pH-Triggered self-assembly and hydrogelation of cyclic peptide nanotubes confined in water micro-droplets

Supramolecular fibrillation in coacervates and other confined systems towards biomimetic function

As in natural cytoskeletons, the cooperative assembly of fibrillar networks can be hosted inside compartments to engineer biomimetic functions, such as mechanical actuation, transport, and reaction templating. Coacervates impose an optimal liquid-liquid phase separation within the aqueous continuum, functioning as membrane-less compartments that can organise such self-assembling processes as well as the exchange of information with their environment. Furthermore, biological fibrillation can often be controlled or assisted by intracellular compartments. Thus, the reconstitution of analogues of natural filaments in simplified artificial compartments, such as coacervates, offer a suitable model to unravel, mimic, and potentially exploit cellular functions. This perspective summarises the latest developments towards assembling fibrillar networks under confinement inside coacervates and related compartments, including a selection of examples ranging from biological to fully synthetic monomers. Comparative analysis between coacervates, lipid vesicles, and droplet emulsions showcases the interplay between supramolecular fibres and the boundaries of the corresponding compartment. Combining inspiration from natural systems and the custom properties of tailored synthetic fibrillators, rational monomer and compartment design will contribute towards engineering increasingly complex and more realistic artificial protocells.

Designer peptide-DNA cytoskeletons regulate the function of synthetic cells

The bottom-up engineering of artificial cells requires a reconfigurable cytoskeleton that can organize at distinct locations and dynamically modulate its structural and mechanical properties. Here, inspired by the vast array of actin-binding proteins and their ability to reversibly crosslink or bundle filaments, we have designed a library of peptide-DNA crosslinkers varying in length, valency and geometry. Peptide filaments conjoint through DNA hybridization give rise to tactoid-shaped bundles with tunable aspect ratios and mechanics. When confined in cell-sized water-in-oil droplets, the DNA crosslinker design guides the localization of cytoskeletal structures at the cortex or within the lumen of the synthetic cells. The tunable spatial arrangement regulates the passive diffusion of payloads within the droplets and complementary DNA handles allow for the reversible recruitment and release of payloads on and off the cytoskeleton. Heat-induced reconfiguration of peptide-DNA architectures triggers shape deformations of droplets, regulated by DNA melting temperatures. Altogether, the modular design of peptide-DNA architectures is a powerful strategy towards the bottom-up assembly of synthetic cells.

Responsive Supramolecular Polymers for Diagnosis and Treatment

Stimuli-responsive supramolecular polymers are ordered nanosized materials that are held together by non-covalent interactions (hydrogen-bonding, metal-ligand coordination, π-stacking and, host-guest interactions) and can reversibly undergo self-assembly. Their non-covalent nature endows supramolecular polymers with the ability to respond to external stimuli (temperature, light, ultrasound, electric/magnetic field) or environmental changes (temperature, pH, redox potential, enzyme activity), making them attractive candidates for a variety of biomedical applications. To date, supramolecular research has largely evolved in the development of smart water-soluble self-assemblies with the aim of mimicking the biological function of natural supramolecular systems. Indeed, there is a wide variety of synthetic biomaterials formulated with responsiveness to control and trigger, or not to trigger, aqueous self-assembly. The design of responsive supramolecular polymers ranges from the use of hydrophobic cores (i.e., benzene-1,3,5-tricarboxamide) to the introduction of macrocyclic hosts (i.e., cyclodextrins). In this review, we summarize the most relevant advances achieved in the design of stimuli-responsive supramolecular systems used to control transport and release of both diagnosis agents and therapeutic drugs in order to prevent, diagnose, and treat human diseases.

Peptide Stereochemistry Effects from pKa-Shift to Gold Nanoparticle Templating in a Supramolecular Hydrogel

The divergent supramolecular behavior of a series of tripeptide stereoisomers was elucidated through spectroscopic, microscopic, crystallographic, and computational techniques. Only two epimers were able to effectively self-organize into amphipathic structures, leading to supramolecular hydrogels or crystals, respectively. Despite the similarity between the two peptides' turn conformations, stereoconfiguration led to different abilities to engage in intramolecular hydrogen bonding. Self-assembly further shifted the pKa value of the C-terminal side chain. As a result, across the pH range 4-6, only one epimer predominated sufficiently as a zwitterion to reach the critical molar fraction, allowing gelation. By contrast, the differing pKa values and higher dipole moment of the other epimer favored crystallization. The four stereoisomers were further tested for gold nanoparticle (AuNP) formation, with the supramolecular hydrogel being the key to control and stabilize AuNPs, yielding a nanocomposite that catalyzed the photodegradation of a dye. Importantly, the AuNP formation occurred without the use of reductants other than the peptide, and the redox chemistry was investigated by LC-MS, NMR, and infrared scattering-type near field optical microscopy (IR s-SNOM). This study provides important insights for the rational design of simple peptides as minimalistic and green building blocks for functional nanocomposites.

