Ultrashort self-assembling Fmoc-peptide gelators for anti-infective biomaterial applications

Rational design of self-assembling ultrashort peptides for the shape- and size-tunable synthesis of metal nanostructures

Peptides have attracted great interest as platforms for the design of nanocomposite hydrogels due to their distinct bioactivity, biofunctionality and biocompatibility. Previously, we have reported on a family of peptides that self-assembled to form stabilised three-dimensional hydrogel networks, displaying potent antimicrobial activity. In this paper, we report on the use of these hydrogelator sequences and their analogues as stabilisers and growth controllers to synthesise anisotropic gold nanoparticles (AuNPs) of different sizes and shapes. In particular, hollow spherical nanoparticles were obtained for HG2.81-AuNPs, whereas hexagonal nanoparticles were observed for TOH_1N-AuNPs and PentaOH-AuNPs in their respective hydrogel networks. The PentaOH-AuNPs' hydrogel exhibited excellent results with high antimicrobial potency against Staphylococcus aureus and Pseudomonas aeruginosa ATCC 27853 and negligible cytotoxicity. On the other hand, TOH_1N-AuNPs showed no antibacterial activity and no cytotoxicity, demonstrating the versatility of these peptides. This work gives credence towards the development of these materials towards further applications such as in tissue culture technology and wound dressing materials.

Antimicrobial therapy based on self-assembling peptides

The emergence of drug-resistant microorganisms has threatened global health, and microbial infections have severely limited the use of medical materials. For example, the attachment and colonization of pathogenic bacteria to medical implant materials can lead to wound infections, inflammation and complications, as well as implant failure, shortening their lifespan and even resulting in patient death. In the era of antibiotic resistance, antimicrobial drug discovery needs to prioritize unconventional therapies that act on new targets or adopt new mechanisms. In this regard, supramolecular antimicrobial peptides have emerged as attractive therapeutic platforms, both as bactericides for combination antibiotics and as delivery vehicles. By taking advantage of their programmable intermolecular and intramolecular interactions, peptides can be modified to form higher-order structures (including nanofibers and nanoparticles) with unique functionality. This paper begins with an analysis of the relationship between peptide self-assembly and antimicrobial activity, describes in detail the research and development of various self-assembled antimicrobial peptides in recent years, and finally explores different combinatorial strategies for self-assembling antimicrobial peptides.

Nanostructured Medical Devices: Regulatory Perspective and Current Applications

Nanomaterials (NMs) are having a huge impact in several domains, including the fabrication of medical devices (MDs). Hence, nanostructured MDs are becoming quite common; nevertheless, the associated risks must be carefully considered in order to demonstrate safety prior to their immission on the market. The biological effect of NMs requires the consideration of methodological issues since already established methods for, e.g., cytotoxicity can be subject to a loss of accuracy in the presence of certain NMs. The need for oversight of MDs containing NMs is reflected by the European Regulation 2017/745 on MDs, which states that MDs incorporating or consisting of NMs are in class III, at highest risk, unless the NM is encapsulated or bound in such a manner that the potential for its internal exposure is low or negligible (Rule 19). This study addresses the role of NMs in medical devices, highlighting the current applications and considering the regulatory requirements of such products.

Evaluation of Pharmacokinetic and Pharmacodynamic (PK/PD) of Novel Fluorenylmethoxycarbonyl- Phenylalanine Antimicrobial Agent

OBJECTIVE

To assess the pharmacokinetic profile, in-vivo toxicity, and efficacy of 9-Fluorenylmethoxycarbonyl-L-phenylalanine (Fmoc-F) as a potential antibacterial agent, with a focus on its suitability for clinical translation.

METHODS

An RP-HPLC-based bio-analytical method was developed and qualified to quantify Fmoc-F levels in mouse plasma for pharmacokinetic analysis. Oral bioavailability was determined, and in-vivo toxicity was evaluated following intra-peritoneal administration. Efficacy was assessed by measuring the reduction in Staphylococcus aureus burden and survival rates in BALB/c mice.

RESULTS

The RP-HPLC method is highly sensitive, detecting as low as 0.8 µg mL-1 (~ 2 µM) of Fmoc-F in blood plasma. This study revealed that Fmoc-F has an oral bioavailability of 65 ± 18% and suitable pharmacokinetic profile. Further, we showed that intra-peritoneal administration of Fmoc-F is well tolerated by BALB/c mice and Fmoc-F treatment (100 mg/kg, i.p.) significantly reduces Staphylococcus aureus burden from visceral organs in BALB/c mice but falls short in enhancing survival rates at higher bacterial loads.

CONCLUSIONS

The study provides crucial insights into the pharmacokinetic and pharmacodynamic properties of Fmoc-F. The compound displayed favourable oral bioavailability and in-vivo tolerance. Its significant reduction of bacterial burden underscores its potential as a treatment for systemic infections. However, limited effectiveness for severe infections, short half-life, and inflammatory response at higher doses need to be addressed for its clinical application.

Tuning conductivity while maintaining mechanical properties in perylene bisimide hydrogels at physiological pH

By using different salts as a method to achieve gelation of two different amino-acid-functionalised perylene bisimides, we were able to tune reduction potentials while maintaining the mechanical and optical properties of the system all at pH 7.4.

Antimicrobial Potency of Fmoc-Phe-Phe Dipeptide Hydrogels with Encapsulated Porphyrin Chromophores Is a Promising Alternative in Antimicrobial Resistance

Antimicrobial resistance (AMR) poses a significant global health risk as a consequence of misuse of antibiotics. Owing to the increasing antimicrobial resistance, it became imperative to develop novel molecules and materials with antimicrobial properties. Porphyrins and metalloporphyrins are compounds which present antimicrobial properties especially after irradiation. As a consequence, porphyrinoids have recently been utilized as antimicrobial agents in antimicrobial photodynamic inactivation in bacteria and other microorganisms. Herein, we report the encapsulation of porphyrins into peptide hydrogels which serve as delivery vehicles. We selected the self-assembling Fmoc-Phe-Phe dipeptide, a potent gelator, as a scaffold due to its previously reported biocompatibility and three different water-soluble porphyrins as photosensitizers. We evaluated the structural, mechanical and in vitro degradation properties of these hydrogels, their interaction with NIH3T3 mouse skin fibroblasts, and we assessed their antimicrobial efficacy against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria. We found out that the hydrogels are cytocompatible and display antimicrobial efficiency against both strains with the zinc porphyrins being more efficient. Therefore, these hydrogels present a promising alternative for combating bacterial infections in the face of growing AMR concerns.

