Single-cell coating with biomimetic extracellular nanofiber matrices

Cell therapies offer great promise in the treatment of diseases and tissue regeneration, but their clinical use has many challenges including survival, optimal performance in their intended function, or localization at sites where they are needed for effective outcomes. We report here on a method to coat a biodegradable matrix of biomimetic nanofibers on single cells that could have specific functions ranging from cell signaling to targeting and helping cells survive when used for therapies. The fibers are composed of peptide amphiphile (PA) molecules that self-assemble into supramolecular nanoscale filaments. The PA nanofibers were able to create a mesh-like coating for a wide range of cell lineages with nearly 100 % efficiency, without interrupting the natural cellular phenotype or functions. The targeting abilities of this system were assessed in vitro using human primary regulatory T (hTreg) cells coated with PAs displaying a vascular cell adhesion protein 1 (VCAM-1) targeting motif. This approach provides a biocompatible method for single-cell coating that does not negatively alter cellular phenotype, binding capacity, or immunosuppressive functionality, with potential utility across a broad spectrum of cell therapies. STATEMENT OF SIGNIFICANCE: Cell therapies hold great promise in the treatment of diseases and tissue regeneration, but their clinical use has been limited by cell survival, targeting, and function. We report here a method to coat single cells with a biodegradable matrix of biomimetic nanofibers composed of peptide amphiphile (PA) molecules. The nanofibers were able to coat cells, such as human primary regulatory T cells, with nearly 100 % efficiency, without interrupting the natural cellular phenotype or functions. The approach provides a biocompatible method for single-cell coating that does not negatively alter cellular phenotype, binding capacity, or immunosuppressive functionality, with potential utility across a broad spectrum of cell therapies.

Autonomous hydrogel locomotion regulated by light and electric fields

Autonomous robotic functions in materials beyond simple stimulus-response actuation require the development of functional soft matter that can complete well-organized tasks without step-by-step control. We report the design of photo- and electroactivated hydrogels that can capture and deliver cargo, avoid obstacles, and return without external, stepwise control. By incorporating two spiropyran monomers with different chemical substituents in the hydrogel, we created chemically random networks that enabled photoregulated charge reversal and autonomous behaviors under a constant electric field. In addition, using perturbations in the electric field induced by a dielectric inhomogeneity, the hydrogel could be attracted to high dielectric constant materials and autonomously bypasses the low dielectric constant materials under the guidance of the electric field vector. The photo- and electroactive hydrogels investigated here can autonomously perform tasks using constant external stimuli, an encouraging observation for the potential development of molecularly designed intelligent robotic materials.

Supramolecular Nanofibers Block SARS-CoV-2 Entry into Human Host Cells

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection relies on its spike protein binding to angiotensin-converting enzyme 2 (ACE2) on host cells to initiate cellular entry. Blocking the interactions between the spike protein and ACE2 offers promising therapeutic opportunities to prevent infection. We report here on peptide amphiphile supramolecular nanofibers that display a sequence from ACE2 in order to promote interactions with the SARS-CoV-2 spike receptor binding domain. We demonstrate that displaying this sequence on the surface of supramolecular assemblies preserves its α-helical conformation and blocks the entry of a pseudovirus and its two variants into human host cells. We also found that the chemical stability of the bioactive structures was enhanced in the supramolecular environment relative to the unassembled peptide molecules. These findings reveal unique advantages of supramolecular peptide therapies to prevent viral infections and more broadly for other targets as well.

Role of supramolecular polymers in photo-actuation of spiropyran hydrogels

Supramolecular-covalent hybrid polymers have been shown to be interesting systems to generate robotic functions in soft materials in response to external stimuli. In recent work supramolecular components were found to enhance the speed of reversible bending deformations and locomotion when exposed to light. The role of morphology in the supramolecular phases integrated into these hybrid materials remains unclear. We report here on supramolecular-covalent hybrid materials that incorporate either high-aspect-ratio peptide amphiphile (PA) ribbons and fibers, or low-aspect-ratio spherical peptide amphiphile micelles into photo-active spiropyran polymeric matrices. We found that the high-aspect-ratio morphologies not only play a significant role in providing mechanical reinforcement to the matrix but also enhance photo-actuation for both light driven volumetric contraction and expansion of spiropyran hydrogels. Molecular dynamics simulations indicate that water within the high-aspect-ratio supramolecular polymers exhibits a faster draining rate as compared to those in spherical micelles, which suggests that the high-aspect-ratio supramolecular polymers effectively facilitate the transport of trapped water molecules by functioning as channels and therefore enhancing actuation of the hybrid system. Our simulations provide a useful strategy for the design of new functional hybrid architectures and materials with the aim of accelerating response and enhancing actuation by facilitating water diffusion at the nanoscopic level.

