Beyond Fmoc: a review of aromatic peptide capping groups

Recent research of peptide-based hydrogel in nervous regeneration

Neurological disorders exert significantly affect the quality of life for patients, necessitating effective strategies for nerve regeneration. Both traditional autologous nerve transplantation and emerging therapeutic approaches encounter scientific challenges due to the complex nature of the nervous system and the unsuitability of the surrounding environment for cell transplantation. Tissue engineering techniques offer a promising path for neurotherapy. Successful neural tissue engineering relies on modulating cell differentiation behavior and tissue repair by developing biomaterials that mimic the natural extracellular matrix (ECM) and establish a three-dimensional microenvironment. Peptide-based hydrogels have emerged as a potent option among these biomaterials due to their ability to replicate the structure and complexity of the ECM. This review aims to explore the diverse range of peptide-based hydrogels used in nerve regeneration with a specific focus on dipeptide hydrogels, tripeptide hydrogels, oligopeptide hydrogels, multidomain peptides (MDPs), and amphiphilic peptide hydrogels (PAs). Peptide-based hydrogels offer numerous advantages, including biocompatibility, structural diversity, adjustable mechanical properties, and degradation without adverse effects. Notably, hydrogels formed from self-assembled polypeptide nanofibers, derived from amino acids, show promising potential in engineering neural tissues, outperforming conventional materials like alginate, poly(ε-caprolactone), and polyaniline. Additionally, the simple design and cost-effectiveness of dipeptide-based hydrogels have enabled the creation of various functional supramolecular structures, with significant implications for nervous system regeneration. These hydrogels are expected to play a crucial role in future neural tissue engineering research. This review aims to highlight the benefits and potential applications of peptide-based hydrogels, contributing to the advancement of neural tissue engineering.

Salt-induced Fmoc-tripeptide supramolecular hydrogels: a combined experimental and computational study of the self-assembly

Delving into the mechanism behind the molecular interactions at the atomic level of short-sequence peptides plays a key role in the development of nanomaterials with specific structure-property-function relationships from a bottom-up perspective. Due to their poor water solubility, the self-assembly of Fmoc-bearing peptides is usually induced by dissolution in an organic solvent, followed by a dilution step in water, pH changes, and/or a heating-cooling process. Herein, we report a straightforward methodology for the gelation of Fmoc-FFpY (F: phenylalanine; Y: tyrosine; and p: PO42-), a negatively charged tripeptide, in NaCl solution. The electrostatic interactions between Fmoc-FFpY and Na+ ions give rise to different nanofibrillar hydrogels with rheological properties and nanofiber sizes modulated by the NaCl concentration in pure aqueous media. Initiated by the electrostatic interactions between the peptide phosphate groups and the Na+ ions, the peptide self-assembly is stabilized thanks to hydrogen bonds between the peptide backbones and the π-π stacking of aromatic Fmoc and phenyl units. The hydrogels showed self-healing and thermo-responsive properties for potential biomedical applications. Molecular dynamics simulations from systems devoid of prior training not only confirm the aggregation of peptides at a critical salt concentration and the different interactions involved, but also corroborate the secondary structure of the hydrogels at the microsecond timescale. It is worth highlighting the remarkable achievement of reproducing the morphological behavior of the hydrogels using atomistic simulations. To our knowledge, this study is the first to report such a correspondence.

Self-assembly of heterochiral, aliphatic dipeptides with Leu

This work describes the self-assembly behavior of heterochiral, aliphatic dipeptides, l-Leu-d-Xaa (Xaa = Ala, Val, Ile, Leu), in green solvents such as acetonitrile (MeCN) and buffered water at neutral pH. Interestingly, water plays a structuring role because at 1% v/v, it enables dipeptide self-assembly in MeCN to yield organogels, which then undergo transition towards crystals. Other organic solvents and oils were tested for gelation, and metastable gels were formed in tetrahydrofuran, although at high peptide concentration (80 mM). Single-crystal X-ray diffraction revealed the dipeptides' supramolecular packing modes in amphipathic layers, as opposed to water channels reported for the homochiral Leu-Leu, or hydrophobic columns reported for homochiral Leu-Val and Leu-Ile.

Biostable hydrogels consisting of hybrid β-sheet fibrils assembled by a pair of enantiomeric peptides

The assembly of chiral peptides facilitates the formation of diverse supramolecular structures with unique physicochemical and biological properties. However, the effects of chirality on peptide assembly and resulting hydrogel properties remain underexplored. In this study, we systematically investigated the assembly propensity, morphology, and biostability of mixture of a pair of enantiomeric peptides LELCLALFLF (ECF-5) and DEDCDADFDF (ecf-5) at various ratios. Results indicate the development of β-sheet fibrils, ultimately leading to the formation of self-supporting hybrid hydrogels. The hydrogel formed at a ratio of 1:1 exhibits a significantly lower storage modulus (G') than of the ratios of 0:1, 1:3, 3:1 and 1:0 (nD/nL; same below). Kink-separated fragments of approximately 100 nm in length predominate at ratios of 1:3 and 3:1, compared with the smooth fibrils at other ratios, probably attributed to an alternating arrangement of the co-assembled and self-assembled peptide fragments. The introduction of ecf-5 to the hybrid hydrogels improves resistance to proteolytic digestion and maintains commendable biocompatibility in both MIN6 and HUVECs cells. These findings provide valuable insights into the development of hydrogels with tailored properties, positing them potential scaffolds for 3D cell culture and tissue engineering.

