Effects of Side Group Functionality and Molecular Weight on the Activity of Synthetic Antimicrobial Polypeptides

Polypeptide-based multilayer nanoarchitectures: Controlled assembly on planar and colloidal substrates for biomedical applications

Polypeptides have shown an excellent potential in nanomedicine thanks to their biocompatibility, biodegradability, high functionality, and responsiveness to several stimuli. Polypeptides exhibit high propensity to organize at the supramolecular level; hence, they have been extensively considered as building blocks in the layer-by-layer (LbL) assembly. The LbL technique is a highly versatile methodology, which involves the sequential assembly of building blocks, mainly driven by electrostatic interactions, onto planar or colloidal templates to fabricate sophisticated multilayer nanoarchitectures. The simplicity and the mild conditions required in the LbL approach have led to the inclusion of biopolymers and bioactive molecules for the fabrication of a wide spectrum of biodegradable, biocompatible, and precisely engineered multilayer films for biomedical applications. This review focuses on those examples in which polypeptides have been used as building blocks of multilayer nanoarchitectures for tissue engineering and drug delivery applications, highlighting the characteristics of the polypeptides and the strategies adopted to increase the stability of the multilayer film. Cross-linking is presented as a powerful strategy to enhance the stability and stiffness of the multilayer network, which is a fundamental requirement for biomedical applications. For example, in tissue engineering, a stiff multilayer coating, the presence of adhesion promoters, and/or bioactive molecules boost the adhesion, growth, and differentiation of cells. On the contrary, antimicrobial coatings should repel and inhibit the growth of bacteria. In drug delivery applications, mainly focused on particles and capsules at the micro- and nano-meter scale, the stability of the multilayer film is crucial in terms of retention and controlled release of the payload. Recent advances have shown the key role of the polypeptides in the adsorption of genetic material with high loading efficiency, and in addressing different pathways of the particles/capsules during the intracellular uptake, paving the way for applications in personalized medicine. Although there are a few studies, the responsiveness of the polypeptides to the pH changes, together with the inclusion of stimuli-responsive entities into the multilayer network, represents a further key factor for the development of smart drug delivery systems to promote a sustained release of therapeutics. The degradability of polypeptides may be an obstacle in certain scenarios for the controlled intracellular release of a drug once an external stimulus is applied. Nowadays, the highly engineered design of biodegradable LbL particles/capsules is oriented on the development of theranostics that, limited to use of polypeptides, are still in their infancy.

Gas-Controlled Self-Assembly of Metallacycle-Cored Supramolecular Star Polymer with Tunable Antibacterial Activity

Herein, a three-armed amphiphilic metallacycle-cored star supramolecular polymer (Por-MOM-PDMAEMA) has been designed and synthesized via highly efficient post-assembly polymerization. This star polymer is further self-assembled into nanoparticles of different sizes depending upon the experimental conditions. The gas-controlled morphology transformation and tunable antibacterial activities of Por-MOM-PDMAEMAis systematically investigated and compared with metallacycle (MOM). The superior antibacterial activity of Por-MOM-PDMAEMA against multidrug-resistant P. aeruginosa implies that the presence of photodynamic photosensitizer (Por) and cationic polymer chain will significantly enhance antibactericidal activity, which is mainly attributed to the synergistic effect of photosensitizer and polymer chain linked in one metallacycle core. By leveraging the unique properties of metallacycle and their dynamic response to gaseous stimuli, the antibacterial properties of the Por-MOM-PDMAEMA can be finely tuned in response to gas triggers.

Pyridinium-Yne Click Polymerization: A Facile Strategy toward Functional Poly(Vinylpyridinium Salt)s with Multidrug-Resistant Bacteria Killing Ability

Polymeric materials with antibacterial properties hold great promise for combating multidrug-resistant bacteria, which pose a significant threat to public health. However, the synthesis of most antibacterial polymers typically involves complicated and time-consuming procedures. In this study, we demonstrate a simple and efficient strategy for synthesizing functional poly(vinylpyridinium salt)s via pyridinium-yne click polymerization. This click polymerization could proceed with high atom economy under mild conditions without any external catalyst, yielding soluble and thermally stable poly(vinylpyridinium salt)s with satisfactory molecular weights and well-defined structures in excellent yields. Additionally, the incorporation of luminescent units such as fluorene, tetraphenylethylene, and triphenylamine into the polymer backbone confers excellent aggregation-enhanced emission properties upon the resulting polymers, rendering them suitable for bacterial staining. Moreover, the existence of pyridinium salt imparts intrinsic antibacterial activity against multidrug-resistant bacteria to the polymers, enabling them to effectively inhibit wound bacterial infection and significantly expedite the healing process. This work not only provides an efficient method to prepare antibacterial polymers, but also opens up the possibility of various applications of polymers in healthcare and other antibacterial fields.

Recent Advances on Antimicrobial Peptides from Milk: Molecular Properties, Mechanisms, and Applications

Traditional antibiotics are facing a tremendous challenge due to increased antimicrobial resistance; hence, there is an urgent need to find novel antibiotic alternatives. Milk protein-derived antimicrobial peptides (AMPs) are currently attracting substantial attention considering that they showcase an extensive spectrum of antimicrobial activities, with slower development of antimicrobial resistance and safety of raw materials. This review summarizes the molecular properties, and activity mechanisms and highlights the applications and limitations of AMPs derived from milk proteins comprehensively. Also the analytical technologies, especially bioinformatics methodologies, applied in the process of screening, identification, and mechanism illustration of AMPs were underlined. This review will give some ideas for further research and broadening of the applications of milk protein-derived AMPs in the food field.