Simvastatin-Releasing Nanofibrous Peptide Hydrogels for Accelerated Healing of Diabetic Wounds

Wound healing is one of the major global health concerns in diabetic patients. Simvastatin (SMV) is a poorly soluble oral cholesterol-lowering drug that may aid diabetic wound healing. In the current study, a thixotropic peptide hydrogel of Fmoc-diphenylalanine (FmocFF) containing SMV was designed to accelerate skin wound healing effectively and safely in diabetic mice. FmocFF hydrogels were prepared at various concentrations by using the solvent-triggering technique and characterized by spectroscopic methods such as attenuated total reflection Fourier transform infrared (FT-IR) spectroscopy and fluorimetry. Mechanical behaviors were explored by oscillatory rheology. In model mice, the regenerative potential of the FmocFF-SMV hydrogel was evaluated in terms of wound contraction and closure, tissue regeneration, acute and chronic inflammation, granulation, and re-epithelization. The results showed that FmocFF-SMV hydrogels had an entangled nanofibrous microstructure and shear-thinning characteristics. FmocFF-SMV demonstrated a sustained drug release over 7 days. Compared to the unloaded FmocFF hydrogel, treatment with FmocFF-SMV led to superior diabetic wound recovery and reduced inflammation. Therefore, the utilization of the sustained-release FmocFF-SMV hydrogel formulation could become an attractive choice for topical wound therapy in diabetes patients.

Hierarchical assembly of tryptophan zipper peptides into stress-relaxing bioactive hydrogels

Soft materials in nature are formed through reversible supramolecular assembly of biological polymers into dynamic hierarchical networks. Rational design has led to self-assembling peptides with structural similarities to natural materials. However, recreating the dynamic functional properties inherent to natural systems remains challenging. Here we report the discovery of a short peptide based on the tryptophan zipper (trpzip) motif, that shows multiscale hierarchical ordering that leads to emergent dynamic properties. Trpzip hydrogels are antimicrobial and self-healing, with tunable viscoelasticity and unique yield-stress properties that allow immediate harvest of embedded cells through a flick of the wrist. This characteristic makes Trpzip hydrogels amenable to syringe extrusion, which we demonstrate with examples of cell delivery and bioprinting. Trpzip hydrogels display innate bioactivity, allowing propagation of human intestinal organoids with apical-basal polarization. Considering these extensive attributes, we anticipate the Trpzip motif will prove a versatile building block for supramolecular assembly of soft materials for biotechnology and medicine.

Enzyme-Triggered l-α/d-Peptide Hydrogels as a Long-Acting Injectable Platform for Systemic Delivery of HIV/AIDS Drugs

Eradicating HIV/AIDS by 2030 is a central goal of the World Health Organization. Patient adherence to complicated dosage regimens remains a key barrier. There is a need for convenient long-acting formulations that deliver drugs over sustained periods. This paper presents an alternative platform, an injectable in situ forming hydrogel implant to deliver a model antiretroviral drug (zidovudine [AZT]) over 28 days. The formulation is a self-assembling ultrashort d or l-α peptide hydrogelator, namely phosphorylated (naphthalene-2-ly)-acetyl-diphenylalanine-lysine-tyrosine-OH (NapFFKY[p]-OH), covalently conjugated to zidovudine via an ester linkage. Rheological analysis demonstrates phosphatase enzyme instructed self-assembly, with hydrogels forming within minutes. Small angle neutron scattering data suggest hydrogels form narrow radius (≈2 nm), large length fibers closely fitting the flexible cylinder elliptical model. d-Peptides are particularly promising for long-acting delivery, displaying protease resistance for 28 days. Drug release, via hydrolysis of the ester linkage, progress under physiological conditions (37 °C, pH 7.4, H2 O). Subcutaneous administration of Napffk(AZT)Y[p]G-OH in Sprague Dawley rats demonstrate zidovudine blood plasma concentrations within the half maximal inhibitory concentration (IC50 ) range (30-130 ng mL-1 ) for 35 days. This work is a proof-of-concept for the development of a long-acting combined injectable in situ forming peptide hydrogel implant. These products are imperative given their potential impact on society.

Self-assembled short peptides: Recent advances and strategies for potential pharmaceutical applications

Self-assembled short peptides have intrigued scientists due to the convenience of synthesis, good biocompatibility, low toxicity, inherent biodegradability and fast response to change in the physiological environment. Therefore, it is necessary to present a comprehensive summary of the recent advances in the last decade regarding the construction, route of administration and application of self-assembled short peptides based on the knowledge on their unique and specific ability of self-assembly. Herein, we firstly explored the molecular mechanisms of self-assembly of short peptides, such as non-modified amino acids, as well as Fmoc-modified, N-functionalized, and C-functionalized peptides. Next, cell penetration, fusion, and peptide targeting in peptide-based drug delivery were characterized. Then, the common administration routes and the potential pharmaceutical applications (drug delivery, antibacterial activity, stabilizers, imaging agents, and applications in bioengineering) of peptide drugs were respectively summarized. Last but not least, some general conclusions and future perspectives in the relevant fields were briefly listed. Although with certain challenges, great opportunities are offered by self-assembled short peptides to the fascinating area of drug development.

Probing the Gelation Synergies and Anti-Escherichia coli Activity of Fmoc-Phenylalanine/Graphene Oxide Hybrid Hydrogel

The N-fluorenyl-9-methyloxycarbonyl (Fmoc)-protected amino acids have shown high antimicrobial application potential, among which the phenylalanine derivative (Fmoc-F) is the most well-known representative. However, the activity spectrum of Fmoc-F is restricted to Gram-positive bacteria only. The demand for efficient antimicrobial materials expanded research into graphene and its derivatives, although the reported results are somewhat controversial. Herein, we combined graphene oxide (GO) flakes with Fmoc-F amino acid to form Fmoc-F/GO hybrid hydrogel for the first time. We studied the synergistic effect of each component on gelation and assessed the material's bactericidal activity on Gram-negative Escherichia coli (E. coli). GO flakes do not affect Fmoc-F self-assembly per se but modulate the elasticity of the gel and speed up its formation. The hybrid hydrogel affects E. coli survival, initially causing abrupt bacterial death followed by the recovery of the surviving ones due to the inoculum effect (IE). The combination of graphene with amino acids is a step forward in developing antimicrobial gels due to their easy preparation, chemical modification, graphene functionalization, cost-effectiveness, and physicochemical/biological synergy of each component.

Effect of number of lysine motifs on the bactericidal and hemolytic activity of short cationic antimicrobial peptides

Antimicrobial peptides (AMPs) are vital components of the nonspecific immune system that represent a promising broad-spectrum alternative to conventional antibiotics. Several short cationic antimicrobial peptides show highly effective antibacterial activity and low hemolytic activity, which are based on the action of a few critical amino acids, such as phenylalanine (F) and lysine (K). Previous studies have reported that Fmoc-based phenylalanine peptides possess appreciable antibacterial potency against Gram-positive bacteria, but their ability to kill Gram-negative bacteria was suboptimal. In this study, we designed and prepared a series of Fmoc-K_nF peptide (n = 1-3) series by adding lysine motifs to strengthen their broad-spectrum antibacterial activity. The effect was investigated that the amount of lysine in Fmoc-F peptides on their antibacterial properties and hemolytic activities. Our results showed that the Fmoc-KKF peptide holds the strongest antimicrobial activity against both Gram-positive and negative bacteria among all designed peptides, as well as low hemolytic activity. These results provide support for the general strategy of enhancing the broad-spectrum antibacterial activity of AMPs through increased lysine content.

Immunomodulatory fibrous hyaluronic acid-Fmoc-diphenylalanine-based hydrogel induces bone regeneration

AIM

To investigate the potential of an ultrashort aromatic peptide hydrogelator integrated with hyaluronic acid (HA) to serve as a scaffold for bone regeneration.