Ultrashort Cationic Peptide Fmoc-FFK as Hydrogel Building Block for Potential Biomedical Applications

Fmoc-diphenylalanine (Fmoc-FF) is a low-molecular-weight peptide hydrogelator. This simple all-aromatic peptide can generate self-supporting hydrogel materials, which have been proposed as novel materials for diagnostic and pharmaceutical applications. Our knowledge of the molecular determinants of Fmoc-FF aggregation is used as a guide to design new peptide-based gelators, with features for the development of improved tools. Here, we enlarge the plethora of Fmoc-FF-based hydrogelated matrices by studying the properties of the Fmoc-FFK tripeptide, alone or in combination with Fmoc-FF. For multicomponent matrices, the relative weight ratios between Fmoc-FFK and Fmoc-FF (specifically, 1/1, 1/5, 1/10, and 1/20 w/w) are evaluated. All the systems and their multiscale organization are studied using different experimental techniques, including rheology, circular dichroism, Fourier transform infrared spectroscopy, and scanning electron microscopy (SEM). Preliminary profiles of biocompatibility for the studied systems are also described by testing them in vitro on HaCaT and 3T3-L1 cell lines. Additionally, the lysine (K) residue at the C-terminus of the Fmoc-FF moiety introduces into the supramolecular material chemical functions (amino groups) which may be useful for modification/derivatization with bioactive molecules of interest, including diagnostic probes, chelating agents, active pharmaceutical ingredients, or peptide nucleic acids.

Self-assembling tripeptide forming water-bound channels and hydrogels

D-Ser(tBu)-L-Phe-L-Trp is described as a self-assembling tripeptide that yields nanofibrillar hydrogels at physiological conditions (phosphate buffer at pH 7.4). The peptide is characterized by several spectroscopic methods, such as circular dichroism and fluorescence, oscillatory rheometry, and transmission electron microscopy. Single-crystal X-ray diffraction reveals supramolecular packing into water-bound channels and allows the visualization of the intermolecular interactions holding together peptide stacks.

Substance P-Derived Extracellular-Matrix-Mimicking Peptide Hydrogel as a Cytocompatible Biomaterial Platform

Self-assembled short peptide-based hydrogel platforms have become widely applicable biomedical therapeutic maneuvers for their soft, tunable architecture, which can influence cellular behavior and morphology to an inordinate extent. In this work, a short supramolecular hydrogelator peptide, substance P, has been designed and synthesized from the C terminus conserved "FFGLM" section of a biologically abundant neuropeptide by using a fusion approach. In addition, to incorporate a good hydrophobic-hydrophilic balance, the truncated pentapeptide segment was further C-terminally modified by the incorporation of an integrin-binding "RGD" motif. Thanks to its N-terminal Fmoc group, this octapeptide ensemble "FFGLMRGD" undergoes rapid self-assembly to give rise to an injectable, pH-responsive, hydrogel-based self-supporting platform that exhibited good cytocompatibility with the cultured mammalian cells under both 2D and 3D culture conditions without exerting any potent cytotoxic effect in a Live/Dead experiment. A rheological experiment demonstrated its hydrogel-like mechanical properties, including thixotropicity. The atomic force microscopy and field emission scanning electron microscopy images of the fabricated hydrogel show a tangled fibrous surface topography owing to the presence of the N-terminal Fmoc-FF residue. Furthermore, an in-vitro scratch assay performed on fibroblast cell lines confirmed the wound-ameliorating potency of this designed hydrogel; this substantiates its future therapeutic prospects.

Cysteine Redox Chemistry in Peptide Self-Assembly to Modulate Hydrogelation

Cysteine redox chemistry is widely used in nature to direct protein assembly, and in recent years it has inspired chemists to design self-assembling peptides too. In this concise review, we describe the progress in the field focusing on the recent advancements that make use of Cys thiol-disulfide redox chemistry to modulate hydrogelation of various peptide classes.

Fmoc-diphenylalanine gelating nanoarchitectonics: A simplistic peptide self-assembly to meet complex applications

9-fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF), has been has been extensively explored due to its ultrafast self-assembly kinetics, inherent biocompatibility, tunable physicochemical properties, and especially, the capability of forming self-sustained gels under physiological conditions. Consequently, various methodologies to develop Fmoc-FF gels and their corresponding applications in biomedical and industrial fields have been extensively studied. Herein, we systemically summarize the mechanisms underlying Fmoc-FF self-assembly, discuss the preparation methodologies of Fmoc-FF hydrogels, and then deliberate the properties as well as the diverse applications of Fmoc-FF self-assemblies. Finally, the contemporary shortcomings which limit the development of Fmoc-FF self-assembly are raised and the alternative solutions are proposed, along with future research perspectives.

Peptide-Based Materials That Exploit Metal Coordination

Metal-ion coordination has been widely exploited to control the supramolecular behavior of a variety of building blocks into functional materials. In particular, peptides offer great chemical diversity for metal-binding modes, combined with inherent biocompatibility and biodegradability that make them attractive especially for medicine, sensing, and environmental remediation. The focus of this review is the last 5 years' progress in this exciting field to conclude with an overview of the future directions that this research area is currently undertaking.

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.

Peptide-Based Supramolecular Hydrogels as Drug Delivery Agents: Recent Advances

Supramolecular peptide hydrogels have many important applications in biomedicine, including drug delivery applications for the sustained release of therapeutic molecules. Targeted and selective drug administration is often preferential to systemic drug delivery, as it can allow reduced doses and can avoid the toxicity and side-effects caused by off-target binding. New discoveries are continually being reported in this rapidly developing field. In this review, we report the latest developments in supramolecular peptide-based hydrogels for drug delivery, focusing primarily on discoveries that have been reported in the last four years (2018-present). We address clinical points, such as peptide self-assembly and drug release, mechanical properties in drug delivery, peptide functionalization, bioadhesive properties and drug delivery enhancement strategies, drug release profiles, and different hydrogel matrices for anticancer drug loading and release.