Membrane-Active Metallopolymers: Repurposing and Rehabilitating Antibiotics to Gram-Negative Superbugs

Among multiple approaches to combating antimicrobial resistance, a combination therapy of existing antibiotics with bacterial membrane-perturbing agents is promising. A viable platform of metallopolymers as adjuvants in combination with traditional antibiotics is reported in this work to combat both planktonic and stationary cells of Gram-negative superbugs and their biofilms. Antibacterial efficacy, toxicity, antibiofilm activity, bacterial resistance propensity, and mechanisms of action of metallopolymer-antibiotic combinations are investigated. These metallopolymers exhibit 4-16-fold potentiation of antibiotics against Gram-negative bacteria with negligible toxicity toward mammalian cells. More importantly, the lead combinations (polymer-ceftazidime and polymer-rifampicin) eradicate preformed biofilms of MDR E. coli and P. aeruginosa, respectively. Further, β-lactamase inhibition, outer membrane permeabilization, and membrane depolarization demonstrate synergy of these adjuvants with different antibiotics. Moreover, the membrane-active metallopolymers enable the antibiotics to circumvent bacterial resistance development. Altogether, the results indicate that such non-antibiotic adjuvants bear the promise to revitalize the efficacy of existing antibiotics to tackle Gram-negative bacterial infections.

Antimicrobial polymer-siRNA polyplexes as a dual-mode platform for the treatment of wound biofilm infections

Treatment of wound biofilm infections faces challenges from both pathogens and uncontrolled host immune response. Treating both issues through a single vector would provide enhanced wound healing. Here, we report the use of a potent cationic antimicrobial polymer to generate siRNA polyplexes for dual-mode treatment of wound biofilms in vivo. These polyplexes act both as an antibiofilm agent and a delivery vehicle for siRNA for the knockdown of biofilm-associated pro-inflammatory MMP9 in host macrophages. The resulting polyplexes were effective in vitro, eradicating MRSA biofilms and efficiently delivering siRNA to macrophages in vitro with concomitant knockdown of MMP9. These polyplexes were likewise effective in an in vivo murine wound biofilm model, significantly reducing bacterial load in the wound (∼99% bacterial clearance) and reducing MMP9 expression by 80% (qRT-PCR). This combination therapeutic strategy dramatically reduced wound purulence and significantly expedited wound healing. Taken together, these polyplexes provide an effective and translatable strategy for managing biofilm-infected wounds.

Global Trends in Research of Antimicrobial Peptides for the Treatment of Drug-Resistant Bacteria from 1995 to 2021: A Bibliometric Analysis

Background

Antimicrobial peptides (AMPs) can act on the bacterial cell membrane to play an antibacterial role in types of drug-resistant bacteria. Therefore, AMPs have attracted more and more attention in the treatment of drug-resistant bacteria.

Methods

Bibliometric analysis was employed to sort out the development and trends in the research of AMPs in the treatment of drug-resistant bacteria and map the knowledge structure for scholars.

Results

Since 2010, the publications and citations in this field have exploded, indicating a growing global interest in the field of AMPs for the treatment of drug-resistant bacteria. And as major countries in this field, China and the USA had conducted very in-depth exchanges and cooperation, which had injected a steady stream of impetus into this field. Both old and new scholars have made efforts, and related fields have developed rapidly, especially in the synthesis and improvement of novel AMPs. In recent years, research directions in the field of AMPs for the treatment of drug-resistant bacteria gradually focused on the practical application, optimization of drug delivery mode, optimization of synthesis mode, screening of new AMPs and other fields, indicating that the relevant research results of AMPs for the treatment of drug-resistant bacteria had entered the actual clinical stage, with higher practical significance.

Conclusion

The research history, global research status, future research hotspots, and trends of the research of AMPs in the treatment of drug-resistant bacteria were discussed in depth in this study, which can provide research references and inspiration for researchers inside and outside the related field.

Biocompatible cationic polypeptoids with antibacterial selectivity depending on hydrophobic carbon chain length

The overuse of antibiotics has triggered a new infection crisis and natural antimicrobial peptides (AMPs) have been extensively studied as an alternative to fight microorganisms. Polypeptoids, or polypeptide-biomimetics, offer similar properties to polypeptides and a highly tunable structure that has been synthesized by various methods such as ring opening polymerization (ROP) using N-carboxyanhydride monomers. Simultaneous high antibacterial activity and biocompatibility of a structure by efficient synthesis is desired in the application of those materials. Herein, a series of cationic polypeptoids (PNBs) with variable side chain lengths was obtained by introducing positive charges to the main chain in one step and preserving the backbone structure, namely polypeptoids (PNBM, PNBE, PNBB) with different end groups (methyl (M), ethyl (E), butyl (B)). To address the issue of infection in interventional biomedical implants, we report cost-effective modified polyurethane (PU) films (PU-PNBM, PU-PNBE, PU-PNBB) as physical-biological synergistic antibacterial surfaces that overcome problems such as steric hindrance and the solubility of the materials. Antibacterial selectivity was achieved by regulating the different side chain lengths. When methyl and ethyl were used as hydrophobic side chains, they can only selectively kill Gram-positive Staphylococcus aureus. PNBB, the most hydrophobic and with a butyl side chain can kill both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus and inhibit the growth of bacterial biofilms. Effective in both solution and modified substrate, its biocompatibility is not compromised while the antibacterial properties are substantially improved. Furthermore, PU-PNBB films demonstrated their potential in vivo antimicrobial efficiency in a model of S. aureus infection established on mouse skin. The synthesis route and the surface modification strategies are convenient, providing a solution to the problem of poor biocompatibility in antimicrobial surface applications and a strategy for the use of peptide polymers for targeted therapy after specific infections in the biomedical field.

