Synthesis of poly-α/β-peptides with tunable sequence via the copolymerization on N-carboxyanhydride and N-thiocarboxyanhydride

Recent Advances and Future Developments in the Preparation of Polypeptides via N-Carboxyanhydride (NCA) Ring-Opening Polymerization

Polypeptides have the same or similar backbone structures as proteins and peptides, rendering them as suitable and important biomaterials. Amino acid N-carboxyanhydrides (NCA) ring-opening polymerization has been the most efficient strategy for polypeptide preparation, with continuous advance in the design of initiators, catalysts and reaction conditions. This Perspective first summarizes the recent progress of NCA synthesis and purification. Subsequently, we focus on various initiators for NCA polymerization, catalysts for accelerating polymerization or enhancing the controllability of polymerization, and recent advances in the reaction approach of NCA polymerization. Finally, we discuss future research directions and open challenges.

Toward Synthetic Intrinsically Disordered Polypeptides (IDPs): Controlled Incorporation of Glycine in the Ring-Opening Polymerization of N-Carboxyanhydrides

Intrinsically disordered proteins (IDPs) do not have a well-defined folded structure but instead behave as extended polymer chains in solution. Many IDPs are rich in glycine residues, which create steric barriers to secondary structuring and protein folding. Inspired by this feature, we have studied how the introduction of glycine residues influences the secondary structure of a model polypeptide, poly(l-glutamic acid), a helical polymer. For this purpose, we carried out ring-opening copolymerization with γ-benzyl-l-glutamate and glycine N-carboxyanhydride (NCA) monomers. We aimed to control the glycine distribution within PBLG by adjusting the reactivity ratios of the two NCAs using different reaction conditions (temperature, solvent). The relationship between those conditions, the monomer distributions, and the secondary structure enabled the design of intrinsically disordered polypeptides when a highly gradient microstructure was achieved in DMSO.

Combating drug-resistant bacteria with sulfonium cationic poly(methionine)

antibiotic resistance and drug-resistant bacterial infections pose significant threats to public health. Antimicrobial peptides (AMPs) are a promising candidate for related-infection therapy, but their clinical application is limited by their high synthesis cost and susceptibility to protease degradation. To address these issues, cationic poly(α-amino acid)s based on lysine have been developed as synthetic mimics of AMPs. In this study, we introduce a new class of cationic AMP synthetic mimics based on functional poly(methionine)s. We synthesized a series of sulfonium cationic poly(d,l-methionine)s with varying chain lengths via a convenient polymerization on α-amino acid thiocarboxyanhydride (α-NTA) using tert-butyl-benzylamine as the initiator, followed by alkylation with iodomethane. Our optimal methionine polymer demonstrated potent and broad-spectrum antibacterial activity against antibiotic-resistant bacteria, as well as excellent biocompatibility with mammalian cells and rapid bactericidal performance. Our findings suggest that sulfonium poly(methionine)s have the potential to address the challenge of drug-resistant bacterial infections.

Convenient and Controllable Synthesis of Poly(2-oxazoline)-Conjugated Doxorubicin for Regulating Anti-Tumor Selectivity

Polyethylene glycol (PEG)-doxorubicin (DOX) conjugation is an important strategy to improve toxicity and enhance clinically therapeutic efficacy. However, with the frequent use of PEG-modified drugs, the accumulation of anti-PEG antibodies has become a tough issue, which limits the application of PEG-drug conjugation. As an alternative solution, poly(2-oxazoline) (POX)-DOX conjugation has shown great potential in the anti-tumor field, but the reported conjugation process of POX with DOX has drawbacks such as complex synthetic steps and purification. Herein, we propose a convenient and controllable strategy for the synthesis of POX-DOX conjugation with different chain lengths and narrow dispersity by N-boc-2-bromoacetohydrazide-initiated 2-ethyl-oxazoline polymerization and the subsequent deprotection of the N-Boc group and direct reaction with DOX. The DOX-PEtOx conjugates were firstly purified, and the successful conjugations were confirmed through various characterization methods. The synthetic DOX-PEtOx_n conjugates reduce the toxicity of DOX and increase the selectivity to tumor cells, reflecting the promising application of this POX-DOX conjugation strategy in drug modification and development.