Self-assembly pathways in a triphenylalanine peptide capped with aromatic groups

Peptide derivatives and, most specifically, their self-assembled supramolecular structures are being considered in the design of novel biofunctional materials. Although the self-assembly of triphenylalanine homopeptides has been found to be more versatile than that of homopeptides containing an even number of residues (i.e. diphenylalanine and tetraphenylalanine), only uncapped triphenylalanine (FFF) and a highly aromatic analog blocked at both the N- and C-termini with fluorenyl-containing groups (Fmoc-FFF-OFm), have been deeply studied before. In this work, we have examined the self-assembly of a triphenylalanine derivative bearing 9-fluorenylmethyloxycarbonyl and benzyl ester end-capping groups at the N- and C-termini, respectively (Fmoc-FFF-OBzl). The antiparallel arrangement clearly dominates in β-sheets formed by Fmoc-FFF-OBzl, whereas the parallel and antiparallel dispositions are almost isoenergetic in Fmoc-FFF-OFm β-sheets and the parallel one is slightly favored for FFF. The effects of both the peptide concentration and the medium on the self-assembly process have been examined considering Fmoc-FFF-OBzl solutions in a wide variety of solvent:co-solvent mixtures. In addition, Fmoc-FFF-OBzl supramolecular structures have been compared to those obtained for FFF and Fmoc-FFF-OFm under identical experimental conditions. The strength of π-π stacking interactions involving the end-capping groups plays a crucial role in the nucleation and growth of supramolecular structures, which determines the resulting morphology. Finally, the influence of a non-invasive external stimulus, ultrasounds, on the nucleation and growth of supramolecular structures has been examined. Overall, FFF-based peptides provide a wide range of supramolecular structures that can be of interest in the biotechnological field.

Chiral Fibers Formation Upon Assembly of Tetraphenylalanine Peptide Conjugated to a PNA Dimer

Self‐assembly of biomolecules such as peptides, nucleic acids or their analogues affords supramolecular objects, exhibiting structures and physical properties dependent on the amino‐acid or nucleobase composition. Conjugation of the peptide diphenylalanine (FF) to peptide nucleic acids triggers formation of self‐assembled structures, mainly stabilized by interactions between FF. In this work we report formation of homogeneous chiral fibers upon self‐assembly of the hybrid composed of the tetraphenylalanine peptide (4F) conjugated to the PNA dimer adenine‐thymine (at). In this case nucleobases seem to play a key role in determining the morphology and chirality of the fibers. When the PNA “at” is replaced by guanine‐cytosine dimer “gc”, disordered structures are observed. Spectroscopic characterization of the self‐assembled hybrids, along with AFM and SEM studies is reported. Finally, a structural model consistent with the experimental evidence has also been obtained, showing how the building blocks of 4Fat arrange to give helical fibers.

Self-Assembled Peptide Nanostructures for ECM Biomimicry

Proteins are functional building blocks of living organisms that exert a wide variety of functions, but their synthesis and industrial production can be cumbersome and expensive. By contrast, short peptides are very convenient to prepare at a low cost on a large scale, and their self-assembly into nanostructures and gels is a popular avenue for protein biomimicry. In this Review, we will analyze the last 5-year progress on the incorporation of bioactive motifs into self-assembling peptides to mimic functional proteins of the extracellular matrix (ECM) and guide cell fate inside hydrogel scaffolds.

An injectable, naproxen-conjugated, supramolecular hydrogel with ultra-low critical gelation concentration-prepared from a known folate receptor ligand

Short peptides capped on the N-terminus with aromatic groups are often able to form supramolecular hydrogels-self-assembled networks of fibrils able to trap water molecules. Typically, these hydrogelators can form stiff gels at concentrations of 0.1 to 1.0 wt%-i.e. they consist of mainly water. The properties of these soft materials mimic those of the extracellular matrix (ECM) of biological tissue and therefore they have found many biomedical uses in tissue engineering, wound healing, drug delivery, biosensing and bioprinting applications. In drug delivery strategies related to cancer therapy, injectable hydrogels can serve as a depot formulation, where a sustained release of the chemotherapeutic from near the tumour site allows reduced doses and, therefore, decreased side effects. To further target cancer cells, folic acid-conjugated hydrogels and nanostructures are often sought, to exploit the overexpression of folate receptors on cancer cells-an approach which can allow the selective cellular uptake of an encapsulated drug. In this present study, two known dipeptide folate receptor ligands (1 and 2) recently identified from a screen of a DNA-encoded compound library, were synthesised and investigated for their hydrogelation ability and cytotoxicity. Compound 1, containing a naproxen capping group, rapidly forms hydrogels at concentrations as low as 0.03 wt%-one of the lowest critical gelation concentrations (CGCs) known for a supramolecular hydrogelator. In contrast, compound 2, which contains a 3-indolepropionic acid capping group, was unable to form hydrogels under a range of conditions and concentrations, instead forming nanospheres with diameters of 0.5 μm. Hydrogels of 1 were characterised by STEM microscopy, rheology, fluorescence spectroscopy and circular dichroism. Both compounds 1 and 2 had no impact on the proliferation of kerotinocytes (HaCaT cells) at concentrations up to 100 μM. Compound 1, containing the NSAID, was tested for anti-inflammatory activity in a human cyclooxygenase-1/2 model. The rate of the release of model drug compounds from within hydrogels of 1 was also investigated.

