Primary ammonium/tertiary amine-mediated controlled ring opening polymerisation of amino acid N-carboxyanhydrides

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.

pH/redox-responsive core cross-linked based prodrug micelle for enhancing micellar stability and controlling delivery of chemo drugs: An effective combination drug delivery platform for cancer therapy

Core-crosslinking of micelles (CCMs) appears to be a favorable strategy to enhance micellar stability and sustained release of the loaded drug. In this study, the DOX-conjugated pH-sensitive polymeric prodrug Methoxy Poly (ethylene oxide)-b-Poly (Aspartate-Hydrazide) (mPEG-P [Asp-(Hyd-DOX)] was created using ring-opening polymerization. To further enhance the micellar system, 3,3'-diselanediyldipropanoic acid (DSeDPA) was applied to link the hydrophobic segment via click reaction to form pH/redox-responsive CCMs. Dual anti-cancer drugs, DOX as a pro-drug and SN-38 as a targeting drug, were used to enhance inhibition. DLS confirmed that the non-cross-linked micelle (NCMs) showed a higher (96.43 nm) particle size compared to the CCMs (72.63 nm). Due to micellar shrinkage after crosslinking, CCMs displayed SN-38 drug loading (7.32 %) and encapsulation efficiency (86.23 %). The mPEG-P(Asp-Hyd) copolymer's in vitro cytotoxicity on HeLa and HaCaT cell lines found that 84.52 % of the cells are alive, and zebrafish (Danio rerio) embryos and larvae are highly biocompatible. The DOX/SN-38@CCMs had a sustained discharge profile in vitro, unlike the DOX/SN-38@NCMs. In DOX/SN-38@CCMs, HeLa cells were inhibited 50.90 % more than HaCaT (14.25 %) at the maximum drug dose (10 μg/mL). The CCMs successfully targeted and supplied DOX/SN-38 in HeLa cells rather than HaCaT cells, based on cellular uptake of 2D cell culture. CCMs, unlike NCMs, inhibit the growth of spheroids for extended periods of time due to the prolonged release of the loaded drug. Overall, CCMs are good-looking for use as regulated delivery of DOX/SN-38 in cancer cells because of all of these appealing characteristics.

Water-Assisted and Protein-Initiated Fast and Controlled Ring-Opening Polymerization of Proline N-Carboxyanhydride

The production of polypeptides via the ring-opening polymerization (ROP) of N-carboxyanhydride (NCA) is usually conducted under stringent anhydrous conditions. The ROP of proline NCA (ProNCA) for the synthesis of poly-_L-proline (PLP) is particularly challenging due to the premature product precipitation as polyproline type I helices, leading to slow reactions for up to one week, poor control of the molar mass and laborious workup. Here, we report the unexpected water-assisted controlled ROP of ProNCA, which affords well-defined PLP as polyproline II helices in 2–5 minutes and almost-quantitative yields. Experimental and theoretical studies together suggest the as-yet-unreported role of water in facilitating proton shift, which significantly lowers the energy barrier of the chain propagation. The scope of initiators can be expanded from hydrophobic amines to encompass hydrophilic amines and thiol-bearing nucleophiles, including complex biomacromolecules such as proteins. Protein-mediated ROP of ProNCA conveniently affords various protein-PLP conjugates via a grafting-from approach. PLP modification not only preserves the biological activities of the native proteins, but also enhances their resistance to extreme conditions. Moreover, PLP modification extends the elimination half-life of asparaginase (ASNase) 18-fold and mitigates the immunogenicity of wt ASNase >250-fold (ASNase is a first-line anticancer drug for lymphoma treatment). This work provides a simple solution to a long-standing problem in PLP synthesis, and offers valuable guidance for the development of water-resistant ROP of other proline-like NCAs. The facile access to PLP can greatly boost the application potential of PLP-based functional materials for engineering industry enzymes and therapeutic proteins.