Heterocyclic Chromophore Amphiphiles and their Supramolecular Polymerization

Supramolecular polymerization of π-conjugated amphiphiles in water is an attractive approach to create functional nanostructures. Here, we report on the synthesis, optoelectronic and electrochemical properties, aqueous supramolecular polymerization, and conductivity of polycyclic aromatic dicarboximide amphiphiles. The chemical structure of the model perylene monoimide amphiphile was modified with heterocycles, essentially substituting one fused benzene ring with thiophene, pyridine or pyrrole rings. All the heterocycle-containing monomers investigated underwent supramolecular polymerization in water. Large changes to the monomeric molecular dipole moments led to nanostructures with low electrical conductivity due to diminished interactions. Although the substitution of benzene with thiophene did not notably change the monomer dipole moment, it led to crystalline nanoribbons with 20-fold higher electrical conductivity, due to enhanced dispersion interactions as a result of the presence of sulfur atoms.

Machine learning overcomes human bias in the discovery of self-assembling peptides

Peptide materials have a wide array of functions, from tissue engineering and surface coatings to catalysis and sensing. Tuning the sequence of amino acids that comprise the peptide modulates peptide functionality, but a small increase in sequence length leads to a dramatic increase in the number of peptide candidates. Traditionally, peptide design is guided by human expertise and intuition and typically yields fewer than ten peptides per study, but these approaches are not easily scalable and are susceptible to human bias. Here we introduce a machine learning workflow-AI-expert-that combines Monte Carlo tree search and random forest with molecular dynamics simulations to develop a fully autonomous computational search engine to discover peptide sequences with high potential for self-assembly. We demonstrate the efficacy of the AI-expert to efficiently search large spaces of tripeptides and pentapeptides. The predictability of AI-expert performs on par or better than our human experts and suggests several non-intuitive sequences with high self-assembly propensity, outlining its potential to overcome human bias and accelerate peptide discovery.

Peptide Sequence Determines Structural Sensitivity to Supramolecular Polymerization Pathways and Bioactivity

Pathways in supramolecular polymerization traverse different regions of the system's energy landscape, affecting not only their architectures and internal structure but also their functions. We report here on the effects of pathway selection on polymerization for two isomeric peptide amphiphile monomers with amino acid sequences AAEE and AEAE. We subjected the monomers to five different pathways that varied in the order they were exposed to electrostatic screening by electrolytes and thermal annealing. We found that introducing electrostatic screening of E residues before annealing led to crystalline packing of AAEE monomers. Electrostatic screening decreased intermolecular repulsion among AAEE monomers thus promoting internal order within the supramolecular polymers, while subsequent annealing brought them closer to thermodynamic equilibrium with enhanced β-sheet secondary structure. In contrast, supramolecular polymerization of AEAE monomers was less pathway dependent, which we attribute to side-chain dimerization. Regardless of the pathway, the internal structure of AEAE nanostructures had limited internal order and moderate β-sheet structure. These supramolecular polymers generated hydrogels with lower porosity and greater bulk mechanical strength than those formed by the more cohesive AAEE polymers. The combination of dynamic, less ordered internal structure and bulk strength of AEAE networks promoted strong cell-material interactions in adherent epithelial-like cells, evidenced by increased cytoskeletal remodeling and cell spreading. The highly ordered AAEE nanostructures formed porous hydrogels with inferior bulk mechanical properties and weaker cell-material interactions. We conclude that pathway sensitivity in supramolecular synthesis, and therefore structure and function, is highly dependent on the nature of dominant interactions driving polymerization.

Hybrid Nanocrystals of Small Molecules and Chemically Disordered Polymers

Organic crystals formed by small molecules can be highly functional but are often brittle or insoluble structures with limited possibilities for use or processing from a liquid phase. A possible solution is the nanoscale integration of polymers into organic crystals without sacrificing long-range order and therefore function. This enables the organic crystals to benefit from the advantageous mechanical and chemical properties of the polymeric component. We report here on a strategy in which small molecules cocrystallize with side chains of chemically disordered polymers to create hybrid nanostructures containing a highly ordered lattice. Synchrotron X-ray scattering, absorption spectroscopy, and coarse-grained molecular dynamics simulations reveal that the polymer backbones form an "exo-crystalline" layer of disordered chains that wrap around the nanostructures, becoming a handle for interesting properties. The morphology of this "hybrid bonding polymer" nanostructure is dictated by the competition between the polymers' entropy and the enthalpy of the lattice allowing for control over the aspect ratio of the nanocrystal by changing the degree of polymer integration. We observed that nanostructures with an exo-crystalline layer of polymer exhibit enhanced fracture strength, self-healing capacity, and dispersion in water, which benefits their use as light-harvesting assemblies in photocatalysis. Guided by computation, future work could further explore these hybrid nanostructures as components for functional materials.

Supramolecular Copolymers of Peptides and Lipidated Peptides and Their Therapeutic Potential

Supramolecular peptide chemistry offers a versatile strategy to create chemical systems useful as new biomaterials with potential to deliver nearly 1000 known candidate peptide therapeutics or integrate other types of bioactivity. We report here on the co-assembly of lipidated β-sheet-forming peptides with soluble short peptides, yielding supramolecular copolymers with various degrees of internal order. At low peptide concentrations, the co-monomer is protected by lodging within internal aqueous compartments and stabilizing internal β-sheets formed by the lipidated peptides. At higher concentrations, the peptide copolymerizes with the lipidated peptide and disrupts the β-sheet secondary structure. The thermodynamic metastability of the co-assembly in turn leads to the spontaneous release of peptide monomers and thus serves as a potential mechanism for drug delivery. We demonstrated the function of these supramolecular systems using a drug candidate for Alzheimer's disease and found that the copolymers enhance neuronal cell viability when the soluble peptide is released from the assemblies.

