N-carboxyanhydride Research Articles

Polypeptide-based materials prepared by ring-opening polymerisation of anionic-based α-amino acid N-carboxyanhydrides: A platform for delivery of bioactive-compounds

Polymerisation of α-amino acid N-carboxyanhydrides (NCAs) is one of the most common techniques to prepare synthetic polypeptides. Of special interest are the NCAs derived from α-amino acids, L-aspartic acid and L-glutamic acid, since most investigations have been focused on their use to synthesise multiblock copolypeptides or hybrid synthetic polypeptide-polymers to design excipients suitable for delivery of bioactive compounds. This perspective highlights advantages of using L-aspartic acid and L-glutamic over other natural α-amino acids in that their pendant carboxyl group serves as a reactive handle for coupling a variety of reactive groups, and because the resulting polypeptides have the ability to adopt secondary structures. In addition, recent progress in the ring-opening polymerisation of NCAs will be discussed. Throughout, we provide representative examples that shed light on the NCAs polymerisation process, and we finally share our perspectives concerning the practical use of anionic α-amino acids as building blocks for future investigations.

Preparation of Synthetic Polypeptide–PolyHIPE Hydrogels with Stimuli-Responsive Behavior

In this work, biodegradable macroporous stimuli-responsive polypeptide hydrogels based on l-glutamic acid (Glu) and its copolymers with equimolar amounts of l-phenylalanine (Phe) or l-lysine (Lys) were prepared by deprotection of the corresponding organogels under acidic conditions. The organogels were synthesized by ring-opening polymerization of N-carboxyanhydride (NCA) derivatives of the corresponding α-amino acids in oil-in-oil high-internal phase emulsions (HIPEs) using the di-NCA derivative of l-cystine as a cross-linker. The organogels exhibit the typical interconnected porous polyHIPE morphology, which is completely preserved in the hydrogels after removal of the protecting groups of the Glu and Lys repeating units. The pH-dependent behavior and mechanical properties of the obtained hydrogels were studied in buffer solutions with different pH values. At pH 7.5, P(Glu) and P(Glu-co-Phe) can be compressed to half their original height and both return to their initial state after unloading. By lowering the pH to 5.5, P(Glu) remains soft, while P(Glu-co-Phe) already becomes much stiffer. In contrast, for the P(Glu-co-Lys) hydrogel, high buffer uptake was observed only at high or low pH values, whereas at intermediate pH values, the low buffer uptake and the impaired ability to return to the original height are attributed to the attractive ionic interaction between the oppositely charged side groups. We have shown that by tuning the chemical composition of the polypeptides, the uptake, in vitro enzymatic degradation, and compression behavior of the hydrogels can be modulated.

Heparin-like anticoagulant polypeptides with tunable activity: Synthesis, characterization, anticoagulative properties and clot solubilities in vitro

Due to the uncontrollable anticoagulant activity and limited source, Heparin, which is commonly used in clinical anticoagulation therapies, faces the risk of spontaneous bleeding and thrombocytopenia. Herein, a series of anionic poly(amino acid) s poly (l-Serine-ran-L-Glutamic acid-ran-L-Cysteine-SO3) (PSEC-SO3) were prepared by the controlled Ring Opening Polymerization (ROP) of N-Carboxyanhydrides (NCAs). The anticoagulant activities of PSEC-SO3 can be regulated by simply adjusting the feeding ratio of monomers. In vitro tests show that these polypeptides can effectively prolong the Activated Partical Thromboplastin Time (APTT) and inhibit Factor IIa and Factor Xa, but has no significant effect on Prothrombin Time (PT) and Thrombin Time (TT), which indicates that PSEC-SO3 mainly act on the intrinsic pathway. In summary, the activity-tunable heparin-like polypeptides are expected to have good application prospects in the anticoagulant field.

Synthesis of α-Amino Acid N-Carboxyanhydrides

A simple phosgene- and halogen-free method for synthesizing α-amino acid N-carboxyanhydrides (NCAs) is described. The reaction between Boc-protected α-amino acids and T3P reagent gave the corresponding NCA derivatives in good yield and purity with no detectable epimerization. The process is safe, is easy-to-operate, and does not require any specific installation. It generates nontoxic, easy to remove byproducts. It can apply to the preparation of NCAs for the on-demand on-site production of either little or large quantities.