Poly(l-Ornithine)-Based Polymeric Micelles as pH-Responsive Macromolecular Anticancer Agents

Anticancer peptides and polymers represent an emerging field of tumor treatment and can physically interact with tumor cells to address the problem of multidrug resistance. In the present study, poly(l-ornithine)-b-poly(l-phenylalanine) (PLO-b-PLF) block copolypeptides were prepared and evaluated as macromolecular anticancer agents. Amphiphilic PLO-b-PLF self-assembles into nanosized polymeric micelles in aqueous solution. Cationic PLO-b-PLF micelles interact steadily with the negatively charged surfaces of cancer cells via electrostatic interactions and kill the cancer cells via membrane lysis. To alleviate the cytotoxicity of PLO-b-PLF, 1,2-dicarboxylic-cyclohexene anhydride (DCA) was anchored to the side chains of PLO via an acid-labile β-amide bond to fabricate PLO(DCA)-b-PLF. Anionic PLO(DCA)-b-PLF showed negligible hemolysis and cytotoxicity under neutral physiological conditions but recovered cytotoxicity (anticancer activity) upon charge reversal in the weakly acidic microenvironment of the tumor. PLO-based polypeptides might have potential applications in the emerging field of drug-free tumor treatment.

Biodegradable Polymers and Polymer Composites with Antibacterial Properties

Antibiotic resistance is one of the greatest threats to global health and food security today. It becomes increasingly difficult to treat infectious disorders because antibiotics, even the newest ones, are becoming less and less effective. One of the ways taken in the Global Plan of Action announced at the World Health Assembly in May 2015 is to ensure the prevention and treatment of infectious diseases. In order to do so, attempts are made to develop new antimicrobial therapeutics, including biomaterials with antibacterial activity, such as polycationic polymers, polypeptides, and polymeric systems, to provide non-antibiotic therapeutic agents, such as selected biologically active nanoparticles and chemical compounds. Another key issue is preventing food from contamination by developing antibacterial packaging materials, particularly based on degradable polymers and biocomposites. This review, in a cross-sectional way, describes the most significant research activities conducted in recent years in the field of the development of polymeric materials and polymer composites with antibacterial properties. We particularly focus on natural polymers, i.e., polysaccharides and polypeptides, which present a mechanism for combating many highly pathogenic microorganisms. We also attempt to use this knowledge to obtain synthetic polymers with similar antibacterial activity.

Functional insights to the development of bioactive material for combating bacterial infections

The emergence of antibiotic-resistant "superbugs" poses a serious threat to human health. Nanomaterials and cationic polymers have shown unprecedented advantages as effective antimicrobial therapies due to their flexibility and ability to interact with biological macromolecules. They can incorporate a variety of antimicrobial substances, achieving multifunctional effects without easily developing drug resistance. Herein, this article discusses recent advances in cationic polymers and nano-antibacterial materials, including material options, fabrication techniques, structural characteristics, and activity performance, with a focus on their fundamental active elements.

Important Roles and Potential Uses of Natural and Synthetic Antimicrobial Peptides (AMPs) in Oral Diseases: Cavity, Periodontal Disease, and Thrush

Numerous epithelial cells and sometimes leukocytes release AMPs as their first line of defense. AMPs encompass cationic histatins, defensins, and cathelicidin to encounter oral pathogens with minimal resistance. However, their concentrations are significantly below the effective levels and AMPs are unstable under physiological conditions due to proteolysis, acid hydrolysis, and salt effects. In parallel to a search for more effective AMPs from natural sources, considerable efforts have focused on synthetic stable and low-cytotoxicy AMPs with significant activities against microorganisms. Using natural AMP templates, various attempts have been used to synthesize sAMPs with different charges, hydrophobicity, chain length, amino acid sequence, and amphipathicity. Thus far, sAMPs have been designed to target Streptococcus mutans and other common oral pathogens. Apart from sAMPs with antifungal activities against Candida albicans, future endeavors should focus on sAMPs with capabilities to promote remineralization and antibacterial adhesion. Delivery systems using nanomaterials and biomolecules are promising to stabilize, reduce cytotoxicity, and improve the antimicrobial activities of AMPs against oral pathogens. Nanostructured AMPs will soon become a viable alternative to antibiotics due to their antimicrobial mechanisms, broad-spectrum antimicrobial activity, low drug residue, and ease of synthesis and modification.

Polymers showing intrinsic antimicrobial activity

Pathogenic microorganisms are considered to a major threat to human health, impinging on multiple sectors including hospitals, dentistry, food storage and packaging, and water contamination. Due to the increasing levels of antimicrobial resistance shown by pathogens, often caused by long-term abuse or overuse of traditional antimicrobial drugs, new approaches and solutions are necessary. In this area, antimicrobial polymers are a viable solution to combat a variety of pathogens in a number of contexts. Indeed, polymers with intrinsic antimicrobial activities have long been an intriguing research area, in part, due to their widespread natural abundance in materials such as chitin, chitosan, carrageen, pectin, and the fact that they can be tethered to surfaces without losing their antimicrobial activities. In addition, since the discovery of the strong antimicrobial activity of some synthetic polymers, much work has focused on revealing the most effective structural elements that give rise to optimal antimicrobial properties. This has often been synthesis targeted, with the generation of either new polymers or the modification of natural antimicrobial polymers with the addition of antimicrobial enhancing modalities such as quaternary ammonium or guanidinium groups. In this review, the growing number of polymers showing intrinsic antimicrobial properties from the past decade are highlighted in terms of synthesis; often based on post-synthesis modification and their utilization. This includes as surface coatings, for example on medical devices, such as intravascular catheters, orthopaedic implants and contact lenses, or directly as antibacterial agents (specifically as eye drops). Surface functionalisation with inherently antimicrobial polymers is highlighted and has been achieved via various techniques, including surface-bound initiators allowing RAFT or ATRP surface-based polymerization, or via physical immobilization such as by layer-by-layer techniques. This article also covers the mechanistic modes of action of intrinsic antimicrobial polymers against bacteria, viruses, or fungi.

