Exploiting enzymatic (reversed) hydrolysis in directed self-assembly of peptide nanostructures

Dynamic supramolecular hydrogels mediated by chemical reactions

Supramolecular self-assembly in a biological system is usually dominated by sophisticated metabolic processes (chemical reactions) such as catalysis of enzymes and consumption of high energy chemicals, leading to groups of biomolecules with unique dynamics and functions in an aqueous environment. In recent years, increasing efforts have been made to couple chemical reactions to molecular self-assembly, with the aim of creating supramolecular materials with lifelike properties and functions. In this feature article, after summarising the work of chemical reaction mediated supramolecular hydrogels, we first focus on a typical example where dynamic self-assembly of molecular hydrogels is activated by in situ formation of a hydrazone bond in water. We discuss how the formation of the hydrazone-based supramolecular hydrogels can be controlled in time and space. After that, we describe transient assembly of supramolecular hydrogels powered by out-of-equilibrium chemical reaction networks regulated by chemical fuels, which show unique properties such as finite lifetime, dynamic structures, and regenerative capabilities. Finally, we provide a perspective on the future investigations that need to be done urgently, which range from fundamental research to real-life applications of dynamic supramolecular hydrogels.

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

Aromatic interactions directing peptide nano-assembly

Self-assembly is a process of spontaneous organization of molecules as a result of non-covalent interactions. Organized self-assembly at the nano level is emerging as a powerful tool in the bottom-up fabrication of functional nanostructures for targeted applications. Aromatic π-π stacking plays a significant role by facilitating the persistent supramolecular association of individual subunits to the self-assembled structures of high stability. Understanding, the supramolecular chemistry of the materials interacting through aromatic interactions, is of tremendous interest in not only constructing functional materials but also in revealing the mechanism of molecular assembly in living organisms. This chapter aims to focus on understanding the potential role of π-π interactions in directing and regulating the self-assembly of peptide nanostructures. The scope of the chapter starts with an outline of the history and mechanism of the aromatic π-π interactions. It progresses through the design strategy for the assembly of peptides containing aromatic rings, the conditions affecting the aromatic stacking interactions, their resulting nanoassemblies, properties, and applications. The properties and applications of the supramolecular materials formed through the aromatic stacking interactions are highlighted to provide an increased understanding of the role of weak interactions in the design and construction of novel functional materials.

Enzyme-Instructed Self-Assembly of Peptides: From Concept to Representative Applications

Enzyme-instructed self-assembly, integrating enzymatic reaction and molecular self-assembly, has drawn noticeable attention over the last decade with the intension of being used in valuable applications. Recent advances in the field allow it possible to spatiotemporally control peptide self-assembly in cellular milieu, broadening the potential applications of peptide assemblies to cancer therapy and subcellular delivery. In this minireview, the concept of enzyme-instructed self-assembly of peptide, containing enzymatic trigger and spatiotemporal control, is described. Representative applications in cells are also discussed, followed by outlook on the field of enzyme-instructed self-assembly.

Highly Tough, Stretchable, and Enzymatically Degradable Hydrogels Modulated by Bioinspired Hydrophobic β-Sheet Peptides

Peptide-based supramolecular hydrogels have attracted great attention due to their good biocompatibility and biodegradability and have become promising candidates for biomedical applications. The bottom-up self-assembly endows the peptides with a highly ordered secondary structure, which has proven to be an effective strategy to improve the mechanical properties of hydrogels through strong physical interactions and energy dissipation. Inspired by the excellent mechanical properties of spider-silk, which can be attributed to the rich β-sheet crystal formation by the hydrophobic peptide fragment, a hydrophobic peptide (HP) that can form a β-sheet assembly was designed and introduced into a poly(vinyl alcohol) (PVA) scaffold to improve mechanical properties of hydrogels by the cooperative intermolecular physical interactions. Compared with hydrogels without peptide grafting (P-HP0), the strong β-sheet self-assembly domain endows the hybrid hydrogels (P-HP20, P-HP29, and P-HP37) with high strength and toughness. The fracture tensile strength increased from 0.3 to 2.1 MPa (7 times), the toughness increased from 0.4 to 21.6 MJ m^-3 (54 times), and the compressive strength increased from 0.33 to 10.43 MPa (31 times) at 75% strain. Moreover, the hybrid hydrogels are enzymatically degradable due to the dominant contribution of the β-sheet assembly for network cross-linking. Combining the good biocompatibility and sustained drug release of the constructed hydrogels, this hydrophobic β-sheet peptide represents a promising candidate for the rational design of hydrogels for biomedical applications.