Dynamic Control of Cyclic Peptide Assembly to Form Higher-Order Assemblies

Chirality correction, asymmetry, ring-chain tautomerism and hierarchical assemblies are fundamental phenomena in nature. They are geometrically related and may impact the biological roles of a protein or other supermolecules. It is challenging to study those behaviors within an artificial system due to the complexity of displaying these features. Herein, we design an alternating D,L peptide to recreate and validate the naturally occurring chirality inversion prior to cyclization in water. The resulting asymmetrical cyclic peptide containing a 4-imidazolidinone ring provides an excellent platform to study the ring-chain tautomerism, thermostability and dynamic assembly of the nanostructures. Different from traditional cyclic D,L peptides, the formation of 4-imidazolidinone promotes the formation of intertwined nanostructures. Analysis of the nanostructures confirmed the left-handedness, representing chirality induced self-assembly. This proves that a rationally designed peptide can mimic multiple natural phenomena and could promote the development of functional biomaterials, catalysts, antibiotics, and supermolecules.

Self-healing cyclic peptide hydrogels

Hydrogels are soft materials of great interest in different areas such as chemistry, biology, and therapy. Gels made by the self-assembly of small molecules are known as supramolecular gels. The modulation of their properties by monomer molecular design is still difficult to predict due to the potential impact of subtle structural modifications in the self-assembly process. Herein, we introduce the design principles of a new family of self-assembling cyclic octapeptides of alternating chirality that can be used as scaffolds for the development of self-healing hydrogelator libraries with tunable properties. The strategy was used in the preparation of an amphiphilic cyclic peptide monomer bearing an alkoxyamine connector, which allowed the insertion of different aromatic aldehyde pendants to modulate the hydrophobic/hydrophilic balance and fine-tune the properties of the resulting gel. The resulting amphiphiles were able to form self-healable hydrogels with viscoelastic properties (loss tangent, storage modulus), which were strongly dependent on the nature and number of aromatic moieties anchored to the hydrophilic peptide. Structural studies by SEM, STEM and AFM indicated that the structure of the hydrogels was based on a dense network of peptide nanotubes. Excellent agreement was established between the peptide primary structure, nanotube length distributions and viscoelastic behaviour.

Hybrid Peptide-Agarose Hydrogels for 3D Immunoassays

Recent advances in biosensing analytical platforms have brought relevant outcomes for novel diagnostic and therapy-oriented applications. In this context, 3D droplet microarrays, where hydrogels are used as matrices to stably entrap biomolecules onto analytical surfaces, potentially provide relevant advantages over conventional 2D assays, such as increased loading capacity, lower nonspecific binding, and enhanced signal-to-noise ratio. Here, we describe a hybrid hydrogel composed of a self-assembling peptide and commercial agarose (AG) as a suitable matrix for 3D microarray bioassays. The hybrid hydrogel is printable and self-adhesive and allows analyte diffusion. As a showcase example, we describe its application in a diagnostic immunoassay for the detection of SARS-CoV-2 infection.

DNA nanotubes in coacervate microdroplets as biomimetic cytoskeletons modulate the liquid fluidic properties of protocells

Coacervate microdroplets, formed via liquid-liquid phase separation, have been proposed as a compartment model for the construction of artificial cells or organelles. However, these microsystems are very fragile and demonstrate liquid-like fluidity. Here, an artificial cytoskeleton based on DNA nanotubes was constructed in coacervate microdroplets to modulate the liquid fluidic properties of the microdroplets. The coacervate microdroplets were obtained from the association of oppositely charged polyelectrolytes through liquid-liquid phase separation, and DNA nanotubes were constructed by molecular tile self-assembly from six clip sequences. The DNA nanotubes were efficiently sequestered in the liquid coacervate microdroplets, and the rigid structure of the DNA nanotubes was capable of modulating the liquid fluidic properties of the coacervate protocell models, as indicated by coalescence imaging and atomic force microscopy analysis. Therefore, artificial cytoskeletons made from DNA nanotubes worked in modulating the liquid fluidic properties of coacervate microdroplets, in a manner akin to the cytoskeleton in the cell. DNA cytoskeletons have the potential to become an ideal platform with which how the liquid fluidic properties of cells are modulated by their cytoskeletons can be investigated, and the cell-sized coacervate microdroplets containing artificial cytoskeletons might be critical in developing a stable liquid-phase protocell model.

