25th anniversary article: supramolecular materials for regenerative medicine

Biopolymers and supramolecular polymers as biomaterials for biomedical applications

Protein- and peptide-based structural biopolymers are abundant building blocks of biological systems. Either in their natural forms, such as collagen, silk or fibronectin, or as related synthetic materials they can be used in various technologies. An emerging area is that of biomimetic materials inspired by protein-based biopolymers, which are made up of small molecules rather than macromolecules and can therefore be described as supramolecular polymers. These materials are very useful in biomedical applications because of their ability to imitate the extracellular matrix both in architecture and their capacity to signal cells. This article describes important features of the natural extracellular matrix and highlight how these features are being incorporated into biomaterials composed of biopolymers and supramolecular polymers. We particularly focus on the structures, properties, and functions of collagen, fibronectin, silk, and the supramolecular polymers inspired by them as biomaterials for regenerative medicine.

Advances in Functional Assemblies for Regenerative Medicine

The ability to synthesise bioresponsive systems and selectively active biochemistries using polymer-based materials with supramolecular features has led to a surge in research interest directed towards their development as next generation biomaterials for drug delivery, medical device design and tissue engineering.

Biocompatible fluorescent supramolecular nanofibrous hydrogel for long-term cell tracking and tumor imaging applications

Biocompatible peptide-based supramolecular hydrogel has recently emerged as a new and promising system for biomedical applications. In this work, Rhodamine B is employed as a new capping group of self-assembling peptide, which not only provides the driving force for supramolecular nanofibrous hydrogel formation, but also endows the hydrogel with intrinsic fluroescence signal, allowing for various bioimaging applications. The fluorescent peptide nanofibrous hydrogel can be formed via disulfide bond reduction. After dilution of the hydrogel with aqueous solution, the fluorescent nanofiber suspension can be obtained. The resultant nanofibers are able to be internalized by the cancer cells and effectively track the HeLa cells for as long as 7 passages. Using a tumor-bearing mouse model, it is also demonstrated that the fluorescent supramolecular nanofibers can serve as an efficient probe for tumor imaging in a high-contrast manner.

Kinetic Analysis as a Tool to Distinguish Pathway Complexity in Molecular Assembly: An Unexpected Outcome of Structures in Competition

While the sensitive dependence of the functional characteristics of self-assembled nanofibers on the molecular structure of their building blocks is well-known, the crucial influence of the dynamics of the assembly process is often overlooked. For natural protein-based fibrils, various aggregation mechanisms have been demonstrated, from simple primary nucleation to secondary nucleation and off-pathway aggregation. Similar pathway complexity has recently been described in synthetic supramolecular polymers and has been shown to be intimately linked to their morphology. We outline a general method to investigate the consequences of the presence of multiple assembly pathways, and show how kinetic analysis can be used to distinguish different assembly mechanisms. We illustrate our combined experimental and theoretical approach by studying the aggregation of chiral bipyridine-extended 1,3,5-benzenetricarboxamides (BiPy-1) in n-butanol as a model system. Our workflow consists of nonlinear least-squares analysis of steady-state spectroscopic measurements, which cannot provide conclusive mechanistic information but yields the equilibrium constants of the self-assembly process as constraints for subsequent kinetic analysis. Furthermore, kinetic nucleation-elongation models based on one and two competing pathways are used to interpret time-dependent spectroscopic measurements acquired using stop-flow and temperature-jump methods. Thus, we reveal that the sharp transition observed in the aggregation process of BiPy-1 cannot be explained by a single cooperative pathway, but can be described by a competitive two-pathway mechanism. This work provides a general tool for analyzing supramolecular polymerizations and establishing energetic landscapes, leading to mechanistic insights that at first sight may seem unexpected and counterintuitive.

Bio-fabrication and physiological self-release of tissue equivalents using smart peptide amphiphile templates

In this study we applied a smart biomaterial formed from a self-assembling, multi-functional synthetic peptide amphiphile (PA) to coat substrates with various surface chemistries. The combination of PA coating and alignment-inducing functionalised substrates provided a template to instruct human corneal stromal fibroblasts to adhere, become aligned and then bio-fabricate a highly-ordered, multi-layered, three-dimensional tissue by depositing an aligned, native-like extracellular matrix. The newly-formed corneal tissue equivalent was subsequently able to eliminate the adhesive properties of the template and govern its own complete release via the action of endogenous proteases. Tissues recovered through this method were structurally stable, easily handled, and carrier-free. Furthermore, topographical and mechanical analysis by atomic force microscopy showed that tissue equivalents formed on the alignment-inducing PA template had highly-ordered, compact collagen deposition, with a two-fold higher elastic modulus compared to the less compact tissues produced on the non-alignment template, the PA-coated glass. We suggest that this technology represents a new paradigm in tissue engineering and regenerative medicine, whereby all processes for the bio-fabrication and subsequent self-release of natural, bio-prosthetic human tissues depend solely on simple template-tissue feedback interactions.

Multifarious facets of sugar-derived molecular gels: molecular features, mechanisms of self-assembly and emerging applications

The remarkable capability of nature to design and create excellent self-assembled nano-structures, especially in the biological world, has motivated chemists to mimic such systems with synthetic molecular and supramolecular systems. The hierarchically organized self-assembly of low molecular weight gelators (LMWGs) based on non-covalent interactions has been proven to be a useful tool in the development of well-defined nanostructures. Among these, the self-assembly of sugar-derived LMWGs has received immense attention because of their propensity to furnish biocompatible, hierarchical, supramolecular architectures that are macroscopically expressed in gel formation. This review sheds light on various aspects of sugar-derived LMWGs, uncovering their mechanisms of gelation, structural analysis, and tailorable properties, and their diverse applications such as stimuli-responsiveness, sensing, self-healing, environmental problems, and nano and biomaterials synthesis.

