Identifying the Hydrolysis of Carbonyl Sulfide as a Side Reaction Impeding the Polymerization of N-Substituted Glycine N-Thiocarboxyanhydride

Easy access to amphiphilic nitrogenous block copolymers via switchable catalysis

A key challenge in polymer synthesis is to develop new methods that enable block copolymers to be prepared from mixed monomer feedstock. The emerging switchable polymerization catalysis can generate block copolymers with well-defined structures and tunable properties from monomer mixtures. However, constrained by the reactivity of monomers and the incompatibility of different polymerization mechanisms, this method is usually confined to oxygenated monomers. In this work, the switchable polymerization was successfully applied to nitrogenous monomers for the first time, achieving the efficient copolymerization of N-substituted N-carboxyanhydrides (NNCAs) with epoxides and cyclic anhydrides. This leads to easy access towards amphiphilic nitrogenous copolymers, such as polyester-b-polypeptoids. Density functional theory calculations demonstrated that the reaction of cyclic anhydrides with the alkoxide terminal is thermodynamically more favorable than that of NNCAs. Characterization, using nuclear magnetic resonance spectroscopy, size exclusion chromatography and in situ infrared spectroscopy, has confirmed the well-defined block structure of the obtained copolymers. This switchable polymerization strategy is applicable to a range of monomer mixtures with different oxygenated monomers and NNCAs, providing a highly efficient synthetic route towards nitrogenous block copolymers. Most importantly, the easily accessed amphiphilic polyester-b-polypeptoids demonstrated excellent anti-protein adsorption capabilities and barely any cytotoxicity, showing great potential in the field of biomedicine.

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.

A Field Guide to Optimizing Peptoid Synthesis

N-Substituted glycines (peptoids) are a class of peptidomimetic molecules used as materials for health, environmental, and drug delivery applications. Automated solid-phase synthesis is the most widely used approach for preparing polypeptoids, with a range of published protocols and modifications for selected synthetic targets. Simultaneously, emerging solution-phase syntheses are being leveraged to overcome limitations in solid-phase synthesis and access high-molecular weight polypeptoids. This Perspective aims to outline strategies for the optimization of both solid- and solution-phase synthesis, provide technical considerations for robotic synthesizers, and offer an outlook on advances in synthetic methodologies. The solid-phase synthesis sections explore steps for protocol optimization, accessing complex side chains, and adaptation to robotic synthesizers; the sections on solution-phase synthesis cover the selection of initiators, side chain compatibility, and strategies for controlling polymerization efficiency and scale. This text acts as a "field guide" for researchers aiming to leverage the flexibility and adaptability of peptoids in their research.

1,1,3,3-Tetramethylguanidine-Mediated Zwitterionic Ring-Opening Polymerization of Sarcosine-Derived N-Thiocarboxyanhydride toward Well-Defined Polysarcosine

Zwitterionic ring-opening polymerization (ZROP) of sarcosine-derived N-thiocarboxyanhydrides (Me-NNTAs) can be induced by using 1,1,3,3-tetramethylguanidine (TMG) initiators in CH₂Cl₂ at 25 °C, rapidly producing well-defined polysarcosine polymers with controlled molecular weights (M_n = 1.9–37 kg/mol) and narrow molecular weight distributions (Đ = 1.01–1.12). The reaction exhibits characteristics of a living polymerization, evidenced by pseudo-first-order polymerization kinetics, the linear increase of polymer molecular weight (M_n) with conversion, and the successful chain extension experiments. The polymerization is proposed to proceed via propagating macro-zwitterions bearing a cationic 1,1,3,3-tetramethylguanidinium and an anionic thiocarbamate chain end. The TMG not only initiates the polymerization but also serves to stabilize the thiocarbamate chain end where the monomer addition occurs. Because of the enhanced hydrolytic stability of Me-NNTA, the polymerization can be conducted without the rigorous exclusion of moisture, further enhancing the appeal of the method to access well-defined polysarcosine.

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.

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.

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.

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.

Prebiotic Peptides: Molecular Hubs in the Origin of Life

The fundamental roles that peptides and proteins play in today's biology makes it almost indisputable that peptides were key players in the origin of life. Insofar as it is appropriate to extrapolate back from extant biology to the prebiotic world, one must acknowledge the critical importance that interconnected molecular networks, likely with peptides as key components, would have played in life's origin. In this review, we summarize chemical processes involving peptides that could have contributed to early chemical evolution, with an emphasis on molecular interactions between peptides and other classes of organic molecules. We first summarize mechanisms by which amino acids and similar building blocks could have been produced and elaborated into proto-peptides. Next, non-covalent interactions of peptides with other peptides as well as with nucleic acids, lipids, carbohydrates, metal ions, and aromatic molecules are discussed in relation to the possible roles of such interactions in chemical evolution of structure and function. Finally, we describe research involving structural alternatives to peptides and covalent adducts between amino acids/peptides and other classes of molecules. We propose that ample future breakthroughs in origin-of-life chemistry will stem from investigations of interconnected chemical systems in which synergistic interactions between different classes of molecules emerge.