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.

Applications of MALDI-TOF-MS in structural characterization of synthetic polymers

In recent years, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been utilized to rapidly and precisely characterize the detailed molecular structures of synthetic polymers. This review summarizes recent progress regarding MALDI-TOF-MS for the characterization of synthetic polymers with a focus on specific important experimental aspects including sample preparation, the choice of matrix, the effects of cationizing agents and solvents, data processing and various applications. Finally, the recent trend of MALDI-TOF-MS development is discussed. We hope this review will be instructive for graduate students and junior users who need to use MALDI-TOF-MS as a necessary characterization technique for new synthetic polymers.

Polypeptoids and Peptoid-Peptide Hybrids by Transition Metal Catalysis

Peptoids have attracted attention for application in biomedicine due to their advantageous properties as compared to peptides. The structural analogues are typically resistant to protease degradation and offer improved biocompatibility. Chemical routes to an impressive variety of short-chain, low-molecular-weight peptoids are well-established. However, synthetic methods for well-defined, high-molecular-weight polypeptoids with side chain diversity are still in their infancy. Here, we report a facile method for synthesis of polypeptoids via transition-metal-catalyzed controlled, living polymerization of N-substituted N-carboxyanhydrides. Our method is amenable to hydrophilic and hydrophobic side chains and yields high-molecular-weight linear polypeptoids of predictable length and low dispersity. Further, the polymer end groups can be tuned for biological targeting, and polypeptide-polypeptoid hybrids are readily prepared in one pot. Our materials are indeed resistant to common proteases and are well-tolerated by human cells. Overall, this work represents a significant stride toward access to tunable polypeptoids.

High Antibacterial Activity and Selectivity of the Versatile Polysulfoniums that Combat Drug Resistance

Sulfonium-ion-containing polymers exhibit significant potential benefits for various applications. An efficient strategy to synthesize a type of antibacterial sulfonium-ion-bearing polypeptoids via a combination of ring-opening polymerization and a post-polymerization functionalization with various functional epoxides is presented. A systematic investigation is further performed in order to explore the influence of the overall hydrophobic/hydrophilic balance on the antimicrobial activity and selectivity of the prepared polysulfoniums. Notably, those chlorepoxypropane-modified polysulfoniums with an optimized amphiphilic balance show higher selectivity toward both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, than to red blood cells. The polymers also show great efficiency in inhibiting S. aureus biofilm formations, as well as in further eradicating the mature biofilms. Remarkably, negligible antibacterial resistance and cross-resistance to commercial antibiotics is shown in these polymers. The polysulfoniums further show their potent in vivo antimicrobial efficacy in a multidrug-resistant S. aureus infection model that is developed on mouse skin. Similar to the antimicrobial peptides, the polysulfoniums are demonstrated to kill bacteria through membrane disruption. The obtained polypeptoid sulfoniums, with high selectivity and potent antibacterial property, are excellent candidates for antibacterial treatment and open up new possibilities for the preparation of a class of innovative antimicrobials.

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.

Solution Self-Assembly of Coil-Crystalline Diblock Copolypeptoids Bearing Alkyl Side Chains

Polypeptoids, a class of synthetic peptidomimetic polymers, have attracted increasing attention due to their potential for biotechnological applications, such as drug/gene delivery, sensing and molecular recognition. Recent investigations on the solution self-assembly of amphiphilic block copolypeptoids highlighted their capability to form a variety of nanostructures with tailorable morphologies and functionalities. Here, we review our recent findings on the solutions self-assembly of coil-crystalline diblock copolypeptoids bearing alkyl side chains. We highlight the solution self-assembly pathways of these polypeptoid block copolymers and show how molecular packing and crystallization of these building blocks affect the self-assembly behavior, resulting in one-dimensional (1D), two-dimensional (2D) and multidimensional hierarchical polymeric nanostructures in solution.

Stimulus-Responsive Polymers Based on Polypeptoid Skeletons

Polypeptoids have attracted a lot of atteSDntion because of their unique structural characteristics and special properties. Polypeptoids have the same main chain structures to polypeptides, making them have low cytotoxicity and excellent biocompatibility. Polypeptoids can also respond to external environmental changes by modifying the configurations of the side chains. The external stimuli can be heat, pH, ions, ultraviolet/visible light and active oxygen or their combinations. This review paper discussed the recent research progress in the field of stimulus-responsive polypeptoids, including the design of new stimulus-responsive polypeptoid structures, controlled actuation factors in response to external stimuli and the application of responsive polypeptoid biomaterials in various biomedical and biological nanotechnology, such as drug delivery, tissue engineering and biosensing.