Secondary Ammonium-Based Hyperbranched Poly(amidoamine) with Excellent Membrane-Active Property for Multidrug-Resistant Bacterial Infection

With the rapid emergence of microbial infections induced by "superbugs", public health and the global economy are threatened by the lack of effective and biocompatible antibacterial agents. Herein, we systematically design a series of secondary ammonium-based hyperbranched poly(amidoamine) (SAHBP) with different alkyl chain lengths for probing high-efficacy antibacterial agents. SAHBP modified with alkyl tails at the hyperbranched core could efficiently kill Escherichia coli and Staphylococcus aureus, two types of clinically important bacteria worldwide. The best SAHBP with 12-carbon-long alkyl tails (SAHBP-12) also showed high activity against problematic multidrug-resistant bacteria, including Pseudomonas aeruginosa and methicillin-resistant S. aureus (MRSA). Based on ζ potential, isothermal titration microcalorimetry (ITC), and membrane integrity assays, it is found that SAHBP-12 could attach to the cell membrane via electrostatic adsorption and hydrophobic interactions, following which the integrity of the bacterial cell wall and the cell membrane is disrupted, resulting in severe cell membrane damage and the leakage of cytoplasmic contents, finally causing bacterial cell death. Impressively, benefiting from excellent membrane-active property, SAHBP-12 exhibited robust therapeutic efficacy in MRSA-infected mice wounds. Moreover, SAHBP-12 also showed excellent biosafety in vitro and in vivo, which undoubtedly distinguished it as a potent weapon in combating the growing threat of problematic multidrug-resistant bacterial infections.

Recent advances in post-polymerization modifications on polypeptides: synthesis and applications

Polypeptides, a kind of very promising biomaterial, have shown a wide range of applications due to their excellent biocompatibility, easy accessibility, and structural variability. To synthesize polypeptides with desired functions, post-polymerization modification (PPM) plays an important role in introducing novel chemical structure on their side-chains. The key of PPM strategy is to develop highly selective and efficient reactions that can couple the additional functional moieties with pre-installed side-chain functionalities on polypeptides. In this minireview, classic PPM reactions and especially their recent progresses are summarized, including different modification approaches for unsaturated alkyl group, oxygen-containing functional group, nitrogen-containing functional group, sulfur-containing functional group and other special functional group on side chains. In addition, this review also highlights the applications of structure-diversified polypeptides generated via PPM strategy in the field of biomaterial.

Peptidomimetic Wet-Adhesive PEGtides with Synergistic and Multimodal Hydrogen Bonding

The remarkable underwater adhesion of mussel foot proteins has long been an inspiration in the design of peptidomimetic materials. Although the synergistic wet adhesion of catechol and lysine has been recently highlighted, the critical role of the polymeric backbone has remained largely underexplored. Here, we present a peptidomimetic approach using poly(ethylene glycol) (PEG) as a platform to evaluate the synergistic compositional relation between the key amino acid residues (i.e., DOPA and lysine), as well as the role of the polyether backbone in interfacial adhesive interactions. A series of PEG-based peptides (PEGtides) were synthesized using functional epoxide monomers corresponding to catechol and lysine via anionic ring-opening polymerization. Using a surface force apparatus, highly synergistic surface interactions among these PEGtides with respect to the relative compositional ratio were revealed. Furthermore, the critical role of the catechol-amine synergy and diverse hydrogen bonding within the PEGtides in the superior adhesive interactions was verified by molecular dynamics simulations. Our study sheds light on the design of peptidomimetic polymers with reduced complexity within the framework of a polyether backbone.

Secondary Amine Pendant β-Peptide Polymers Displaying Potent Antibacterial Activity and Promising Therapeutic Potential in Treating MRSA-Induced Wound Infections and Keratitis

Interest in developing antibacterial polymers as synthetic mimics of host defense peptides (HPDs) has accelerated in recent years to combat antibiotic-resistant bacterial infections. Positively charged moieties are critical in defining the antibacterial activity and eukaryotic toxicity of HDP mimics. Most examples have utilized primary amines or guanidines as the source of positively charged moieties, inspired by the lysine and arginine residues in HDPs. Here, we explore the impact of amine group variation (primary, secondary, or tertiary amine) on the antibacterial performance of HDP-mimicking β-peptide polymers. Our studies show that a secondary ammonium is superior to either a primary ammonium or a tertiary ammonium as the cationic moiety in antibacterial β-peptide polymers. The optimal polymer, a homopolymer bearing secondary amino groups, displays potent antibacterial activity and the highest selectivity (low hemolysis and cytotoxicity). The optimal polymer displays potent activity against antibiotic-resistant bacteria and high therapeutic efficacy in treating MRSA-induced wound infections and keratitis as well as low acute dermal toxicity and low corneal epithelial cytotoxicity. This work suggests that secondary amines may be broadly useful in the design of antibacterial polymers.

Antimicrobial Peptides and Macromolecules for Combating Microbial Infections: From Agents to Interfaces

Bacterial resistance caused by the overuse of antibiotics and the shelter of biofilms has evolved into a global health crisis, which drives researchers to continuously explore antimicrobial molecules and strategies to fight against drug-resistant bacteria and biofilm-associated infections. Cationic antimicrobial peptides (AMPs) are considered to be a category of potential alternative for antibiotics owing to their excellent bactericidal potency and lesser likelihood of inducing drug resistance through their distinctive antimicrobial mechanisms. In this review, the hitherto reported plentiful action modes of AMPs are systematically classified into 15 types and three categories (membrane destructive, nondestructive membrane disturbance, and intracellular targeting mechanisms). Besides natural AMPs, cationic polypeptides, synthetic polymers, and biopolymers enable to achieve tunable antimicrobial properties by optimizing their structures. Subsequently, the applications of these cationic antimicrobial agents at the biointerface as contact-active surface coatings and multifunctional wound dressings are also emphasized here. At last, we provide our perspectives on the development of clinically significant cationic antimicrobials and related challenges in the translation of these materials.

Polymers as advanced antibacterial and antibiofilm agents for direct and combination therapies

The growing prevalence of antimicrobial drug resistance in pathogenic bacteria is a critical threat to global health. Conventional antibiotics still play a crucial role in treating bacterial infections, but the emergence and spread of antibiotic-resistant micro-organisms are rapidly eroding their usefulness. Cationic polymers, which target bacterial membranes, are thought to be the last frontier in antibacterial development. This class of molecules possesses several advantages including a low propensity for emergence of resistance and rapid bactericidal effect. This review surveys the structure-activity of advanced antimicrobial cationic polymers, including poly(α-amino acids), β-peptides, polycarbonates, star polymers and main-chain cationic polymers, with low toxicity and high selectivity to potentially become useful for real applications. Their uses as potentiating adjuvants to overcome bacterial membrane-related resistance mechanisms and as antibiofilm agents are also covered. The review is intended to provide valuable information for design and development of cationic polymers as antimicrobial and antibiofilm agents for translational applications.