Electrofabrication of large volume di- and tripeptide hydrogels via hydroquinone oxidation

The fabrication of protected peptide-based hydrogels on electrode surfaces can be achieved by employing the electrochemical oxidation of hydroquinone to benzoquinone, liberating protons at the electrode-solution interface. The localised reduction in pH below the dipeptide gelator molecules pK_a initiates the neutralisation, self-assembly and formation of self-supporting hydrogels exclusively at the electrode surface. Previous examples have been on a nanometre to millimetre scale, using deposition times ranging from seconds to minutes. However, the maximum size to which these materials can grow and their subsequent mechanical properties have not yet been investigated. Here, we report the fabrication of the largest reported di- and tri-peptide based hydrogels using this electrochemical method, employing deposition times of two to five hours. To overcome the oxidation of hydroquinone in air, the fabrication process was performed under an inert nitrogen atmosphere. We show that this approach can be used to form multilayer gels, with the mechanical properties of each layer determined by gelator composition. We also describe examples where gel-to-crystal transitions and syneresis occur within the material.

Recent Advances in Zwitterionic Hydrogels: Preparation, Property, and Biomedical Application

Nonspecific protein adsorption impedes the sustainability of materials in biologically related applications. Such adsorption activates the immune system by quick identification of allogeneic materials and triggers a rejection, resulting in the rapid failure of implant materials and drugs. Antifouling materials have been rapidly developed in the past 20 years, from natural polysaccharides (such as dextran) to synthetic polymers (such as polyethylene glycol, PEG). However, recent studies have shown that traditional antifouling materials, including PEG, still fail to overcome the challenges of a complex human environment. Zwitterionic materials are a class of materials that contain both cationic and anionic groups, with their overall charge being neutral. Compared with PEG materials, zwitterionic materials have much stronger hydration, which is considered the most important factor for antifouling. Among zwitterionic materials, zwitterionic hydrogels have excellent structural stability and controllable regulation capabilities for various biomedical scenarios. Here, we first describe the mechanism and structure of zwitterionic materials. Following the preparation and property of zwitterionic hydrogels, recent advances in zwitterionic hydrogels in various biomedical applications are reviewed.

Self-Assembling Peptides: From Design to Biomedical Applications

Self-assembling peptides could be considered a novel class of agents able to harvest an array of micro/nanostructures that are highly attractive in the biomedical field. By modifying their amino acid composition, it is possible to mime several biological functions; when assembled in micro/nanostructures, they can be used for a variety of purposes such as tissue regeneration and engineering or drug delivery to improve drug release and/or stability and to reduce side effects. Other significant advantages of self-assembled peptides involve their biocompatibility and their ability to efficiently target molecular recognition sites. Due to their intrinsic characteristics, self-assembled peptide micro/nanostructures are capable to load both hydrophobic and hydrophilic drugs, and they are suitable to achieve a triggered drug delivery at disease sites by inserting in their structure's stimuli-responsive moieties. The focus of this review was to summarize the most recent and significant studies on self-assembled peptides with an emphasis on their application in the biomedical field.

Self-Assembly of Unprotected Dipeptides into Hydrogels: Water-Channels Make the Difference

Unprotected dipeptides are attractive building blocks for environmentally friendly hydrogel biomaterials by virtue of their low‐cost and ease of preparation. This work investigates the self‐assembling behaviour of the distinct stereoisomers of Ile‐Phe and Phe‐Ile in phosphate buffered saline (PBS) to form hydrogels, using transmission electron microscopy (TEM), attenuated total reflectance infrared spectroscopy (ATR‐IR), circular dichroism (CD), and oscillatory rheometry. Each peptide purity and identity was also confirmed by ¹H‐ and ¹³C‐NMR spectroscopy and HPLC‐MS. Finally, single‐crystal XRD data allowed the key interactions responsible for the supramolecular packing into amphipathic layers or water‐channels to be revealed. The presence of the latter in the crystal structure is a distinctive feature of the only gelator of this work that self‐organizes into stable hydrogels, with fast kinetics and the highest elastic modulus amongst its structural isomers and stereoisomers.

Self-assembly of amino acids toward functional biomaterials

Biomolecules, such as proteins and peptides, can be self-assembled. They are widely distributed, easy to obtain, and biocompatible. However, the self-assembly of proteins and peptides has disadvantages, such as difficulty in obtaining high quantities of materials, high cost, polydispersity, and purification limitations. The difficulties in using proteins and peptides as functional materials make it more complicate to arrange assembled nanostructures at both microscopic and macroscopic scales. Amino acids, as the smallest constituent of proteins and the smallest constituent in the bottom-up approach, are the smallest building blocks that can be self-assembled. The self-assembly of single amino acids has the advantages of low synthesis cost, simple modeling, excellent biocompatibility and biodegradability in vivo. In addition, amino acids can be assembled with other components to meet multiple scientific needs. However, using these simple building blocks to design attractive materials remains a challenge due to the simplicity of the amino acids. Most of the review articles about self-assembly focus on large molecules, such as peptides and proteins. The preparation of complicated materials by self-assembly of amino acids has not yet been evaluated. Therefore, it is of great significance to systematically summarize the literature of amino acid self-assembly. This article reviews the recent advances in amino acid self-assembly regarding amino acid self-assembly, functional amino acid self-assembly, amino acid coordination self-assembly, and amino acid regulatory functional molecule self-assembly.