Organobase 1,1,3,3-tetramethyl guanidine catalyzed rapid ring-opening polymerization of α-amino acid N-carboxyanhydrides adaptive to amine, alcohol and carboxyl acid initiators

For amine, hydroxyl and carboxyl terminated initiators, the organobase 1,1,3,3-tetramethylguanidine (TMG) catalyzes the rapid polymerization to afford polypeptides with controllable molecular weights and dispersities.

An Inspection into Multifarious Ways to Synthesize Poly(Amino Acid)s

Poly(α-amino acid)s (PAAs) attract growing attention due to their essential role in the application as biomaterials. To synthesize PAAs with desired structures and properties, scientists have developed various synthetic techniques with respective advantages. Here, different approaches to preparing PAAs are inspected. Basic features and recent progresses of these methods are summarized, including polymerizations of amino acid N-carboxyanhydrides (NCAs), amino acid N-thiocarboxyanhydrides (NTAs), and N-phenoxycarbonyl amino acids (NPCs), as well as other synthetic routes. NCA is the most classical monomer to prepare PAAs with high molecular weights (MWs). NTA polymerizations are promising alternative pathways to produce PAAs, which can tolerate nucleophiles including alcohols, mercaptans, carboxyl acids, and water. By various techniques including choosing appropriate solvents or using organic acids as promoters, NTAs polymerize to produce polypeptoids and polypeptides with narrow dispersities and designed MWs up to 55.0 and 57.0 kg mol^-1 , respectively. NPC polymerizations are phosgene-free ways to synthesize polypeptides and polypeptoids. For the future prospects, detail investigations into polymerization mechanisms of NTA and NPC are expected. The synthesis of PAAs with designed topologies and assembly structures is another intriguing topic. The advantages and unsettled problems in various synthetic ways are discussed for readers to choose appropriate approaches for PAAs.

Poly(α-l-lysine)-based nanomaterials for versatile biomedical applications: Current advances and perspectives

Poly(α-l-lysine) (PLL) is a class of water-soluble, cationic biopolymer composed of α-l-lysine structural units. The previous decade witnessed tremendous progress in the synthesis and biomedical applications of PLL and its composites. PLL-based polymers and copolymers, till date, have been extensively explored in the contexts such as antibacterial agents, gene/drug/protein delivery systems, bio-sensing, bio-imaging, and tissue engineering. This review aims to summarize the recent advances in PLL-based nanomaterials in these biomedical fields over the last decade. The review first describes the synthesis of PLL and its derivatives, followed by the main text of their recent biomedical applications and translational studies. Finally, the challenges and perspectives of PLL-based nanomaterials in biomedical fields are addressed.

α-Amino acid N-carboxyanhydride (NCA)-derived synthetic polypeptides for nucleic acids delivery

In recent years, gene therapy has come into the spotlight for the prevention and treatment of a wide range of diseases. Polypeptides have been widely used in mediating nucleic acid delivery, due to their versatilities in chemical structures, desired biodegradability, and low cytotoxicity. Chemistry plays an essential role in the development of innovative polypeptides to address the challenges of producing efficient and safe gene vectors. In this Review, we mainly focused on the latest chemical advances in the design and preparation of polypeptide-based nucleic acid delivery vehicles. We first discussed the synthetic approach of polypeptides via ring-opening polymerization (ROP) of N-carboxyanhydrides (NCAs), and introduced the various types of polypeptide-based gene delivery systems. The extracellular and intracellular barriers against nucleic acid delivery were then outlined, followed by detailed review on the recent advances in polypeptide-based delivery systems that can overcome these barriers to enable in vitro and in vivo gene transfection. Finally, we concluded this review with perspectives in this field.

Biological applications of water-soluble polypeptides with ordered secondary structures

Water-soluble polypeptides are a class of synthetic polymers with peptide bond frameworks imitating natural proteins and have broad prospects in biological applications. The regulation and dynamic transition of the secondary structures of water-soluble polypeptides have a great impact on their physio-chemical properties and biological functions. In this review article, we briefly introduce the current strategies to synthesize polypeptides and modulate their secondary structures. We then discuss the factors affecting the conformational stability/transition of polypeptides and the potential impact of side-chain functionalization on the ordered secondary structures, such as α-helix and β-sheet. We then summarize the biological applications of water-soluble polypeptides such as cell penetration, gene delivery, and antimicrobial treatment, highlighting the important roles of ordered secondary structures therein.