De novo design of self-assembly hydrogels based on Fmoc-diphenylalanine providing drug release

Short peptides with self-assembled nanostructures are widely applied in the areas of drug delivery systems and biomaterials. In this article, we create a new peptide-based hydrogelator (Fmoc-FFRRVR) based on N-fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) through an approach to improve its hydrophilicity. Compared to Fmoc-FF, Fmoc-FFRRVR prefers to form a hydrogel under mild conditions, and the gelation time is only 2 s. Fmoc-FFRRVR self-assembles into organized arrays of β-sheets in nanofibers via π-stacking of Fmoc-FF, which are supported by circular dichroism and fluorescence emission spectroscopy. Rheology results confirm that the hydrogel of Fmoc-FFRRVR is elastic, reversible and injectable. The newly discovered hydrogel not only retains some excellent performances of Fmoc-FF, but also can be used as a drug carrier for biomedical applications.

Self-Assembling Peptidic Bolaamphiphiles for Biomimetic Applications

Bolaamphiphile, which is a class of amphiphilic molecules, has a unique structure of two hydrophilic head groups at the ends of the hydrophobic center. Peptidic bolaamphiphiles that employ peptides or amino acids as their hydrophilic groups exhibit unique biochemical activities when they self-organize into supramolecular structures, which are not observed in a single molecule. The self-assembled peptidic bolaamphiphiles hold considerable promise for imitating proteins with biochemical activities, such as specific affinity toward heterogeneous substances, a catalytic activity similar to a metalloenzyme, physicochemical activity from harmonized amino acid segments, and the capability to encapsulate genes like a viral vector. These diverse activities give rise to large research interest in biomaterials engineering, along with the synthesis and characterization of the assembled structures. This review aims to address the recent progress in the applications of peptidic bolaamphiphile assemblies whose densely packed peptide motifs on their surface and their stacked hydrophobic centers exhibit unique protein-like activity and designer functionality, respectively.

Enzyme-Induced Supramolecular Order in Pyrene Dipeptide Hydrogels for the Development of an Efficient Energy-Transfer Template

Peptide self-assembly is gathering much attention due to the precise control it provides for the arrangement of functional moieties for the fabrication of advanced functional materials. It is desirable to use a physical, chemical, or biological trigger that can control the self-assembly process. In the current article, we have applied an enzyme to induce the peptide self-assembly of an aromatic peptide amphiphile, which modulates the supramolecular order in the final gel phase material. We accessed diverse peptide hydrogels from identical gelator concentrations by simply changing the enzyme concentration, which controlled the reaction kinetics and influenced the dynamics of self-assembly. Depending upon the concentration of the enzyme, a bell-shaped relationship was observed in terms of intermolecular interactions, morphology, and properties of the final gel phase material. The access of non-equilibrium structures was further demonstrated by fluorescence emission spectroscopy, circular dichroism spectroscopy, atomic force microscopy, transmission electron microscopy, and rheology. This strategy is applied to construct a charge-transfer hydrogel by doping the donor hydrogel with an acceptor moiety, which exhibits efficient energy transfer. Interestingly, such structural control at the nanoscopic level can further tune the energy-transfer efficiency by simply modulating the enzyme concentration.

A transient non-covalent hydrogel by a supramolecular gelator with dynamic covalent bonds

In aqueous solution, equilibrium self-assembly and gelation occur at higher concentration but on addition of EDC non-equilibrium self-assembly and transient hydrogels are formed at low concentration, which dissolve upon anhydride hydrolysis.