An Inspection into Multifarious Ways to Synthesize Poly(Amino Acid)s

Poly(α-amino acid)s (PAAs) attract growing attention due to their essential role in the application as biomaterials. To synthesize PAAs with desired structures and properties, scientists have developed various synthetic techniques with respective advantages. Here, different approaches to preparing PAAs are inspected. Basic features and recent progresses of these methods are summarized, including polymerizations of amino acid N-carboxyanhydrides (NCAs), amino acid N-thiocarboxyanhydrides (NTAs), and N-phenoxycarbonyl amino acids (NPCs), as well as other synthetic routes. NCA is the most classical monomer to prepare PAAs with high molecular weights (MWs). NTA polymerizations are promising alternative pathways to produce PAAs, which can tolerate nucleophiles including alcohols, mercaptans, carboxyl acids, and water. By various techniques including choosing appropriate solvents or using organic acids as promoters, NTAs polymerize to produce polypeptoids and polypeptides with narrow dispersities and designed MWs up to 55.0 and 57.0 kg mol^-1 , respectively. NPC polymerizations are phosgene-free ways to synthesize polypeptides and polypeptoids. For the future prospects, detail investigations into polymerization mechanisms of NTA and NPC are expected. The synthesis of PAAs with designed topologies and assembly structures is another intriguing topic. The advantages and unsettled problems in various synthetic ways are discussed for readers to choose appropriate approaches for PAAs.

Self-Assembly Dipeptide Hydrogel: The Structures and Properties

Self-assembly peptide-based hydrogels are well known and popular in biomedical applications due to the fact that they are readily controllable and have biocompatibility properties. A dipeptide is the shortest self-assembling motif of peptides. Due to its small size and simple synthesis method, dipeptide can provide a simple and easy-to-use method to study the mechanism of peptides' self-assembly. This review describes the design and structures of self-assembly linear dipeptide hydrogels. The strategies for preparing the new generation of linear dipeptide hydrogels can be divided into three categories based on the modification site of dipeptide: 1) COOH-terminal and N-terminal modified dipeptide, 2) C-terminal modified dipeptide, and 3) uncapped dipeptide. With a deeper understanding of the relationship between the structures and properties of dipeptides, we believe that dipeptide hydrogels have great potential application in preparing minimal biocompatible materials.

Understanding gel-to-crystal transitions in supramolecular gels

Most supramolecular gels are stable or assumed to be stable over time, and aging effects are often not studied. However, some gels do show clear changes on aging, and a small number of systems exhibit gel-to-crystal transitions. In these cases, crystals form over time, typically at the expense of the network underpinning the gel; this leads to the gel falling apart. These systems are rare, and little is known about how these gel-to-crystal transitions occur. Here, we use a range of techniques to understand in detail a gel-to-crystal transition for a specific functionalised dipeptide based gelator. We show that the gel-to-crystal transition depends on the final pH of the medium which we control by varying the amount of glucon-δ-lactone (GdL) added. In the gel phase, at low concentrations of GdL, and at early time points with high concentrations of GdL, we are able to show the nanometre scale dimensions of the self-assembled fibre using SAXS; however there is no evidence of molecular ordering of the gel fibres in the WAXS. At low concentrations of GdL, these self-assembled fibres stiffen with time but do not crystallise over the timescale of the SAXS experiment. At high concentrations of GdL, the fibres are already stiffened, and then, as the pH drops further, give way to the presence of crystals which appear to grow preferentially along the direction of the fibre axis. We definitively show therefore that the gel and crystal phase are not the same. Our work shows that many assumptions in the literature are incorrect. Finally, we also show that the sample holder geometry is an important parameter for these experiments, with the rate of crystallisation depending on the holder in which the experiment is carried out.

Exploiting and controlling gel-to-crystal transitions in multicomponent supramolecular gels

Multicomponent supramolecular gels provide opportunities to form materials that are not accessible when using the single components alone. Different scenarios are possible when mixing multiple components, from complete co-assembly (mixing of the components within the self-assembled structures formed) to complete self-sorting such that each structure contains only one of the components. Most examples of multicomponent gels that currently exist form stable gels. Here, we show that this can be used to control the mechanical properties of the gels, but what is probably most exciting is that we show that we can use a magnetic field to control the shape of the crystals. The gelling component aligns in a magnetic field and so results in anisotropic crystals being formed.

Metal Cation Triggered Peptide Hydrogels and Their Application in Food Freshness Monitoring and Dye Adsorption

Metal-ligand interactions have emerged as an important tool to trigger and modulate self-assembly, and to tune the properties of the final supramolecular materials. Herein, we report the metal-cation induced self-assembly of a pyrene-peptide conjugate to form hydrogels. The peptide has been rationally designed to favor the formation of β-sheet 1D assemblies and metal coordination through the Glu side chains. We studied in detail the self-assembly process in the presence of H+, Li+, Na+, K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Co2+, Fe3+, and Cr3+ and found that the morphology and mechanical properties of the hydrogels are ion-dependent. Moreover, thanks to the presence of the metal, new applications could be explored. Cu2+ metallogels could be used for amine sensing and meat freshness monitoring, while Zn2+ metallogels showed good selectivity for cationic dye adsorption and separation.

Nicotinamide-based supergelator self-assembling via asymmetric hydrogen bonding NH⋯OC and H⋯Br- pattern for reusable, moldable and self-healable nontoxic fuel gels

HYPOTHESIS

Development of highly efficient low-molecular weight gelators (LMWGs) for safe energy storage materials is of great demand. Energy storage materials as fuel gels are often achieved by construction of hybrid organic frameworks capable of multiple noncovalent interactions in self-assembly, which allow tuning required properties at the molecular level by altering individual building blocks of the LMWG. However, LMWGs have limited rechargeable capability due to their chemical instability.

EXPERIMENTS

We designed, synthesized and characterized a novel, bio-inspired chiral gemini amphiphile derivative 1 containing N-hexadecyl aliphatic tails from quaternized nicotinamide-based segment and bromide anion showing supergelation ability in water, alcohols, aprotic polar and aromatic solvents, with critical gel concentrations as low as 0.1 and 0.035 wt% in isopropanol and water, respectively.