A convenient approach for antibacterial polypeptoids featuring sulfonium and oligo(ethylene glycol) subunits

Bioinspired polypeptoids show great potential in many applications. Here, we report a convenient approach to synthesize a novel type of polypeptoid containing both sulfonium and oligo(ethylene glycol) (OEG) moieties by ring-opening polymerization (ROP) and a post-modification strategy. Three types of epoxides with (OEG)n (n = 1-3) groups are involved to offer various functionalities. The obtained polypeptoid sulfonium salts show positive ζ potential, irrespective of the solution pH and the degree of polymerization (DP). We demonstrate that the polypeptoids exhibit excellent antibacterial activity against Staphylococcus aureus (S. aureus) with MIC (minimal inhibitory concentration) in the range of 3.9-7.8 μg mL-1. In addition, the polypeptoids have a low hemolysis property and good in vitro biocompatibility. Remarkably, the as-prepared polypeptoids show rapid and potent antibacterial activity within 5 min. These features suggest that the obtained polypeptoids offer great potential for antimicrobial agents.

Controlled ring-opening polymerization of N-(3-tert-butoxy-3-oxopropyl) glycine derived N-carboxyanhydrides towards well-defined peptoid-based polyacids

Polypeptoids bearing carboxylic acid groups on the N-substituent are useful building blocks for the construction of peptidomimetic supramolecular assemblies with stimuli-responsive properties.

Synthesis and self-assembly of photo-responsive polypeptoid-based copolymers containing azobenzene side chains

Photo-responsive polypeptoid-based copolymers containing azobenzene side chains have been well synthesized and they could self-assemble into tunable nanostructures with reversible light-switched behaviors.

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.

Tunable LCST/UCST-Type Polypeptoids and Their Structure-Property Relationship

Bioinspired thermoresponsive polymeric materials with tunable phase-transition behaviors are highly desirable for biomedical applications. Here, we reported a facile approach for the synthesis of both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) types of thermoresponsive polypeptoids with tunable phase-transition temperature in the range of 29--55 °C. The introduction of alkyl groups and ethylene glycol (EG) units results in a controlled phase-transition behavior under fairly mild conditions. A very sharp transition (ΔT ≤ 1.5 °C) is observed by simply adjusting pH and the alkyl chain length. In particular, the carboxyl-containing polypeptoids display designable UCST behavior, which can be finely tuned in both water and methanol. All these features make the obtained polymers beneficial for practical applications. More interestingly, we demonstrate that the hydrophilic EG group behaves as an excellent regulator to tune the UCST behavior, while the hydrophobic alkyl residues show remarkable capability to regulate the LCST behavior of the system. We hope that such systematic structure-property studies will enable the design of smart polymer materials to meet the specific needs of future applications.

Transiently thermoresponsive polymers and their applications in biomedicine

The focus of this review is on the class of transiently thermoresponsive polymers. These polymers are thermoresponsive, but gradually lose this property upon chemical transformation - often a hydrolysis reaction - in the polymer side chain or backbone. An overview of the different approaches used for the design of these polymers along with their physicochemical properties is given. Their amphiphilic properties and degradability into fully soluble compounds make this class of responsive polymers attractive for drug delivery and tissue engineering applications. Examples of these are also provided in this review.

Schiff base and reductive amination reactions of α-amino acids: a facile route toward N-alkylated amino acids and peptoid synthesis

Polypeptoids are a promising class of peptidomimetic polymers for applications in biotechnology, but the polymers prepared by solution polymerization have limited side-chain functionalities due to synthetic difficulty.