Acid-Base Equilibrium and Dielectric Environment Regulate Charge in Supramolecular Nanofibers

Peptide amphiphiles are a class of molecules that can self-assemble into a variety of supramolecular structures, including high-aspect-ratio nanofibers. It is challenging to model and predict the charges in these supramolecular nanofibers because the ionization state of the peptides are not fixed but liable to change due to the acid-base equilibrium that is coupled to the structural organization of the peptide amphiphile molecules. Here, we have developed a theoretical model to describe and predict the amount of charge found on self-assembled peptide amphiphiles as a function of pH and ion concentration. In particular, we computed the amount of charge of peptide amphiphiles nanofibers with the sequence C₁₆ − V₂A₂E₂. In our theoretical formulation, we consider charge regulation of the carboxylic acid groups, which involves the acid-base chemical equilibrium of the glutamic acid residues and the possibility of ion condensation. The charge regulation is coupled with the local dielectric environment by allowing for a varying dielectric constant that also includes a position-dependent electrostatic solvation energy for the charged species. We find that the charges on the glutamic acid residues of the peptide amphiphile nanofiber are much lower than the same functional group in aqueous solution. There is a strong coupling between the charging via the acid-base equilibrium and the local dielectric environment. Our model predicts a much lower degree of deprotonation for a position-dependent relative dielectric constant compared to a constant dielectric background. Furthermore, the shape and size of the electrostatic potential as well as the counterion distribution are quantitatively and qualitatively different. These results indicate that an accurate model of peptide amphiphile self-assembly must take into account charge regulation of acidic groups through acid–base equilibria and ion condensation, as well as coupling to the local dielectric environment.

Modeling Interactions within and between Peptide Amphiphile Supramolecular Filaments

Many peptides are able to self-assemble into one-dimensional (1D) nanostructures, such as cylindrical fibers or ribbons of variable widths, but the relationship between the morphology of 1D objects and their molecular structure is not well understood. Here, we use coarse-grained molecular dynamics (CG-MD) simulations to study the nanostructures formed by self-assembly of different peptide amphiphiles (PAs). The results show that ribbons are hierarchical superstructures formed by laterally assembled cylindrical fibers. Simulations starting from bilayer structures demonstrate the formation of filaments, whereas other simulations starting from filaments indicate varying degrees of interaction among them depending on chemical structure. These interactions are verified by observations using atomic force microscopy of the various systems. The interfilament interactions are predicted to be strongest in supramolecular assemblies that display hydrophilic groups on their surfaces, while those with hydrophobic ones are predicted to interact more weakly as confirmed by viscosity measurements. The simulations also suggest that peptide amphiphiles with hydrophobic termini bend to reduce their interfacial energy with water, which may explain why these systems do not collapse into superstructures of bundled filaments. The simulations suggest that future experiments will need to address mechanistic questions about the self-assembly of these systems into hierarchical structures, namely, the preformation of interactive filaments vs equilibration of large assemblies into superstructures.

Supramolecular Interactions and Morphology of Self-Assembling Peptide Amphiphile Nanostructures

The morphology of supramolecular peptide nanostructures is difficult to predict given their complex energy landscapes. We investigated peptide amphiphiles containing β-sheet forming domains that form twisted nanoribbons in water. We explained the morphology based on a balance between the energetically favorable packing of molecules in the center of the nanostructures, the unfavorable packing at the edges, and the deformations due to packing of twisted β-sheets. We find that morphological polydispersity of PA nanostructures is determined by peptide sequences, and the twisting of their internal β-sheets. We also observed a change in the supramolecular chirality of the nanostructures as the peptide sequence was modified, although only amino acids with l-configuration were used. Upon increasing charge repulsion between molecules, we observed a change in morphology to long cylinders and then rodlike fragments and spherical micelles. Understanding the self-assembly mechanisms of peptide amphiphiles into nanostructures should be useful to optimize their well-known functions.

Hybrid gels via bulk interfacial complexation of supramolecular polymers and polyelectrolytes

Hierarchical self-assembly leading to organized supramolecular structures across multiple length scales has been of great recent interest. Earlier work from our laboratory reported the complexation of peptide amphiphile (PA) supramolecular polymers with oppositely charged polyelectrolytes into a single solid membrane at a macroscopic interface. We report here the formation of bulk gels with many internal interfaces between the covalent and supramolecular polymer components formed by the rapid chaotic mixing of solutions, one containing negatively charged PA nanofibers and the other the positively charged biopolymer chitosan. We found that formation of a contact layer at the interface of the solutions locks the formation of hydrogels with lamellar microstructure. The nanofiber morphology of the supramolecular polymer is essential to this process since gels do not form when solutions of supramolecular assemblies form spherical micelles. We found that rheological properties of the gels can be tuned by changing the relative amounts of each component. Furthermore, both positively and negatively charged proteins are easily encapsulated within the contact layer of the gel, which provides an interesting biomedical function for these systems.