FINDINGS

Nanostructural architecture of the network depended on the solvent used and showed variations in size and shape of 1D nanofibers. Supergelation is attributed to a unique asymmetric NH⋯OC, H⋯Br^- hydrogen bonding pattern between H-2 hydrogens from nicotinamide-based segment, amide functional groups from chiral trans-cyclohexane-1,2-diamide-based segment and bromide ions, supporting the intermolecular amide-amide interactions appearing across one strand of the self-assembly. Gels formed from 1 exhibit high stiffness, self-healing, moldable and colorable properties. In addition, isopropanol gels of 1 are attractive as reusable, shape-persistent non-toxic fuels maintaining the chemical structure with gelation efficiency for at least five consecutive burning cycles.

Electrochemical Peptide-Based Sensors for Foodborne Pathogens Detection

Food safety and quality control pose serious issues to food industry and public health domains, in general, with direct effects on consumers. Any physical, chemical, or biological unexpected or unidentified food constituent may exhibit harmful effects on people and animals from mild to severe reactions. According to the World Health Organization (WHO), unsafe foodstuffs are especially dangerous for infants, young children, elderly, and chronic patients. It is imperative to continuously develop new technologies to detect foodborne pathogens and contaminants in order to aid the strengthening of healthcare and economic systems. In recent years, peptide-based sensors gained much attention in the field of food research as an alternative to immuno-, apta-, or DNA-based sensors. This review presents an overview of the electrochemical biosensors using peptides as molecular bio-recognition elements published mainly in the last decade, highlighting their possible application for rapid, non-destructive, and in situ analysis of food samples. Comparison with peptide-based optical and piezoelectrical sensors in terms of analytical performance is presented. Methods of foodstuffs pretreatment are also discussed.

Fmoc-PEG Coated Single-Wall Carbon Nanotube Carriers by Non-covalent Functionalization: An Experimental and Molecular Dynamics Study

Due to their structural characteristics at the nanoscale level, single-walled carbon nanotubes (SWNTs), hold great promise for applications in biomedicine such as drug delivery systems. Herein, a novel single-walled carbon nanotube (SWNT)-based drug delivery system was developed by conjugation of various Fmoc-amino acid bearing polyethylene glycol (PEG) chains (Mw = 2,000, 5,000, and 12,000). In the first step, full-atom molecular dynamics simulations (MD) were performed to identify the most suitable Fmoc-amino acid for an effective surface coating of SWNT. Fmoc-glycine, Fmoc-tryptophan, and Fmoc-cysteine were selected to attach to the PEG polymer. Here, Fmoc-cysteine and -tryptophan had better average interaction energies with SWNT with a high number of aromatic groups, while Fmoc-glycine provided a non-aromatic control. In the experimental studies, non-covalent modification of SWNTs was achieved by Fmoc-amino acid-bearing PEG chains. The remarkably high amount of Fmoc-glycine-PEG, Fmoc-tryptophan-PEG, and Fmoc-cysteine-PEG complexes adsorbed onto the SWNT surface, as was assessed via thermogravimetric and UV-vis spectroscopy analyses. Furthermore, Fmoc-cysteine-PEG5000 and Fmoc-cysteine-PEG12000 complexes displayed longer suspension time in deionized water, up to 1 and 5 week, respectively, underlying the ability of these surfactants to effectively disperse SWNTs in an aqueous environment. In vitro cell viability assays on human dermal fibroblast cells also showed the low cytotoxicity of these two samples, even at high concentrations. In conclusion, synthesized nanocarriers have a great potential for drug delivery systems, with high loading capacity, and excellent complex stability in water critical for biocompatibility.

Designing a Transparent and Fluorine Containing Hydrogel

Physical hydrogels are supramolecular materials obtained by self-assembly of small molecules called gelators. Aromatic amino acids and small peptides containing aromatic rings are good candidates as gelators due to their ability to form weak bonds as π-π interactions and hydrogen bonds between NH and CO of the peptide chain. In this paper we show our results in the preparation of a transparent hydrogel that was obtained by self-assembly of a fluorine-containing dipeptide that relies on the additional formation of halogen bonds due to the fluorine atoms contained in the dipeptide. We used Boc-D-F2Phe-L-Oxd-OH (F2Phe = 3,4-difluorophenylalainine; Oxd = 4-methyl-5-carboxy-oxazolidin-2-one) that formed a strong and transparent hydrogel in 0.5% w/w concentration at pH = 4.2. The formation of a hydrogel made of unnatural fluorinated amino acids may be of great interest in the evaluation of patients with parkinsonian syndromes and may be used for controlled release.

Evaluation of a Model Photo-Caged Dehydropeptide as a Stimuli-Responsive Supramolecular Hydrogel

Short peptides capped on the N-terminus with aromatic groups are often able to form supramolecular hydrogels, via self-assembly, in aqueous media. The rheological properties of these readily tunable hydrogels resemble those of the extracellular matrix (ECM) and therefore have potential for various biological applications, such as tissue engineering, biosensors, 3D bioprinting, drug delivery systems and wound dressings. We herein report a new photo-responsive supramolecular hydrogel based on a "caged" dehydropeptide (CNB-Phe-ΔPhe-OH 2), containing a photo-cleavable carboxy-2-nitrobenzyl (CNB) group. We have characterized this hydrogel using a range of techniques. Irradiation with UV light cleaves the pendant aromatic capping group, to liberate the corresponding uncaged model dehydropeptide (H-Phe-ΔPhe-OH 3), a process which was investigated by ¹H NMR and HPLC studies. Crucially, this cleavage of the capping group is accompanied by dissolution of the hydrogel (studied visually and by fluorescence spectroscopy), as the delicate balance of intramolecular interactions within the hydrogel structure is disrupted. Hydrogels which can be disassembled non-invasively with temporal and spatial control have great potential for specialized on-demand drug release systems, wound dressing materials and various topical treatments. Both 2 and 3 were found to be non-cytotoxic to the human keratinocyte cell line, HaCaT. The UV-responsive hydrogel system reported here is complementary to previously reported related UV-responsive systems, which are generally composed of peptides formed from canonical amino acids, which are susceptible to enzymatic proteolysis in vivo. This system is based on a dehydrodipeptide structure which is known to confer proteolytic resistance. We have investigated the ability of the photo-activated system to accelerate the release of the antibiotic, ciprofloxacin, as well as some other small model drug compounds. We have also conducted some initial studies towards skin-related applications. Moreover, this model system could potentially be adapted for on-demand "self-delivery", through the uncaging of known biologically active dehydrodipeptides.