Enzyme-manipulated hydrogelation of small molecules for biomedical applications

Enzyme-manipulated hydrogelation based on self-assembly of small molecules is an attractive methodology for development of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. Such approach has numerous advantages of region- and enantioselectivity, and mild reaction conditions for encapsulation of biomedications or cells that are fragile to environmental change. In addition to the common applications as drug reservoirs or cell scaffolds, the utilization of endogenous enzymes as stimuli to initiate self-assembly in the living cells and tissue is considered as an intelligent spatiotemporally controllable hydrogelation strategy for biomedical applications. The enzyme-instructed in situ self-assembly and hydrogelation can modulate the cell behavior, and even present therapeutic bioactivities, which provides a new perspective in the field of disease treatment. In this review, we categorize distinct enzymatic stimuli and elaborate substrate design, catalytic characteristics, and mechanisms of self-assembly and hydrogelation. The biomedical applications in drug delivery, tissue engineering, bioimaging, and in situ gelation-produced bioactivity are outlined. Advantages and limitations regarding the state-of-the-art enzyme-driven hydrogelation technologies and future perspectives are also discussed. STATEMENT OF SIGNIFICANCE: Hydrogel is a semi-solid soft material containing a large amount of water. Due to the features of adjustable flexibility, extremely porous architecture, and the high similarity of structure to natural extracellular matrices, the hydrogel has broad application prospects in biomedicine. In recent 20 years, enzyme-manipulated hydrogelation based on self-assembly of small molecules has developed rapidly as an attractive methodology for the construction of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. This review summarized the characteristics of enzymatic hydrogel, as well as the traditional application and emerging prospect of enzyme-instructed self-assembly and hydrogelation.

The Dynamic Range of Acidity: Tracking Rules for the Unidirectional Penetration of Cellular Compartments

Labeled ammonium cations with pKa ∼7.4 accumulate in acidic organelles because they can be neutralized transiently to cross the membrane at cytosolic pH 7.2 but not at their internal pH<5.5. Retention in early endosomes with less acidic internal pH was achieved recently using weaker acids of up to pKa 9.8. We report here that primary ammonium cations with higher pKa 10.6, label early endosomes more efficiently. This maximized early endosome tracking coincides with increasing labeling of Golgi networks with similarly weak internal acidity. Guanidinium cations with pKa 13.5 cannot cross the plasma membrane in monomeric form and label the plasma membrane with selectivity for vesicles embarking into endocytosis. Self-assembled into micelles, guanidinium cations enter cells like arginine-rich cell-penetrating peptides and, driven by their membrane potential, penetrate mitochondria unidirectionally despite their high inner pH. The resulting tracking rules with an approximated dynamic range of pKa change ∼3.5 are expected to be generally valid, thus enabling the design of chemistry tools for biology research in the broadest sense. From a practical point of view, most relevant are two complementary fluorescent flipper probes that can be used to image the mechanics at the very beginning of endocytosis.

pH-Responsive Self-Assembling Peptide-Based Biomaterials: Designs and Applications

Stimuli-responsive peptide-based biomaterials are increasingly gaining interest for various specific and targeted treatments, including drug delivery and tissue engineering. Among all stimuli, pH can be especially useful because endogenous pH changes are often associated with abnormal microenvironments. pH-Responsive amino acids and organic linkers can be easily incorporated into peptides that self-assemble into various nanostructures. Thus, these largely biocompatible and easily tunable platforms are ideal candidates for drug release and as fibrous materials capable of mimicking the native extracellular matrix. In this review, we highlight common design motifs and mechanisms of pH-responsiveness in self-assembling peptide-based biomaterials, focusing on recent advances of these biomaterials applied in drug delivery and tissue engineering. Finally, we suggest future challenges and areas for potential development in pH-responsive self-assembling peptide-based biomaterials.

Hierarchical self-assembly of aromatic peptide conjugates into supramolecular polymers: it takes two to tango

Supramolecular polymers are self-assembled materials displaying adaptive and responsive "life-like" behaviour which are often made of aromatic compounds capable of engaging in π-π interactions to form larger assemblies. Major advances have been made recently in controlling their mode of self-assembly, from thermodynamically-controlled isodesmic to kinetically-controlled living polymerization. Dynamic covalent chemistry has been recently implemented to generate dynamic covalent polymers which can be seen as dynamic analogues of biomacromolecules. On the other hand, peptides are readily-available and structurally-rich building blocks that can lead to secondary structures or specific functions. In this context, the past decade has seen intense research activity in studying the behaviour of aromatic-peptide conjugates through supramolecular and/or dynamic covalent chemistries. Herein, we review those impressive key achievements showcasing how aromatic- and peptide-based self-assemblies can be combined using dynamic covalent and/or supramolecular chemistry, and what it brings in terms of the structure, self-assembly pathways, and function of supramolecular and dynamic covalent polymers.