Polysarcosine brush stabilized gold nanorods for in vivo near-infrared photothermal tumor therapy

Gold nanorods (AuNRs) are suitable candidates for photothermal therapy in vivo, because of their excellent ability to transfer near-infrared (NIR) light into heat. However, appropriate surface should be generated on AuNRs before their in vivo application because of the low colloidal stability in complicate biological environment and relatively strong toxicity compared to their pristine stabilizer cetyltrimethylammonium bromide. In the current study, polysarcosine (PS), a non-ionic hydrophilic polypeptoid whose structure is similar to polypeptides, bearing repeating units of natural α-amino acid, was used to stabilize AuNRs due to its excellent hydrophilicity and biocompatibility. Polysarcosine with optimized molecular weight was synthesized and used to modify AuNRs by traditional ligand exchange. The grafting of PS on AuNRs was evidenced by fourier transform infrared (FTIR) spectroscopy and the alternation of surface zeta potential. The polysarcosine coated AuNRs (Au@PS) showed good stabilities in wide pH range and simulated physiological buffer with the ligand competition of dithiothreitol (DTT). The Au@PS NRs had neglectable cytotoxicity and showed efficient ablation of tumor cells in vitro. Moreover, Au@PS NRs had a longer circulation time in body that resulted in a higher accumulation in solid tumors after intravenous injection, compared to AuNRs capped with polyethylene glycol (PEG). Photothermal therapy in vivo demonstrated that the tumors were completely destroyed by single-time irradiation of NIR laser after one-time injection of the polysarcosine capped AuNRs. The Au@PS NRs did not cause obvious toxicity in vivo, suggesting promising potential in cancer therapy.

STATEMENT OF SIGNIFICANCE

In current study, polysarcosine (PS), a non-ionic hydrophilic polypeptoid whose structure is similar to polypeptides, bearing repeating units of natural α-amino acid, was used to stabilize AuNRs due to its excellent hydrophilicity and biocompatibility. The polysarcosine coated AuNRs (Au@PS) showed good stabilities in wide pH range and simulated physiological buffer. The Au@PS NRs had very low cytotoxicity and showed high efficacy for the ablation of cancer cells in vitro. Moreover, Au@PS NRs had a longer circulation time in blood that led to a higher accumulation in tumors after intravenous injection, compared to AuNRs capped with polyethylene glycol (PEG). In vivo photothermal therapy showed that tumors were completely cured without reoccurrence by one-time irradiation of NIR laser after a single injection of the polysarcosine modified AuNRs.

Thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides

Recent advances in thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides, with a specific focus on structure–property relationships, self-assembly, and applications, are reviewed.

Synthesis and Characterization of Cleavable Core-Cross-Linked Micelles Based on Amphiphilic Block Copolypeptoids as Smart Drug Carriers

Amphiphilic block copolypeptoids consisting of a hydrophilic poly(N-ethyl glycine) segment and a hydrophobic poly[(N-propargyl glycine)-r-(N-decyl glycine)] random copolymer segment [PNEG-b-P(NPgG-r-NDG), EPgD] have been synthesized by sequential primary amine-initiated ring-opening polymerization (ROP) of the corresponding N-alkyl N-carboxyanhydride monomers. The block copolypeptoids form micelles in water and the micellar core can be cross-linked with a disulfide-containing diazide cross-linker by copper-mediated alkyne-azide cycloaddition (CuAAC) in aqueous solution. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis revealed the formation of spherical micelles with uniform size for both the core-cross-linked micelles (CCLMs) and non-cross-linked micelles (NCLMs) precursors for selective block copolypeptoid polymers. The CCLMs exhibited increased dimensional stability relative to the NCLMs in DMF, a nonselective solvent for the core and corona segments. Micellar dissociation of CCLMs can be induced upon addition of a reducing agent (e.g., dithiothreitol) in dilute aqueous solutions, as verified by a combination of fluorescence spectroscopy, size exclusion chromatography (SEC), and (1)H NMR spectroscopic measurement. Doxorubicin (DOX), an anticancer drug, can be loaded into the hydrophobic core of CCLMs with a maximal 23% drug loading capacity (DLC) and 37% drug loading efficiency (DLE). In vitro DOX release from the CCLMs can be triggered by DTT (10 mM), in contrast to significantly reduced DOX release in the absence of DTT, attesting to the reductively responsive characteristic of the CCLMs. While the CCLMs exhibited minimal cytotoxicity toward HepG2 cancer cells, DOX-loaded CCLMs inhibited the proliferation of the HepG2 cancer cells in a concentration and time dependent manner, suggesting the controlled release of DOX from the DOX-loaded CCLMS in the cellular environment.

Phosgene-free synthesis of non-ionic hydrophilic polyserine

Non-ionic water-soluble poly-dl-serine (PSer) was synthesized from an activated urethane-type derivative of serine.