Rapid Hemostasis Resulting from the Synergism of Self-Assembling Short Peptide and O-Carboxymethyl Chitosan

The development of novel hemostatic agents with distinct modes of action from traditional ones remains a formidable challenge. Self-assembling peptide hydrogels have emerged as a new hemostatic material, not only because of their inherent biocompatibility and biodegradability but also their designability. Especially, rational molecular design can make peptides and their hydrogelation responsive to biological cues. In this study, we demonstrated that transglutaminase-catalyzed reactions not only occurred among designed short peptide I₃QGK molecules but also between the peptide and a natural polysaccharide O-carboxymethyl chitosan. Because Factor XIII in the blood can rapidly convert into activated transglutaminase (Factor XIIIa) upon bleeding, these enzymatic reactions, together with the electrostatic attraction between the two hemostatic agents, induced a strong synergetic effect in promoting hydrogelation, blood coagulation, and platelet adhesion, eventually leading to rapid hemostasis. The study presents a promising strategy for developing alternative hemostatic materials and methods.

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.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Water-Alcohol Bigels from Fatty Acylated Dipeptides

Peptide-based gels are emerging as an interesting class of biocompatible soft materials. 9-Fluorenylmethoxycarbonyl-protected amino acids and short peptides have gained considerable attention as promising gelators. Peptide amphiphiles, wherein an alkyl chain is appended to a polar peptidic moiety, are another important class of peptide-based gelators. Here, we report the alcohol/water bigels formed by the rather simple fatty acylated dipeptides wherein the peptidic moiety is made up of hydrophobic amino acids, viz., Val, Ile, and Leu. Lauroyl, myristoyl, and palmitoyl were investigated as the N-terminal fatty acyl groups. None of the lauroylated peptides caused gelation of methanol/water and ethanol/water mixtures up to 2 wt % peptide concentration. Eight out of the 27 peptides resulted in distinct bigels. The gels are composed of fibrous aggregates as characterized by electron microscopy. Infrared spectroscopy suggests the β-sheet conformation of the peptidic region in the gels. Using the Ma-IV ethanol/water bigel as the representative gel, entrapment and steady release of the anticancer drug docetaxel are demonstrated. Such bigels from rather simple amphipathic peptides that are easily synthesized and purified through solvent extraction could be attractive gelator candidates with potential application in drug delivery.

Mechanoresponsive, proteolytically stable and biocompatible supergelators from ultra short enantiomeric peptides with sustained drug release propensity

This report describes the discovery of a set of decanoic acid based amphiphilic derivatives that serves as a template for the stabilization of hydrogel nanoparticles for the sustained release of model drugs.

Modeling the assembly of oppositely charged lock- and key-colloids

The interaction of oppositely charged lock- and key-colloids is investigated using computer simulations. We show that indented spheres, i.e., lock-particles, can be specifically assembled with spherical key-particles using solely electrostatic interactions in addition to a hard overlap potential. An analytic description of the entropic and energetic contributions is derived and supported by simulations and explicit energy calculations, respectively. The analytic expression of the electrostatic contribution is further employed to build up a schematic model allowing for efficient large-scale Monte Carlo simulations. The influence of the charge/ionic strength, the degree of indentation, and the size/number ratio is discussed by analyzing the specific and unspecific associations from the simulations. Herein, both particle design and mixing conditions lead to the formation of stable specific clusters analogous to colloidal molecules whose valence is defined by the number of lock-particles associated with a key-particle. In addition, the approach is extended to the encapsulation of an excess of small key-particles in largely indented lock-particles. These two examples exemplify that highly specific pairwise interactions can be implemented by using solely oppositely charged particles with complementary geometries, which opens the road for a rational design of complex hierarchical self-assemblies of complementary building blocks.

