Assessing the utility of infrared spectroscopy as a structural diagnostic tool for β-sheets in self-assembling aromatic peptide amphiphiles

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 and Cytocompatibility of Amino Acid Conjugates Containing a Novel Water-Soluble Aromatic Protecting Group

There has been considerable interest in peptides in which the Fmoc (9-fluorenylmethoxycarbonyl) protecting group is retained at the N-terminus, since this bulky aromatic group can drive self-assembly, and Fmoc-peptides are biocompatible and have applications in cell culture biomaterials. Recently, analogues of new amino acids with 2,7-disulfo-9-fluorenylmethoxycarbonyl (Smoc) protecting groups have been developed for water-based peptide synthesis. Here, we report on the self-assembly and biocompatibility of Smoc-Ala, Smoc-Phe and Smoc-Arg as examples of Smoc conjugates to aliphatic, aromatic, and charged amino acids, respectively. Self-assembly occurs at concentrations above the critical aggregation concentration (CAC). Cryo-TEM imaging and SAXS reveal the presence of nanosheet, nanoribbon or nanotube structures, and spectroscopic methods (ThT fluorescence circular dichroism and FTIR) show the presence of β-sheet secondary structure, although Smoc-Ala solutions contain significant unaggregated monomer content. Smoc shows self-fluorescence, which was used to determine CAC values of the Smoc-amino acids from fluorescence assays. Smoc fluorescence was also exploited in confocal microscopy imaging with fibroblast cells, which revealed its uptake into the cytoplasm. The biocompatibility of these Smoc-amino acids was found to be excellent with zero cytotoxicity (in fact increased metabolism) to fibroblasts at low concentration.

Computation meets experiment: identification of highly efficient fibrillating peptides

Self-assembling peptides are of huge interest for biological, medical and nanotechnological applications. The enormous chemical variety that is available from the 20 amino acids offers potentially unlimited peptide sequences, but it is currently an issue to predict their supramolecular behavior in a reliable and cheap way. Herein we report a computational method to screen and forecast the aqueous self-assembly propensity of amyloidogenic pentapeptides. This method was found also as an interesting tool to predict peptide crystallinity, which may be of interest for the development of peptide based drugs.

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

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

1D Supramolecular Assemblies That Crystallize and Form Gels in Response to the Shape-Complementarity of Alcohols

N-9-Fluorenylmethyloxycarbonyl (Fmoc)- and C-tertiary butyl (t-Bu)-protected glutamate (L-2), bearing a phenanthroline moiety at the side residue, forms 1D supramolecular assemblies via H-bonding as well as undergoing π-stacking interactions to afford crystals or gels that depend on the shape-complementarity of coexisting alcohols, as demonstrated by structural analyses on these assemblies by means of single-crystal X-ray diffractometry and supplemented with small- and wide-angle X-ray scattering data. Moreover, the rheological measurements on the gels help to define a model for when gels and crystals are expected and found. These observations and conclusions highlight an important, but not very appreciated, aspect of solute-solvent interactions within supramolecular assemblies that can allow the constituent-aggregating molecules in some systems to exhibit high selectivity toward the structures of their solvents. The consequences of this selectivity, as demonstrated here by single-crystal and powder X-ray diffraction data, can lead to self-assembled structures which alter completely the bulk phase properties and morphology of the materials. In that regard, rheological measurements have helped to develop a model to explain when gels and phase-separated mixtures of crystals and solvents are expected.

Biocompatible Short-Peptides Fibrin Co-assembled Hydrogels

Fibrin hydrogels made by self-assembly of fibrinogen obtained from human plasma have shown excellent biocompatible and biodegradable properties and are widely used in regenerative medicine. The fibrinogen self-assembly process can be triggered under physiological conditions by the action of thrombin, allowing the injection of pregel mixtures that have been used as cell carriers, wound-healing systems, and bio-adhesives. However, access to fibrinogen from human plasma is expensive and fibrin gels have limited mechanical properties, which make them unsuitable for certain applications. One solution to these problems is to obtain composite gels made of fibrin and other polymeric compounds that improve their mechanical properties and usage. Herein, we prepared composite hydrogels made by the self-assembly of fibrinogen together with Fmoc-FF (Fmoc-diphenylalanine) and Fmoc-RGD (Fmoc-arginine-glycine-aspartic acid). We have shown that the mixture of these three peptides co-assembles and gives rise to a unique type of supramolecular fiber, whose morphology and mechanical properties can be modulated. We have carried out a complete characterization of these materials from chemical, physical, and biological points of view. Composite gels have improved mechanical properties compared to pure fibrin gels, as well as showing excellent biocompatibility ex vivo. In vivo experiments have shown that these gels do not cause any type of inflammatory response or tissue damage and are completely resorbed in short time, which would enable their use as vehicles for cell, drug, or growth factor release.