Nanoengineered polypeptides from tetraphenylethylene-functionalized N-carboxyanhydride: Synthesis, self-assembly and intrinsic aggregation-induced emission

Ring-opening polymerization of N-carboxyanhydrides (NCAs) bearing pendant groups creates functional polypeptides. In this paper, we report the design, synthesis and polymerization of tetraphenylethylene (TPE)-modified NCA, which is used to incorporate aggregation-induced emission (AIE)-active segments into polypeptides. Specifically, we attempted the synthesis of amphiphilic methoxy poly(ethyleneglycol)-block-poly(γ-benzyl-l-glutamate)-block-poly(γ-4-(1,2,2-triphenylvinyl)benzyl-l-glutamate) (PEG-PBLG-PTPELG), which had well-controlled molecular weight and narrow polydispersity. The hydrophilic PEG and hydrophobic PBLG-PTPELG assembled into micelles in aqueous solution with diameter around 70 ​nm, displaying AIE at 480 ​nm. Our work demonstrates that the TPE-modified NCA is applicable for the preparation of the AIE-active polypeptides, integrating the AIE luminogens into the polypeptides which may offer unique opportunities for tuning AIE properties by adjusting the composition and conformation of polypeptides. We preliminarily explored the use of the amphiphilic polypeptides for the formulation of doxorubicin-containing micelles that can potentially be used for real-time monitoring of the nanomedicine distribution.

Bifunctional Fluoroalcohol Catalysts Enabled Sustainable Synthesis of Poly(amino acid)s†

Main observation and conclusionA series of novel bifunctional single‐molecule hydrogen‐bonding organocatalysts based on fluorinated tertiary alcohol were synthesized to mediate the metal‐free polymerization of α‐amino acid N‐carboxyanhydride (NCA) monomer. Through the screening of catalysts, the polymerization catalyzed by the preferred bifunctional fluoroalcohol catalyst proceeded in a fast and controlled manner, affording high molecular weight polypeptide up to 35.2 × 104 Da with relatively narrow molecular weight distribution (Đ < 1.20). A phenomenon of self‐accelerating effect of polymerization induced by α‐helical structure of polypeptides was observed, and a possible polymerization pathway has been discussed.

PEG-polyaminoacid based micelles for controlled release of doxorubicin: Rational design, safety and efficacy study

A library of amphiphilic monomethoxypolyethylene glycol (mPEG) terminating polyaminoacid co-polymers able to self-assemble into colloidal systems was screened for the delivery and controlled release of doxorubicin (Doxo). mPEG-Glu/Leu random co-polymers were generated by Ring Opening Polymerization from 5 kDa mPEG-NH2 macroinitiator using 16:0:1, 8:8:1, 6:10:1, 4:12:1 γ-benzyl glutamic acid carboxy anhydride monomer/leucine N-carboxy anhydride monomer/PEG molar ratios. Glutamic acid was selected for chemical conjugation of Doxo, while leucine units were introduced in the composition of the polyaminoacid block as spacer between adjacent glutamic repeating units to minimize the steric hindrance that could impede the Doxo conjugation and to promote the polymer self-assembly by virtue of the aminoacid hydrophobicity.The benzyl ester protecting the γ-carboxyl group of glutamic acid was quantitatively displaced with hydrazine to yield mPEG5kDa-b-(hydGlum-r-Leun). Doxo was conjugated to the diblock co-polymers through pH-sensitive hydrazone bond. The Doxo derivatized co-polymers obtained with a 16:0:1, 8:8:1, 6:10:1 Glu/Leu/PEG ratios self-assembled into 30–40 nm spherical nanoparticles with neutral zeta-potential and CMC in the range of 4-7 μM. At pH 5.5, mimicking endosome environment, the carriers containing leucine showed a faster Doxo release than at pH 7.4, mimicking the blood conditions. Doxo-loaded colloidal formulations showed a dose dependent cytotoxicity on two cancer cell lines, CT26 murine colorectal carcinoma and 4T1 murine mammary carcinoma with IC50 slightly higher than those of free Doxo. The carrier assembled with the polymer containing 6:10:1 hydGlu/Leu/PEG molar ratio {mPEG5kDa-b-[(Doxo-hydGlu)6-r-Leu10]} was selected for subsequent in vitro and in vivo investigations. Confocal imaging on CT26 cell line showed that intracellular fate of the carrier involves a lysosomal trafficking pathway. The intratumor or intravenous injection to CT26 and 4T1 subcutaneous tumor bearing mice yielded higher antitumor activity compared to free Doxo. Furthermore, mPEG5kDa-b-[(Doxo-hydGlu)6-r-Leu10] displayed a better safety profile when compared to commercially available Caelyx®.