Dehydropeptide Supramolecular Hydrogels and Nanostructures as Potential Peptidomimetic Biomedical Materials

Supramolecular peptide hydrogels are gaining increased attention, owing to their potential in a variety of biomedical applications. Their physical properties are similar to those of the extracellular matrix (ECM), which is key to their applications in the cell culture of specialized cells, tissue engineering, skin regeneration, and wound healing. The structure of these hydrogels usually consists of a di- or tripeptide capped on the N-terminus with a hydrophobic aromatic group, such as Fmoc or naphthalene. Although these peptide conjugates can offer advantages over other types of gelators such as cross-linked polymers, they usually possess the limitation of being particularly sensitive to proteolysis by endogenous proteases. One of the strategies reported that can overcome this barrier is to use a peptidomimetic strategy, in which natural amino acids are switched for non-proteinogenic analogues, such as D-amino acids, β-amino acids, or dehydroamino acids. Such peptides usually possess much greater resistance to enzymatic hydrolysis. Peptides containing dehydroamino acids, i.e., dehydropeptides, are particularly interesting, as the presence of the double bond also introduces a conformational restraint to the peptide backbone, resulting in (often predictable) changes to the secondary structure of the peptide. This review focuses on peptide hydrogels and related nanostructures, where α,β-didehydro-α-amino acids have been successfully incorporated into the structure of peptide hydrogelators, and the resulting properties are discussed in terms of their potential biomedical applications. Where appropriate, their properties are compared with those of the corresponding peptide hydrogelator composed of canonical amino acids. In a wider context, we consider the presence of dehydroamino acids in natural compounds and medicinally important compounds as well as their limitations, and we consider some of the synthetic strategies for obtaining dehydropeptides. Finally, we consider the future direction for this research area.

Revisiting the Self-Assembly of Highly Aromatic Phenylalanine Homopeptides

Diphenylalanine peptide (FF), which self-assembles into rigid tubular nanostructures, is a very short core recognition motif in Alzheimer's disease β-amyloid (Aβ) polypeptide. Moreover, the ability of the phenylalanine (F or Phe)-homopeptides to self-assemble into ordered nanostructures has been proved. Within this context it was shown that the assembly preferences of this family of compounds is altered by capping both the N- and C-termini using highly aromatic fluorenyl groups (i.e., fluorenyl-9-methoxycarbonyl and 9-fluorenylmethyl ester, named Fmoc and OFm, respectively). In this article the work performed in the field of the effect of the structure and incubation conditions on the morphology and polymorphism of short (from two to four amino acid residues) Phe-homopeptides is reviewed and accompanied by introducing some new results for completing the comparison. Special attention has been paid to the influence of solvent: co-solvent mixture used to solubilize the peptide, the peptide concentration and, in some cases, the temperature. More specifically, uncapped (FF, FFF, and FFFF), N-capped with Fmoc (Fmoc-FF, Fmoc-FFF, and Fmoc-FFFF), C-capped with OFm (FF-OFm), and doubly capped (Fmoc-FF-OFm, Fmoc-FFF-OFm, and Fmoc-FFFF-OFm) Phe-homopeptides have been re-measured. Although many of the experienced assembly conditions have been only revisited as they were previously reported, other experimental conditions have been examined by the first time in this work. In any case, pooling the effect of highly aromatic blocking groups in a single study, using a wide variety of experimental conditions, allows a perspective of how the disappearance of head-to-tail electrostatic interactions and the gradual increase in the amount of π-π stacking interactions, affects the morphology of the assemblies. Future technological applications of Phe-homopeptides can be envisaged by choosing the most appropriate self-assemble structure, defining not only the length of the peptide but also the amount and the position of fluorenyl capping groups.

Exploring the properties and potential biomedical applications of NSAID-capped peptide hydrogels

The development of strategies to minimise the adverse side-effects of non-steroidal anti-inflammatory drugs (NSAIDs) remains a challenge for medicinal chemists. One such strategy is the development of NSAID-peptide prodrug conjugates and this conjugation to a peptide often confers the additional property of hydrogelation. This review summarises the work published by our research group, alongside other research groups, on supramolecular hydrogels consisting of short peptides conjugated to NSAIDs. Generally, supramolecular low molecular weight hydrogels (LMWHs) are composed of amphiteric molecules, usually consisting of short peptides attached to an aromatic capping group. When the aromatic capping group is switched for an NSAID to afford hybrid gelators, some conjugates exhibit retained or improved anti-inflammatory properties of the parent drug, and sometimes new and unexpected biological activities are observed. Conjugation to peptides often provides selective COX-2 inhibition over COX-1 inhibtion, which is key to retaining the anti-inflammatory benefits of NSAIDs whilst minimising gastric side-effects. Naproxen is the most commonly employed NSAID capping group, partly due to its similarity in structure to commonly employed naphthalene capping groups. Biomimetic approaches, where canonical amino acids are switched for non-natural amino acids such as d-amino acids or dehydroamino acids, are often employed, to tune the stability. The future direction for this area of research is discussed.