Reactions Coupled Self- and Co-Assembly: A Highly Dynamic Process and the Resultant Spatially Inhomogeneous Structure

Reactions coupled self-assembly represents a step forward towards biomimetic behavior in the field of supramolecular research. Here, two pH-dependent reactions of thiol-disulfide exchange and ligand exchange were used to couple with the self-assembly of an Au^I -thiolate coordination polymer consisting of two ligands. Thanks to the comparable rates between the reactions and self-assembly, the compositions of the assemblies change continuously with time, resulting in a highly dynamic assembly process and spatially inhomogeneous structure that are very common in life systems but cannot be easily obtained with one-pot artificial methods.

Synergistic Tricolor Emission-Based White Light from Supramolecular Organic-Inorganic Hybrid Gel

The development of engineered hybrid systems by encapsulating nanoparticles in gel scaffolds and their synergistic effects are highly crucial for the fabrication of advanced functional materials. Herein, a series of dipeptides containing an aromatic amino acid at the N-terminal and an aliphatic amino acid at the C-terminal were synthesized and studied. Among them, only the dipeptide l-Phe-l-Val can form both hydro- and organogelator, depending on the N- and C-terminal protecting groups. The organogel shows bright blue emission under 366 nm UV irradiation; however, the hydrogel does not show such blue emission. Such kind of emission may be due to the self-assembly and high degree of aggregation in the gel state of the phenyl ring. The blue-emitting organogel efficiently encapsulates green emission source CdSe quantum dots and red emission source LD 700 perchlorate dye. The resulting organic-inorganic hybrid gel exhibits white light emission due to the synergistic effect under 366 nm UV irradiation.

Unraveling the Design Rules in Ultrashort Amyloid-Based Peptide Assemblies toward Shape-Controlled Synthesis of Gold Nanoparticles

The fundamental understanding of the detailed relationship between molecular structure and material function remains a challenging task, until now. In order to understand the relative contribution of aromatic moieties and hydrophobicity of amino acid chains, we designed a library of ultrashort amyloid-like peptides based on Ar-Phe-X (where "Ar" represents different aromatic moieties and "X" represents amino acids having varied side-chain functionalities). Our research clearly indicated that the alteration in the size and hydrophobicity of the aromatic capping play a crucial role compared to the subtle change in the amino acid sequence of the dipeptide in dictating the final self-assembled structure and properties of these short peptide amphiphiles. Further, we explored our detailed understanding toward the controlled synthesis of bioinspired organic-inorganic hybrids. For the first time, we established the differential role of aliphatic and aromatic hydroxyl moieties toward the in situ shape-controlled synthesis of gold nanoparticles in three-dimensional nanostructures of hydrogels. To the best of our knowledge, it is the first report which demonstrated the formation of rectangular platonic gold nanoparticles using simple dipeptide hydrogels, exhibiting pH-dependent size control. Our study shows promising implications in bottom-up nanofabrication of next-generation nanomaterials with emergent properties.

Poly(vinyl alcohol)-induced thixotropy of an l-carnosine-based cytocompatible, tripeptidic hydrogel

The generally poor mechanical stability of hydrogels limits their use as functional materials for many biomedical applications. In this work, a poly(vinyl alcohol) (PVA) embedded hybrid hydrogel of a β-amino acid-containing Fmoc-protected tripeptide was produced at physiological pH (7.4) and room temperature. The hydrogel system was characterized by a number of techniques, including UV-vis, fluorescence, circular dichroism, FT-IR spectroscopy, electron microscopy, and rheology. While the tripeptide-based pure hydrogel was found to be unstable after ca. half an hour, addition of PVA, a water soluble polymer, increased the temporal and mechanical stability of the hydrogel. A rheological step-strain experiment demonstrates that the peptide-polymer hydrogel is thixotropic. Results from a fluorescence probe study and transmission electron microscopy reveal that addition of PVA increases both the fibre diameter and entanglement. Circular dichroism spectra of the hydrogels confirm the formation of aggregates with supramolecular chirality. The thixotropic nature of the hydrogel has been exploited to entrap and release doxorubicin, an anticancer drug, under physiological conditions. Furthermore, an MTT assay of the Fmoc-tripeptide using AH927 cells confirmed its cytocompatibility, which broadens the utility of the hybrid gel for biomedical applications.