π-System Functionalization Transforms Amyloidogenic Peptide Fragment of Human Islet Amyloid Polypeptide into a Super Hydrogelator

While a considerable number of ultra-short/short amyloid peptides have been reported to form 3D supramolecular hydrogels, they all possess high minimum gelation concentration (MGC) (≥1 wt%), which preclude their applications. In this context, we demonstrate that functionalisation of a well-known amyloidogenic ultra-short peptide fragment NFGAIL (IAPf) of human Islet amyloid polypeptide with a π-system (Fluorenyl, Fm) at the N-terminus of the peptide (Fm-IAPf) yield not only highly thermostable hydrogel at physiological pH but also exhibited super gelator nature as the MGC (0.08 wt%) falls below 0.1 wt%. Various experimental results confirmed that aromatic π-π interactions from fluorenyl moieties and hydrogen bonding interactions between the IAPf drive the self-assembly/fibril formation. Fm-IAPf is the first super hydrogelator derived from amyloid-based ultra-short peptides, to the best of our knowledge. We strongly believe that this report, i. e., functionalization of an amyloid peptide with π-system, provides a lead to develop super hydrogelators from other amyloid-forming peptide fragments for their potential applications.

Forbidden Secondary Structures Found in Gel-Forming Fibrils of Glycylphenylalanylglycine

The zwitterionic l-tripeptide glycylphenylalanylglycine self-assembles into very long crystalline fibrils in an aqueous solution, which causes the formation of an exceptionally strong gel phase (G' ∼ 5 × 10⁶ Pa). The Rietveld refinement analysis of its powder X-ray diffraction (PXRD) pattern reveals a unit cell with four peptides forming a P2₁2₁2₁ space group and adopting an inverse polyproline II conformation, that is, a right-handed helical structure that occupies the "forbidden" region of the Ramachandran plot. This unusual structure is stabilized by a plethora of intermolecular interactions facilitated by the large number of different functional groups of the unblocked tripeptide. Comparisons of simulated and experimental Fourier transform infrared and vibrational circular dichroism (VCD) amide I' profiles corroborate the PXRD structure. Our experimental setup reduces the sample to a quasi-two-dimensional network of fibrils. We exploited the influence of this reduced dimensionality on the amide I VCD to identify the main fibril axis. We demonstrate that PXRD, vibrational spectroscopy, and amide I simulations provide a powerful toolset for secondary structure and fibril axis determination.

Smart Peptide Defense Web In Situ Connects for Continuous Interception of IgE against Allergic Rhinitis

Allergic rhinitis (AR) is a chronic inflammatory reaction by immunoglobulin E (IgE) mediators after individual contact with allergens. It affects 10-40% of the world's population and reduces the quality of life. Long-term symptoms of rhinitis can cause inflammation to spread and trigger asthma, which can harm human health. Herein, we develop a Smart PeptIde defeNse (SPIN) web technique, which in situ constructs a peptide web, trapping IgE against AR. Two candidate SPINs, SPIN-1 and SPIN-2, are designed with different IgE-binding sequences. The SPIN-1 or SPIN-2 is able to bind to IgE and transform from nanoparticles into entangled nanofibers. In turn, the web of SPIN-1 or SPIN-2 acts as a long-term trap of IgE to prevent the IgE from binding to mast cells. SPIN-1 or SPIN-2 (10 mg/kg) is able to treat AR model Balb/c mice with high efficiency and reduced symptoms of rhinitis and inflammatory factors, even better than a first-line clinical drug, cetirizine (10 mg/kg). For example, the amount of IL-4 released in the AR group (185.5 ± 6.8 pg/mL) is significantly reduced after the treatment with SPIN-1 (70.4 ± 14.1 pg/mL), SPIN-2 (86.0 ± 9.3 pg/mL), or cetirizine (112.8 ± 19.3 pg/mL). More importantly, compared with the cetirizine group (1 day), the SPIN-1 or SPIN-2 group shows long-term therapeutic effects (1 week). The SPIN web technique shows the great potential for blocking IgE binding to mast cells in vivo, attenuating AR or other allergic reactions.

Gelation of a π-Decorated Glutamate as a Homochiral Selective Self-assembly to Emerge Macroscopic Chiral Symmetry Breaking

An N -Fmoc and C -tBu-protected glutamate ( 1 ) bearing a phenanthrene moiety at the side residue crystalizes and gels to afford hetero- and homochiral assemblies, respectively, depending on its optical purity or solvent. When a non-stoichiometric mixture of enantiomers of 1 in acetonitrile was treated with the conditions that leave a mixture of gel and supernatant, it exhibited the selfdisproportionation of enantiomers with an enrichment of the major enantiomer in the gel. Under the similar conditions, a racemic mixture of 1 also provided a gel/supernatant mixture, where the gel was enriched in either of L or D-form of 1 stochastically as the result of macroscopic chiral symmetry breaking in its gelation process.

Self-assembled dipeptide based fluorescent nanoparticles as a platform for developing cellular imaging probes and targeted drug delivery chaperones