Growth of Extra-Large Chromophore Supramolecular Polymers for Enhanced Hydrogen Production

The control of morphology in bioinspired chromophore assemblies is key to the rational design of functional materials for light harvesting. We investigate here morphological changes in perylene monoimide chromophore assemblies during thermal annealing in aqueous environments of high ionic strength to screen electrostatic repulsion. We found that annealing under these conditions leads to the growth of extra-large ribbon-shaped crystalline supramolecular polymers of widths from about 100 nm to several micrometers and lengths from 1 to 10 μm while still maintaining a unimolecular thickness. This growth process was monitored by variable-temperature absorbance spectroscopy, synchrotron X-ray scattering, and confocal microscopy. The extra-large single-crystal-like supramolecular polymers are highly porogenic, thus creating loosely packed hydrogel scaffolds that showed greatly enhanced photocatalytic hydrogen production with turnover numbers as high as 13 500 over ∼110 h compared to 7500 when smaller polymers are used. Our results indicate great functional opportunities in thermally and pathway-controlled supramolecular polymerization.

Self-sorting in supramolecular assemblies

Supramolecular self-assembly enables living organisms to form highly functional hierarchical structures with individual components self-organized across multiple length scales. This has inspired work on multicomponent supramolecular materials to understand factors behind co-assembly versus self-sorting of molecules. We report here on a supramolecular system comprised of negatively charged peptide amphiphile (PA) molecules, in which only a tiny fraction of the molecules (0.7 mol%) were covalently conjugated to one of two different fluorophores, half to fluorescein isothiocyanate (FTIC) and the other half to tetramethylrhodamine (TAMRA). Confocal microscopy of the system revealed self-sorting of the two different fluorescent PA molecules, where TAMRA PA is concentrated in micron-scale domains while FITC PA remains dispersed throughout the sample. From Förster resonance energy transfer and fluorescence recovery experiments, we conclude that conjugation of the negatively charged FITC to PA significantly disrupts its co-assembly with the 99.3 mol% of unlabeled molecules, which are responsible for formation of micron-scale domains. Conversely, conjugation of the zwitterionic TAMRA causes no such disruption. Interestingly, this dissimilar behavior between FITC and TAMRA PA causes them to self-sort at large length scales in the supramolecular system, mediated not by specific interactions among the individual fluorophores but instead by their different propensities to co-assemble with the majority component. We also found that greater ionic strength in the aqueous environment of the system promotes mixing by lowering the electrostatic barriers involved in self-sorting. Our results demonstrate great thermodynamic subtlety in the driving forces that mediate self-sorting versus co-assembly in supramolecular peptide assemblies.

Supramolecular-covalent hybrid polymers for light-activated mechanical actuation

The development of synthetic structures that mimic mechanical actuation in living matter such as autonomous translation and shape changes remains a grand challenge for materials science. In living systems the integration of supramolecular structures and covalent polymers contributes to the responsive behaviour of membranes, muscles and tendons, among others. Here we describe hybrid light-responsive soft materials composed of peptide amphiphile supramolecular polymers chemically bonded to spiropyran-based networks that expel water in response to visible light. The supramolecular polymers form a reversibly deformable and water-draining skeleton that mechanically reinforces the hybrid and can also be aligned by printing methods. The noncovalent skeleton embedded in the network thus enables faster bending and flattening actuation of objects, as well as longer steps during the light-driven crawling motion of macroscopic films. Our work suggests that hybrid bonding polymers, which integrate supramolecular assemblies and covalent networks, offer strategies for the bottom-up design of soft matter that mimics living organisms.

Development of Optimized Tissue-Factor-Targeted Peptide Amphiphile Nanofibers to Slow Noncompressible Torso Hemorrhage

Noncompressible torso hemorrhage accounts for a significant portion of preventable trauma deaths. We report here on the development of injectable, targeted supramolecular nanotherapeutics based on peptide amphiphile (PA) molecules that are designed to target tissue factor (TF) and, therefore, selectively localize to sites of injury to slow hemorrhage. Eight TF-targeting sequences were identified, synthesized into PA molecules, coassembled with nontargeted backbone PA at various weight percentages, and characterized via circular dichroism spectroscopy, transmission electron microscopy, and X-ray scattering. Following intravenous injection in a rat liver hemorrhage model, two of these PA nanofiber coassemblies exhibited the most specific localization to the site of injury compared to controls (p < 0.05), as quantified using immunofluorescence imaging of injured liver and uninjured organs. To determine if the nanofibers were targeting TF in vivo, a mouse saphenous vein laser injury model was performed and showed that TF-targeted nanofibers colocalized with fibrin, demonstrating increased levels of nanofiber at TF-rich sites. Thromboelastograms obtained using samples of heparinized rat whole blood containing TF demonstrated that no clots were formed in the absence of TF-targeted nanofibers. Lastly, both PA nanofiber coassemblies decreased blood loss in comparison to sham and backbone nanofiber controls by 35-59% (p < 0.05). These data demonstrate an optimal TF-targeted nanofiber that localizes selectively to sites of injury and TF exposure, and, interestingly, reduces blood loss. This research represents a promising initial phase in the development of a TF-targeted injectable therapeutic to reduce preventable deaths from hemorrhage.