In search of bioinspired hydrogels from amphiphilic peptides: a template for nanoparticle stabilization for the sustained release of anticancer drugs

This report presents the efficiency of palmitic acid-based proteolytically stable, biocompatible hydrogelators for the sustained release of anticancer drugs.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

Instructed-Assembly (iA): A Molecular Process for Controlling Cell Fate

Instructed-assembly (iAssembly or iA) refers to the formation of ordered superstructures of molecules as the consequence of at least one trigger event (e.g., a reaction or a ligand-receptor interaction). As a biomimetic process that transforms from an equilibrium to another equilibrium, iA has emerging as a powerful approach to provide spatiotemporal control for a range of potential biomedical applications, including molecular imaging, cancer therapy, and tissue engineering. This account introduces the general concept of iA in the context of cells and illustrates how to achieve iA for applications. By mainly describing the representative examples of iA and its applications in complex environment, such as cells or animals, and providing the perspectives of the future development of iA, we intend to show that, as a process that bridges self-assembly and self-organization, iA offers chemists a facile mean to explore the emergent properties of molecular assemblies and the dynamics of molecular processes to control cell fate. Particularly, iA promises many wonderful surprises and useful applications in physical and/or life sciences when multiple processes (e.g., self-assembly, instructed-assembly, and self-organization) are taking place simultaneously.

Secondary structures in synthetic polypeptides from N-carboxyanhydrides: design, modulation, association, and material applications

This article highlights the conformation-specific properties and functions of synthetic polypeptides derived from N-carboxyanhydrides.

An auxin–tyrosine derivative based biocompatible supergelator: a template for fabrication of nanoparticles for sustained release of model drugs

This report reveals the supergelating ability of an auxin-tyrosine derivative based biocompatible hydrogel in drug delivery.

A Substrate-Selective Enzyme-Catalysis Assembly Strategy for Oligopeptide Hydrogel-Assisted Combinatorial Protein Delivery

Oligopeptide hydrogels for localized protein delivery have considerable potential to reduce systemic side effects but maximize therapeutic efficacy. Although enzyme catalysis to induce formation of oligopeptide hydrogels has the merits of unique regio- and enantioselectivity and mild reaction conditions, it may cause the impairment of function and activity of the encapsulated proteins by proteolytic degradation during gelation. Here we report a novel enzyme-catalysis strategy for self-assembly of oligopeptide hydrogels using an engineered protease nanocapsule with tunable substrate selectivity. The protease-encapsulated nanocapsule shielded the degradation activity of protease on the laden proteins due to the steric hindrance by the polymeric shell weaved around the protease, whereas the small-molecular precursors were easier to penetrate across the polymeric network and access the catalytic pocket of the protease to convert to the gelators for self-assembling hydrogel. The resulting oligopeptide hydrogels supported a favorable loading capacity without inactivation of both an antiangiogenic protein, hirudin and an apoptosis-inducing cytokine, TRAIL as model proteins. The hirudin and TRAIL coloaded oligopeptide hydrogel for combination cancer treatment showed enhanced synergistic antitumor effects both in vitro and in vivo.

Enzyme-Instructed Self-Assembly of Peptides Containing Phosphoserine to Form Supramolecular Hydrogels as Potential Soft Biomaterials