Self-assembled peptide-based nanostructures, comprised of naturally occurring amino acids, display excellent biocompatibility, biodegradability, flexible responsiveness, and synthetic feasibility and can be customized for various biomedical applications. However, the lack of inherent optical properties of peptide-based nanoparticles is a limitation on their use as imaging probes or drug delivery vehicles. To overcome this impediment, we generated Boc protected tyrosine-tryptophan dipeptide-based nanoparticles (DPNPs) with structure rigidification by Zn(ii), which shifted the peptide's intrinsic fluorescent properties from the ultraviolet to the visible range. These DPNPs are photostable, biocompatible and have visible fluorescence signals that allow for real-time monitoring of their entry into cells. We further show that two DPNPs (PS1-Zn and PS2-Zn) can encapsulate the chemotherapeutic drug doxorubicin (Dox) and facilitate intracellular drug delivery resulting in cancer cell killing actions comparable to the unencapsulated drug. Finally, we chemically modified our DPNPs with an aptamer directed toward the epithelial cell surface marker EPCAM, which improved Dox delivery to the lung cancer epithelial cell line A549. In contrast, the aptamer conjugated DPNPs failed to deliver Dox into the cardiomyocyte cell line AC16. Theoretically, this strategy could be employed in vivo to specifically deliver Dox to cancer cells while sparing the myocardium, a major source of dose-limiting adverse events in the clinic. Our work represents an important proof-of-concept exercise demonstrating that ultra-short peptide-based fluorescent nanostructures have great promise for the development of new imaging probes and targeted drug delivery vehicles.

Injectable Tripeptide/Polymer Nanoparticles Supramolecular Hydrogel: A Candidate for the Treatment of Inflammatory Pathologies

Supramolecular peptide-based hydrogels attract great attention in several fields, i.e., biomedicine, catalysis, energy, and materials chemistry, due to the noncovalent nature of the self-assembly and functional tunable properties defined by the amino acid sequence. In this work, we developed an injectable hybrid supramolecular hydrogel whose formation was triggered by electrostatic interactions between a phosphorylated tripeptide, Fmoc-FFpY (F: phenylalanine, pY: phosphorylated tyrosine), and cationic polymer nanoparticles made of vinylimidazole and ketoprofen (poly(HKT-co-VI) NPs). Hydrogel formation was assessed through inverted tube tests, and its fibrillary structure, around polymer NPs, was observed by transmission electron microscopy. Interestingly, peptide self-assembly yields the formation of nontwisted and twisted fibers, which could be attributed to β-sheets and α-helix structures, respectively, as characterized by circular dichroism and infrared spectroscopies. An increase of the elastic modulus of the Fmoc-FFpY/polymer NPs hybrid hydrogels was observed with peptide concentration as well as its injectability property, due to its shear thinning behavior and self-healing ability. After checking their stability under physiological conditions, the cytotoxicity properties of these hybrid hydrogels were evaluated in contact with human dermal fibroblasts (FBH) and murine macrophages (RAW 264.7). Finally, the Fmoc-FFpY/polymer NPs hybrid hydrogels exhibited a great nitric oxide reduction (∼67%) up to basal values of pro-inflammatory RAW 264.7 cells, thus confirming their excellent anti-inflammatory properties for the treatment of localized inflammatory pathologies.

Self-assembly driven chiral transfer from a dipeptide to the twist and stacking handedness of cyanobiphenylyl groups

The chiral transfer phenomenon was studied on four Ala–Ala lipodipeptides with a cyanobiphenylyl group at the terminal alkyl chain.

Chiral Nematic Cellulose Nanocrystal Films Cooperated with Amino Acids for Tunable Optical Properties

The exploration of functional materials relies greatly on the understanding of material structures and nanotechnologies. In the present work, chiral nematic cellulose nanocrystal (CNC) films were prepared by incorporation with four types of amino acids (AAs, glycine, histidine, phenylalanine, and serine) via evaporation-induced self-assembly. The films present ideal iridescence and birefringence that can be tuned by the amount of AAs added. The intercalation of AAs enlarged the pitch values, contributing to the red-shift trend of the reflective wavelength. Among the AAs, serine presented the most compatible intercalation into cellulose crystals. Interestingly, histidine and phenylalanine composite films showed high shielding capabilities of UV light in diverse wavelength regions, exhibiting multi-optical functions. The sustainable preparation of chiral nematic CNC films may provide new strategies for materials production from biocompatible lignocellulose.

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.

Enhancement of the mechanical properties of lysine-containing peptide-based supramolecular hydrogels by chemical cross-linking

Exposure of lysine-containing peptide-based gelators to the cross-linking agent glutaraldehyde allows tuning of gel mechanical properties. The effect of cross-linking depends on the position of the lysine residue in the peptide chain, the concentration of gelator and the conditions under which cross-linking takes place. Through control of these factors, cross-linking leads to increased gel strength.

Stimuli-Responsive Nucleotide-Amino Acid Hybrid Supramolecular Hydrogels

The ability to assemble chemically different gelator molecules into complex supramolecular hydrogels provides excellent opportunities to construct functional soft materials. Herein, we demonstrate the formation of hybrid nucleotide–amino acid supramolecular hydrogels. These are generated by the silver ion (Ag⁺)-triggered formation of silver–guanosine monophosphate (GMP) dimers, which undergo self-assembly through non-covalent interactions to produce nanofilaments. This process results in a concomitant pH reduction due to the abstraction of a proton from the guanine residue, which triggers the in situ gelation of a pH-sensitive amino acid, N-fluorenylmethyloxycarbonyl tyrosine (FY), to form nucleotide–amino acid hybrid hydrogels. Alterations in the supramolecular structures due to changes in the assembly process are observed, with the molar ratio of Ag:GMP:FY affecting the assembly kinetics, and the resulting supramolecular organisation and mechanical properties of the hydrogels. Higher Ag:GMP stoichiometries result in almost instantaneous gelation with non-orthogonal assembly of the gelators, while at lower molar ratios, orthogonal assembly is observed. Significantly, by increasing the pH as an external stimulus, nanofilaments comprising FY can be selectively disassembled from the hybrid hydrogels. Our results demonstrate a simple approach for the construction of multicomponent stimuli-responsive supramolecular hydrogels with adaptable network and mechanical properties.

