Hydroxyl Group Tolerated Polymerization of N-Substituted Glycine N-Thiocarboxyanhydride Mediated by Aminoalcohols: A Simple Way to α-Hydroxyl-ω-aminotelechelic Polypeptoids

Bioactive poly(amino acid)s for multi-modal cancer therapy

The interplay between the tumor cells and their microenvironments is as inseparable as the relationship between "seeds" and "soil." The tumor microenvironments (TMEs) exacerbate malignancy by enriching malignant cell subclones, generating extracellular matrices, and recruiting immunosuppressive cells, thereby diminishing the efficacy of clinical therapies. Modulating TMEs has emerged as a promising strategy to enhance cancer therapy. However, the existing drugs used in clinical settings do not target the TMEs specifically, underscoring the urgent need for advanced strategies. Bioactive materials present unique opportunities for modulating TMEs. Poly(amino acid)s with precisely controllable structures and properties offer exceptional characteristics, such as diverse structural units, excellent biosafety, ease of modification, sensitive biological responsiveness, and unique secondary structures. These attributes hold significant potential for the modulation of TMEs and clinical applications further. Consequently, developing bioactive poly(amino acid)s capable of modulating the TMEs by elucidating structure-activity relationships and mechanisms is a promising approach for innovative clinical oncology therapy. This review summarizes the recent progress of our research team in developing bioactive poly(amino acid)s for multi-modal tumor therapy. First, a brief overview of poly(amino acid) synthesis and their advantages as nanocarriers is provided. Subsequently, the pioneering research of our research group on synthesizing the biologically responsive, dynamically allosteric, and immunologically effective poly(amino acid)s are highlighted. These poly(amino acid)s are designed to enhance tumor therapy by modulating the intracellular, extracellular matrix, and stromal cell microenvironments. Finally, the future development of poly(amino acid)s is discussed. This review will guide and inspire the construction of bioactive poly(amino acid)s with promising clinical applications in cancer therapy. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Biology-Inspired Nanomaterials > Peptide-Based Structures.

A Review on the Synthesis of Polypeptoids

Polyeptoids are a promising class of polypeptide mimetic biopolymers based on N-substituted glycine backbones. Because of the high designability of their side chains, polypeptoids have a wide range of applications in surface antifouling, biosensing, drug delivery, and stimuli-responsive materials. To better control the structures and properties of polypeptoids, it is necessary to understand different methods for polypeptoid synthesis. This review paper summarized and discussed the main synthesis methods of polypeptoids: the solid-phase submonomer synthesis method, ring-opening polymerization method and Ugi reaction method.

Research Progress in Polypeptoids Prepared by Controlled Ring-Opening Polymerizations

Polypeptoids, structural mimics of polypeptides, have attracted considerable attention due to their biocompatibility, proteolytic stability, thermal processability, good solubility, synthetic accessibility, and structural diversity. Polypeptoids have emerged as an interesting material in both polymer science and biological field. This review primarily discusses the research progress of polypeptoids prepared by controlled ring-opening polymerizations in the past decade, including synthetic strategies of monomers, polymerizations by different initiators, postfunctionalization, fundamental properties, crystallization-driven self-assembly, and potential biological applications.

Biodegradable Polysarcosine with Inserted Alanine Residues: Synthesis and Enzymolysis

Polysarcosine (PSar), a water-soluble polypeptoid, is gifted with biodegradability via the random ring-opening copolymerization of sarcosine- and alanine-N-thiocarboxyanhydrides catalyzed by acetic acid in controlled manners. Kinetic investigation reveals the copolymerization behavior of the two monomers. The random copolymers, named PaS, with high molecular weights between 5.3 and 43.6 kg/mol and tunable Ala molar fractions varying from 6 to 43% can be degraded by porcine pancreatic elastase within 50 days under mild conditions (pH = 8.0 at 37 °C). Both the biodegradation rate and water solubility of PaS depend on the content of Ala residues. PaS with Ala fractions below 43% are soluble in water, while the one with 43% Ala self-assembles in water into nanoparticles. Moreover, PaS are noncytotoxic at the concentration of 5 mg/mL. The biodegradability and biocompatibility endow the Ala-containing PSar with the potential to replace poly(ethylene glycol) as a protective shield in drug-delivery.