Cyclization and Self-Assembly of Cyclic Peptides

Cyclic peptides are a fascinating class of molecules that can be programmed to fold or self-assemble into diverse mono- and multidimensional structures with potential applications in biomedicine, nanoelectronics, or catalysis. Herein we describe on-resin procedures to carry out head-to-tail peptide cyclization based on orthogonal protected linear structures. We also present essential characterization tools for obtaining dynamic and structural information, including the visualization cyclic peptide assembly into nanotubes (AFM, TEM) as well as the use of fluorescence microscopy.

1D alignment of proteins and other nanoparticles by using reversible covalent bonds on cyclic peptide nanotubes

Peptide nanotubes deposit on mica surface can be used for the alignment of proteins thank to the use of dynamic covalent bonds that allow the incorporation of appropriate ligands on nanotube surface.

Molecular Self-Assembly and Supramolecular Chemistry of Cyclic Peptides

This Review focuses on the establishment and development of self-assemblies governed by the supramolecular interactions between cyclic peptides. The Review first describes the type of cyclic peptides able to assemble into tubular structures to form supramolecular cyclic peptide nanotubes. A range of cyclic peptides have been identified to have such properties, including α-peptides, β-peptides, α,γ-peptides, and peptides based on δ- and ε-amino acids. The Review covers the design and functionalization of these cyclic peptides and expands to a recent advance in the design and application of these materials through their conjugation to polymer chains to generate cyclic peptide-polymer conjugates nanostructures. The Review, then, concentrates on the challenges in characterizing these systems and presents an overview of the various analytical and characterization techniques used to date. This overview concludes with a critical survey of the various applications of the nanomaterials obtained from supramolecular cyclic peptide nanotubes, with a focus on biological and medical applications, ranging from ion channels and membrane insertion to antibacterial materials, anticancer drug delivery, gene delivery, and antiviral applications.

A mini-review on peptide-based self-assemblies and their biological applications

Peptide-based supramolecular self-assembly from peptide monomers into well-organized nanostructures, has attracted extensive attentions towards biomedical and biotechnological applications in recent decades. This spontaneous and reversible assembly process involving non-covalent bonding interactions can be artificially regulated. In this review, we have elaborated different strategies to modulate the peptide self-assembly through tuning the physicochemical and environmental conditions, includingpH, light, temperature, solvent, and enzyme. Detailed introduction of biological applications and future potential of the peptide-based nano-assemblies will also be given.

Supramolecular fibrillation of peptide amphiphiles induces environmental responses in aqueous droplets

One-dimensional (1D) supramolecular polymers are commonly found in natural and synthetic systems to prompt functional responses that capitalise on hierarchical molecular ordering. Despite amphiphilic self-assembly being significantly studied in the context of aqueous encapsulation and autopoiesis, very little is currently known about the physico-chemical consequences and functional role of 1D supramolecular polymerisation confined in aqueous compartments. Here, we describe the different phenomena that resulted from the chemically triggered supramolecular fibrillation of synthetic peptide amphiphiles inside water microdroplets. The confined connection of suitable dormant precursors triggered a physically autocatalysed chemical reaction that resulted in functional environmental responses such as molecular uptake, fusion and chemical exchange. These results demonstrate the potential of minimalistic 1D supramolecular polymerisation to modulate the behaviour of individual aqueous entities with their environment and within communities.

Hybrid Self-Assembled Gel Beads for Tuneable pH-Controlled Rosuvastatin Delivery

This article describes the fabrication of new pH-responsive hybrid gel beads combining the polymer gelator calcium alginate with two different low-molecular-weight gelators (LMWGs) based on 1,3 : 2,4-dibenzylidene-d-sorbitol: pH-responsive DBS-COOH and thermally responsive DBS-CONHNH2 , thus clearly demonstrating that different classes of LMWG can be fabricated into gel beads by using this approach. We also demonstrate that self-assembled multicomponent gel beads can be formed by using different combinations of these gelators. The different gel bead formulations exhibit different responsiveness - the DBS-COOH network can disassemble within those beads in which it is present upon raising the pH. To exemplify preliminary data for a potential application for these hybrid gel beads, we explored aspects of the delivery of the lipid-lowering active pharmaceutical ingredient (API) rosuvastatin. The release profile of this statin from the hybrid gel beads is pH-dependent, with greater release at pH 7.4 than at pH 4.0 - primary control of this process results from the pKa of the API. The extent of pH-mediated API release is also significantly further modified according to gel bead composition. The DBS-COOH/alginate beads show rapid, highly effective drug release at pH 7.4, whereas the three-component DBS-COOH/DBS-CONHNH2 /alginate system shows controlled slow release of the API under the same conditions. These initial results indicate that such gel beads constitute a promising, versatile and easily tuned platform suitable for further development for controlled drug-delivery applications.