MATERIALS AND METHODS

Fluorenylmethyloxycarbonyl-diphenylalanine (FmocFF)/HA hydrogel was prepared and characterized using microscopy and rheology. Osteogenic differentiation of MC3T3-E1 preosteoblasts was investigated using Alizarin red, alkaline phosphatase and calcium deposition assays. In vivo, 5-mm-diameter calvarial critical-sized defects were prepared in 20 Sprague-Dawley rats and filled with either FmocFF/HA hydrogel, deproteinized bovine bone mineral, FmocFF/Alginate hydrogel or left unfilled. Eight weeks after implantation, histology and micro-computed tomography analyses were performed. Immunohistochemistry was performed in six rats to assess the hydrogel's immunomodulatory effect.

RESULTS

A nanofibrous FmocFF/HA hydrogel with a high storage modulus of 46 KPa was prepared. It supported osteogenic differentiation of MC3T3-E1 preosteoblasts and facilitated calcium deposition. In vivo, the hydrogel implantation resulted in approximately 93% bone restoration. It induced bone deposition not only around the margins, but also generated bony islets along the defect. Elongated M2 macrophages lining at the periosteum-hydrogel interface were observed 1 week after implantation. After 3 weeks, these macrophages were dispersed through the regenerating tissue surrounding the newly formed bone.

CONCLUSIONS

FmocFF/HA hydrogel can serve as a cell-free, biomimetic, immunomodulatory scaffold for bone regeneration.

Antimicrobial Natural Hydrogels in Biomedicine: Properties, Applications, and Challenges-A Concise Review

Natural hydrogels are widely used as biomedical materials in many areas, including drug delivery, tissue scaffolds, and particularly wound dressings, where they can act as an antimicrobial factor lowering the risk of microbial infections, which are serious health problems, especially with respect to wound healing. In this review article, a number of promising strategies in the development of hydrogels with biocidal properties, particularly those originating from natural polymers, are briefly summarized and concisely discussed. Common strategies to design and fabricate hydrogels with intrinsic or stimuli-triggered antibacterial activity are exemplified, and the mechanisms lying behind these properties are also discussed. Finally, practical antibacterial applications are also considered while discussing the current challenges and perspectives.

Rational design of potent ultrashort antimicrobial peptides with programmable assembly into nanostructured hydrogels

Microbial resistance to common antibiotics is threatening to cause the next pandemic crisis. In this context, antimicrobial peptides (AMPs) are receiving increased attention as an alternative approach to the traditional small molecule antibiotics. Here, we report the bi-functional rational design of Fmoc-peptides as both antimicrobial and hydrogelator substances. The tetrapeptide Fmoc-WWRR-NH₂-termed Priscilicidin-was rationally designed for antimicrobial activity and molecular self-assembly into nanostructured hydrogels. Molecular dynamics simulations predicted Priscilicidin to assemble in water into small oligomers and nanofibrils, through a balance of aromatic stacking, amphiphilicity and electrostatic repulsion. Antimicrobial activity prediction databases supported a strong antimicrobial motif via sequence analogy. Experimentally, this ultrashort sequence showed a remarkable hydrogel forming capacity, combined to a potent antibacterial and antifungal activity, including against multidrug resistant strains. Using a set of biophysical and microbiology techniques, the peptide was shown to self-assemble into viscoelastic hydrogels, as a result of assembly into nanostructured hexagonal mesophases. To further test the molecular design approach, the Priscilicidin sequence was modified to include a proline turn-Fmoc-WPWRR-NH₂, termed P-Priscilicidin-expected to disrupt the supramolecular assembly into nanofibrils, while predicted to retain antimicrobial activity. Experiments showed P-Priscilicidin self-assembly to be effectively hindered by the presence of a proline turn, resulting in liquid samples of low viscosity. However, assembly into small oligomers and nanofibril precursors were evidenced. Our results augur well for fast, adaptable, and cost-efficient antimicrobial peptide design with programmable physicochemical properties.

Hydrogels with intrinsic antibacterial activity prepared from naphthyl anthranilamide (NaA) capped peptide mimics

In this study, we prepared antibacterial hydrogels through the self-assembly of naphthyl anthranilamide (NaA) capped amino acid based cationic peptide mimics. These ultra-short cationic peptide mimics were rationally designed with NaA as a capping group, L-phenylalanine, a short aliphatic linker, and a cationic group. The synthesized peptide mimics efficiently formed hydrogels with minimum gel concentrations between 0.1 and 0.3%w/v. The resulting hydrogels exhibited desirable viscoelastic properties which can be tuned by varying the cationic group, electronegative substituent, or counter anion. Importantly, nanofibers from the NaA-capped cationic hydrogels were found to be the source of hydrogels' potent bacteriacidal actvity against both Gram-positive and Gram-negative bacteria while remaining non-cytotoxic. These intrinsically antibacterial hydrogels are ideal candidates for further development in applications where bacterial contamination is problematic.

Antibacterial hydrogels of aromatic tripeptides

Self-assembled peptide hydrogels have emerged as alternatives to the conventional approaches employed in controlled drug release, wound-healing, and drug delivery, and as anti-infective agents. However, peptide hydrogels possessing antibacterial properties are less explored. In this work, we have designed three ultrashort antibacterial peptide hydrogels: Fmoc-FFH-CONH₂, Fmoc-FHF-CONH₂, and Fmoc-HFF-CONH₂. The rheological study showed the higher storage modulus of Fmoc-FFH-CONH₂ (30.43 kPa) compared to Fmoc-FHF-CONH₂ and Fmoc-HFF-CONH₂, which may be attributed to the enhanced aromatic interaction in Fmoc-FFH-CONH₂ compared to the other two variants, resulting in more mechanical rigidity. Further, the prepared hydrogels were evaluated for their inherent antibacterial potency against Gram-positive (Staphylococcus aureus, strain MTCC 96) and Gram-negative (Pseudomonas aeruginosa, strain PA01) bacteria. Antibacterial experiments demonstrated the potency of the hydrogels in the order of Fmoc-FFH-CONH₂ > Fmoc-FHF-CONH₂ > Fmoc-HFF-CONH₂. The antibacterial effect of the hydrogels was predominantly due to the osmotic stress and membrane disruption, which was verified by reactive oxygen species (ROS) generation and outer membrane permeabilization assays. Our findings point to the scope of using the synthesized peptide hydrogels as agents for topical applications.

Fmoc-Diphenylalanine Hydrogels: Optimization of Preparation Methods and Structural Insights

Hydrogels (HGs) are tri-dimensional materials with a non-Newtonian flow behaviour formed by networks able to encapsulate high amounts of water or other biological fluids. They can be prepared using both synthetic or natural polymers and their mechanical and functional properties may change according to the preparation method, the solvent, the pH, and to others experimental parameters. Recently, many short and ultra-short peptides have been investigated as building blocks for the formulation of biocompatible hydrogels suitable for different biomedical applications. Due to its simplicity and capability to gel in physiological conditions, Fmoc-FF dipeptide is one of the most studied peptide hydrogelators. Although its identification dates to 15 ago, its behaviour is currently studied because of the observation that the final material obtained is deeply dependent on the preparation method. To collect information about their formulation, here are reported some different strategies adopted until now for the Fmoc-FF HG preparation, noting the changes in the structural arrangement and behaviour in terms of stiffness, matrix porosity, and stability induced by the different formulation strategy on the final material.