One-Shot Synthesis of Peptide Amphiphiles with Applications in Directed Graphenic Assembly

High molecular weight, synthetic block copolypeptides that self-assemble are in high demand for biomedical applications. The current standard method for synthesis of block copolypeptides is the controlled ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydride (NCA) monomers, where block architectures can be created by sequential NCA monomer addition. Recently, researchers have focused on developing reaction conditions and initiation systems that make NCA ROP more convenient, particularly for interdisciplinary labs without designated polypeptide facilities. In an effort to further simplify and increase the convenience of polypeptide synthesis, we developed a one-shot copolymerization strategy that allows access to block copolypeptides by capitalizing on the inherently faster reactivity of NCA monomers, compared to NTA (N-thiocarboxyanhydride) monomers. For the first time, we combine an NCA and NTA monomer in one reaction to kinetically promote block copolypeptide formation, providing a convenient alternative to sequential monomer addition. The controlled nature of this copolymerization technique is supported by a molecular weight that is modulated by the concentration of the initiator and low dispersities. We used this one-shot copolymerization to synthesize p(lysine)-b-p(leucine), a known peptide amphiphile (PA). Our one-shot PAs are antimicrobial and can spontaneously form ordered, micron-scale assemblies. Covalent conjugation of one-shot PAs to a graphenic backbone results in a functional graphenic material (FGM) with a self-assembled morphology, paving the way for creation of sophisticated FGM scaffolds with polypeptide-templated, hierarchical order. Overall, we demonstrate that this novel, one-shot copolymerization strategy produces functional copolypeptides with macroscopic sequence control.

Ring opening polymerization of α-amino acids: advances in synthesis, architecture and applications of polypeptides and their hybrids

This review provides a comprehensive overview of the latest advances in the synthesis, architectural design and biomedical applications of polypeptides and their hybrids.

Organobase triggered controlled supramolecular ring opening polymerization and 2D assembly

A carboxylic acid appended naphthalene-diimide (NDI) derivative spontaneously aggregates in decane to generate a kinetically controlled product with irregular fibrillar morphology. By fine-tuning the sample preparation conditions, the carboxylic acid group can be trapped by intra-molecular H-bonds with the adjacent imide carbonyl, which retards the spontaneous aggregation. In the presence of a catalytic amount of a non-nucleophilic organic base (DBU or DMAP), the meta-stable monomer exhibits supramolecular polymerization through a thermodynamically controlled pathway involving simultaneous H-bonding and π-stacking and generates ultra-thin 2D nano-sheets. DMAP/DBU helps in ring-opening of the intra-molecularly H-bonded monomer and in situ breeds the free acid, which, beyond a critical concentration, initiates controlled supramolecular ring opening polymerization (SROP) via the chain-growth mechanism. The 2D polymer acts as a macro-initiator for subsequent two cycles of SROP and produces laterally extended ultra-thin nano-sheets.

Lithium hexamethyldisilazide initiated superfast ring opening polymerization of alpha-amino acid N-carboxyanhydrides

Polypeptides have broad applications and can be prepared via ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs). Conventional initiators, such as primary amines, give slow NCA polymerization, which requires multiple days to reach completion and can result in substantial side reactions, especially for very reactive NCAs. Moreover, current NCA polymerizations are very sensitive to moisture and must typically be conducted in a glove box. Here we show that lithium hexamethyldisilazide (LiHMDS) initiates an extremely rapid NCA polymerization process that is completed within minutes or hours and can be conducted in an open vessel. Polypeptides with variable chain length (DP = 20–1294) and narrow molecular weight distribution (Mw/Mn = 1.08–1.28) were readily prepared with this approach. Mechanistic studies support an anionic ring opening polymerization mechanism. This living NCA polymerization method allowed rapid synthesis of polypeptide libraries for high-throughput functional screening.