Enzyme-instructed self-assembly (EISA) offers a facile approach to explore the supramolecular assemblies of small molecules in cellular milieu for a variety of biomedical applications. One of the commonly used enzymes is phosphatase, but the study of the substrates of phosphatases mainly focuses on the phosphotyrosine containing peptides. In this work, we examine the EISA of phosphoserine containing small peptides for the first time by designing and synthesizing a series of precursors containing only phosphoserine or both phosphoserine and phosphotyrosine. Conjugating a phosphoserine to the C-terminal of a well-established self-assembling peptide backbone, (naphthalene-2-ly)-acetyl-diphenylalanine (NapFF), affords a novel hydrogelation precursor for EISA. The incorporation of phosphotyrosine, another substrate of phosphatase, into the resulting precursor, provides one more enzymatic trigger on a single molecule, and meanwhile increases the precursors' propensity to aggregate after being fully dephosphorylated. Exchanging the positions of phosphorylated serine and tyrosine in the peptide backbone provides insights on how the specific molecular structures influence self-assembling behaviors of small peptides and the subsequent cellular responses. Moreover, the utilization of D-amino acids largely enhances the biostability of the peptides, thus providing a unique soft material for potential biomedical applications.

Fmoc-modified amino acids and short peptides: simple bio-inspired building blocks for the fabrication of functional materials

Amino acids and short peptides modified with the 9-fluorenylmethyloxycarbonyl (Fmoc) group possess eminent self-assembly features and show distinct potential for applications due to the inherent hydrophobicity and aromaticity of the Fmoc moiety which can promote the association of building blocks. Given the extensive study and numerous publications in this field, it is necessary to summarize the recent progress concerning these important bio-inspired building blocks. Therefore, in this review, we explore the self-organization of this class of functional molecules from three aspects, i.e., Fmoc-modified individual amino acids, Fmoc-modified di- and tripeptides, and Fmoc-modified tetra- and pentapeptides. The relevant properties and applications related to cell cultivation, bio-templating, optical, drug delivery, catalytic, therapeutic and antibiotic properties are subsequently summarized. Finally, some existing questions impeding the development of Fmoc-modified simple biomolecules are discussed, and corresponding strategies and outlooks are suggested.

Can self-assembled hydrogels composed of aromatic amino acid derivatives function as drug delivery carriers?

This work reflects the efficient candidature of the hydrogelators and nanoparticles generated therefrom as excellent carriers for drug delivery.

Self-assembly of diphenylalanine with preclick components as capping groups

Alkyne and azide, which are commonly used in the cycloaddition reaction recognized as “click chemistry”, have been used as capping groups of two engineered diphenylalanine (FF) derivatives due to their ability to form weak intermolecular interactions (i.e. dipole–π and π–π stacking).

Hydrogelation of the Short Self-Assembling Peptide I3QGK Regulated by Transglutaminase and Use for Rapid Hemostasis

The self-assembly of short peptides is a promising route to the creation of smart biomaterials. To combine peptide self-assembly with enzymatic catalysis, we design an amphiphilic short peptide I3QGK that can self-assemble into long nanoribbons in aqueous solution. Upon addition of transglutaminase (TGase), the peptide solution undergoes a distinct sol-gel transition to form a rigid hydrogel, which shows strong shear-thinning and immediate recovery properties. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements indicate the occurrence of considerable nanofibers in addition to the original nanoribbons. Liquid chromatography and mass spectrometry analyses reveal the enzymatic formation of peptide dimers from monomers through intermolecular ε-(γ-glutamyl)lysine isopeptide bonding. The dimers rapidly self-assemble into flexible and entangled nanofibers, and the coexistence of the original nanoribbons and the newly created nanofibers is responsible for hydrogelation. Factor XIII in blood is converted by thrombin to an active TGase (Factor XIIIa) during bleeding, so the peptide solution shows a more rapid and effective hemostasis via a combination of gelling blood and promoting platelet adhesion, relative to other hemostasis methods or materials. These features of I3QGK, together with its low cytotoxicity against normal mammalian cells and noninduction of nonspecific immunogenic responses, endow it with great potential for future clinical hemostasis applications.

Design of Catalytic Peptides and Proteins Through Rational and Combinatorial Approaches

This review focuses on recent progress in noncomputational methods to introduce catalytic function into proteins, peptides, and peptide assemblies. We discuss various approaches to creating catalytic activity and classification of noncomputational methods into rational and combinatorial classes. The section on rational design covers recent progress in the development of short peptides and oligomeric peptide assemblies for various natural and unnatural reactions. The section on combinatorial design describes recent advances in the discovery of catalytic peptides. We present the future prospects of these and other new approaches in a broader context, including implications for functional material design.