Molecular Plumbing to Bend Self-Assembling Peptide Nanotubes

Light-induced molecular piping of cyclic peptide nanotubes to form bent tubular structures is described. The process is based on the [4+4] photocycloaddition of anthracene moieties, whose structural changes derived from the interdigitated flat disposition of precursors to the corresponding cycloadduct moieties, induced the geometrical modifications in nanotubes packing that provokes their curvature. For this purpose, we designed a new class of cyclic peptide nanotubes formed by β- and α-amino acids. The presence of the former predisposes the peptide to stack in a parallel fashion with the β-residues aligned along the nanotube and the homogeneous distribution of anthracene pendants.

Effect of Water Models on Transmembrane Self-Assembled Cyclic Peptide Nanotubes

Self-assembling cyclic peptide nanotubes can form nanopores when they are inserted in lipid bilayers, acting as ion and/or water permeable channels. In order to improve the versatility of these systems, it is possible to specifically design cyclic peptides with a combination of natural and non-natural amino acids, enabling the control of the nature of the inner cavity of the channels. Here, the behavior of two types of self-assembling peptide motifs, alternating α-amino acids with γ- or δ-aminocycloalkanecarboxylic acids, is studied via molecular dynamics (MD) simulations. The behavior of water molecules in nanopores is expected to affect the properties of these channels and therefore merits detailed examination. A number of water models commonly used in MD simulations have been validated by how well they reproduce bulk water properties. However, it is less clear how these water models behave in the nanoconfined condition inside a channel. The behavior of four different water models-TIP3P, TIP4P, TIP4P/2005, and OPC-are evaluated in MD simulations of self-assembled cyclic peptide nanotubes of distinct composition and diameter. The dynamic behavior of the water molecules and ions in these designed artificial channels depends subtly on the water model used. TIP3P water molecules move faster than those of TIP4P, TIP4P/2005, and OPC. This demeanor is clearly observed in the filling of the nanotube, in water diffusion within the pore, and in the number and stability of hydrogen bonds of the peptides with water. It was also shown that the water model influences the simulated ion flux through the nanotubes, with TIP3P producing the greatest ion flux. Additionally, the two more recent models, TIP4P/2005 and OPC, which are known to reproduce the experimental self-diffusion coefficient of bulk water quite well, exhibit very similar results under the nanoconfined conditions studied here. Because none of these models have been parametrized specifically for waters confined in peptide nanotubes, this study provides a point of reference for further validation.

Smart Nanocages as a Tool for Controlling Supramolecular Aggregation

An important aspect in the field of supramolecular chemistry is the control of the composition and aggregation state of supramolecular polymers and the possibility of stabilizing out-of-equilibrium states. The ability to freeze metastable systems and release them on demand, under spatiotemporal control, to allow their thermodynamic evolution toward the most stable species is a very attractive concept. Such temporal blockage could be realized using stimuli-responsive "boxes" able to trap and redirect supramolecular polymers. In this work, we report the use of a redox responsive nanocontainer, an organosilica nanocage (OSCs), for controlling the dynamic self-assembly pathway of supramolecular aggregates of a luminescent platinum compound (Pt_AC). The aggregation of the complexes leads to different photoluminescent properties that allow visualization of the different assemblies and their evolution. We discovered that the nanocontainers can encapsulate kinetically trapped species characterized by an orange emission, preventing their evolution into the thermodynamically stable aggregation state characterized by blue-emitting fibers. Interestingly, the out-of-equilibrium trapped Pt species (Pt_AC@OSCs) can be released on demand by the redox-triggered degradation of OSCs, re-establishing their self-assembly toward the thermodynamically stable state. To demonstrate that control of the self-assembly pathway occurs also in complex media, we followed the evolution of the supramolecular aggregates inside living cells, where the destruction of the cages allows the intracellular release of Pt_AC aggregates, followed by the formation of microscopic blue emitting fibers. Our approach highlights the importance of "ondemand" confinement as a tool to temporally stabilize transient species which modulate complex self-assembly pathways in supramolecular polymerization.