Advance in the Polymerization Strategy for the Synthesis of β-Peptides and β-Peptoids

Peptide mimics, possessing excellent biocompatibility and protease stability, have attracted broad attention and research in the biomedical field. β-Peptides and β-peptoids, as two types of vital peptide mimics, have demonstrated great potential in the field of foldamers, antimicrobials and protein binding, etc. Currently, the main synthetic strategies for β-peptides and β-peptoids include solid-phase synthesis and polymerization. Among them, polymerization in one-pot can minimize the repeated separation and purification used in solid-phase synthesis, and has the advantages of high efficiency and low cost, and can synthesize β-peptides and β-peptoids with high molecular weight. This review summarizes the polymerization methods for β-peptides and β-peptoids. Moreover, future developments of the polymerization method for the synthesis of β-peptides and β-peptoids will be discussed.

Beyond nylon 6: polyamides via ring opening polymerization of designer lactam monomers for biomedical applications

Ring opening polymerization (ROP) of lactams is a highly efficient and versatile method to synthesize polyamides. Within the last ten years, significant advances in polymerization methodology and monomer diversity are ushering in a new era of polyamide chemistry. We begin with a discussion of polymerization techniques including the most widely used anionic ring opening polymerization (AROP), and less prevalent cationic ROP and enzyme-catalyzed ROP. Next, we describe new monomers being explored for ROP with increased functionality and stereochemistry. We emphasize the relationships between composition, structure, and properties, and how chemists can control composition and structure to dictate a desired property or performance. Finally, we discuss biomedical applications of the synthesized polyamides, specifically as biomaterials and pharmaceuticals, with examples to include as antimicrobial agents, cell adhesion substrates, and drug delivery scaffolds.

Advance in Hybrid Peptides Synthesis

Hybrid peptides with heterogeneous backbone are a class of peptide mimics with adjustable proteolytic stability obtained from incorporating unnatural amino acid residues into peptide backbone. α/β-peptides and peptide/peptoid hybrids are two types of hybrid peptides that are widely studied for diverse applications, and several synthetic methods have been developed. In this mini review, the advance in hybrid peptide synthesis is summarized, including solution-phase method, solid-phase method, and novel polymerization method. Conventional solution-phase method and solid-phase method generally result in oligomers with defined sequences, while polymerization methods have advantages in preparing peptide hybrid polymers with high molecular weight with simple operation and low cost. In addition, the future development of polymerization method to realize the control of the peptide hybrid polymer sequence is discussed.

Host defense peptide mimicking cyclic peptoid polymers exerting strong activity against drug-resistant bacteria

Extensive use of antibiotics accelerates the emergence of drug-resistant bacteria and related infections. Host defense peptides (HDPs) have been studied as promising and potential therapeutic candidates. However, their clinical applications of HDPs are limited due to their high cost of synthesis and low stability upon proteolysis. Therefore, HDP mimics have become a new approach to address the challenge of bacterial resistance. In this work, we design the amphiphilic peptoid polymers by mimicking the positively charged and hydrophobic structures of HDPs and synthesize a series of cyclic peptoid polymers efficiently via the polymerization on α-amino acid N-substituted glycine N-carboxyanhydrides (α-NNCAs) using 1,8-diazabicycloundec-7-ene (DBU) as the initiator. The optimal cyclic peptoid polymer, poly(Naeg_0.7Npfbg_0.3)₂₀, displays strong antibacterial activities against drug-resistant bacteria, but low hemolysis and cytotoxicity. In addition, the mode-of-action study indicates that the antibacterial mechanism is associated with bacterial membrane interaction. Our study implies that HDP mimicking cyclic peptoid polymers have potential application in treating drug-resistant bacterial infections.