Gram-selective antibacterial peptide hydrogels

The human microbiome plays fundamental roles in human health and disease. However, widely used broad-spectrum antibiotics severely disrupt human-related microbial communities, eventually leading to resistant bacteria, posing a growing threat to global medical health. Antimicrobial peptides (AMPs) are promising antimicrobial agents that barely cause bacterial resistance. Excellent broad-spectrum antimicrobial activities have been achieved using hydrogels self-assembled from AMPs, but there is still a lack of AMP hydrogels that can target Gram-positive and Gram-negative bacteria. Herein, several hydrogels self-assembled from AMPs, termed IK1, IK3, and IK4, were designed and synthesized. In vitro antibacterial results indicated that the IK1 and IK4 hydrogels specifically targeted Gram-positive and Gram-negative bacteria, respectively, while the IK3 hydrogel targeted both Gram-positive and Gram-negative bacteria. The desired broad-spectrum or Gram-selective AMP hydrogels are believed to be obtained through the rational design of the hydrophilicity, hydrophobicity, and charge properties of the peptide molecules. Good in vivo Gram-selective antibacterial properties and the ability to promote wound healing have been demonstrated via treating mouse wound models with these AMP hydrogels. We believe that these Gram-selective AMP hydrogels could potentially have important applications in treating common recurring infections.

A Self-Assembled Fmoc-Diphenylalanine Hydrogel-Encapsulated Pt Nanozyme as Oxidase- and Peroxidase-Like Breaking pH Limitation for Potential Antimicrobial Application

Nanomaterials with oxidase- and peroxidase-like activities have potential in antibacterial therapy. The optimal activity of most nanozymes occurred in acidic pH (3.0-5.0), while the pH in biological systems is mostly near neutral. Herein, a general system using 9-fluorenylmethoxycarbonyl-modified diphenylalanine (Fmoc-FF) hydrogel for enhancing oxidase- and peroxidase-like activities of Pt NPs and other typical enzyme-like nanomaterials at neutral or even alkaline pH is proposed. In this system, Fmoc-FF hydrogel provides an acidic microenvironment for Pt NPs due to hydrogen protons (H⁺ ) produced by the dissociation of F at neutral pH. As a result, Pt NPs exhibits 6-fold enhanced oxidase-like and 26-fold peroxidase-like activity after being encapsulated into Fmoc-FF hydrogel at pH 7.0. Based on outstanding enzymatic activities and the antibacterial activity of Fmoc-FF hydrogel itself, Pt-Fmoc-FF hydrogel realizes excellent antibacterial effect. This design provides a universal strategy to break pH limitation of nanozymes and promotes the biological applications of nanozymes.

Self-Assembling Peptide-Based Hydrogels for Wound Tissue Repair

Wound healing is a long-term, multistage biological process that includes hemostasis, inflammation, proliferation, and tissue remodeling and requires intelligent designs to provide comprehensive and convenient treatment. The complexity of wounds has led to a lack of adequate wound treatment materials, which must systematically regulate unique wound microenvironments. Hydrogels have significant advantages in wound treatment due to their ability to provide spatiotemporal control over the wound healing process. Self-assembling peptide-based hydrogels are particularly attractive due to their innate biocompatibility and biodegradability along with additional advantages including ligand-receptor recognition, stimulus-responsive self-assembly, and the ability to mimic the extracellular matrix. The ability of peptide-based materials to self-assemble in response to the physiological environment, resulting in functionalized microscopic structures, makes them conducive to wound treatment. This review introduces several self-assembling peptide-based systems with various advantages and emphasizes recent advances in self-assembling peptide-based hydrogels that allow for precise control during different stages of wound healing. Moreover, the development of multifunctional self-assembling peptide-based hydrogels that can regulate and remodel the wound immune microenvironment in wound therapy with spatiotemporal control has also been summarized. Overall, this review sheds light on the future clinical and practical applications of self-assembling peptide-based hydrogels.

Antimicrobial peptide-based materials: opportunities and challenges

The multifunctional properties of antimicrobial peptides (AMPs) make them attractive candidates for the treatment of various diseases. AMPs are considered alternatives to antibiotics due to the rising number of multidrug-resistant...

A Heterochiral Diphenylalanine Auxin Derivative empowers Remarkable Mechanical Integrity with promising Antiinflammatory and Antimicrobial Performances

The emergence of drug-resistant microbes in conjunction with antiinflammatory responses are one of the universal challenges in health care avenues. Although scientists have dedicated enormous efforts to discover potential constructs,...

Self-assembled di- and tripeptide gels for the passive entrapment and pH-responsive, sustained release of an antidiabetic drug, glimepiride

Diabetes is a global epidemic that poses a severe challenge to public health. The characteristic features of this disease are hyperglycemia and deterioration of the function of pancreatic β-cells, which...

Antimicrobial and Bioactive Silk Peptide Hybrid Hydrogel with a Heterogeneous Double Network Formed by Orthogonal Assembly

Hydrogels mimic the natural extracellular matrix in terms of their nanofibrous structure and large water content. However, the lack of a combination of properties including sufficient heterogeneity in the gel structure, intrinsic antimicrobial activity, and bioactivity limits the efficiency of hydrogels for tissue engineering applications. In this work, a hydrogel with a combination of these properties was fabricated by hybridizing silk fibroin with a low-molecular-weight peptide gelator. It was observed that silk fibroin and the peptide gelator assembled orthogonally in sequence. While the morphology of silk fibroin nanofibrils was not affected by the peptide gelator, silk fibroin promoted the formation of wider nanoribbons of the peptide gelator by modulating its nucleation and growth. Orthogonal assembly maintained the antimicrobial activity of the peptide gelator and the excellent biocompatibility of silk fibroin in the hybrid gel. The hybrid gel also demonstrated improved interactions with cells, an indicator of a higher bioactivity, possibly due to the heterogeneous double network structure.

Fabrication of Supramolecular Antibacterial Nanofibers with Membrane-Disruptive Mechanism

By studying the principles of self-assembly and combining the structural parameters required for the asymmetric distribution of antimicrobial peptides (AMPs), we newly designed and screened the high-activity and low-toxicity AMP F2I-LL. This peptide can form a supramolecular hydrogel with a nanofiber microstructure in a simulated physiological environment (phosphate buffered saline), which exhibits broad-spectrum antibacterial activity. Compared with monomeric peptides, the introduction of a self-assembly strategy not only improved the bactericidal titer but also enhanced the serum stability of AMPs. Mechanistic studies showed that the positive charge enriched on the surface of the nanofiber was conducive to its rapid binding to the negatively charged part of the outer membrane of bacteria and further entered the inner membrane, increasing its permeability and ultimately leading to cell membrane rupture and death. This work provides insights into the design of nanopeptides with broad-spectrum antibacterial activity and provides new results for the development of biomedicine.

