protein Chemistry

Peptides and proteins are important functional biomolecules both inside and outside of living organisms. The ability to prepare various types of functionalized peptides and proteins is essential for understanding fundamental biological processes, such as protein folding and post-translational modifications (PTMs), and for developing new therapeutics for many diseases, such as cancers and neurodegenerative diseases. The o-aminoaniline moiety was first proposed for activation to a thioester precursor and used for native chemical ligation to prepare large peptides and proteins. In the past decade, the function of o-aminoaniline has been greatly expanded to facilitate the preparation of homogeneously modified peptide and protein samples, where the modifications can include cyclization, C-terminus diversification, etc. Many o-aminoaniline derivatives have also been developed to overcome the inherent limitations of previous versions. In this review, we attempt to summarize the recent developments of different o-aminoaniline derivatives, focusing on their application to the preparation of functional peptide and protein molecules.

Safety-Catch Linkers for Solid-Phase Peptide Synthesis

Solid-phase peptide synthesis (SPPS) is the preferred strategy for synthesizing most peptides for research purposes and on a multi-kilogram scale. One key to the success of SPPS is the continual evolution and improvement of the original method proposed by Merrifield. Over the years, this approach has been enhanced with the introduction of new solid supports, protecting groups for amino acids, coupling reagents, and other tools. One of these improvements is the use of the so-called "safety-catch" linkers/resins. The linker is understood as the moiety that links the peptide to the solid support and protects the C-terminal carboxylic group. The "safety-catch" concept relies on linkers that are totally stable under the conditions needed for both α-amino and side-chain deprotection that, at the end of synthesis, can be made labile to one of those conditions by a simple chemical reaction (e.g., an alkylation). This unique characteristic enables the simultaneous use of two primary protecting strategies: tert-butoxycarbonyl (Boc) and fluorenylmethoxycarbonyl (Fmoc). Ultimately, at the end of synthesis, either acids (which are incompatible with Boc) or bases (which are incompatible with Fmoc) can be employed to cleave the peptide from the resin. This review focuses on the most significant "safety-catch" linkers.

A versatile o-aminoanilide linker for native chemical ligation

Chemical protein synthesis (CPS) is a consolidated field founded on the high chemospecificity of amide-forming reactions, most notably the native chemical ligation (NCL), but also on new technologies such as the Ser/Thr ligation of C-terminal salicylaldehyde esters and the α-ketoacid-hydroxylamine (KAHA) condensation. NCL was conceptually devised for the ligation of peptides having a C-terminal thioester and an N-terminal cysteine. The synthesis of C-terminal peptide thioesters has attracted a lot of interest, resulting in the invention of a wide diversity of different methods for their preparation. The N-acylurea (Nbz) approach relies on the use of the 3,4-diaminobenzoic (Dbz-COOH) and the 3-amino-(4-methylamino)benzoic (MeDbz-COOH) acids; the latter disclosed to eliminate the formation of branching peptides. Dbz-COOH has been also used for the development of the benzotriazole (Bt)-mediated NCL, in which the peptide-Dbz-CONH2 precursor is oxidized to a highly acylating peptide-Bt-CONH2 species. Here, we have brought together the Nbz and Bt approaches in a versatile linker, the 1,2-diaminobenzene (Dbz). The Dbz combines the robustness of MeDbz-COOH and the flexibility of Dbz-COOH: it can be converted into the Nbz or Bt C-terminal peptides. Both are ligated in high yields, and the reaction intermediates can be conveniently characterized. Our results show that the Bt precursors have faster NCL kinetics that is reflected by a rapid transthioesterification (<5 min). Taking advantage of this major acylating capacity, peptide-Bt can be transselenoesterified in the presence of selenols to afford peptide selenoesters which hold enormous potential in NCL.

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

Chemical Synthesis of Phosphorylated Insulin-like Growth Factor Binding Protein 2

Chemical protein synthesis is a powerful avenue for accessing homogeneously modified proteins. While a significant number of small modified proteins bearing native post-translational modifications and non-natural modifications have been generated to date, access to larger targets has proved challenging. Herein, we describe the use of two ligation manifolds, namely, diselenide-selenoester ligation and native chemical ligation, to assemble a 31.5 kDa phosphorylated insulin-like growth factor binding protein (IGFBP-2) that comprises 290 amino acid residues, a phosphoserine post-translational modification, and nine disulfide bonds.

Recent advances in the synthesis of C-terminally modified peptides

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

Benzimidazolinone-Free Peptide o-Aminoanilides for Chemical Protein Synthesis

The thioester surrogate 3,4-diaminobenzoic acid (Dbz) facilitates the efficient synthesis of peptide thioesters by Fmoc chemistry solid phase peptide synthesis and the optional attachment of a solubility tag at the C-terminus. The protection of the partially deactivated ortho-amine of Dbz is necessary to obtain contamination-free peptide synthesis. The reported carbamate protecting groups promote a serious side reaction, benzimidazolinone formation. Herein we introduce the Boc-protected Dbz that prevents the benzimidazolinone formation, leading to clean peptide o-aminoanilides suitable for the total chemical synthesis of proteins.

N,N-Dimethylaminoxy Carbonyl, a Polar Protecting Group for Efficient Peptide Synthesis

Peptide coupling with minimal protection is one of the desired methods for the synthesis of peptides and proteins. To achieve regioselective amide bond formation, side chain protection is often essential; however, protecting groups potentially diminish peptide solubility and render the polar polyamide chain amphipathic due to their apolar nature. In this manuscript, we describe a new protecting group, N,N-dimethylaminoxy carbonyl (Dmaoc), and its use in peptide coupling reactions. The Dmaoc group has a relatively polar character compared to the Boc group, which is a conventional protecting group for the N^ε-amine of Lys residues. This polar protecting group is removable by reduction in the buffer containing (±)-dithiothreitol (DTT). Furthermore, the Dmaoc group proved compatible with peptide ligation strategies featuring the activation of N-acyl diaminobenzamides (Dbz) with sodium nitrate to generate the respective benzotriazole leaving group. The Dmaoc/Dbz strategy described in this manuscript provides a new method for the chemical synthesis of peptides.

Expedient on-resin modification of a peptide C-terminus through a benzotriazole linker

Peptides with various C-terminal functionalization, including peptides and dendrimers, were prepared via SPPS and an efficient on-resin modification.

A convenient and efficient chemical toolbox was developed for the on-resin C-terminal functionalization of various peptides. By transforming resin-bound 3,4-diaminobenzoic acid species with isoamyl nitrite, the resulting resin-bound benzotriazole entity can be efficiently displaced by nucleophiles during cleavage of the peptide–resin connection in a short reaction time. The resin cleavage step allowed for the use of various nucleophiles including water, EtOH, amines, thiol, and G5 poly(amidoamino) dendrimers with yields ranging from 66% to 82% within 5 h. This method was successfully applied to prepare the elastin sequence (VPGVG)₄ through on-resin ligation in 77% yield in one day and a head-to-tail cyclic peptide, sunflower trypsin inhibitor-1, in 42% yield.