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.

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.

Understanding Acid-Promoted Polymerization of the N-Substituted Glycine N-Thiocarboxyanhydride in Polar Solvents

Polymerization of N-substituted glycine N-thiocarboxyanhydrides (NNTAs) is a promising pathway to prepare functional polypeptoids benefiting from their tolerance to nucleophilic impurities. However, controlled NNTA polymerization is hard to achieve in amide polar solvents, including N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methyl pyrrolidone (NMP), the only aprotic solvents for many biomacromolecules and polypeptoids. In the present work, we successfully achieve controlled NNTA polymerization in amide polar solvents by adding acetic acid as a promoter. The promotion is applied to the polymerization of sarcosine NTA, N-ethyl glycine NTA, and N-butyl glycine NTA. DMAc, DMF, and NMP are suitable solvents to prepare polypeptoids with designable molecular weights and low dispersities (1.06-1.21). The polysarcosines with high molecular weights are prepared up to 35.2 kg/mol. A kinetic investigation quantitatively reveals that the presence of acetic acid not only accelerates the polymerization, but also suppresses H₂S-catalyzed decomposition of NNTAs by decreasing the concentration of H₂S dissolved in polar solvents. Benzoic acid is also able to promote the polymerization, while trifluoroacetic acid, phosphoric acid, and phenol are not appropriate promoters. The moderate acidity of acids is essential. l-Methionine, l-tryptophan, and l-phenylalanine, which are dissolved in DMF, initiate the controlled polymerization of sarcosine-NTA in the presence of acetic acid and introduce functional end groups to polysarcosines quantitatively. In DMAc, hydrophilic vancomycin is grafted by poly(N-butyl glycine). The amphiphilic product dissolves in dichloromethane and stabilizes water-in-oil emulsion.

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.

Thermoresponsive Polypeptoids

Stimuli-responsive polymers have been widely studied in many applications such as biomedicine, nanotechnology, and catalysis. Temperature is one of the most commonly used external triggers, which can be highly controlled with excellent reversibility. Thermoresponsive polymers exhibiting a reversible phase transition in a controlled manner to temperature are a promising class of smart polymers that have been widely studied. The phase transition behavior can be tuned by polymer architectures, chain-end, and various functional groups. Particularly, thermoresponsive polypeptoid is a type of promising material that has drawn growing interest because of its excellent biocompatibility, biodegradability, and bioactivity. This paper summarizes the recent advances of thermoresponsive polypeptoids, including the synthetic methods and functional groups as well as their applications.

Direct N-substituted N-thiocarboxyanhydride polymerization towards polypeptoids bearing unprotected carboxyl groups

Synthesis of poly(α-amino acid)s bearing carboxyl groups is a critical pathway to prepare biomaterials to simulate functional proteins. The traditional approaches call for carboxyl-protected monomers to prevent degradation of monomers or wrong linkage. In this contribution, we synthesize N-carboxypentyl glycine N-thiocarboxyanhydride (CPG-NTA) and iminodiacetic acid N-thiocarboxyanhydride (IDA-NTA) without protection. Initiated by amines, CPG-NTA directly polymerizes into polyCPG bearing unprotected carboxyl groups with controlled molecular weight (2.8-9.3 kg mol^-1) and low dispersities (1.08-1.12). Block and random copolymerizations of CPG-NTA with N-ethyl glycine N-thiocarboxyanhydride (NEG-NTA) demonstrate its versatile construction of complicated polypeptoids. On the contrary, IDA-NTA transforms amines into cyclic IDA dimer-capped species with carboxyl end group in decent yields (>89%) regio-selectively. Density functional theory calculation elucidates that IDA repeating unit is prone to cyclize to be the six-membered ring product with low ΔG. The polymer is a good adhesive reagent to various materials with adhesive strength of 33-229 kPa.