Antioxidant activity of linear and star-shaped polypeptides modified with dopamine and glutathione

The aim of this study was to investigate the effect of topology on the antioxidative activities of antioxidant-grafted polypeptides. The linear and star-shaped poly(L-glutamic acid) (PLG) polypeptides were synthesized by N-carboxyanhydrides (NCAs) ring opening polymerization (ROP) using primary amine and polyols as initiators, followed by deprotection. Graft copolypeptides (PLG-Dopa, PLG-GSH and PLG-Dopa-GSH) were obtained by EDC/NHS coupling chemistry using dopamine (Dopa) and glutathione (GSH). Successful synthesis of these polypeptides was confirmed by 1H NMR, MALDI TOF, GPC, UV–vis and IR spectra analyses. The conformation of polypeptides was investigated by circular dichroism (CD) spectroscopy, demonstrating the grafting of Dopa and GSH onto PLG was accompanied by a conformational change from a random coil to α-helical structure. The results showed that the antioxidant activities of PLG-Dopa and PLG-Dopa-GSH were mainly ascribed to Dopa. It was found that the conjugation of Dopa resulted in the decrease in its antioxidant activity and the antioxidant-grafted polypeptides exhibited comparable antioxidant activity regardless the polypeptide arm number. The conjugation of GSH could prolong polypeptide antioxidant activity by preventing Dopa from oxidation. It was found that both PLG-Dopa and PLG-Dopa-GSH exhibited good hemocompatibility and biocompatibility.

Density Functional Theory Studies on the Synthesis of Poly(α-Amino Acid)s Via the Amine-Mediated Ring Opening Polymerizations of N-Carboxyanhydrides and N-Thiocarboxyanhydrides.

To synthesize well-defined poly (α-amino acid)s (PAAs), ring opening polymerizations (ROP) of cyclic monomers of α-amino acid N-carboxyanhydrides (NCAs) and N-thiocarboxyanhydrides (NTAs) are most widely used. In this mini-review, we summarize the mechanism details of the monomer preparation and ROP. The present study used density functional theory calculations to reveal the mechanisms together with experimental phenomena in the past decades. Detailed discussion includes normal amine mechanism and the selectivity of the initiators bearing various nucleophilic groups.

Accelerated polymerization of N-carboxyanhydrides catalyzed by crown ether

The recent advances in accelerated polymerization of N-carboxyanhydrides (NCAs) enriched the toolbox to prepare well-defined polypeptide materials. Herein we report the use of crown ether (CE) to catalyze the polymerization of NCA initiated by conventional primary amine initiators in solvents with low polarity and low hydrogen-bonding ability. The cyclic structure of the CE played a crucial role in the catalysis, with 18-crown-6 enabling the fastest polymerization kinetics. The fast polymerization kinetics outpaced common side reactions, enabling the preparation of well-defined polypeptides using an α-helical macroinitiator. Experimental results as well as the simulation methods suggested that CE changed the binding geometry between NCA and propagating amino chain-end, which promoted the molecular interactions and lowered the activation energy for ring-opening reactions of NCAs. This work not only provides an efficient strategy to prepare well-defined polypeptides with functionalized C-termini, but also guides the design of catalysts for NCA polymerization.

On‐POM Ring‐Opening Polymerisation of N‐Carboxyanhydrides

AbstractThe ring‐opening polymerisation of α‐amino acid N‐carboxyanhydrides (NCAs) offers a simple and scalable route to polypeptides with predicted and narrow molecular weight distributions. Here we show how polyoxometalates (POMs)—redox‐active molecular metal‐oxide anions—can serve as inorganic scaffold initiators for such NCA polymerisations. This “On‐POM polymerisation” strategy serves as an innovative platform to design hybrid materials with additive or synergistic properties stemming from the inorganic and polypeptide component parts. We have used this synthetic approach to synthesise a library of bactericidal poly(lysine)–POM hybrid derivatives that can be used to prevent biofilm formation. This versatile “On‐POM polymerisation” method provides a flexible synthetic approach for combining inorganic scaffolds with amino acids, and the potential to tailor and improve the specificity and performance of hybrid antimicrobial materials.

On‐POM Ring‐Opening Polymerisation of N‐Carboxyanhydrides

The ring-opening polymerisation of α-amino acid N-carboxyanhydrides (NCAs) offers a simple and scalable route to polypeptides with predicted and narrow molecular weight distributions. Here we show how polyoxometalates (POMs)-redox-active molecular metal-oxide anions-can serve as inorganic scaffold initiators for such NCA polymerisations. This "On-POM polymerisation" strategy serves as an innovative platform to design hybrid materials with additive or synergistic properties stemming from the inorganic and polypeptide component parts. We have used this synthetic approach to synthesise a library of bactericidal poly(lysine)-POM hybrid derivatives that can be used to prevent biofilm formation. This versatile "On-POM polymerisation" method provides a flexible synthetic approach for combining inorganic scaffolds with amino acids, and the potential to tailor and improve the specificity and performance of hybrid antimicrobial materials.