Self-Assembly of Peptide Hierarchical Helical Arrays with Sequence-Encoded Circularly Polarized Luminescence

Self-assembled peptide materials with sequence-encoded properties have attracted great interest. Despite their intrinsic chirality, the generation of circularly polarized luminescence (CPL) based on the self-assembly of simple peptides has been rarely reported. Here, we report the self-assembly of peptides into hierarchical helical arrays (HHAs) with controlled supramolecular handedness. The HHAs can emit full-color CPL signals after the incorporation of various achiral fluorescent molecules, and the g_lum value is 40 times higher than that of the CPL signal from the solutions. By simply changing the amino acid sequence of the peptides, CPL signals with opposite handedness can be generated within the HHAs. The peptide HHAs can provide hydrophobic pockets to accommodate the fluorescent molecules with helical arrangement through strong aromatic stacking interactions, which are responsible for the CPL signals. This work provides a pathway to construct highly ordered chiral materials, which have broad applications in the chiroptical field.

Rapid discovery of self-assembling peptides with one-bead one-compound peptide library

Self-assembling peptides have shown tremendous potential in the fields of material sciences, nanoscience, and medicine. Because of the vast combinatorial space of even short peptides, identification of self-assembling sequences remains a challenge. Herein, we develop an experimental method to rapidly screen a huge array of peptide sequences for self-assembling property, using the one-bead one-compound (OBOC) combinatorial library method. In this approach, peptides on beads are N-terminally capped with nitro-1,2,3-benzoxadiazole, a hydrophobicity-sensitive fluorescence molecule. Beads displaying self-assembling peptides would fluoresce under aqueous environment. Using this approach, we identify eight pentapeptides, all of which are able to self-assemble into nanoparticles or nanofibers. Some of them are able to interact with and are taken up efficiently by HeLa cells. Intracellular distribution varied among these non-toxic peptidic nanoparticles. This simple screening strategy has enabled rapid identification of self-assembling peptides suitable for the development of nanostructures for various biomedical and material applications.

Self-assembling peptides have a range of potential applications but developing self-assembling sequences can be challenging. Here, the authors report on a one-bead one-compound combinatorial library where fluorescence is used to detect the potential for self-assembly and identified candidates are evaluated.

Nanoengineered Peptide-Based Antimicrobial Conductive Supramolecular Biomaterial for Cardiac Tissue Engineering

Owing to their dynamic nature and ordered architecture, supramolecular materials strikingly resemble organic components of living systems. Although short-peptide self-assembled nanostructured hydrogels are regarded as intriguing supramolecular materials for biotechnology, their application is often limited due to their low stability and considerable challenge of combining other desirable properties. Herein, a di-Fmoc-based hydrogelator containing the cell-adhesive Arg-Gly-Asp (RGD) fragment that forms a mechanically stable, self-healing hydrogel is designed. Molecular dynamics simulation reveals the presence of RGD segments on the surface of the hydrogel fibers, highlighting their cell adherence capacity. Aiming to impart conductivity, the 3D network of the hydrogel is further nanoengineered by incorporating polyaniline (PAni). The composite hydrogels demonstrate semiconductivity, excellent antibacterial activity, and DNA binding capacity. Cardiac cells grown on the surface of the composite hydrogels form functional synchronized monolayers. Taken together, the combination of these attributes in a single hydrogel suggests it as an exceptional candidate for functional supramolecular biomaterial designed for electrogenic tissue engineering.

Continuous flow fabrication of Fmoc-cysteine based nanobowl infused core-shell like microstructures for pH switchable on-demand anti-cancer drug delivery

Asymmetric nanostructures such as nanobowls (NBs) can exhibit superior drug delivery performances owing to their concave structure and interior asymmetric cavities. Here, we present a facile one-step method for the fabrication of NB like structures from a mere single amino acid mimetic, N-(9-fluorenylmethoxycarbonyl)-S-triphenylmethyl-l-cysteine following continuous-flow microfluidics enabled supramolecular self-assembly. Following fabrication, NBs were further infused into a vesicular shell consisting of the amino acid N-(tert-butoxycarbonyl)-S-triphenylmethyl-l-cysteine, carrying dual acid labile groups, the triphenylmethyl and the tert-butyloxycarbonyl groups. The NB infused core-shell like microstructures formed after the shell coating will now be addressed as NB-shells. Presence of pH-responsive shells bestowed the core-shell NB like structures with the ability to actively tune their surface pore opening and closing in response to environmental pH switch. To illustrate the potential use of the NB-shells in the field of anticancer drug delivery, the particles were loaded with doxorubicin (Dox) with an encapsulation efficiency of 42% and Dox loaded NB-shells exhibited enhanced efficacy in C6 glioma cells. Additionally, when tested in an animal model of glioblastoma, the nanoformulations demonstrated significantly higher retardation of tumour growth as compared to free Dox. Thus, this work strives to provide a new research area in the development of well turned-out and neatly fabricated pH switchable on/off anti-cancer drug delivery systems with significant translational potential.