Versatile fully-substituted triazole-functionalized polypeptides with a stable α-helical conformation for gene delivery

A library of polypeptides bearing fully-substituted triazoles (FT) was developed via a Cu-catalyzed multicomponent reaction (MCR), which avoided the undesired hydrogen bonding and stabilized the α-helix in a broad pH range.

Uniform, Length-Tunable Antibacterial 1D Diblock Copolymer Nanofibers

The rapid increase in antibiotic resistant strains of bacteria has led to an urgent need to develop new methods of treating bacterial infections. Antibacterial polymeric nanoparticles are of interest for...

Crucial Impact of Residue Chirality on the Gelation Process and Biodegradability of Thermoresponsive Polypeptide Hydrogels

Thermosensitive polypeptide hydrogels have gained considerable attention in potential biomedical applications, of which the polymer structure may be tuned by residue chirality. In this study, polypeptide-based block copolymers with different chiralities were synthesized by ring-opening polymerization of γ-ethyl-l-glutamate N-carboxyanhydride and/or γ-ethyl-d-glutamate N-carboxyanhydride using amino-terminated monomethoxy poly(ethylene glycol) as a macroinitiator. All mPEG-polypeptide copolymers underwent sol-gel transition with an increase in temperature. The block copolymers with mixed enantiomeric residues of γ-ethyl-l-glutamate (ELG) and γ-ethyl-d-glutamate (EDG) in the polypeptide blocks exhibited lower critical gelation concentrations and lower critical gelation temperatures compared with those composed of pure ELG or EDG residues. We established that the difference in gelation properties between the copolymers was derived from the distinction of the secondary structures. We further demonstrated the influence of polypeptide chirality on the degradability and biocompatibility of hydrogels in vivo. Our findings provide insights into the design of hydrogels having tailored secondary conformation, gelation property, and biodegradability.

Facile Synthesis of Imidazolium-Based Block Copolypeptides with Excellent Antimicrobial Activity

Antimicrobial polypeptides are promising mimics of antimicrobial peptides (AMPs) with low risks of antimicrobial resistance (AMR). Polypeptides with facile and efficient production, high antimicrobial activity, and low toxicity toward mammalian cells are highly desirable for practical applications. Herein, triblock copolypeptides with chloro groups (PPG_n-PCPBLG_m) and different main-chain lengths were synthesized via an ultrafast ring-opening polymerization (ROP) using a macroinitiator, namely poly(propylene glycol) bis(2-aminopropyl ether), and purified or nonpurified monomer (i.e., CPBLG-NCA). PPG_n-PCPBLG_m with 90 amino acid residues can be readily prepared within 300 s. Imidazolium-based block copolypeptides (PPG_n-PIL_m) were facilely prepared via nucleophilic substitution of PPG_n-PCPBLG_m with NaN₃ and subsequent "click" chemistry. α-Helical PPG_n-PIL_m can self-assemble into nanostructured and cationic micelles which displayed highly potent antimicrobial activity and low hemolysis. The top-performing material, namely PPG₃₄-PIL₇₀, showed low minimum inhibitory concentration (MIC) against both Gram-positive S. aureus and Gram-negative E. coli (25 μg mL^-1). It also displayed low toxicity against mouse embryonic fibroblast (NIH 3T3) and human embryonic kidney (293T) cells at 2× MIC.

Recent Progress in Bile Acid-Based Antimicrobials

With the emergence of drug-resistant bacteria and the formation of biofilms by bacteria and fungi, microbial infections gradually threaten global health. Natural antimicrobial peptides (AMPs) have low susceptibility for developing resistance due to the membrane targeted mechanism, but instability and high manufacturing cost limit their applications in clinic. Bile acids, a group of steroids in the human body, with high stability, biocompatibility, and inherent facial amphiphilic structure similar to the characteristics of AMPs, have been applied to the biological field, such as drug delivery systems, self-healing hydrogels, antimicrobials, and so on. In this review, we mainly focus on the different classes of bile acid-based antimicrobials in recent years. Various designs and methods for the preparation of unimolecular antimicrobials with bile acid skeletons are first introduced, including coupling of primary amine, quaternary ammonium, and amino acid units with bile acid skeletons. Some representative oligomeric antimicrobials, including dimers of bile acids, are summarized. Finally, macromolecular antimicrobials bearing some positive charges at the main chain or side chain and interaction mechanisms of these bile acid-based antimicrobials are discussed.

Recent advances in design of antimicrobial peptides and polypeptides toward clinical translation

The recent outbreaks of infectious diseases caused by multidrug-resistant pathogens have sounded a piercing alarm for the need of new effective antimicrobial agents to guard public health. Among different types of candidates, antimicrobial peptides (AMPs) and the synthetic mimics of AMPs (SMAMPs) have attracted significant enthusiasm in the past thirty years, due to their unique membrane-active antimicrobial mechanism and broad-spectrum antimicrobial activity. The extensive research has brought many drug candidates into clinical and pre-clinical development. Despite tremendous progresses have been made, several major challenges inherent to current design strategies have slowed down the clinical translational development of AMPs and SMAMPs. However, these challenges also triggered many efforts to redesign and repurpose AMPs. In this review, we will first give an overview on AMPs and their synthetic mimics, and then discuss the current status of their clinical translation. Finally, the recent advances in redesign and repurposing AMPs and SMAMPs are highlighted.