Unravelling the antimicrobial activity of peptide hydrogel systems: current and future perspectives

The use of hydrogels has garnered significant interest as biomaterial and drug delivery platforms for anti-infective applications. For decades antimicrobial peptides have been heralded as a much needed new class of antimicrobial drugs. Self-assembling peptide hydrogels with inherent antimicrobial ability have recently come to the fore. However, their fundamental antimicrobial properties, selectivity and mechanism of action are relatively undefined. This review attempts to establish a link between antimicrobial efficacy; the self-assembly process; peptide-membrane interactions and mechanical properties by studying several reported peptide systems: β-hairpin/β-loop peptides; multidomain peptides; amphiphilic surfactant-like peptides and ultrashort/low molecular weight peptides. We also explore their role in the formation of amyloid plaques and the potential for an infection etiology in diseases such as Alzheimer's. We look briefly at innovative methods of gel characterization. These may provide useful tools for future studies within this increasingly important field.

Single Amino-Acid Based Self-Assembled Biomaterials with Potent Antimicrobial Activity

The design and development of soft biomaterials based on amino acid and short-peptide have gained much attention due to their potent biomedical applications. A slight alteration in the side-chain of single amino acid in a peptide or protein sequence has a huge impact on the structure and function. Phenylalanine is one of the most studied amino acids, which contains an aromatic phenyl group connected through a flexible -CH₂ - unit. In this work, we have examined whether flexibility and aromatic functionality of phenylalanine (Phe) are important in gel formation of model gelator Fmoc-Phe-OH or not. To examine this hypothesis, we synthesized Fmoc-derivatives of three analogues unnatural amino acids including cyclohexylalanine, phenylglycine, and homophenylalanine; which are slightly varied from Phe. Interestingly, all these three new analogues formed hydrogels in phosphate buffer at pH 7.0 having different gelation efficacy and kinetics. This study suggests that the presence of aromatic side-chain and flexibility are not mandatory for the gelation of this model gelator. Newly synthesized unnatural amino acid derivatives have also exhibited promising antimicrobial activity towards gram-positive bacteria by inhibiting cellular oxygen consumption. We further determined the biocompatibility of these amino acid derivatives by using a hemolysis assay on human blood cells. Overall studies described the development of single amino acid-based new injectable biomaterials with improved antimicrobial activity by the slight alteration in the side-chain of amino acid.

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

Antibiofilm activity of Fmoc-phenylalanine against Gram-positive and Gram-negative bacterial biofilms

Biofilm associated infections are the major contributor of mortality, morbidity and financial burden in patients with a bacterial infection. About 65% of all bacterial infections are associated with the information of bacterial biofilms. Bacterial biofilms not only reduce the efficacy of antibacterial treatment but also increases the threat of developing antibacterial resistance. Recently, our group has discovered the antibacterial activity of Fmoc-phenylalanine (Fmoc-F) and other Fmoc-amino acids (Fmoc-AA). Fmoc-F and other Fmoc-AA showed antibacterial activity due to their surfactant properties. Surfactants are known to eradicate biofilm and enhance antimicrobial activity in biofilm. Thus, in the present study, we evaluated the anti-biofilm activity of Fmoc-F against clinically relevant bacteria. We found that Fmoc-F not only inhibits the biofilm formation in Staphylococcus aureus and Pseudomonas aeruginosa, but also eradicates the already formed biofilms over the surface. Further, Fmoc-F coated glass surface resists S. aureus and P. aeruginosa biofilm formation and attachment, when biofilm is grown over the surface. The mechanistic investigation suggests that Fmoc-F reduces the extracellular matrix (ECM) components such as proteins, carbohydrates and eDNA in the biofilm and affect its stability via direct interactions with ECM components and/ or indirectly through reducing bacterial cell population. Finally, we showed that Fmoc-F treatment in combination with vancomycin and ampicillin synergistically inhibit biofilm formation. Overall, the study demonstrates the potential application of Fmoc-F and other Fmoc-AA molecules individually as well as in combination as anti-biofilm coating material for treating biofilm associated infections.

Development of mechanism-based antibacterial synergy between Fmoc-phenylalanine hydrogel and aztreonam

Recently, fluorenylmethyloxycarbonyl (Fmoc) conjugated amino acids (Fmoc-AA), especially Fmoc-phenylalanine (Fmoc-F), have been discovered to have antimicrobial properties specific to Gram-positive bacteria including MRSA. Their weak antibacterial activity against Gram-negative bacteria is due to their inability to cross the bacterial membrane. Here in order to increase the antibacterial spectrum of Fmoc-F, we prepared a formulation of Fmoc-F with the Gram-negative specific antibiotic aztreonam (AZT). This formulation displayed antibacterial activity against both Gram-positive and Gram-negative bacteria and significantly reduced the bacterial load in a mouse wound infection model. The combination produced a synergistic effect and higher efficacy against P. aeruginosa due to the increased Fmoc-F permeability by AZT through the bacterial membrane. This combinatorial approach could be an effective strategy for other Fmoc-AA having a Gram-positive specific antibacterial effect for the better management of bacterial wound infections.

Surface Triggered Self-Assembly of Fmoc-Tripeptide as an Antibacterial Coating

In western countries, one patient on twenty will develop a nosocomial infection during his hospitalization at health care facilities. Classical antibiotics being less and less effective, this phenomenon is expanding year after year. Prevention of bacteria colonization of implantable medical devices constitutes a major medical and financial issue. In this study, we developed an antibacterial coating based on self-assembled Fmoc-tripeptide. Fmoc-FFpY peptides (F: phenylalanine; Y: tyrosine; p: PO4 2-) are dephosphorylated enzymatically into Fmoc-FFY by action of alkaline phosphatase functionalized silica nanoparticles (NPs@AP), previously deposited on a surface. Fmoc-FFY peptides then self-assemble through π-π stacking interactions, hydrogen bonds and hydrophobic interactions adopting β-sheets secondary structures. The obtained hydrogel coatings show fibrillary structures observed by cryo-scanning electron microscopy with a thickness of few micrometers. At low concentration (≤0.5 mg.mL-1), self-assembled Fmoc-FFY has a superior antibacterial activity than Fmoc-FFpY peptide in solution. After 24 h of incubation, Fmoc-FFY hydrogel coatings fully inhibit the development of Gram-positive Staphylococcus aureus (S. aureus). The antibacterial effect is maintained on an in vitro model of repetitive infection in the case of S. aureus. This coating could serve in infections were Gram positive bacteria are prevalent, e.g., intravascular catheter infections. This work gives new insights toward the design of an alternative antimicrobial coating.