Self-Repair of Structure and Bioactivity in a Supramolecular Nanostructure

Supramolecular nanostructures formed through self-assembly can have energy landscapes, which determine their structures and functions depending on the pathways selected for their synthesis and processing and on the conditions they are exposed to after their initial formation. We report here on the structural damage that occurs in supramolecular peptide amphiphile nanostructures, during freezing in aqueous media, and the self-repair pathways that restore their functions. We found that freezing converts long supramolecular nanofibers into shorter ones, compromising their ability to support cell adhesion, but a single heating and cooling cycle reverses the damage and rescues their bioactivity. Thermal energy in this cycle enables noncovalent interactions to reconfigure the nanostructures into the thermodynamically preferred long nanofibers, a repair process that is impeded by kinetic traps. In addition, we found that nanofibers disrupted during freeze-drying also exhibit the ability to undergo thermal self-repair and recovery of their bioactivity, despite the extra disruption caused by the dehydration step. Following both freezing and freeze-drying, which shorten the 1D nanostructures, their self-repair capacity through thermally driven elongation is inhibited by kinetically trapped states, which contain highly stable noncovalent interactions that are difficult to rearrange. These states decrease the extent of thermal nanostructure repair, an observation we hypothesize applies to supramolecular systems in general and is mechanistically linked to suppressed molecular exchange dynamics.

Peptide supramolecular materials for therapeutics

Supramolecular assembly of peptide-based monomers into nanostructures offers many promising applications in advanced therapies. In this Tutorial Review, we introduce molecular designs to control the structure and potential biological function of supramolecular assemblies. An emphasis is placed on peptide-based supramolecular nanostructures that are intentionally designed to signal cells, either directly through the incorporation of amino acid sequences that activate receptors or indirectly by recruiting native signals such as growth factors. Additionally, we describe the use and future potential of hierarchical structures, such as single molecules that assemble into nanoscale fibers which then align to form macroscopic strings; the strings can then serve as scaffolds for cell growth, proliferation, and differentiation.

Covalent-supramolecular hybrid polymers as muscle-inspired anisotropic actuators

Skeletal muscle provides inspiration on how to achieve reversible, macroscopic, anisotropic motion in soft materials. Here we report on the bottom-up design of macroscopic tubes that exhibit anisotropic actuation driven by a thermal stimulus. The tube is built from a hydrogel in which extremely long supramolecular nanofibers are aligned using weak shear forces, followed by radial growth of thermoresponsive polymers from their surfaces. The hierarchically ordered tube exhibits reversible anisotropic actuation with changes in temperature, with much greater contraction perpendicular to the direction of nanofiber alignment. We identify two critical factors for the anisotropic actuation, macroscopic alignment of the supramolecular scaffold and its covalent bonding to polymer chains. Using finite element analysis and molecular calculations, we conclude polymer chain confinement and mechanical reinforcement by rigid supramolecular nanofibers are responsible for the anisotropic actuation. The work reported suggests strategies to create soft active matter with molecularly encoded capacity to perform complex tasks.

Supramolecular Assembly of Peptide Amphiphiles

Peptide amphiphiles (PAs) are small molecules that contain hydrophobic components covalently conjugated to peptides. In this Account, we describe recent advances involving PAs that consist of a short peptide sequence linked to an aliphatic tail. The peptide sequence can be designed to form β-sheets among the amino acids near the alkyl tail, while the residues farthest from the tail are charged to promote solubility and in some cases contain a bioactive sequence. In water, β-sheet formation and hydrophobic collapse of the aliphatic tails induce assembly of the molecules into supramolecular one-dimensional nanostructures, commonly high-aspect-ratio cylindrical or ribbonlike nanofibers. These nanostructures hold significant promise for biomedical functions due to their ability to display a high density of biological signals on their surface for targeting or to activate pathways, as well as for biocompatibility and biodegradable nature.

Recent studies have shown that supramolecular systems, such as PAs, often become kinetically trapped in local minima along their self-assembly reaction coordinate, not unlike the pathways associated with protein folding. Furthermore, the assembly pathway can influence the shape, internal structure, and dimension of nanostructures and thereby affect their bioactivity. We discuss methods to map the energy landscape of a PA structure as a function of thermal energy and ionic strength and vary these parameters to convert between kinetically trapped and thermodynamically favorable states. We also demonstrate that the pathway-dependent morphology of the PA assembly can determine biological cell adhesion and survival rates.