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.

Understanding ring-closing and racemization to prepare α-amino acid NCA and NTA monomers: a DFT study

Polypeptides and polypeptoids are promising materials in biomedical applications bearing α-amino acid repeating units, which are prepared from ring-opening polymerizations of α-amino acid N-carboxyanhydride (NCA) or N-thiocarboxyanydride (NTA) monomers. Detailed studies on monomer synthetic routes are essential to explore new α-amino acid NCA and NTA monomers as well as the corresponding poly(α-amino acid) materials. In this contribution, density functional theory (DFT) is applied to investigate the mechanism of the Leuchs approach including two possible pathways, precursor structure and racemization in the ring-closing reaction. According to DFT calculations, pathway 2 is preferred with lower ΔG than pathway 1, and the rate-determining step is recognized as an SN2 substitution with releasing equivalent halogenated hydrocarbon, which explains our experimental observations. Racemization results from the reaction between the NTA monomer and a strong protonic acid, which can be suppressed by low temperature and short reaction time. Racemization is inhibited by steric hindrance in those NTAs of α-amino acids containing high bulkiness at the β-carbon, such as leucine-NTA.

Fe3+@polyDOPA-b-polysarcosine, a T1-Weighted MRI Contrast Agent via Controlled NTA Polymerization

α-Amino acid N-thiocarboxyanhydrides (NTAs) are promising cyclic monomers to synthesize polypeptides and polypeptoids via controlled ring-opening polymerizations. Superior to N-carboxyanhydrides requiring protection on hydroxyl groups, NTAs are able to tolerate such nucleophiles. In this work, we report the synthesis of NTA monomers containing unprotected phenolic hydroxyl groups of 3,4-dihydroxy-l-phenylalanine (DOPA) and l-tyrosine (Tyr). Their controlled ROPs and sequential copolymerizations with polysarcosine (PSar) yield PDOPA, PTyr, and PDOPA-b-polysarcosine (PDOPA-b-PSar) products quantitatively with designable degrees of polymerization. Micellar nanoparticles of Fe³⁺@PDOPA-b-PSar have been prepared thanks to the strong chelation of iron(III) cation by catechol ligands that act as T1-weighted magnetic resonance imaging (MRI) contrast agents. For instance, Fe³⁺@PDOPA₁₀-b-PSar₅₀ exhibits higher longitudinal relaxivity (r₁ = 5.6 mM^-1 s^-1) than commercial Gd³⁺-based compounds. Effective MRI contrast enhancement in vivo of nude mice with a moderate duration (150 min) and 3D magnetic resonance angiography in rabbit illustrated by using volume rendering and maximal intensity projection techniques ignite the clinical application of Fe³⁺-based polypept(o)ides in diagnostic radiology as Gd-free MRI contrast agents.

Synthesis and Properties of Alternating Polypeptoids and Polyampholytes as Protein-Resistant Polymers

Alternating polypeptoids are particularly appealing because alternating sequence may impart highly ordered structure and special functions, while their simple synthesis still remains a key challenge. We describe that natural amino acid monomers can be polymerized via Ugi reaction in a step-growth fashion as an AA' BB' system, which leads to alternating polypeptoids with molecular weight up to 15 kg/mol. These alternating polypeptoids are thermally responsive and exhibit cloud points ( T_cp) between 27 and 37 °C. Importantly, the marriage of high functionality of amino acids with Ugi reaction also enables the preparation of polypeptoids encoding both protected amino and carboxyl groups in the side chains with alternating arrangement. The cleavage of the protecting groups leads to alternating polyampholytes without any compositional drift. Such alternating polyampholytes not only exhibit high water solubility (>100 mg/mL) but also demonstrate the ability to resist aggregation with proteins. Moreover, the cell viability measurements reveal that these materials have minimal cytotoxicity to HeLa cells. Overall, this study offers us a simple way to prepare a variety of polypeptoids and polyampholytes as new protein-resistant materials for bioapplications.