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.

Synthesis and Characterization of the Novel Nε-9-Fluorenylmethoxycarbonyl-l-Lysine N-Carboxy Anhydride. Synthesis of Well-Defined Linear and Branched Polypeptides.

The synthesis of well-defined polypeptides exhibiting complex macromolecular architectures requires the use of monomers that can be orthogonally deprotected, containing primary amines that will be used as the initiator for the Ring Opening Polymerization (ROP) of N-carboxy anhydrides. The synthesis and characterization of the novel monomer Nε-9-Fluorenylmethoxycarbonyl-l-Lysine N-carboxy anhydride (Nε-Fmoc-l-Lysine NCA), as well as the novel linear Poly(Nε-Fmoc-l-Lys)n homopolypeptide and Poly(l-Lysine)78–block–[Poly(l-Lysine)10–graft–Poly(l-Histidine)15] block-graft copolypeptide, are presented. The synthesis of the graft copolypeptide was conducted via ROP of the Nε-Boc-l-Lysine NCA while using n-hexylamine as the initiator, followed by the polymerization of Nε-Fmoc-l-Lysine NCA. The last block was selectively deprotected under basic conditions, and the resulting ε-amines were used as the initiating species for the ROP of Nim-Trityl-l-Histidine NCA. Finally, the Boc- and Trt- groups were deprotected by TFA. High Vacuum Techniques were applied to achieve the conditions that are required for the synthesis of well-defined polypeptides. The molecular characterization indicated that the polypeptides exhibited high degree of molecular and compositional homogeneity. Finally, Dynamic Light Scattering, ζ-potential, and Circular Dichroism measurements were used in order to investigate the ability of the polypeptide to self-assemble in different conditions. This monomer opens avenues for the synthesis of polypeptides with complex macromolecular architectures that can define the aggregation behavior, and, therefore, can lead to the synthesis of “smart” stimuli-responsive nanocarriers for controlled drug delivery applications.

Sterically Stabilized Polyionic Complex Nanogels of Chitosan Lysate and PEG-b-Polyglutamic Acid Copolymer for the Delivery of Irinotecan Active Metabolite (SN-38).

Poly Ionic Complex (PIC) nanogels are promising delivery systems with numerous attractions such as simple, fast, and organic solvent-free particle formation and mild drug loading conditions. Among polyelectrolytes, poly (L-amino acid) copolymers, such as poly (ethylene glycol)-block-poly (L-glutamic acid) copolymers (PEG-b-PGlu) are interesting biocompatible and biodegradable candidates bearing carboxylic acid functional groups. Aiming to solubilize and to preserve short-acting irinotecan active metabolite (SN38), sterically stabilized PIC nanogels were prepared through electrostatic charge neutralization between PEG-b-PGlu and chitosan lysate, a polycationic natural polymer obtained through digestion of chitosan by hydrogen peroxide oxidation and is soluble in a wide range of pH. Synthesis of PEG-b-PGlu was accomplished by N-carboxy anhydride polymerization of γ -benzyl L-glutamic acid, which is initiated by methoxy PEG-NH2 and successive debenzylation reaction. The resulting block copolymer was characterized by FTIR, 1H-NMR, and Size Exclusion Chromatography (SEC). Self-assembling properties of the PIC nanogels were investigated by pyrene assay, Dynamic Light Scattering (DLS), and Transmission Electron Microscopy (TEM), indicating the formation of homogeneous spherical particles with a mean size of 28 nm at the PEGb- PGlu concentrations/LMWC weight ratio of 5:1. Upon direct loading of SN38, the drug solubility enhanced more than 4×103 folds with a mean loading efficiency of 89% and the drug loading of 30%. PIC nanogels exhibited zeta potential of +1 mV, acceptable biocompatibility, and superior cytotoxicity in murine colorectal carcinoma (CT26 cell line) compared to free drug. In addition, the PIC nanogels provided SN38 protection against hydrolytic degradation in physiologic conditions. Conclusively, the well-tuned PIC nanogels are suggested as a potentially biocompatible nanocarrier for SN38 delivery.

Drug Delivery Through Multifunctional Polypeptidic Hydrogels.