The dynamics associated with the nanostructures are also critical to their function, and techniques are now available to probe the internal dynamics of these nanostructures. For example, by conjugating radical electron spin labels to PAs, electron paramagnetic resonance spectroscopy can be used to study the rotational diffusion rates within the fiber, showing a liquidlike to solidlike transition through the cross section of the nanofiber. PAs can also be labeled with fluorescent dyes, allowing the use of super-resolution microscopy techniques to study the molecular exchange dynamics between PA fibers. For a weak hydrogen-bonding PA, individual PA molecules or clusters exchange between fibers in time scales as short as minutes. The amount of hydrogen bonding within PAs that dictates the dynamics also plays an important role in biological function. In one case, weak hydrogen bonding within a PA resulted in cell death through disruption of lipid membranes, while in another example reduced hydrogen bonding enhanced growth factor signaling by increasing lipid raft mobility.

PAs are a promising platform for designing advanced hybrid materials. We discuss a covalent polymer with a rigid aromatic imine backbone and alkylated peptide side chains that simultaneously polymerizes and interacts with a supramolecular PA structure with identical chemistry to that of the side chains. The covalent polymerization can be “catalyzed” by noncovalent polymerization of supramolecular monomers, taking advantage of the dynamic nature of supramolecular assemblies. These novel hybrid structures have potential in self-repairing materials and as reusable scaffolds for delivery of drugs or other chemicals. Finally, we highlight recent biomedical applications of PAs and related structures, ranging from bone regeneration to decreasing blood loss during internal bleeding.

Programmable Assembly of Peptide Amphiphile via Noncovalent-to-Covalent Bond Conversion

Controlling the number of monomers in a supramolecular polymer has been a great challenge in programmable self-assembly of organic molecules. One approach has been to make use of frustrated growth of the supramolecular assembly by tuning the balance of attractive and repulsive intermolecular forces. We report here on the use of covalent bond formation among monomers, compensating for intermolecular electrostatic repulsion, as a mechanism to control the length of a supramolecular nanofiber formed by self-assembly of peptide amphiphiles. Circular dichroism spectroscopy in combination with dynamic light scattering, size-exclusion chromatography, and transmittance electron microscope analyses revealed that hydrogen bonds between peptides were reinforced by covalent bond formation, enabling the fiber elongation. To examine these materials for their potential biomedical applications, cytotoxicity of nanofibers against C2C12 premyoblast cells was tested. We demonstrated that cell viability increased with an increase in fiber length, presumably because of the suppressed disruption of cell membranes by the fiber end-caps.

Crystal-Phase Transitions and Photocatalysis in Supramolecular Scaffolds

The energy landscape of a supramolecular material can include different molecular packing configurations that differ in stability and function. We report here on a thermally driven crystalline order transition in the landscape of supramolecular nanostructures formed by charged chromophore amphiphiles in salt-containing aqueous solutions. An irreversible transition was observed from a metastable to a stable crystal phase within the nanostructures. In the stable crystalline phase, the molecules end up organized in a short scroll morphology at high ionic strengths and as long helical ribbons at lower salt content. This is interpreted as the result of the competition between electrostatic repulsive forces and attractive molecular interactions. Only the stable phase forms charge-transfer excitons upon exposure to visible light as indicated by absorbance and fluorescence features, second-order harmonic generation microscopy, and femtosecond transient absorbance spectroscopy. Interestingly, the supramolecular reconfiguration to the stable crystalline phase nanostructures enhances photosensitization of a proton reduction catalyst for hydrogen production.

Electrostatic Control of Polymorphism in Charged Amphiphile Assemblies

Stimuli-induced structural transformations of molecular assemblies in aqueous solutions are integral to nanotechnological applications and biological processes. In particular, pH responsive amphiphiles as well as proteins with various degrees of ionization can reconfigure in response to pH variations. Here, we use in situ small and wide-angle X-ray scattering (SAXS/WAXS), transmission electron microscopy (TEM), and Monte Carlo simulations to show how charge regulation via pH induces morphological changes in the assembly of a positively charged peptide amphiphile (PA). Monte Carlo simulations and pH titration measurements reveal that ionic correlations in the PA assemblies shift the ionizable amine pK ∼ 8 from pK ∼ 10 in the lysine headgroup. SAXS and TEM show that with increasing pH, the assembly undergoes spherical micelle to cylindrical nanofiber to planar bilayer transitions. SAXS/WAXS reveal that the bilayer leaflets are interdigitated with the tilted PA lipid tails crystallized on a rectangular lattice. The details of the molecular packing in the membrane result from interplay between steric and van der Waals interactions. We speculate that this packing motif is a general feature of bilayers comprised of amphiphilic lipids with large ionic headgroups. Overall, our studies correlate the molecular charge and the morphology for a pH-responsive PA system and provide insights into the Å-scale molecular packing in such assemblies.

Asymmetric Peptide Nanoribbons

Asymmetry in chemical structure or shape at molecular, nanoscale, or microscopic levels is essential to a vast number of functionalities in both natural and artificial systems. Bottom-up approaches to create asymmetric supramolecular nanostructures are considered promising but this strategy suffers from the potentially dynamic nature of noncovalent interactions. We report here on supramolecular self-assembly of asymmetric peptide amphiphiles consisting of two different molecularly linked domains. We found that strong noncovalent interactions and a high degree of internal order among the asymmetric amphiphiles lead to nanoribbons with asymmetric faces due to the preferential self-association of the two domains. The capture of gold nanoparticles on only one face of the nanoribbons demonstrates symmetry breaking in these supramolecular structures.