Hierarchical self-assembly of di-, tri- and tetraphenylalanine peptides capped with two fluorenyl functionalities: from polymorphs to dendrites

Homopeptides with 2, 3 and 4 phenylalanine (Phe) residues and capped with fluorenylmethoxycarbonyl and fluorenylmethyl esters at the N-terminus and C-terminus, respectively, have been synthesized to examine their self-assembly capabilities. Depending on the conditions, the di- and triphenylalanine derivatives self-organize into a wide variety of stable polymorphic structures, which have been characterized: stacked braids, doughnut-like shapes, bundled arrays of nanotubes, corkscrew-like shapes and spherulitic microstructures. These highly aromatic Phe-based peptides also form incipient branched dendritic microstructures, even though they are highly unstable, making their manipulation very difficult. Conversely, the tetraphenylalanine derivative spontaneously self-assembles into stable dendritic microarchitectures made of branches growing from nucleated primary frameworks. The fractal dimension of these microstructures is ∼1.70, which provides evidence for self-similarity and two-dimensional diffusion controlled growth. DFT calculations at the M06L/6-31G(d) level have been carried out on model β-sheets since this is the most elementary building block of Phe-based peptide polymorphs. The results indicate that the antiparallel β-sheet is more stable than the parallel one, with the difference between them growing with the number of Phe residues. Thus, the cooperative effects associated with the antiparallel disposition become more favorable when the number of Phe residues increases from 2 to 4, while those of the parallel disposition remained practically constant.

Peptidomics of Peptic Digest of Selected Potato Tuber Proteins: Post-Translational Modifications and Limited Cleavage Specificity

Bioinformatic tools are useful in predicting bioactive peptides from food proteins. This study was focused on using bioinformatics and peptidomics to evaluate the specificity of peptide release and post-translational modifications (PTMs) in a peptic digest of potato protein isolate. Peptides in the protein hydrolysate were identified by LC-MS/MS and subsequently aligned to their parent potato tuber proteins. Five major proteins were selected for further analysis, namely, lipoxygenase, α-1,4-glucan phosphorylase, annexin, patatin, and polyubiquitin, based on protein coverage, abundance, confidence levels, and function. Comparison of the in silico peptide profile generated with ExPASy PeptideCutter and experimental peptidomics data revealed several differences. The experimental peptic cleavage sites were found to vary in number and specificity from PeptideCutter predictions. Average peptide chain length was also found to be higher than predicted with hexapeptides as the smallest detected peptides. Moreover, PTMs, particularly Met oxidation and Glu/Asp deamidation, were observed in some peptides, and these were unaccounted for during in silico analysis. PTMs can be formed during aging of potato tubers, or as a result of processing conditions during protein isolation and hydrolysis. The findings provide insights on the limitations of current bioinformatics tools for predicting bioactive peptide release from proteins, and on the existence of structural modifications that can alter the peptide bioactivity and functionality.

Enzyme–substrate interactions promote the self-assembly of amino acid derivatives into supramolecular hydrogels

The first example of enzyme-substrate interaction-promoted self-assembly was reported for the synthesis of supramolecular hydrogels from Fmoc-amino acids and amino acid esters in the presence of α-chymotrypsin.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

The Phe-Phe Motif for Peptide Self-Assembly in Nanomedicine

Since its discovery, the Phe-Phe motif has gained in popularity as a minimalist building block to drive the self-assembly of short peptides and their analogues into nanostructures and hydrogels. Molecules based on the Phe-Phe motif have found a range of applications in nanomedicine, from drug delivery and biomaterials to new therapeutic paradigms. Here we discuss the various production methods for this class of compounds, and the characterization, nanomorphologies, and application of their self-assembled nanostructures. We include the most recent findings on their remarkable properties, which hold substantial promise for the creation of the next generation nanomedicines.