Enzyme mimics based membrane reactor for di(2-ethylhexyl) phthalate degradation

Di(2-ethylhexyl) phthalate (DEHP), the most abundantly used plasticizer, was considered to be a hazardous chemical that was difficult to be degraded naturally. In this study, inspired by the "catalytic triad'' in serine proteases, an enzyme mimic material was developed by combining the proteases's active sites of serine, histidine and aspartate (S-H-D) with the self-assembling sequence of LKLKLKL and the aromatic group of fluorenylmethyloxycarbonyl (Fmoc). By mixing the monomer of peptides containing separate S, H and D residues with a ratio of 2:1:1, the enzyme mimics were found to co- assemble into nanofibers (Co-HSD) and showed the highest activity towards DEHP degradation because of the synergistic effects of active sites, orderly secondary structure and stable molecular conformation. To further improve ability and applicability, the high active mimetic enzyme was immobilized onto regenerated cellulose (RC) membranes for DEHP degradation in a continuous recycling mode. The RC membranes were first functionalized by the NaIO₄ oxidation method to form aldehyde groups and then conjugated with the enzyme mimics via Schiff-base reaction. As a biocatalytic membrane, this membrane could not only effectively degrade DEHP, but also showed good stability, thus establishing a promising biomaterial for large scale biodegradation of DEHP in water decontamination and liquid food depollution.

Supramolecular tripeptide self-assembly initiated at the surface of coacervates by polyelectrolyte exchange

Spatial control of supramolecular self-assembly can yield compartmentalized structures, a key feature for the design of artificial cells. Inducing self-assembly from and on compartments is still a challenge. Polyelectrolyte complex coacervates are simple model droplet systems able to reproduce the basic features of membrane-less organelles, appearing in cells. Here, we demonstrate the supramolecular self-assembly of a phosphorylated tripeptide, Fmoc-FFpY (Fmoc: fluorenyl-methoxycarbonyl; F: phenyl alanine, pY: phosphorylated tyrosine), on the surface of poly(l-glutamic acid)/poly(allylamine hydrochloride) (PGA/PAH) complex coacervate microdroplets. The phosphorylated peptides self-assemble, without dephosphorylation, through ion pairing between the phosphate groups of Fmoc-FFpY and the amine groups of PAH. This process provides spontaneous capsules formed by an amorphous polyelectrolyte complex core surrounded by a structured peptide/PAH shell. Similar fibrillar Fmoc-FFpY self-assembled structures are obtained at the interface between the peptide solution and a PGA/PAH polyelectrolyte multilayer, a complex coacervate in the thin film or "multilayer" format. In contact with the peptide solution, PAH chains diffuse out of the coacervate or multilayer film and complex with Fmoc-FFpY at the solution interface, exchanging any PGA with which they were associated. Self-assembly of Fmoc-FFpY, now concentrated by complexation with PAH, follows quickly.

Insights into the co-assemblies formed by different aromatic short-peptide amphiphiles

The mechanism of co-assembly of different aromatic dipeptides has been studied using a combination of microscopy and spectroscopy techniques. At an equimolar ratio, the kinetics of the process is favored giving rise to alternate copolymers.

Acid-responsive fibrillation and urease-assisted defibrillation of phenylalanine: a transient supramolecular hydrogel

The aggregation of proteins and peptides into fibrils is associated with many neurodegenerative diseases in humans, including Alzheimer's disease, Parkinson's disease and non-neurological type-II diabetes. A better understanding of the fibril formation process and defibrillation using biochemical tools is highly important for therapeutics. Under physiological conditions, acidic pH promotes the formation of toxic fibrils. Here, a mimic of living systems has been achieved by the acid-responsive assembly of benzyloxycarbonyl-l-phenylalanine to fibrils, as well as the urease-assisted disassembly of the said fibrils. The simultaneous incorporation of the two triggers helped to prepare a transient supramolecular hydrogel from benzyloxycarbonyl-l-phenylalanine-entangled fibrils with a high degree of control over the self-assembly lifetime and mechanical properties. Further, under acidic pH, the compound formed the O-HO[double bond, length as m-dash]C hydrogen-bonded dimer. The dimers were further self-assembled by intermolecular N-HO[double bond, length as m-dash]C hydrogen bonds and π-π stacking interactions to form fibrils with high mechanical properties, from this simple molecule. However, the self-assembly process is dynamic. Hence, the in situ-generated NH3 uniformly increased the pH and led to the homogeneous disassembly of the fibrils. Thus, this report provides a valuable approach to defibrillation.