Energy landscapes and functions of supramolecular systems

By means of two supramolecular systems--peptide amphiphiles engaged in hydrogen-bonded β-sheets, and chromophore amphiphiles driven to assemble by π-orbital overlaps--we show that the minima in the energy landscapes of supramolecular systems are defined by electrostatic repulsion and the ability of the dominant attractive forces to trap molecules in thermodynamically unfavourable configurations. These competing interactions can be selectively switched on and off, with the order of doing so determining the position of the final product in the energy landscape. Within the same energy landscape, the peptide-amphiphile system forms a thermodynamically favoured product characterized by long bundled fibres that promote biological cell adhesion and survival, and a metastable product characterized by short monodisperse fibres that interfere with adhesion and can lead to cell death. Our findings suggest that, in supramolecular systems, functions and energy landscapes are linked, superseding the more traditional connection between molecular design and function.

Simultaneous covalent and noncovalent hybrid polymerizations

Covalent and supramolecular polymers are two distinct forms of soft matter, composed of long chains of covalently and noncovalently linked structural units, respectively. We report a hybrid system formed by simultaneous covalent and supramolecular polymerizations of monomers. The process yields cylindrical fibers of uniform diameter that contain covalent and supramolecular compartments, a morphology not observed when the two polymers are formed independently. The covalent polymer has a rigid aromatic imine backbone with helicoidal conformation, and its alkylated peptide side chains are structurally identical to the monomer molecules of supramolecular polymers. In the hybrid system, covalent chains grow to higher average molar mass relative to chains formed via the same polymerization in the absence of a supramolecular compartment. The supramolecular compartments can be reversibly removed and re-formed to reconstitute the hybrid structure, suggesting soft materials with novel delivery or repair functions.

Supramolecular Packing Controls H₂ Photocatalysis in Chromophore Amphiphile Hydrogels

Light harvesting supramolecular assemblies are potentially useful structures as components of solar-to-fuel conversion materials. The development of these functional constructs requires an understanding of optimal packing modes for chromophores. We investigated here assembly in water and the photocatalytic function of perylene monoimide chromophore amphiphiles with different alkyl linker lengths separating their hydrophobic core and the hydrophilic carboxylate headgroup. We found that these chromophore amphiphiles (CAs) self-assemble into charged nanostructures of increasing aspect ratio as the linker length is increased. The addition of salt to screen the charged nanostructures induced the formation of hydrogels and led to internal crystallization within some of the nanostructures. For linker lengths up to seven methylenes, the CAs were found to pack into 2D crystalline unit cells within ribbon-shaped nanostructures, whereas the nine methylene CAs assembled into long nanofibers without crystalline molecular packing. At the same time, the different molecular packing arrangements after charge screening led to different absorbance spectra, despite the identical electronic properties of all PMI amphiphiles. While the crystalline CAs formed electronically coupled H-aggregates, only CAs with intermediate linker lengths showed evidence of high intermolecular orbital overlap. Photocatalytic hydrogen production using a nickel-based catalyst was observed in all hydrogels, with the highest turnovers observed for CA gels having intermediate linker lengths. We conclude that the improved photocatalytic performance of the hydrogels formed by supramolecular assemblies of the intermediate linker CA molecules likely arises from improved exciton splitting efficiencies due to their higher orbital overlap.

Internal dynamics of a supramolecular nanofibre

A large variety of functional self-assembled supramolecular nanostructures have been reported over recent decades. The experimental approach to these systems initially focused on the design of molecules with specific interactions that lead to discrete geometric structures, and more recently on the kinetics and mechanistic pathways of self-assembly. However, there remains a major gap in our understanding of the internal conformational dynamics of these systems and of the links between their dynamics and function. Molecular dynamics simulations have yielded information on the molecular fluctuations of supramolecular assemblies, yet experimentally it has been difficult to obtain analogous data with subnanometre spatial resolution. Using site-directed spin labelling and electron paramagnetic resonance spectroscopy, we measured the conformational dynamics of a self-assembled nanofibre in water through its 6.7 nm cross-section. Our measurements provide unique insight for the design of supramolecular functional materials.

Self-assembly of biomolecular soft matter

Self-assembly programmed by molecular structure and guided dynamically by energy dissipation is a ubiquitous phenomenon in biological systems that build functional structures from the nanoscale to macroscopic dimensions. This paper describes examples of one-dimensional self-assembly of peptide amphiphiles and the consequent biological functions that emerge in these systems. We also discuss here hierarchical self-assembly of supramolecular peptide nanostructures and polysaccharides, and some new results are reported on supramolecular crystals formed by highly charged peptide amphiphiles. Reflecting on presentations at this Faraday Discussion, the paper ends with a discussion of some of the future opportunities and challenges of the field.