Bi-functional peptide-based 3D hydrogel-scaffolds

Over the last few years, hydrogels have been proposed for many biomedical applications, including drug delivery systems and scaffolds for tissue engineering. In particular, peptides have been envisioned as excellent candidates for the development of hydrogel materials, due to their intrinsic biocompatibility, ease of handling, and intrinsic biodegradability. Recently, we developed a novel hybrid polymer-peptide conjugate, PEG8-(FY)3, which is able to self-assemble into a self-supporting soft hydrogel over dry and wet surfaces as demonstrated by molecular dynamics simulation. Here, we describe the synthesis and supramolecular organization of six novel hexapeptides rationally designed by punctual chemical modification of the primary peptide sequence of the ancestor peptide (FY)3. Non-coded amino acids were incorporated by replacing the phenylalanine residue with naphthylalanine (Nal) and tyrosine with dopamine (Dopa). We also studied the effect of the modification of the side chain and the corresponding PEGylated peptide analogues, on the structural and mechanical properties of the hydrogel. Secondary structure, morphology and rheological properties of all the peptide-based materials were assessed by various biophysical tools. The in vitro biocompatibility of the supramolecular nanostructures was also evaluated on fibroblast cell lines. We conclude that the PEG8-(Nal-Dopa)3 hydrogel possesses the right properties to serve as a scaffold and support cell growth.

Nanostructured antimicrobial peptides: The last push towards clinics

Peptide drugs hold great potential for the treatment of infectious diseases due to their unconventional mechanisms of action, biocompatibility, biodegradability and ease of synthesis and modification. The increasing rising of bacterial strains resistant to classical antibiotics have pushed the development of new peptide-based antimicrobial therapies. In this context, over the past few years, different approaches have reached a clinical approval. Furthermore, the application of nanotechnological principles to the design of antimicrobial peptide-based composites increases even more the already known benefits of antimicrobial peptides as competent protein drugs. Then, we provide here an overview of the current strategies for antimicrobial peptide discovery and modification and the status of such peptides already under clinical development. In addition, we summarize the innovative formulation strategies for their application, focusing on the controlled self-assembly for the fabrication of antimicrobial nanostructures without the assistance of external nanocarriers, and with emphasis on bioengineering, design of ultra-short peptides and rising insights in bacterial selectivity.

Supramolecular Peptide Assemblies as Antimicrobial Scaffolds

Antimicrobial discovery in the age of antibiotic resistance has demanded the prioritization of non-conventional therapies that act on new targets or employ novel mechanisms. Among these, supramolecular antimicrobial peptide assemblies have emerged as attractive therapeutic platforms, operating as both the bactericidal agent and delivery vector for combinatorial antibiotics. Leveraging their programmable inter- and intra-molecular interactions, peptides can be engineered to form higher ordered monolithic or co-assembled structures, including nano-fibers, -nets, and -tubes, where their unique bifunctionalities often emerge from the supramolecular state. Further advancements have included the formation of macroscopic hydrogels that act as bioresponsive, bactericidal materials. This systematic review covers recent advances in the development of supramolecular antimicrobial peptide technologies and discusses their potential impact on future drug discovery efforts.

Effect of polar amino acid incorporation on Fmoc-diphenylalanine-based tetrapeptides

Peptide hydrogels show great promise as extracellular matrix mimics due to their tuneable, fibrous nature. Through incorporation of polar cationic, polar anionic or polar neutral amino acids into the Fmoc-diphenylalanine motif, we show that electrostatic charge plays a key role in the properties of the subsequent gelators. Specifically, we show that an inverse relationship exists for biocompatibility in the solution state versus the gel state for cationic and anionic peptides. Finally, we use tethered bilayer lipid membrane (tBLM) experiments to suggest a likely mode of cytotoxicity for tetrapeptides which exhibit cytotoxicity in the solution state.

Structure-Dependent Antibacterial Activity of Amino Acid-Based Supramolecular Hydrogels

Bacterial infections are currently a major concern to human health. Amino acid-based supramolecular polymer hydrogels, which boast intrinsic antibacterial activity, are an important solution due to their good biocompatibility, cost effectiveness, and tunable structural properties. Herein, we reported three types of transparent supramolecular hydrogel with intrinsic antibacterial activity from self-assembly of commercially available Fmoc-tryptophan (Fmoc-W), Fmoc-methionine (Fmoc-M), and Fmoc-tyrosine (Fmoc-Y). The resulting hydrogels selectively inhibited the growth of Gram-positive bacteria. Moreover, the order of antibacterial activity was Fmoc-W hydrogel > Fmoc-M hydrogel > Fmoc-Y hydrogel. The critical aggregation concentration (CAC) values were found at concentrations of approximately 0.0293, 0.1172, and 0.4688 mM for Fmoc-W, Fmoc-M, and Fmoc-Y, respectively. Transmission electron microscope (TEM) images revealed rigid and aligned nanofibers for Fmoc-W hydrogel, while flexible nanofibers for Fmoc-M hydrogel and Fmoc-Y hydrogel. The results indicated that stronger aggregation capability of the gelator and the synergistic nanostructural morphology with more rigid and aligned nanofibers can lead to higher antibacterial activity of its corresponding hydrogel. In addition, the molecular arrangements of Fmoc-amino acids in the hydrogels may also contribute to their antibacterial activity. These results can guide the rational design, fabrication, and future application of other self-assembled amino acid-based hydrogels with excellent antibacterial activity.

Peptide Self-Assembly Is Linked to Antibacterial, but Not Antifungal, Activity of Histatin 5 Derivatives

Antimicrobial peptides are important modulators of host defense against bacterial, fungal, and viral pathogens in humans and other multicellular organisms. Two converging paradigms point to a link between antimicrobial peptides that self-assemble into amyloid-like nanoassemblies and classical amyloidogenic peptides that often have potent broad-spectrum antimicrobial activity, suggesting that antimicrobial and amyloidogenic peptides may represent two sides of the same coin. Here, we asked if the ability of an antifungal peptide to self-assemble affects its antifungal or antibacterial activity. We found that modifications of classical antifungal peptide derivative allowed it to self-assemble and did not alter its antifungal activity, and yet self-assembly substantially increased the antibacterial activity of the peptide. These results support the idea that peptide self-assembly can enhance antibacterial activities and emphasize a distinction between the action of antifungal peptides and that of antibacterial peptides. Accordingly, we suggest that the possible generality of this distinction should be widely tested.

The rise of multidrug-resistant pathogens has awakened interest in new drug candidates such as antimicrobial peptides and their derivatives. Recent work suggests that some antimicrobial peptides have the ability to self-assemble into ordered amyloid-like nanostructures which facilitate their antibacterial activity. Here, we evaluate a histatin-based antimicrobial peptide, and its self-assembling derivative, in the interplay between self-assembly, membrane interactions, and antibacterial and antifungal activities. We demonstrate substantial membrane targeting by both peptides, as well as mechanistic insights into this mode of action, which correlates to their antifungal activity and is not affected by their self-assembling state. The ability to self-assemble does, however, significantly affect peptide antibacterial activity against both Gram-negative and Gram-positive bacteria. These results are surprising and hint at important distinctions between antifungal and antibacterial peptide activities in prokaryotes and eukaryotic microbes.

IMPORTANCE Antimicrobial peptides are important modulators of host defense against bacterial, fungal, and viral pathogens in humans and other multicellular organisms. Two converging paradigms point to a link between antimicrobial peptides that self-assemble into amyloid-like nanoassemblies and classical amyloidogenic peptides that often have potent broad-spectrum antimicrobial activity, suggesting that antimicrobial and amyloidogenic peptides may represent two sides of the same coin. Here, we asked if the ability of an antifungal peptide to self-assemble affects its antifungal or antibacterial activity. We found that modifications of classical antifungal peptide derivative allowed it to self-assemble and did not alter its antifungal activity, and yet self-assembly substantially increased the antibacterial activity of the peptide. These results support the idea that peptide self-assembly can enhance antibacterial activities and emphasize a distinction between the action of antifungal peptides and that of antibacterial peptides. Accordingly, we suggest that the possible generality of this distinction should be widely tested.