Aggregation propensity of therapeutic fibrin-homing pentapeptides: insights from experiments and molecular dynamics simulations

CREKA (Cys-Arg-Glu-Lys-Ala) and its engineered analogue CRMeEKA, in which Glu has been replaced by N-methyl-Glu to provide resistance against proteolysis, are emerging pentapeptides that were specifically designed to bind fibrin-fibronectin complexes accumulated in the walls of tumour vessels. However, many of the intrinsic properties of CREKA and CRMeEKA, which are probably responsible for their different behaviour when combined with other materials (such as polymers) for diagnosis and therapeutics, remain unknown yet. The intrinsic tendency of these pentapeptides to form aggregates has been analysed by combining experimental techniques and atomistic Molecular Dynamics (MD) simulations. Dynamic light scattering assays show the formation of nanoaggregates that increase in size with the peptide concentration, even though aggregation occurs sooner for CRMeEKA, independently of the peptide concentration. FTIR and circular dichroism spectroscopy studies suggest that aggregated pentapeptides do not adopt any secondary structure. Atomistic MD trajectories show that CREKA aggregates faster and forms bigger molecular clusters than CRMeEKA. This behaviour has been explained by stability of the conformations adopted by un-associated peptide strands. While CREKA molecules organize by forming intramolecular backbone - side chain hydrogen bonds, CRMeEKA peptides display main chain - main chain hydrogen bonds closing very stable γ- or β-turns. Besides, energetic analyses reveal that CRMeEKA strands are better solvated in water than CREKA ones, independent of whether they are assembled or un-associated.

Tryptophan-Based Self-Assembling Peptides with Bacterial Flocculation and Antimicrobial Properties

Tryptophan as an aromatic amino acid with a hydrophobic indole group plays important roles in stabilizing protein structures and enhancing molecular bindings in nature, but was rarely used in the molecular design of self-assembling peptides or gelators. Therefore, we prepared a series of short peptides from Trp amino acids and examined the potential roles of Trp residues for regulating peptide self-assembly and gelation. The introduced Trp amino acids not only diversify the molecular structures of peptide gelators, but also promote aromatic and hydrogen-bonding interactions for supramolecular self-assembling and gelation, which generates self-assembled nanostructures with twisted helical morphologies and supramolecular hydrogels with low minimal gelation concentrations. More importantly, the self-assembling peptides with Trp residues displayed strong preference for interacting with the lipidic membranes of bacteria, which resulted in bacterial flocculation and the death of E. coli and S. aureus.

Self-Assembly of Peptide Chiral Nanostructures with Sequence-Encoded Enantioseparation Capability

Biopolymers such as polysaccharides and proteins have been widely used for the chiral separation of various components due to the intrinsic chirality of the polymers. Amyloid-like short peptides can also self-assemble into diverse chiral supramolecular nanostructures or polymers with precisely tailored architectures driving by noncovalent interactions. However, the use of such supramolecular nanostructures for the resolution and separation of chiral components remains largely unexplored. Here, we report that the self-assembled peptide supramolecular nanostructures can be used for the highly efficient chiral separation of various enantiomers. By rationally designing the constituent amino acid sequence of the peptides and the self-assembling environment, we can fabricate supramolecular polymers with distinct surface charges and architectures, including nanohelices, nanoribbons, nanosheets, nanofibrils, and nanospheres. The various supramolecular nanostructures were then used to resolve the racemic mixtures of α-methylbenzylamine, 2-phenylpropionic acid, and 1-phenylethanol. The results indicated that the self-assembled peptide polymers showed excellent enantioselective separation efficiency for different chiral molecules. The enantioselective separation efficiency of the peptide nanostructures can be tailored by changing their surface charges, morphology, and the constituent amino acid sequences of the peptides.

Controlled Supramolecular Assembly Inside Living Cells by Sequential Multistaged Chemical Reactions

Synthetic assembly within living cells represents an innovative way to explore purely chemical tools that can direct and control cellular behavior. We use a simple and modular platform that is broadly accessible and yet incorporates highly intricate molecular recognition, immolative, and rearrangement chemistry. Short bimodular peptide sequences undergo a programmed sequence of events that can be tailored within the living intracellular environment. Each sequential stage of the pathways beginning with the cellular uptake, intracellular transport, and localization imposes distinct structural changes that result in the assembly of fibrillar architectures inside cells. The observation of apoptosis, which is characterized by the binding of Annexin V, demonstrates that programmed cell death can be promoted by the peptide assembly. Higher complexity of the assemblies was also achieved by coassembly of two different sequences, resulting in intrinsically fluorescent architectures. As such, we demonstrate that the in situ construction of architectures within cells will broaden the community's perspective toward how structure formation can impact a living system.

Formation of peptide-based oligomers in dimethylsulfoxide: identifying the precursor of fibril formation

The well-studied dipeptide fluorenylmethyloxycarbonyl-di-phenylalanine (FmocFF) forms a rigid hydrogel upon dissolving in dimethylsulfoxide (DMSO) and dilution in H2O. Here, we explored the pre-aggregation of the peptide in pure DMSO by vibrational spectroscopies, X-ray powder diffraction and dynamic light scattering. Our results show an equilibrium between a dominant population of amorphous oligomers (on a length scale of 2 nm) and a small number of protofibrils/fibrils (on a length scale of 30 nm in the centimolar and of 200 nm in the sub-molar region). To probe the mechanism underlying the formation of these protofilaments, we measured the 1H-NMR, IR and visible Raman spectra of DMSO containing different FmocFF concentrations, ranging between 10 and 300 mM. Our data reveal that interpeptide hydrogen bonding leads to the self-assembly of FmocFF in the centimolar region, while π-π stacking between Fmoc-groups is observed above 100 mM. The high 3J(HNHCα) coupling constant of the N-terminal amide proton indicates that the Fmoc end-cap of the peptide locks the N-terminal residue into a conformational ensemble centered at a φ-value of ca. -120°, which corresponds to a parallel β-sheet type conformation. The 3J(HNHCα) coupling constant of the C-terminal residue is indicative of a polyproline II (pPII)/βt mixture. Our results suggest that the gelation of FmocFF caused by the addition of a small amount of water to DMSO mixtures is facilitated by the formation of disordered protofibrils in pure DMSO.