Unnatural Amino-Acid-Based Star-Shaped Poly(l-Ornithine)s as Emerging Long-Term and Biofilm-Disrupting Antimicrobial Peptides to Treat Pseudomonas aeruginosa-Infected Burn Wounds

Peptide-based antimicrobial materials are recognized as promising alternatives to antibiotics to circumvent the emergence of antibiotic-resistant bacteria or to combat multiple resistant bacteria by targeting the bacterial cell membrane. The components and conformations of antimicrobial peptides are extensively explored to achieve broad-spectrum and effective antimicrobial activity. Here, star-shaped antimicrobial polypeptides are fabricated by employing homologs of poly(l-lysine)s (i.e., poly(l-ornithine)s, poly(l-lysine)s, and poly(l-α,ζ-diaminoheptylic acid)s) with the aim of modulating their charge/hydrophobicity balance and rationalizing their structure-antimicrobial property relationships. The in vitro antibacterial investigation reveals that unnatural amino-acid-based star-shaped poly(l-ornithine)s have remarkable proteolytic stability, excellent biofilm-disrupting capacity, and broad-spectrum antimicrobial activity, even against difficult-to-kill Gram-negative Pseudomonas aeruginosa. Furthermore, star-shaped poly(l-ornithine)s significantly reduce the microbial burden and improve the burn wound healing of mouse skin infected with P. aeruginosa. These results demonstrate that unnatural amino-acid-based star-shaped poly(l-ornithine)s can serve as emerging long-term and biofilm-disrupting antimicrobial agents to treat biofilm-related infections in burn, especially caused by notorious P. aeruginosa.

Synthetic Polypeptide Polymers as Simplified Analogues of Antimicrobial Peptides

Antimicrobial peptides (AMPs) are naturally occurring macromolecules made of amino acids that are potent broad-spectrum antibiotics with potential as novel therapeutic agents. This review aims to summarize the fundamental principles concerning the structure and mechanism of action of these AMPs, in order to guide the design of polymeric analogues that organic chemistry can generate. Among those simplified analogues, this review particularly focuses on those made of amino acids called polypeptide polymers: they are showing great potential by providing one of the best biomimetic and bioactive structures for further biomaterials science applications.

Antimicrobial peptide polymers: no escape to ESKAPE pathogens-a review

Antimicrobial resistance (AMR) is one of the significant clinical challenges and also an emerging area of concern arising from nosocomial infections of ESKAPE pathogens, which has been on the rise in both the developed and developing countries alike. These pathogens/superbugs can undergo rapid mutagenesis, which helps them to generate resistance against antimicrobials in addition to the patient's non-adherence to the antibiotic regimen. Sticking to the idea of a 'one-size-fits-all' approach has led to the inappropriate administration of antibiotics resulting in augmentation of antimicrobial resistance. Antimicrobial peptides (AMPs) are the natural host defense peptides that have gained attention in the field of AMR, and recently, synthetic AMPs are well studied to overcome the drawbacks of natural counterparts. This review deals with the novel techniques utilizing the bacteriolytic activity of natural AMPs. The effective localization of these peptides onto the negatively charged bacterial surface by using nanocarriers and structurally nanoengineered antimicrobial peptide polymers (SNAPPs) owing to its smaller size and better antimicrobial activity is also described here.

Helical polymers for biological and medical applications

Helices are the most prevalent secondary structure in biomolecules and play vital roles in their activity. Chemists have been fascinated with mimicking this molecular conformation with synthetic materials. Research has now been devoted to the synthesis and characterization of helical materials, and to understand the design principles behind this molecular architecture. In parallel, work has been done to develop synthetic polymers for biological and medical applications. We now have access to materials with controlled size, molecular conformation, multivalency or functionality. As a result, synthetic polymers are being investigated in areas such as drug and gene delivery, tissue engineering, imaging and sensing, or as polymer therapeutics. Here, we provide a critical view of where these two fields, helical polymers and polymers for biological and medical applications, overlap. We have selected relevant polymer families and examples to illustrate the range of applications that can be targeted and the impact of the helical conformation on the performance. For each family of polymers, we briefly describe how they can be prepared, what helical conformations are observed and what parameters control helicity. We close this Review with an outlook of the challenges ahead, including the characterization of helicity through the process and the identification of biocompatibility.

Synthetic macromolecules as therapeutics that overcome resistance in cancer and microbial infection

Synthetic macromolecular antimicrobials have shown efficacy in the treatment of multidrug resistant (MDR) pathogens. These synthetic macromolecules, inspired by Nature's antimicrobial peptides (AMPs), mitigate resistance by disrupting microbial cell membrane or targeting multiple intracellular proteins or genes. Unlike AMPs, these polymers are less prone to degradation by proteases and are easier to synthesize on a large scale. Recently, various studies have revealed that cancer cell membrane, like that of microbes, is negatively charged, and AMPs can be used as anticancer agents. Nevertheless, efforts in developing polymers as anticancer agents has remained limited. This review highlights the recent advancement in the development of synthetic biodegradable antimicrobial polymers (e.g. polycarbonates, polyesters and polypeptides) and anticancer macromolecules including peptides and polymers. Additionally, strategies to improve their in vivo bioavailability and selectivity towards bacteria and cancer cells are examined. Lastly, future perspectives, including use of artificial intelligence or machine learning, in the development of antimicrobial and anticancer macromolecules are discussed.

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.

Structural design and antimicrobial properties of polypeptides and saccharide–polypeptide conjugates

The development and progress of antimicrobial polypeptides and saccharide–polypeptide conjugates in regards to their structural design, biological functions and antimicrobial mechanism.