Anthranilamide-based Short Peptides Self-Assembled Hydrogels as Antibacterial Agents

In this study, we describe the synthesis and molecular properties of anthranilamide-based short peptides which were synthesised via ring opening of isatoic anhydride in excellent yields. These short peptides were incorporated as low molecular weight gelators (LMWG), bola amphiphile, and C3-symmetric molecules to form hydrogels in low concentrations (0.07-0.30% (w/v)). The critical gel concentration (CGC), viscoelastic properties, secondary structure, and fibre morphology of these short peptides were influenced by the aromaticity of the capping group or by the presence of electronegative substituent (namely fluoro) and hydrophobic substituent (such as methyl) in the short peptides. In addition, the hydrogels showed antibacterial activity against S. aureus 38 and moderate toxicity against HEK cells in vitro.

Design, characterization, and evaluation of antibacterial gels, Boc-D-Phe-γ4-L-Phe-PEA/chitosan and Boc-L-Phe-γ4-L-Phe-PEA/chitosan, for biomaterial-related infections

Self-assembled peptide gels have generated interest as antibacterial materials to prevent biomaterial-related infections but these peptides are often associated with poor proteolytic stability. Efforts have been made to stabilize peptides by incorporating non-natural amino acids and/or linkages but complexation with polymers have not been explored. Therefore, we developed self-assembled peptide/chitosan gels, Boc-D-Phe-γ⁴-L-Phe-PEA (NH007)/chitosan and Boc-L-Phe-γ⁴-L-Phe-PEA (NH009)/chitosan, by complexing dipeptide NH007 or NH009 with chitosan in DMSO:acetic acid. The gels were characterized using SEM, FTIR, contact angle, and rheology data and found to exhibit excellent viscoelastic and self-healing characteristics. Complexation with chitosan led to an increase in stability against proteolytic degradation. Peptide/chitosan gels showed broad spectrum antibacterial activities against Gram-negative and Gram-positive bacteria, such as Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus subtilis at a high inoculum of 10⁷-10⁸ cfu/mL. NH007/chitosan gels showed 70-75% inhibition, whereas NH009/chitosan showed 78-81% inhibition and NH009/chitosan gels, in particular, showed strong antibacterial activity against pathogenic strain of P. aeruginosa. A unique feature of these gels is that the antibacterial activities did not decrease gradually but were sustained for up to 48 h. The mechanistic studies using SEM and HR-TEM indicated interaction of gels with bacterial membrane components, leading to cell lysis. The MTT and LDH assays indicated >90% cell viability and only 8-10% toxicity towards NIH 3T3 fibroblast cells. Thus, peptide/chitosan gels developed in the present work showed improved proteolytic stability and sustained antibacterial activities and, therefore, may be used for preventing biomaterial-related infections.

Fmoc-diphenylalanine as a suitable building block for the preparation of hybrid materials and their potential applications

Due to its capability to self-assemble in self-supporting hydrogels (HG) under physiological conditions, Fmoc-FF is one of the most studied ultra-short peptide. The structural properties of the resulting hydrogel (mechanical rigidity, entanglement of the fibrillary network, and the thickness of the fibers) strictly depend on the experimental conditions used during the preparation. In the past few years, a broad range of applications in different fields, such as biomedical and industrial fields, have been proposed. However, the research on novel materials with enhanced mechanical properties, stability, and biocompatibility has brought about the development of novel Fmoc-FF-based hybrid systems, in which the ultra-short hydrogelator is combined with others entities such as polysaccharides, polymers, peptides, or organic molecules. The structural features and the potential applications of these novel hybrid materials, with particular attention to tissue engineering, drug delivery, and catalysis, are described here. The aim is to give the readers a tool to design new hybrid nanomaterials based on the Fmoc-FF dipeptide hydrogelator, with appropriate properties for specific applications.

Expanding the Functional Scope of the Fmoc-Diphenylalanine Hydrogelator by Introducing a Rigidifying and Chemically Active Urea Backbone Modification

Peptidomimetic low-molecular-weight hydrogelators, a class of peptide-like molecules with various backbone amide modifications, typically give rise to hydrogels of diverse properties and increased stability compared to peptide hydrogelators. Here, a new peptidomimetic low-molecular-weight hydrogelator is designed based on the well-studied N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) peptide by replacing the amide bond with a frequently employed amide bond surrogate, the urea moiety, aiming to increase hydrogen bonding capabilities. This designed ureidopeptide, termed Fmoc-Phe-NHCONH-Phe-OH (Fmoc-FuF), forms hydrogels with improved mechanical properties, as compared to those formed by the unmodified Fmoc-FF. A combination of experimental and computational structural methods shows that hydrogen bonding and aromatic interactions facilitate Fmoc-FuF gel formation. The Fmoc-FuF hydrogel possesses properties favorable for biomedical applications, including shear thinning, self-healing, and in vitro cellular biocompatibility. Additionally, the Fmoc-FuF, but not Fmoc-FF, hydrogel presents a range of functionalities useful for other applications, including antifouling, slow release of urea encapsulated in the gel at a high concentration, selective mechanical response to fluoride anions, and reduction of metal ions into catalytic nanoparticles. This study demonstrates how a simple backbone modification can enhance the mechanical properties and functional scope of a peptide hydrogel.

Enhanced Nanoassembly-Incorporated Antibacterial Composite Materials

The rapid advancement of peptide- and amino-acid-based nanotechnology offers new approaches for the development of biomedical materials. The utilization of fluorenylmethyloxycarbonyl (Fmoc)-decorated self-assembling building blocks for antibacterial and anti-inflammatory purposes represents promising advancements in this field. Here, we present the antibacterial capabilities of the nanoassemblies formed by Fmoc-pentafluoro-l-phenylalanine-OH, their substantial effect on bacterial morphology, as well as new methods developed for the functional incorporation of these nanoassemblies within resin-based composites. These amalgamated materials inhibit and hinder bacterial growth and viability and are not cytotoxic toward mammalian cell lines. Importantly, due to the low dosage required to confer antibacterial activity, the integration of the nanoassemblies does not affect their mechanical and optical properties. This approach expands on the growing number of accounts on the intrinsic antibacterial capabilities of self-assembling building blocks and serves as a basis for further design and development of enhanced composite materials for biomedical applications.