Folding and self-assembly of short intrinsically disordered peptides and protein regions

Proteins and peptide fragments are highly relevant building blocks in self-assembly for nanostructures with plenty of applications. Intrinsically disordered proteins (IDPs) and protein regions (IDRs) are defined by the absence of a well-defined secondary structure, yet IDPs/IDRs show a significant biological activity. Experimental techniques and computational modelling procedures for the characterization of IDPs/IDRs are discussed. Directed self-assembly of IDPs/IDRs allows reaching a large variety of nanostructures. Hybrid materials based on the derivatives of IDPs/IDRs show a promising performance as alternative biocides and nanodrugs. Cell mimicking, in vivo compartmentalization, and bone regeneration are demonstrated for IDPs/IDRs in biotechnological applications. The exciting possibilities of IDPs/IDRs in nanotechnology with relevant biological applications are shown.

Nanoscale Assembly of Functional Peptides with Divergent Programming Elements

Self-assembling peptides are being applied both in the biomedical area and as building blocks in nanotechnology. Their applications are closely linked to their modes of self-assembly, which determine the functional nanostructures that they form. This work brings together two structural elements that direct nanoscale self-association in divergent directions: proline as a β-breaker and the β-structure-associated diphenylalanine motif, into a single tripeptide sequence. Amino acid chirality was found to resolve the tension inherent to these conflicting self-assembly instructions. Stereoconfiguration determined the ability of each of the eight possible Pro-Phe-Phe stereoisomers to self-associate into diverse nanostructures, including nanoparticles, nanotapes, or fibrils, which yielded hydrogels with gel-to-sol transition at a physiologically relevant temperature. Three single-crystal structures and all-atom molecular dynamics simulations elucidated the ability of each peptide to establish key interactions to form long-range assemblies (i,e., stacks leading to gelling fibrils), medium-range assemblies (i.e., stacks yielding nanotapes), or short-range assemblies (i.e., dimers or trimers that further associated into nanoparticles). Importantly, diphenylalanine is known to serve as a binding site for pathological amyloids, potentially allowing these heterochiral systems to influence the fibrillization of other biologically relevant peptides. To probe this hypothesis, all eight Pro-Phe-Phe stereoisomers were tested in vitro on the Alzheimer's disease-associated Aβ(1-42) peptide. Indeed, one nonfibril-forming stereoisomer effectively inhibited Aβ fibrillization through multivalent binding between diphenylalanine motifs. This work thus defined heterochirality as a useful feature to strategically develop future therapeutics to interfere with pathological processes, with the additional value of resistance to protease-mediated degradation and biocompatibility.

Self-assembled low-molecular-weight gelator injectable microgel beads for delivery of bioactive agents

We report the preparation of hybrid self-assembled microgel beads by combining the low molecular weight gelator (LMWG) DBS-CONHNH2 and the natural polysaccharide calcium alginate polymer gelator (PG). Microgel formulations based on LMWGs are extremely rare due to the fragility of the self-assembled networks and the difficulty of retaining any imposed shape. Our hybrid beads contain interpenetrated LMWG and PG networks, and are obtained by an emulsion method, allowing the preparation of spherical gel particles of controllable sizes with diameters in the mm or μm range. Microgels based on LMWG/alginate can be easily prepared with reproducible diameters <1 μm (ca. 800 nm). They are stable in water at room temperature for many months, and survive injection through a syringe. The rapid assembly of the LMWG on cooling plays an active role in helping control the diameter of the microgel beads. These LMWG microbeads retained the ability of the parent gel to deliver the bioactive molecule heparin, and in cell culture medium this enhanced the growth of human mesenchymal stem cells. Such microgels may therefore have future applications in tissue repair. This approach to fabricating LMWG microgels is a platform technology, which could potentially be applied to a variety of different functional LMWGs, and hence has wide-ranging potential.

Self-Assembling Peptide Hydrogels for 3D Microarrays

Recent advances in biosensing analytical platforms have brought relevant outcomes for novel diagnostic and therapy-oriented applications. In this context, hydrogels have emerged as appealing matrices to locally confine biomolecules onto sensing surfaces under solution mimetic conditions, preserving their structural integrity and function. Here, we describe the application of a self-assembling peptide hydrogel as a suitable matrix for 3D microarray bioassays. The hydrogel is printable and self-adhesive and allows for fast analyte diffusion. As a showcase example, we describe its application in a diagnostic immunoassay for the detection of arbovirus infection.

Cyclic γ-Peptides With Transmembrane Water Channel Properties

Self-assembling peptides can be used to design new materials for medical and biological applications. Here we synthesized and characterized two novel cyclic γ-peptides (γ-CPs) with hydrophobic inner surfaces. The NMR and FT-IR studies confirmed that the CPs could self-assemble into parallel stacking structures via intermolecular H-bonds and π-π interactions. The morphologies of the self-assembly CPs showed bundles of nanotubes via transmission electron microscopy (TEM); these nanotubes form water channels to transport water across the lipid membrane. The properties of blocking the transport of protons like natural water channels showed that the hydrophobic inner surfaces are important in artificial transmembrane water channel designs. These studies also showed that water transport was a function of pore size and length of the assemblies.