Statistic Copolymers Working as Growth Factor-Binding Mimics of Fibronectin

Growth factors (GFs) play important roles in biological system and are widely used in tissue regeneration. However, their application is greatly hindered by short in vivo lifetime of GFs. GFs are bound to fibronectin dynamically in the extracellular matrix, which inspired the authors to mimic the GF binding domain of fibronectin and design GF-binding amphiphilic copolymers bearing positive charges. The optimal amino acid polymer can bind to a variety of representative GFs, such as bone morphogenetic protein-2 (BMP-2) and TGF-β1 from the transforming growth factor-β superfamily, PDGF-AA and PDGF-BB from the platelet-derived growth factor family, FGF-10 and FGF-21 from the fibroblast growth factor family, epidermal growth factor from the EGF family and hepatocyte growth factor from the plasminogen-related growth factor family, with binding affinities up to the nanomolar level. 3D scaffolds immobilized with the optimal copolymer enable sustained release of loaded BMP-2 without burst release and significantly enhances the in vivo function of BMP-2 for bone formation. This strategy opens new avenues in designing GF-binding copolymers as synthetic mimics of fibronectin for diverse applications.

Recent advances in post-polymerization modifications on polypeptides: synthesis and applications

Polypeptides, a kind of very promising biomaterial, have shown a wide range of applications due to their excellent biocompatibility, easy accessibility, and structural variability. To synthesize polypeptides with desired functions, post-polymerization modification (PPM) plays an important role in introducing novel chemical structure on their side-chains. The key of PPM strategy is to develop highly selective and efficient reactions that can couple the additional functional moieties with pre-installed side-chain functionalities on polypeptides. In this minireview, classic PPM reactions and especially their recent progresses are summarized, including different modification approaches for unsaturated alkyl group, oxygen-containing functional group, nitrogen-containing functional group, sulfur-containing functional group and other special functional group on side chains. In addition, this review also highlights the applications of structure-diversified polypeptides generated via PPM strategy in the field of biomaterial.

Secondary Amine Pendant β-Peptide Polymers Displaying Potent Antibacterial Activity and Promising Therapeutic Potential in Treating MRSA-Induced Wound Infections and Keratitis

Interest in developing antibacterial polymers as synthetic mimics of host defense peptides (HPDs) has accelerated in recent years to combat antibiotic-resistant bacterial infections. Positively charged moieties are critical in defining the antibacterial activity and eukaryotic toxicity of HDP mimics. Most examples have utilized primary amines or guanidines as the source of positively charged moieties, inspired by the lysine and arginine residues in HDPs. Here, we explore the impact of amine group variation (primary, secondary, or tertiary amine) on the antibacterial performance of HDP-mimicking β-peptide polymers. Our studies show that a secondary ammonium is superior to either a primary ammonium or a tertiary ammonium as the cationic moiety in antibacterial β-peptide polymers. The optimal polymer, a homopolymer bearing secondary amino groups, displays potent antibacterial activity and the highest selectivity (low hemolysis and cytotoxicity). The optimal polymer displays potent activity against antibiotic-resistant bacteria and high therapeutic efficacy in treating MRSA-induced wound infections and keratitis as well as low acute dermal toxicity and low corneal epithelial cytotoxicity. This work suggests that secondary amines may be broadly useful in the design of antibacterial polymers.

Organocatalytic Copolymerization of Cyclic Lysine Derivative and ε-Caprolactam toward Antibacterial Nylon-6 Polymers

Functional polymers of nylon-6, particularly those with sustained antibacterial functions, have many practical applications. However, the development of functional ε-caprolactam monomers for the subsequent ring-opening copolymerization (ROCOP) formation of these materials remains a challenge. Here we report a t-BuP₄-mediated ROCOP of dimethyl-protected cyclic lysine with ε-caprolactam, followed by quaternization, affording antibacterial nylon-6 polymers bearing quaternary ammonium functionality with high molecular weight (up to 77.4 kDa). The antibacterial nylon-6 polymers exhibited good physical and mechanical properties and strong antimicrobial activities. At 25 mol % quaternary ammonium group incorporation, the nylon-6 polymer demonstrated complete killing of Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). The results from this study may provide a strategy for the facile preparation of antibacterial nylon-6 polymers to addressing the public health and safety challenges.

Controlled copolymerization of α-NCAs and α-NNTAs for preparing peptide/peptoid hybrid polymers with adjustable proteolysis

The living and controlled copolymerization of α-NCAs and α-NNTAs enables the facile synthesis of peptide/peptoid hybrid polymers with an alternating-like distribution of residues and adjustable proteolysis by varying the proportion of peptoid residues.