Enhancing the Potency of Antimicrobial Peptides through Molecular Engineering and Self-Assembly

Healthcare-associated infections resulting from bacterial attachment and biofilm formation on medical implants are posing significant challenges in particular with the emergence of bacterial resistance to antibiotics. Here, we report the design, synthesis and characterization of self-assembled nanostructures, which integrate on their surface antibacterial peptides. The antibacterial WMR peptide, which is a modification of the native sequence of the myxinidin, a marine peptide isolated from the epidermal mucus of hagfish, was used considering its enhanced activity against Gram-negative bacteria. WMR was linked to a peptide segment of aliphatic residues (AAAAAAA) containing a lipidic tail (C₁₉H₃₈O₂) attached to the ε-amino of a terminal lysine to generate a peptide amphiphile (WMR PA). The self-assembly of the WMR PA alone, or combined with coassembling shorter PAs, was studied using spectroscopy and microscopy techniques. The designed PAs were shown to self-assemble into stable nanofiber structures and these nanoassemblies significantly inhibit biofilm formation and eradicate the already formed biofilms of Pseudomonas aeruginosa (Gram-negative bacteria) and Candida albicans (pathogenic fungus) when compared to the native WMR peptide. Our results provide insights into the design of peptide based supramolecular assemblies with antibacterial activity, and establish an innovative strategy to develop self-assembled antimicrobial materials for biomedical applications.

Ultrashort Self-Assembling Peptide Hydrogel for the Treatment of Fungal Infections

The threat of antimicrobial resistance to society is compounded by a relative lack of new clinically effective licensed therapies reaching patients over the past three decades. This has been particularly problematic within antifungal drug development, leading to a rise in fungal infection rates and associated mortality. This paper highlights the potential of an ultrashort peptide, (naphthalene-2-ly)-acetyl-diphenylalanine-dilysine-OH (NapFFKK-OH), encompassing hydrogel-forming and antifungal properties within a single peptide motif, thus overcoming formulation (e.g., solubility, drug loading) issues associated with many currently employed highly hydrophobic antifungals. A range of fungal susceptibility (colony counts) and cell cytotoxicity (MTS cell viability, LIVE/DEAD staining^® with fluorescent microscopy, haemolysis) assays were employed. Scanning electron microscopy confirmed the nanofibrous architecture of our self-assembling peptide, existing as a hydrogel at concentrations of 1% w/v and above. Broad-spectrum activity was demonstrated against a range of fungi clinically relevant to infection (Aspergillus niger, Candida glabrata, Candida albicans, Candida parapsilosis and Candida dubliniensis) with greater than 4 log₁₀ CFU/mL reduction at concentrations of 0.5% w/v and above. We hypothesise antifungal activity is due to targeting of anionic components present within fungal cell membranes resulting in membrane disruption and cell lysis. NapFFKK-OH demonstrated reduced toxicity against mammalian cells (NCTC 929, ARPE-19) suggesting increased selectivity for fungal cells. However, further studies relating to safety for systemic administration is required, given the challenges toxicity has presented in the wider context of antimicrobial peptide drug development. Overall this study highlights the promise of NapFFKK-OH hydrogels, particularly as a topical formulation for the treatment of fungal infections relating to the skin and eyes, or as a hydrogel coating for the prevention of biomaterial related infection.

Fmoc-phenylalanine displays antibacterial activity against Gram-positive bacteria in gel and solution phases

In the quest for new antimicrobial materials, hydrogels of Fmoc-protected peptides and amino acids have gained momentum due to their ease of synthesis and cost effectiveness; however, their repertoire is currently limited, and the mechanistic details of their function are not well understood. Herein, we report the antibacterial activity of the hydrogel and solution phases of Fmoc-phenylalanine (Fmoc-F) against a variety of Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA). Fmoc-F, a small molecule hydrogelator, reduces the bacterial load both in vitro and in the skin wound infections of mice. The antibacterial activity of Fmoc-F is predominantly due to its release from the hydrogel. Fmoc-F shows surfactant-like properties with critical micelle concentration nearly equivalent to its minimum bactericidal concentration. Similar to Fmoc-F, some Fmoc-conjugated amino acids (Fmoc-AA) have also shown antibacterial effects that are linearly correlated with their surfactant properties. At low concentrations, where Fmoc-F does not form micelles, it inhibits bacterial growth by entering the cell and reducing the glutathione levels. However, at higher concentrations, Fmoc-F triggers oxidative and osmotic stress and, alters the membrane permeabilization and integrity, which kills Gram-positive bacteria. Herein, we proposed the use of the Fmoc-F hydrogel and its solution for several biomedical applications. This study will open up new avenues to enhance the repertoire of Fmoc-AA to act as antimicrobial agents and improve their structure-activity relationship.

Broad-Spectrum Antibacterial Activity of Proteolytically Stable Self-Assembled αγ-Hybrid Peptide Gels

Bacterial infections pose a serious threat to mankind, and there is immense interest in the design and development of self-assembled peptide gels using ultrashort peptides for antibacterial applications. The peptide gels containing natural amino acids suffer from poor stability against proteolytic enzymes. Therefore, there is a need to design and develop peptide gels with improved stability against proteolytic enzymes. In the present work, we report the synthesis and characterization of α/γ hybrid peptides Boc-D-Phe-γ⁴-L-Phe-PEA (NH007) and Boc-L-Phe-γ⁴-L-Phe-PEA (NH009) to improve the proteolytic stability. Both of the dipeptides were found to self-assemble into gels in aqueous DMSO (3-5% w/v), and the self-assembly process was studied using FTIR and CD, which indicated antiparallel β-sheet formation with random coils in NH007 gels and random or unordered conformation in NH009. The rheological studies indicated viscoelastic characteristics for both gels; the storage modulus ( G') for NH007 and NH009 gels (3% w/v) was estimated as 0.2 and 0.5 MPa, higher than the loss modulus ( G''). Also, both gels demonstrated self-healing characteristics for six consecutive cycles when subjected to varying strains of 0.1 and 30% (200 s each). The peptide gels were incubated with a mocktail of proteolytic enzymes, proteinase K, pepsin, and chymotrypsin, and stability was monitored using RP HPLC. Up to 23 and 40% degradation was observed for NH007 (3%, w/v) in 24 and 36 h, and 77 and 94% degradation was observed for NH009 (3%, w/v), within the same period. Thus α/γ hybrid peptide gels containing D-Phe exhibited higher stability than gels fabricated using L-Phe. The use of D-residue in α/γ hybrid peptide significantly enhanced the stability of peptides against proteolytic enzymes, as the stability data reported in this work are possibly the best in class. Both peptide gels exhibited broad-spectrum antibacterial activity against Gram-negative and Gram-positive bacteria, such as Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus. The Pseudomonas aeruginosa and Staphylococcus aureus, in particular, are known to develop resistance. The NH007 (3%, w/v) demonstrated 65% inhibition, whereas NH009 (3%, w/v) showed 78% inhibition, with potent activity against Pseudomonas aeruginosa. Mechanistic studies, using SEM, HR-TEM, and bacterial live-dead assay, indicated entrapment of bacteria in gel networks, followed by interaction with cell membrane components and lysis. Cell viability (MTT assay) and toxicity (LDH assay) studies showed that both gels are not toxic to NIH 3T3 mouse embryonic fibroblast cells (mammalian). MTT assay showed >85% cell viability, and LDH assay exhibited not more than 15% cytotoxicity, even at higher concentrations (5%, w/v) and prolonged exposures (48 h). Overall, studies indicate the potential application of gels developed from the α/γ hybrid peptides in preventing biomaterial-related infections.