Amino Acid Conjugated Polymers: Antibacterial Agents Effective against Drug-Resistant Acinetobacter baumannii with No Detectable Resistance

An optimum hydrophilic/hydrophobic balance has been recognized as a crucial parameter in designing cationic polymers that mimic antimicrobial peptides (AMPs). To date, this balance was achieved either by hydrophilicity variation through altering the nature and the number of cationic charges or by hydrophobicity modulation through incorporation of alkyl groups of different chain lengths. However, how the hydrophobicity variation through AMPs' building blocks-amino acids-influences the antibacterial efficacy of AMP-mimicking cationic polymers has rarely been explored. Toward this goal, herein we report a class of amino acid conjugated polymers (ACPs) with tunable antibacterial activity through a simple post-polymer-functionalization strategy. Our polymeric design comprised a permanent cationic charge in every repeating unit, whereby the hydrophobicity was tuned through incorporation of different amino acids. Our results revealed that the amino acid alteration has a strong influence on antibacterial efficacy. Upon increasing the amino acid side-chain hydrophobicity, both the antibacterial activity (against broad spectrum of bacteria) and toxicity increased. However, the distinct feature of this class of polymers was their good activity against Acinetobacter baumannii-the top most critical pathogen according to WHO, which has created an alarming situation worldwide, causing the majority of infections in humans. A nontoxic (no hemolysis even at 1000 μg/mL) ACP including a glycine residue (ACP-1 (Gly)) showed very good activity (MIC = 8-16 μg/mL) against both drug-sensitive and drug-resistant strains of A. baumannii, including clinical isolates. This polymer not only was rapidly bactericidal against growing planktonic A. baumannii but also killed nondividing stationary-phase cells instantaneously (<2 min). Moreover, it eradicated the established biofilm formed by drug-resistant A. baumannii clinical isolates. No propensity for bacterial resistance development against this polymer was seen even after 14 continuous passages. Taken together, the results highlight that hydrophobicity modulation through incorporation of amino acids in cationic polymers will provide a significant opportunity in designing new ACPs with potent antibacterial activity and minimum toxicity toward mammalian cells. More importantly, the excellent anti-A. baumannii efficacy of the optimized lead polymer indicates its immense potential for being developed as therapeutic agent.

Thermo- and oxidation-responsive homopolypeptide: synthesis, stimuli-responsive property and antimicrobial activity

We present the design and synthesis of a side-chain functional polypeptide with a thioether spacer that is readily decorated to produce thermo- and oxidation-responsive polymers with antimicrobial activity.

Nanoengineered Peptide-Grafted Hyperbranched Polymers for Killing of Bacteria Monitored in Real Time via Intrinsic Aggregation-Induced Emission

Facing the global health crisis caused by drug-resistant bacteria, antimicrobial peptides and their analogues offer exciting solutions to this widespread problem. Without additionally introducing a fluorescent probe, novel nanoengineered peptide-grafted hyperbranched polymers (NPGHPs) are constructed for their combined outstanding antimicrobial activity and sensitive bacterial detection in real time. Hyperbranched polyamide amine (H-PAMAM) that exhibits aggregation-induced emission (AIE) effects is synthesized. Then, NPGHPs are prepared by ring-opening polymerization of α-amino acid N-carboxyanhydrides on the periphery of the H-PAMAM. The NPGHPs exhibit high-efficiency antibacterial properties against a wide spectrum of bacteria, especially against Gram-negative bacteria. On the basis of the AIE effect of NPGHPs, the interaction between NPGHPs and Escherichia coli is explored and the fluorescence intensity of NPGHPs is dependent on the number of E. coli present. Thus, a method for monitoring E. coli concentration is developed, and the detection limit is 1 × 10⁴ CFU mL^-1. Furthermore, NPGHPs are used as fluorescent probes to visualize antibacterial process via lighting-up bacteria. NPGHPs can penetrate the membrane of bacteria and cause cell rupture and apoptosis. In addition, the excellent selectivity of NPGHPs toward bacteria over mammalian cells makes them bright prospects for clinical applications.

Theoretical Insights into the Interactions between Star-Shaped Antimicrobial Polypeptides and Bacterial Membranes

A structurally nanoengineered antimicrobial polypeptide consisting of lysine and valine residues is a new class of antimicrobial agent with superior antibacterial activity against multidrug-resistant bacteria and low toxicity toward mammalian cells. Utilizing coarse-grained models, we studied the interactions of microbial cytoplasmic membranes with polypeptides of either (K₂V₁)₅ (star-KV) or CM15 (star-CM15). Our computational results verify the low toxicity of polypeptides of (K₂V₁)₅ toward the dipalmitoyl phosphatidylcholine bilayer. This low toxicity is demonstrated to originate from weakened hydrophobicity combined with its random coil conformation for (K₂V₁)₅ because of the highly abundant valine residues, compared with the typical antimicrobial peptides, such as CM15. In the interactions with a palmitoyl-oleoyl-phosphatidylethanolamine/palmitoyl-oleoyl-phosphatidylglycerol bilayer, star-KV has greater ability in phase separation and generation of phase boundary defects not only in lipid redistribution but also in lateral dynamic movements, although both star-KV and star-CM15 can extract the phosphatidylglycerol lipids and purify the phosphatidylethanolamine lipids into continuum domains. We suggest that the polypeptide of (K₂V₁)₅ can nondisruptively kill bacteria by hampering bacterial metabolism through reorganizing lipid domain distribution and simultaneously "freezing" lipid movement.

Architectural Effects of Star-Shaped "Structurally Nanoengineered Antimicrobial Peptide Polymers" (SNAPPs) on Their Biological Activity

In this work, the effect of two key structural parameters, number of arms and arm length, of star-shaped "structurally nanoengineered antimicrobial peptide polymers" (SNAPPs) on their antimicrobial activity and biocompatibility, is investigated. A library of star-shaped SNAPPs is prepared, containing varying arm numbers and arm lengths. Antimicrobial assays are then performed to assess the capacity of the SNAPPs to disrupt the membrane, inhibit the growth, and kill pathogenic bacteria. A major finding of the study is that increasing arm number and length of SNAPPs enhanced antimicrobial activity, which can be respectively attributed to the higher local concentrations of polypeptide arms and increased α-helical content. SNAPP architecture is shown to affect the bacteria membrane state and therefore mechanism of killing. Two more potent structures with up to twice the antimicrobial activity of the previously reported SNAPP are discovered in this process. Toxicities of the SNAPPs also increase with arm number and arm length, however therapeutic index calculations identified a 16-arm SNAPP and an easier to prepare 4-arm SNAPP as the best therapeutic agents. The biocompatibility of the SNAPP with the best biological activity is also evaluated in vivo, showing no markers of systemic damage in mice.