Collagen-Inspired Helical Peptide Coassembly Forms a Rigid Hydrogel with Twisted Polyproline II Architecture

Collagen, the most abundant protein in mammals, possesses notable cohesion and elasticity properties and efficiently induces tissue regeneration. The Gly-Pro-Hyp canonical tripeptide repeating unit of the collagen superhelix has been well-characterized. However, to date, the shortest tripeptide repeat demonstrated to attain a helical conformation contained 3-10 peptide repeats. Here, taking a minimalistic approach, we studied a single repeating unit of collagen in its protected form, Fmoc-Gly-Pro-Hyp. The peptide formed single crystals displaying left-handed polyproline II superhelical packing, as in the native collagen single strand. The crystalline assemblies also display head-to-tail H-bond interactions and an "aromatic zipper" arrangement at the molecular interface. The coassembly of this tripeptide, with Fmoc-Phe-Phe, a well-studied dipeptide hydrogelator, produced twisted helical fibrils with a polyproline II conformation and improved hydrogel mechanical rigidity. The design of these peptides illustrates the possibility to assemble superhelical nanostructures from minimal collagen-inspired peptides with their potential use as functional motifs to introduce a polyproline II conformation into hybrid hydrogel assemblies.

Fmoc-diphenylalanine-based hydrogels as a potential carrier for drug delivery

Self-assembled hydrogels from 9-fluorenylmethoxycarbonyl-modified diphenylalanine (Fmoc-FF) peptides were evaluated as potential vehicles for drug delivery. During self-assembly of Fmoc-FF, high concentrations of indomethacin (IDM) drugs were shown to be incorporated into the hydrogels. The β-sheet arrangement of peptides was found to be predominant in Fmoc-FF–IDM hydrogels regardless of the IDM content. The release mechanism for IDM displayed a biphasic profile comprising an initial hydrogel erosion-dominated stage followed by the diffusion-controlled stage. Small amounts of polyamidoamine dendrimer (PAMAM) added to the hydrogel (Fmoc-FF 0.5%–IDM 0.5%–PAMAM 0.03%) resulted in a more prolonged IDM release compared with Fmoc-FF 0.5%–IDM 0.5% hydrogel. Furthermore, these IDM-loaded hydrogels demonstrated excellent thixotropic response and injectability, which make them suitable candidates for use as injectable self-healing matrices for drug delivery.

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

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

Controlling Neuronal Cell Growth through Composite Laminin Supramolecular Hydrogels

Designing an extracellular matrix mimic by biofunctionalization of polymeric scaffolds is a popular strategy and extremely crucial for facilitating the interactions between cells and the matrix. To this direction, supramolecular gels are gaining exponential attention over the last few years, owing to their potential biocompatibility and biodegradability. In spite of diverse biological roles of native laminin, the bioactivities of self-assembling laminin-derived short peptides were less explored. In this work, we have explored the minimalist design to develop hydrogel scaffolds based on IKVAV and YIGSR peptides individually and their composite matrix, which can provide structurally and functionally relevant materials for tissue engineering. Till date, composite supramolecular gels solely made up of self-assembling IKVAV and YIGSR peptides have never been reported. Such composite gels can be a closer mimic of natural laminin protein, which could mimic the essential functions of the short peptide fragments present on different chains of the extracellular matrix protein, laminin. Interestingly, we used a unique strategy of simple mixing of the two laminin mimetic peptides, which tend to induce coassembly with a self-sorted nanofibrous network with relatively enhanced mechanical strength. The physicochemical properties of the biofunctional hydrogels were studied using different microscopic, spectroscopic, and rheology techniques. To assess the bioactivity of laminin-derived scaffolds in controlling neuronal cell growth, its biocompatibility, cellular growth, and proliferation were quantified using C6 glial cells and SHSY5Y neuroblastoma cells. The live/dead staining further confirmed the adhesion and proliferation of the cells. A significant increase in neurite length provides clear evidence on mimicking the neurite extension function of native laminin protein by its short derivatives. Interestingly, similar β-III tubulin expression and cell cycle phases were observed, in comparison to control, which indicated normal cellular functioning of the cells cultured over short laminin hydrogel scaffolds. All bioassays suggested that Fmoc YIGSR promotes growth of neural cells to a greater extent and maintains healthier morphology, in comparison to hydrophobic Fmoc IKVAV, owing to the entangled longer fibrous network formed by YIGSR peptide. It is expected that thinner long fibers provide a more uniform surface and are more supportive for cell adhesion in comparison to hydrophobic, shorter fibers IKVAV peptide. However, in composite gels, the detrimental effect of hydrophobic IKVAV peptide could be reduced and better adhesion and proliferation could be achieved along with enhanced cell survival. These observations demonstrate the high potential of the laminin-derived hydrogels in tissue engineering and neuronal stem cell differentiation in future.