Photo-dissociation of self-assembled (anthracene-2-carbonyl)amino acid hydrogels

Amino acids modified with an N-terminal anthracene group self-assemble into supramolecular hydrogels upon the addition of a range of salts or cell culture medium. Gel-phase photo-dimerisation of gelators results in hydrogel disassembly and was used to recover cells from 3D culture.

Low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, wound healing, anticancer, drug delivery, bioimaging and 3D bioprinting applications

In this review, we have focused on the design and development of low molecular weight self-assembling peptide-based materials for various applications including cell proliferation, tissue engineering, antibacterial, antifungal, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting. The first part of the review describes about stimuli and various noncovalent interactions, which are the key components of various self-assembly processes for the construction of organized structures. Subsequently, the chemical functionalization of the peptides has been discussed, which is required for the designing of self-assembling peptide-based soft materials. Various low molecular weight self-assembling peptides have been discussed to explain the important structural features for the construction of defined functional nanostructures. Finally, we have discussed various examples of low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting applications.

Amyloids and their untapped potential as hydrogelators

Amyloid fibrils are cross-β-sheet-rich fibrous aggregates. They were originally identified as disease-associated protein/peptide deposits. The cross-β motif was consequently labelled as an alien and pathogenic fold. Subsequent research revealed that the fibrillar aggregates were benign, and the cytotoxicity in the amyloid diseases was attributed to the pre-fibrillar structures. Research in the past two decades has identified the native functional amyloids in organisms ranging from bacteria to human. The amyloid-like fibrils, therefore, are not necessarily pathogenic, and the cross-β motif is very much native. This premise makes way for the amyloids to be used as biocompatible materials. Many naturally occurring amyloidogenic proteins/peptides or their fragments have been reported in the literature to form hydrogels. Hydrogels constitute one of the most interesting classes of soft materials that find application in diverse fields such as environmental, electronic, and biomedical engineering. Applications of hydrogels in medicine are particularly extensive. Among various classes of peptides that form hydrogels, the potential of amyloids is largely untapped. In this review, we have attempted to compile the literature on amyloid hydrogels and discuss their potential applications.

Constitutionally Isomeric Aromatic Tripeptides: Self-Assembly and Metal-Ion-Modulated Transformations

Self-assembling peptides based on aromatic amino acids can adopt diverse nanostructures which primarily depend on their molecular structures. Therefore, to understand the nature of self-assembly on the molecular level we rationally designed two constitutional isomers of short aromatic peptides. The first isomer consists of a tyrosine moiety at the N-terminus and the second isomer consists of a tyrosine moiety at the C-terminus of the FF peptide, a core recognition motif of Amyloid β peptides. Therefore, it can be considered that both the designed tripeptides are the analogues of the FFF peptide with only atomic(-H) level replacement by -OH functional group on the first and last phenyl ring, respectively. The first isomer self-assembled into 2D porous nanosheets ("Nanowebs"), however the second isomers produced toroidal shapes with central spheres ("Nano-Saturn" like assemblies). Interestingly, the presence of the transition-metal ions (copper, zinc and iron) triggered the self-assembly of both the peptides into fibrous circular discs, nanomats and nanoplates like assembly.

PAINT-ing Fluorenylmethoxycarbonyl (Fmoc)-Diphenylalanine Hydrogels

Self-assembly of fluorenylmethoxycarbonyl-protected diphenylalanine (FmocFF) in water is widely known to produce hydrogels. Typically, confocal microscopy is used to visualize such hydrogels under wet conditions, that is, without freezing or drying. However, key aspects of hydrogels like fiber diameter, network morphology and mesh size are sub-diffraction limited features and cannot be visualized effectively using this approach. In this work, we show that it is possible to image FmocFF hydrogels by Points Accumulation for Imaging in Nanoscale Topography (PAINT) in native conditions and without direct gel labelling. We demonstrate that the fiber network can be visualized with improved resolution (≈50 nm) both in 2D and 3D. Quantitative information is extracted such as mesh size and fiber diameter. This method can complement the existing characterization tools for hydrogels and provide useful information supporting the design of new materials.

Heterochirality Restricts the Self-Assembly of Phenylalanine Dipeptides Capped with Highly Aromatic Groups

The influence of stereochemistry on the self-assembly of phenylalanine (Phe) dipeptides bearing aromatic fluorenyl groups at both the N- and C-termini (Fmoc, OFm) has been investigated. For this purpose, Fmoc-d-Phe-l-Phe-OFm and Fmoc-l-Phe-l-Phe-OFm have been examined considering a wide variety of solvents, which differ in dielectric constant and volatility. Results reveal that replacement of l-Phe by d-Phe has a major impact on the self-assembly propensities, restricting drastically the structural diversity and polymorphism shown by the homochiral dipeptide. Thus, the analogous heterochiral dipeptide shows a great propensity to form micro/nanofibers, independently of the environmental conditions. Theoretical calculations revealed that the stability of antiparallel disposition is much higher (a factor of ca. 15) for Fmoc-d-Phe-l-Phe-OFm than that for Fmoc-l-Phe-l-Phe-OFm, which has been attributed to the hydrophobic core formed in the former. Overall, results suggest that control of the backbone chirality is a potent and versatile strategy to drive and finely tune the self-assembly propensities of highly aromatic peptides.

Construction of self-assembled nanostructure-based tetraphenylethylene dipeptides: supramolecular nanobelts as biomimetic hydrogels for cell adhesion and proliferation

A novel TPE-YY peptide hydrogelator self-assembled to form twisted nanobelts at neutral pH, upon cultured with 3A6 cells showed selective cell adhesion and growth.