Self-assembly of highly ordered peptide amphiphile metalloporphyrin arrays

Long fibers assembled from peptide amphiphiles capable of binding the metalloporphyrin zinc protoporphyrin IX ((PPIX)Zn) have been synthesized. Rational peptide design was employed to generate a peptide, c16-AHL(3)K(3)-CO(2)H, capable of forming a β-sheet structure that propagates into larger fibrous structures. A porphyrin-binding site, a single histidine, was engineered into the peptide sequence in order to bind (PPIX)Zn to provide photophysical functionality. The resulting system indicates control from the molecular level to the macromolecular level with a high order of porphyrin organization. UV/visible and circular dichroism spectroscopies were employed to detail molecular organization, whereas electron microscopy and atomic force microscopy aided in macromolecular characterization. Preliminary picosecond transient absorption data are also reported. Reduced hemin, (PPIX)Fe(II), was also employed to highlight the material's versatility and tunability.

Grooved nanowires from self-assembling hairpin molecules for solar cells

One of the challenges facing bulk heterojunction organic solar cells is obtaining organized films during the phase separation of intimately mixed donor and acceptor components. We report here on the use of hairpin-shaped sexithiophene molecules to generate by self-assembly grooved nanowires as the donor component in bulk heterojunction solar cells. Photovoltaic devices were fabricated via spin-casting to produce by solvent evaporation a percolating network of self-assembled nanowires and fullerene acceptors. Thermal annealing was found to increase power conversion efficiencies by promoting domain growth while still maintaining this percolating network of nanostructures. The benefits of self-assembly and grooved nanowires were examined by building devices from a soluble sexithiophene derivative that does not form one-dimensional structures. In these systems, excessive phase separation caused by thermal annealing leads to the formation of defects and lower device efficiencies. We propose that the unique hairpin shape of the self-assembling molecules allows the nanowires as they form to interact well with the fullerenes in receptor-ligand type configurations at the heterojunction of the two domains, thus enhancing device efficiencies by 23%.

A self-assembly pathway to aligned monodomain gels

Aggregates of charged amphiphilic molecules have been found to access a structure at elevated temperature that templates alignment of supramolecular fibrils over macroscopic scales. The thermal pathway leads to a lamellar plaque structure with fibrous texture that breaks on cooling into large arrays of aligned nanoscale fibres and forms a strongly birefringent liquid. By manually dragging this liquid crystal from a pipette onto salty media, it is possible to extend this alignment over centimetres in noodle-shaped viscoelastic strings. Using this approach, the solution of supramolecular filaments can be mixed with cells at physiological temperatures to form monodomain gels of aligned cells and filaments. The nature of the self-assembly process and its biocompatibility would allow formation of cellular wires in situ that have any length and customized peptide compositions for use in biological applications.

Molecular self-assembly into one-dimensional nanostructures

Self-assembly of small molecules into one-dimensional nanostructures offers many potential applications in electronically and biologically active materials. The recent advances discussed in this Account demonstrate how researchers can use the fundamental principles of supramolecular chemistry to craft the size, shape, and internal structure of nanoscale objects. In each system described here, we used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to study the assembly morphology. Circular dichroism, nuclear magnetic resonance, infrared, and optical spectroscopy provided additional information about the self-assembly behavior in solution at the molecular level. Dendron rod-coil molecules self-assemble into flat or helical ribbons. They can incorporate electronically conductive groups and can be mineralized with inorganic semiconductors. To understand the relative importance of each segment in forming the supramolecular structure, we synthetically modified the dendron, rod, and coil portions. The self-assembly depended on the generation number of the dendron, the number of hydrogen-bonding functions, and the length of the rod and coil segments. We formed chiral helices using a dendron-rod-coil molecule prepared from an enantiomerically enriched coil. Because helical nanostructures are important targets for use in biomaterials, nonlinear optics, and stereoselective catalysis, researchers would like to precisely control their shape and size. Tripeptide-containing peptide lipid molecules assemble into straight or twisted nanofibers in organic solvents. As seen by AFM, the sterics of bulky end groups can tune the helical pitch of these peptide lipid nanofibers in organic solvents. Furthermore, we demonstrated the potential for pitch control using trans-to-cis photoisomerization of a terminal azobenzene group. Other molecules called peptide amphiphiles (PAs) are known to assemble in water into cylindrical nanostructures that appear as nanofiber bundles. Surprisingly, TEM of a PA substituted by a nitrobenzyl group revealed assembly into quadruple helical fibers with a braided morphology. Upon photocleavage of this the nitrobenzyl group, the helices transform into single cylindrical nanofibers. Finally, inspired by the tobacco mosaic virus, we used a dumbbell-shaped, oligo(phenylene ethynylene) template to control the length of a PA nanofiber self-assembly (<10 nm). AFM showed complete disappearance of long nanofibers in the presence of this rigid-rod template. Results from quick-freeze/deep-etch TEM and dynamic light scattering demonstrated the templating behavior in aqueous solution. This strategy could provide a general method to control size the length of nonspherical supramolecular nanostructures.