Parallel Versus Antiparallel β-Sheet Structure in Cyclic Peptide Hybrids Containing γ- or δ-Cyclic Amino Acids

Cyclic peptides with disc-shaped structures have emerged as potent building blocks for the preparation of new biomaterials in fields ranging from biological to material science. In this work, we analyze in depth the self-assembling properties of a new type of cyclic peptides based on the alternation of α-residues and cyclic δ-amino acids (α,δ-CPs). To examine the preferred stacking properties adopted by cyclic peptides bearing this type of amino acids, we carried out a synergistic in vitro/in silico approximation by using simple dimeric models and then extended to nanotubes. Although these new cyclic peptides (α,δ-CPs) can interact either in a parallel or antiparallel fashion, our results confirm that although the parallel β-sheet is more stable, it can be switched to the antiparallel stacking by choosing residues that can establish favorable cross-strand interactions. Moreover, the subsequent comparison by using the same methodology but applied to α,γ-CPs models, up to the moment assumed as antiparallel-like d,l-α-CPs, led to unforeseen conclusions that put into question preliminary conjectures about these systems. Surprisingly, they tend to adopt a parallel β-sheet directed by the skeleton interactions. These results imply a change of paradigm with respect to cyclic peptide designs that should be considered for dimers and nanotubes.

Bioadhesive supramolecular hydrogel from unprotected, short d,l-peptides with Phe-Phe and Leu-Asp-Val motifs

The uncapped tripeptide DPhe-Phe-Leu acts as self-assembly template to yield supramolecular hydrogel biomaterials. As an example, self-assembling DPhe-Phe-Leu-Asp-Val contains the LDV bioadhesive motif for β1 integrin activation. Hydrogels made of the two peptides successfully mimic fibronectin of the extracellular matrix and lead to high cell viability, adhesion, and spreading.

Mechanoresponsive Alignment of Molecular Self-Assembled Negatively Charged Nanofibrils

Inspired by the mechanoresponsive orientation of actin filaments in cell, we introduce a design paradigm of synthetic molecular self-assembling fibrils that respond to external mechanical force by transforming from a macroscopically disorder state to a highly ordered uniaxial aligned state. The incorporation of aromatic-containing amino acids and negatively charged amino acids lead to self-assembly motifs that transform into uniform nanofibrils in acidic solution. Adjusting the pH level of aqueous solution introduces optimal negative charge to the surface of self-assembling nanofibrils inducing long-range electrostatic repulsion forming a nematic phase. Upon external mechanical force, nanofibrils align in the force direction. Via evaporation casting in capillary confinement, the solvated synthetic self-assembling nanofibrils transform into scalable lamellar domains. Adjusting capillary geometry and drying procedure offers further parameters for tuning the mesoscale alignment of nanofibrils generating a variety of interference colors. The design paradigm of mechanoresponsive alignment of self-assembled nanofibrils as an addition of nanofabrication techniques is potentially employable for realizing biomimetic optical structures.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

A minimalistic catalytically-active cell mimetic made of a supra-molecular hydrogel encapsulated into a polymersome

A minimalistic multicomponent cell mimetic is constructed from a catalytic low molecular weight fibrillar network and a polymersome compartment.

Dual self-assembly of supramolecular peptide nanotubes to provide stabilisation in water

Self-assembling peptides have the ability to spontaneously aggregate into large ordered structures. The reversibility of the peptide hydrogen bonded supramolecular assembly make them tunable to a host of different applications, although it leaves them highly dynamic and prone to disassembly at the low concentration needed for biological applications. Here we demonstrate that a secondary hydrophobic interaction, near the peptide core, can stabilise the highly dynamic peptide bonds, without losing the vital solubility of the systems in aqueous conditions. This hierarchical self-assembly process can be used to stabilise a range of different β-sheet hydrogen bonded architectures.

Embedding and Positioning of Two FeII4 L4 Cages in Supramolecular Tripeptide Gels for Selective Chemical Segregation

An unreported d,l-tripeptide self-assembled into gels that embedded FeII4 L4 metal-organic cages to form materials that were characterized by TEM, EDX, Raman spectroscopy, rheometry, UV/Vis and NMR spectroscopy, and circular dichroism. The cage type and concentration modulated gel viscoelasticity, and thus the diffusion rate of molecular guests through the nanostructured matrix, as gauged by 19 F and 1 H NMR spectroscopy. When two different cages were added to spatially separated gel layers, the gel-cage composite material enabled the spatial segregation of a mixture of guests that diffused into the gel. Each cage selectively encapsulated its preferred guest during diffusion. We thus present a new strategy for using nested supramolecular interactions to enable the separation of small molecules.