Programmable enzymatic oxidation of tyrosine-lysine tetrapeptides

The ability to control the response of self-assembled systems upon exposure to external stimuli has been a long-standing goal of supramolecular chemistry. Short peptides are an attractive platform to realise this objective due to their chemical diversity and modular nature. Here, we synthesise a library of Fmoc-capped tetrapeptides, each containing two tyrosine and two lysine residues and varying in their amino acid sequence. Despite having similar secondary structure, these tetrapeptides form structures which are highly sequence dependent, yielding aggregates, nanofibres or monomers. This in turn highly affects the rate and degree of oxidative polymerisation by the enzyme tyrosinase, with self-assembled nanofibres exhibiting a greater degree of polymerisation. We monitor the formation of tyrosine oxidation products over time, finding that the precipitation of polymers is driven by quinone-based species. This affects the electrochemical properties of the oxidised peptide polymers, as determined through electrical impedance spectroscopy. Finally, intrinsic fluorescence microscale thermophoresis studies confirm that the degree of oxidative polymerisation is highly dependent on tyrosine solvent accessibility and the presence of peptide monomers. The ability to tune the kinetics of enzymatically active substrates and understand their polymerisation pathways on a molecular level is important for the creation of programmable, enzyme responsive biomaterials.

Unusual Two-Step Assembly of a Minimalistic Dipeptide-Based Functional Hypergelator

Self-assembled peptide hydrogels represent the realization of peptide nanotechnology into biomedical products. There is a continuous quest to identify the simplest building blocks and optimize their critical gelation concentration (CGC). Herein, a minimalistic, de novo dipeptide, Fmoc-Lys(Fmoc)-Asp, as an hydrogelator with the lowest CGC ever reported, almost fourfold lower as compared to that of a large hexadecapeptide previously described, is reported. The dipeptide self-assembles through an unusual and unprecedented two-step process as elucidated by solid-state NMR and molecular dynamics simulation. The hydrogel is cytocompatible and supports 2D/3D cell growth. Conductive composite gels composed of Fmoc-Lys(Fmoc)-Asp and a conductive polymer exhibit excellent DNA binding. Fmoc-Lys(Fmoc)-Asp exhibits the lowest CGC and highest mechanical properties when compared to a library of dipeptide analogues, thus validating the uniqueness of the molecular design which confers useful properties for various potential applications.

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.

Phase Separation in Supramolecular Hydrogels Based on Peptide Self-Assembly from Enzyme-Coated Nanoparticles

Spatial localization of biocatalysts, such as enzymes, has recently proven to be an effective process to direct supramolecular self-assemblies in a spatiotemporal way. In this work, silica nanoparticles (NPs) functionalized covalently by alkaline phosphatase (NPs@AP) induce the localized growth of self-assembled peptide nanofibers from NPs by dephosphorylation of Fmoc-FFpY peptides (Fmoc: fluorenylmethyloxycarbonyl; F: phenylalanine; Y: tyrosine; p: phosphate group). The fibrillary nanoarchitecture around NPs@AP underpins a homogeneous hydrogel, which unexpectedly undergoes a macroscopic shape change over time. This macroscopic change is due to a phase separation leading to a dense phase (in NPs and nanofibers) in the center of the vial and surrounded by a dilute one, which still contains NPs and peptide self-assemblies. We thus hypothesize that the phase separation is not a syneresis process. Such a change is only observed when the enzymes are localized on the NPs. The dense phase contracts with time until reaching a constant volume after several days. For a given phosphorylated peptide concentration, the dense phase contracts faster when the NPs@AP concentration is increased. For a given NPs@AP concentration, it condenses faster when the peptide concentration increases. We hypothesize that the appearance of a dense phase is not only due to attractive interactions between NPs@AP but also to the strong interactions of self-assembled peptide nanofibers with the enzymes, covalently fixed on the NPs.

Controlling the Assembly and Properties of Low-Molecular-Weight Hydrogelators

Low-molecular-weight gels are formed by the self-assembly of small molecules into fibrous networks that can immobilize a significant amount of solvent. Here, we focus on our work with a specific class of gelator, the functionalized dipeptide. We discuss the current state of the art in the area, focusing on how these materials can be controlled. We also highlight interesting and unusual observations and unanswered questions in the field.

Photo- and Aromatic Stacking-Induced Green Emissive Peptidyl Nanoparticles for Cell Imaging and Monitoring of Nucleic Acid Delivery

Owing to their potential applications in biomedicine and biotechnology, peptide nanostructures that exhibit stable intrinsic fluorescence in the visible range are highly desired. This research proposes a facile strategy to construct peptidyl virus-like nanoparticles (NVPs) that show green luminescence by coassembly of two bioactive ferrocene-diphenylalanine-based (Fc-FF) peptides. The green fluorescence of NVPs was originated from the highly ordered structures assembled by the amphiphilic Fc-FF-based peptides via strong π-π stacking interactions. In the assemblies, Fc-FF chromophore can be hydrolyzed under the natural light irradiation, which eliminates the fluorophore quenching effect of Fc and increases the aromatic stacking interactions, thereby giving rise to strong fluorescent nanoparticles. The NVPs could cross cytomembrane barriers by virtue of the HIV V3 peptide and the nuclear localization signal, and could thus be used for long-term cell imaging with excellent photostability and biocompatibility in physiological condition. In addition, NVPs could package DNA and be used to monitor the delivery of DNA, indicating great potential in the tracking and monitoring of genetic biological processes.