Functional biodegradable polymers via ring-opening polymerization of monomers without protective groups

Biodegradable polymers are of current interest and chemical functionality in such materials is often demanded in advanced biomedical applications. Functional groups often are not tolerated in the polymerization process of ring-opening polymerization (ROP) and therefore protective groups need to be applied. Advantageously, several orthogonally reactive functions are available, which do not demand protection during ROP. We give an insight into available, orthogonally reactive cyclic monomers and the corresponding functional synthetic and biodegradable polymers, obtained from ROP. Functionalities in the monomer are reviewed, which are tolerated by ROP without further protection and allow further post-modification of the corresponding chemically functional polymers after polymerization. Synthetic concepts to these monomers are summarized in detail, preferably using precursor molecules. Post-modification strategies for the reported functionalities are presented and selected applications highlighted.

From Antimicrobial Peptides to Antimicrobial Poly(α-amino acid)s

Conventional small-molecule antibiotics are facing a significant challenge of the rapidly developed drug resistance of pathogens. In contrast, antimicrobial peptides (AMPs), an important component for innate host defenses, are now under intensive investigation as a promising antimicrobial agent for combating drug resistant pathogens. Most AMPs can effectively kill a broad spectrum of pathogens via physical disruption of microbial cellular membranes, which is identified to be difficult to develop resistance. However, the clinical applications of AMPs are still greatly limited by several inherent impediments, such as high cost of production, potential hemolysis or toxicity, and liability to proteinase degradation. Recently, cationic poly(α-amino acid)s with structures mimicking the AMPs are found to have excellent antimicrobial activity. These polymers, termed "antimicrobial poly(α-amino acid)s (APAAs)," have some advantages over AMPs, such as easy production and modification, prolonged antimicrobial activity, low cytotoxicity, and enhanced stability to protease degradation. Here, a brief introduction of mechanisms and affecting factors of microbial killing by AMPs is first presented, followed by a systematic illustration of recent advances in design and preparation of biomimetic APAAs and a perspective in this field.

Antibacterial and Potentiation Properties of Charge-Optimized Polyrotaxanes for Combating Opportunistic Bacteria

Bacteria are now becoming more resistant to most conventional antibiotics. Approaches for the treatment of multidrug-resistant bacterial infections are urgently required. Cationic polymers have broad-spectrum antibacterial activity but can also induce non-specific damage to mammalian cells. Herein, we report on the design of cationic polyrotaxanes (cPRs) with variable charge densities. cPRs were prepared by conjugating neutral ethanolamine and cationic ethylenediamine at various ratios onto threaded alpha-cyclodextrins and their antimicrobial and cytocompatible properties were investigated in vitro. In contact with Gram-negative bacteria, cPRs can disrupt the bacterial outer membrane integrity via electrostatic interactions and penetrate into the cytosol. The ability of cPRs to serve as potentiators at sub-MIC concentrations, to enhance the permeability and activity of poorly permeable antibiotics such as vancomycin, erythromycin and rifampicin, was also investigated against Gram-negative P. aeruginosa PAO1 and E. coli ATCC 25922.

Chemo-Enzymatic Synthesis of Poly(4-piperidine lactone- b-ω-pentadecalactone) Block Copolymers as Biomaterials with Antibacterial Properties

With increasing troubles in bacterial contamination and antibiotic-resistance, new materials possessing both biocompatibility and antimicrobial efficacy are supposed to be developed for future biomedical application. Herein, we demonstrated a chemo-enzymatic ring opening polymerization (ROP) approach for block copolyester, that is, poly(4-benzyl formate piperidine lactone- b-ω-pentadecalactone) (PNPIL- b-PPDL), in a one-pot two-step process. Afterward, cationic poly(4-piperidine lactone- b-ω-pentadecalactone) (PPIL- b-PPDL) with pendent secondary amino groups was obtained via acidic hydrolysis of PNPIL- b-PPDL. The resulting cationic block copolyester exhibited high antibacterial activity against Gram negative E. coli and Gram positive S. aureus, while showed low toxicity toward NIH-3T3 cells. Moreover, the antibacterial property, cytotoxicity and degradation behavior could be tuned simply by variation of PPIL content. Therefore, we anticipate that such cationic block copolymers could potentially be applied as biomaterials for medicine or implants.

Pyrene-Terminated, Amphiphilic Polypeptide and Its Hydrogen-Bonded Interpolymer Complex as Delivery Systems of Doxorubicin

The intensity ratio between the first (373 nm) and the third (383 nm) vibronic peaks [I ₁/I ₃, as the pyrene (Py) scale] of fluorescent Py was used to monitor the critical concentration, drug-loading, and -releasing behaviors of a Py-terminated, amphiphilic polypeptide PPM and its hydrogen-bonded interpolymer complex (HIPC) with poly(acrylic acid) (PAA). Primarily, an amphiphilic PPM with a hydrophobic Py terminal and hydrophilic methoxy-bis(ethylene oxide) pendant groups was synthesized through multiple preparative steps, and the resultant PPM was thoroughly mixed with PAA through a preferable hydrogen bond (H bond) interaction to form HIPC. The emission study suggested that the I ₁/I ₃ ratio and the quantum yield (Φ_F) are effective in determining the critical concentrations of the aqueous PPM and PPM/PAA solutions. Moreover, the I ₁/I ₃ ratio and Φ_F were found to be convenient measures for determining the amounts of doxorubicin drugs loaded by and released from the aqueous PPM and PPM/PAA solutions.

Secondary structures of synthetic polypeptide polymers

Synthetic peptide-based polymers can fold into different secondary structures in the same way as do proteins. This review article presents how tuning the polypeptide secondary structure could be a key step to modulate various properties in advanced polymeric materials (size, rigidity, self-assembly,etc.).