Self-assembled micro-fibres by oxime connection of linear peptide amphiphiles

Linear peptide amphiphiles are excellent biocompatible scaffolds for the hierarchical self-assembly of one-dimensional nano-structures in aqueous media. However, their structural exploration and screening of self-assembling properties are often limited by time-consuming synthesis and purification steps. We here describe the application of an oxime bond as a powerful synthetic tool towards the conjugation of peptide heads bearing a hydroxylamine group with hydrophobic aldehyde tails. This methodology allowed the quick preparation of a small library of oxime-connected peptide amphiphiles, whose supramolecular screening revealed nano-to-micro-fibrillation with dependency on their chemical structure. These results demonstrate the simplicity and the synthetic potential of the oxime conjugation for the preparation of peptide amphiphiles with improved self-assembling capabilities.

A self-assembling peptide hydrogel for ultrarapid 3D bioassays

Biosensing analytical platforms rely on the intimate structure-function relationship of immobilized probes. In this context, hydrogels are appealing semi-wet systems to locally confine biomolecules while preserving their structural integrity and function. Yet, limitations imposed by biomolecule diffusion rates or fabrication difficulties still hamper their broad application. Here, using a self-assembling peptide, a printable and self-adhesive hydrogel was obtained and applied to fabricate arrays of localized bio-functional 3D microenvironments on analytical interfaces. This soft matrix represents a robust and versatile material, allowing fast and selective tuning of analyte diffusion, which is exploited here to run in-gel immunoassays under solution-like conditions in an unprecedented (<10 min) time frame. The developed material overcomes major limitations associated with hydrogels for bioassays, widening the prospects for easy fabrication of multifunctional bio-interfaces for high-throughput, molecular recognition assays.

Chiral and random arrangements of flavin chromophores along cyclic peptide nanotubes on gold influencing differently on surface potential and piezoelectricity

Two kinds of peptide nanotubes are prepared from cyclo(β-Asp(flavin)-β-alanine-β-alanine) (C3FAA) and cyclo(β-Asp(flavin)-ethylenediamine-succinic acid) (C3FES). The flavin chromophores are protruding on the C3FAA and C3FES peptide nanotube surfaces in random and chiral ways, respectively. The surface potentials of the C3FAA nanotube bundles on a gold substrate become larger than the C3FES nanotube bundles of the corresponding thicknesses. The converse piezoelectric coefficients are as small as less than 1 pm V⁻¹. The peptide nanotube bundles are subjected to a thermal anneal treatment which raises up all the surface potentials and also the converse piezoelectricity of the C3FES nanotube bundles of 3 pm V⁻¹. The macrodipole of the C3FAA nanotube and the chiral arrangement of the flavin groups in the C3FES nanotube are considered to contribute influentially to the surface potential and the piezoelectricity, respectively.

Two kinds of peptide nanotubes are prepared from cyclo(β-Asp(flavin)-β-alanine-β-alanine) (C3FAA) and cyclo(β-Asp(flavin)-ethylenediamine-succinic acid) (C3FES).

Supramolecular assembly of functional peptide–polymer conjugates

The following review gives an overview about synthetic peptide–polymer conjugates as macromolecular building blocks and their self-assembly into a variety of supramolecular architectures, from supramolecular polymer chains, to anisotropic 1D arrays, 2D layers, and more complex 3D networks.

New self-assembling peptide nanotubes of large diameter using δ-amino acids

Here we show that 4-aminocyclohexanecarboxylic acid is a rigid stretcher building block for the preparation of cyclic peptides that self-assemble to form peptide nanotubes with large diameter and hydrophobic pores. The hydrophobic properties of the resulting nanotubes provided by the two methylene groups per δ-residue allow the encapsulation of C60 moieties forming a new type of bionanopeapod structure.

Hydrogel and Organogel Formation by Hierarchical Self-Assembly of Cyclic Peptides Nanotubes

Breaking away from the linear structure of previously reported peptide-based gelators, this study reports the first example of gel formation based on the use of cyclic peptides made of alternating d- and l-amino acids, known to self-assemble in solution to form long nanotubes. Herein, a library of cyclic peptides was systemically studied for their gelation properties in various solvents, uncovering key parameters driving both organogel and hydrogel formation. The hierarchical nature of the self-assembly process in water was characterised by a combination of electron microscopy imaging and small-angle X-ray scattering, revealing a porous network of entangled nanofibres composed by the aggregation of several cyclic peptide nanotubes. Rheology measurements then confirmed the formation of soft hydrogels.