Ring-opening polymerizations of α-amino acid N-carboxyanhydrides to form polypeptides are usually sensitive to moisture, slow and can undergo side reactions. Here the authors use lithium hexamethyldisilazide to initiate α-amino acid N-carboxyanhydride polymerizations that is very fast and can be conducted in an open vessel.

Brønsted base mediated one-pot synthesis of catechol-ended amphiphilic polysarcosine-b-poly(N-butyl glycine) diblock copolypeptoids

Catechol moiety offers a versatile platform in the preparation of functionalized polymers, but it is not usually compatible with catalysis in polymerizations. To address these challenges, we suggest employment of one Brønsted base in masking the activity of catechol moiety and to modulate the polymerization. Based on this strategy, the ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides (Sar-NCA) was carried out using dopamine hydrochloride as an initiator and triethylamine as a Brønsted base. PSar with predicted molecular weights (M n,NMR=3.7 kg mol−1) and narrow dispersities (Đ<1.13) was prepared. Catechol initiator was successfully linked to PSar end as confirmed by MALDI-ToF MS. Subsequently, copolymerization of N-butyl glycine N-carboxyanhydrides (Bu-Gly-NCA) from the PSar in one-pot produced catechol end-functionalized amphiphilic polysarcosine-block-poly(N-butyl glycine) diblock copolypeptoids (cat-PSar-b-PGlyBu). Further, cat-PSar-b-PGlyBu enabled the aqueous dispersion of manganese oxide nanoparticles which was attributable to the anchor of the diblock copolymers onto the surface of the nanoparticles. The strategy for catechol masking and polymerization mediating by one Brønsted base offered a new avenue into the synthesis of catechol-ended block copolymers.

Ring-Opening Polymerization of N-Carboxyanhydrides Initiated by a Hydroxyl Group

We report on a method for preparation of well-defined synthetic polypeptides by ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCA) initiated by a hydroxyl group. To overcome the issue of slow initiation by hydroxyl group, an acid catalyst was used in the initiation step to catalyze opening of the NCA ring by the hydroxyl group and to simultaneously suppress further chain propagation by protonation of the formed amine group. In this way, we have separated slow initiation from the fast chain propagation, since such a combination leads to poorly defined products, and instead performed them in a successive manner. Only after completion of the initiation, the propagation was started by the addition of a base to deprotonate the ammonium group. This method was successfully applied for the synthesis of homopolypeptides by using alcohol as an initiator as well as polypeptide-based block copolymers by using poly(ethylene glycol) or poly(styrene) macroinitiator terminated with the hydroxyl group. This approach not only expands the pool of possible initiators, but also significantly facilities the preparation of polypeptide-based hybrid polymers.

Methacrylate-ended polypeptides and polypeptoids for antimicrobial and antifouling coatings

Methacrylate-terminated polypept(o)ides were directly synthesized via NCA-ROP, and then surface-grafted to form a polymer brush coating with infection-resistant efficacy.

Targeted delivery of a guanidine-pendant Pt(iv)-backboned poly-prodrug by an anisamide-functionalized polypeptide

A guanidine-pendant Pt(iv)-backboned prodrug-like polymer was synthesized and formulated with an anisamide-functionalized polypeptide for targeted delivery and enhanced cellular uptake.

Investigation of the triazolinedione (TAD) reaction with tryptophan as a direct route to copolypeptide conjugation and cross-linking

The TAD reaction with tryptophan permits the modification of polypeptides omitting protection/deprotection routes or the use on non-natural amino acids.

Synthesis of polysarcosine from air and moisture stable N-phenoxycarbonyl-N-methylglycine assisted by tertiary amine base

Well-defined polysarcosines are synthesized from a stable urethane derivative ofN-methylglycine, avoiding the direct use of NCA.