Over the last two decades, remarkable progress has been made to the discovery of novel drugs as well as their delivery systems for the treatment of cancer, the major challenge in medicine. Pharmaceutical scientists are trying to shift from traditional to novel drug delivery systems by applying nanotechnology and, in particular, polymeric carriers to medicine. In complex diseases, very sophisticated nanocarriers should be designed to encapsulate a significant quantity of drugs and bypass biological barriers with minimum cargo loss to effectively and directly deliver the encapsulated drug to the desired pathological site. One of the most promising classes of polymeric materials for drug delivery applications is polypeptides, combining the properties of the traditional polymers with the 3D structure of natural proteins, i.e., a-helices and β-sheets. In this chapter, we present the recent progress in the synthesis of polymers that form hydrogels in aqueous solutions, based on polypeptides prepared through ring-opening polymerization of N-carboxy anhydrides and which have been loaded with anticancer drugs and studied for their functionality. Advancements in drug design and improvement of multifunctional nanocarriers from the combination of well-defined macromolecular architectures and smart materials are the future for the successful treatment of numerous lethal diseases.

Synthesis of poly(Asparagine-co-phenylalanine) copolymers, analogy with thermosensitive poly(acrylamide-co-styrene) copolymers and formation of PEGylated nanoparticles

We presented optimized synthetic pathways to high purity L-phenylalanine (Phe) and L-asparagine (Asn) derived N-carboxyanhydrides (NCA) in one-pot processes and in a highly reproducible manner. Ring-opening homopolymerizations of Asn-NCA and Phe-NCA were successfully achieved together with their copolymerization in an attempt to mimic upper critical solution temperature (UCST) vinylic poly(acrylamide-co-styrene) (P(Aam-co-St)) copolymers. Poly(asparagine-co-phenylalanine) (P(Asn-co-Phe)) copolypeptides of different Asn-to-Phe ratio were prepared and their thermoresponsive behavior was investigated in aqueous solution. Unfortunately, polymer aggregation imparted by strong hydrophobic interactions favored due to presence of β-sheet structures prevented achieving thermosensitive response in aqueous media in the 50–62 °C range conversely to its vinylic counterpart. However, by taking advantage of those strong hydrophobic interactions, PEG-b-PAsn and PEG-b-PPhe diblock copolymers were prepared from amino-PEG initiators and PEG-b-PAsn was successfully formulated into PEGylated nanoparticles of ~270 nm.

Well-Defined Construction of Functional Macromolecular Architectures Based on Polymerization of Amino Acid Urethanes.

Polypeptide synthesis was accomplished using the urethane derivatives of amino acids as monomers, which can be easily prepared, purified, and stored at ambient temperature without the requirement for special precautions. The urethanes of amino acids are readily synthesized by the N-carbamoylation of onium salts of amino acids using diphenyl carbonate (DPC). The prepared urethanes are then efficiently cyclized to produce amino acid N-carboxyanhydrides (NCAs). Thereafter, in the presence of primary amines, the ring-opening polymerization (ROP) of NCAs is initiated using the amines, to yield polypeptides with controlled molecular weights. The polypeptides have propagating chains bearing reactive amino groups and initiating chain ends endowed with functional moieties that originate from the amines. Aiming to benefit from these interesting characteristics of the polypeptide synthesis using the urethanes of amino acids, various macromolecular architectures containing polypeptide components have been constructed and applied as biofunctional materials in highly efficient antifouling coatings against proteins and cells, as biosensors for specific molecules, and in targeted drug delivery.

Facile Synthesis of High Molecular Weight Polypeptides via Fast and Moisture Insensitive Polymerization of α-Amino Acid N-Carboxyanhydrides

Polypeptides have been widely utilized in the fields of biomaterials and biomedicine. Ever since N-carboxyanhydride (NCA) was reported by Hermann Leuchs in 1906, ring-opening polymerization of NCAs has been extensively used to prepare polypeptides. Despite continuous innovations, it is still challenging to synthesize polypeptides in high molecular weight efficiently. To address this challenge, we developed KHMDS/NaHMDS initiated fast NCA polymerization that is also moisture tolerant, open-flask amenable and terminal tunable. This NCA polymerization was able to proceed in most common solvents and meet the solubility requirement of variable NCA monomers and corresponding polypeptides. KHMDS can initiate γ-benzyl-L-glutamate-N-carboxyanhydride (BLG NCA) polymerization in a reaction rate 92 times faster than does hexylamine and 80 times faster than does triethylamine. This NCA polymerization also demonstrated easy and fast synthesis of gram-scale long chain polypeptides in an open flask.

Streamlined Synthesis of PEG-Polypeptides Directly from Amino Acids

The application of synthetic polypeptides is greatly limited by the difficulty of the purification and polymerization of N-carboxyanhydrides (NCAs). Here, we report a streamlined, controlled synthe...