Biotin-Avidin Based Universal Cell-Matrix Interaction for Promoting Three-Dimensional Cell Adhesion

To promote cell adhesion in three-dimensional (3D) extracellular matrix (ECM) is crucial for avoiding cell anoikis, which is one of the most important issues for fundamental cell biology. Herein, a biotin-avidin based universal cell-matrix interaction for different types of cells is developed in order to achieve the promoted adhesion in 3D ECM. For the purpose, biotinylated nanofibrous hydrogels are constructed by coassembling 1,4-benzyldicarboxamide (C2) based non-biotinylated and biotinylated supramolecular gelators. The used cells are modified by avidin (AV-cells) through biotinylating cells and then interacting with avidin. After in situ encapsulating AV-cells in the hydrogels, the adhered amount can be increased by tens of percent even with adding several percentages of the biotinylated C2 gelators in the coassembly due to the specific biotin-avidin interaction. Reverse transcription polymerase chain reaction (RT-PCR) confirms that AV-cells can proliferate without varying gene expression and denaturation. Compared with the interaction between RGD and cells, this avidin-biotin interaction should be much more universal and it is feasible to be employed to promote cell adhesion for most types of cells in 3D matrix.

Supramolecular Surface Immobilization of Knottin Derivatives for Dynamic Display of High Affinity Binders

Knottins are known as a robust and versatile class of miniprotein scaffolds for the presentation of high-affinity binding peptides; however, to date their application in biomaterials, biological coatings, and surface applications have not been explored. We have developed a strategy to recombinantly synthesize a β-trypsin inhibitory knottin with supramolecular guest tags that enable it to adhere to self-assembled monolayers of the supramolecular host cucurbit[8]uril (CB[8]). We have described a strategy to easily express knottins in E. coli by conjugating them to a fluorescent protein after which they are cleaved and purified. Knottin constructs that varied in the number and position of the supramolecular tag at either the N- or C-termini or at both ends have been verified for their trypsin inhibitory function and CB[8]-binding properties in solution and on surfaces. All of the knottin constructs showed strong inhibition of trypsin with inhibition constants between 10 and 30 nM. Using microscale thermophoresis, we determined that the supramolecular guest tags on the knottins bind CB[8] with a Kd of ∼6 μM in solution. At the surface, strong divalent binding has been determined with a Kd of 0.75 μM in the case of the knottin with two supramolecular guest tags, whereas only weak monovalent binding occurred when only one guest tag was present. We also show successful supramolecular surface immobilization of the knottin using CB[8] and prove that they can be used to immobilize β-trypsin at the surface.

Peptide self-assembly for nanomaterials: the old new kid on the block

Peptide self-assembly is an increasingly attractive tool for nanomaterials. Perfected in biology peptide self-assembling systems have impacted on nearly any conceivable nanomaterial type. However, with all the information available to us commercialisation of peptide materials remains in its infancy. In an attempt to better understand the reasons behind this shortcoming we categorise peptide self-assembled materials in relation to their non-peptide counterparts. A particular emphasis is placed on the versatility of peptide self-assembly in terms of modularity, responsiveness and functional diversity, which enables direct comparisons with more traditional material chemistries.

Beta-Sheet-Forming, Self-Assembled Peptide Nanomaterials towards Optical, Energy, and Healthcare Applications

Peptide self-assembly is an attractive route for the synthesis of intricate organic nanostructures that possess remarkable structural variety and biocompatibility. Recent studies on peptide-based, self-assembled materials have expanded beyond the construction of high-order architectures; they are now reporting new functional materials that have application in the emerging fields such as artificial photosynthesis and rechargeable batteries. Nevertheless, there have been few reviews particularly concentrating on such versatile, emerging applications. Herein, recent advances in the synthesis of self-assembled peptide nanomaterials (e.g., cross β-sheet-based amyloid nanostructures, peptide amphiphiles) are selectively reviewed and their new applications in diverse, interdisciplinary fields are described, ranging from optics and energy storage/conversion to healthcare. The applications of peptide-based self-assembled materials in unconventional fields are also highlighted, such as photoluminescent peptide nanostructures, artificial photosynthetic peptide nanomaterials, and lithium-ion battery components. The relation of such functional materials to the rapidly progressing biomedical applications of peptide self-assembly, which include biosensors/chips and regenerative medicine, are discussed. The combination of strategies shown in these applications would further promote the discovery of novel, functional, small materials.

Hydrogen-Bonded Multifunctional Supramolecular Copolymers in Water

We have investigated the self-assembly in water of molecules having a single hydrophobic bis-urea domain linked to different hydrophilic functional side chains, i.e., bioactive peptidic residues and fluorescent cyanine dyes. By using a combination of spectroscopy, scattering, and microscopy techniques, we show that each one of these molecules can individually produce well-defined nanostructures such as twisted ribbons, two-dimensional plates, or branched fibers. Interestingly, when these monomers of different functionalities are mixed in an equimolar ratio, supramolecular copolymers are preferred to narcissistic segregation. Radiation scattering and imaging techniques demonstrate that one of the molecular units dictates the formation of a preferential nanostructure, and optical spectroscopies reveal the alternated nature of the copolymerization process. This work illustrates how social self-sorting in H-bond supramolecular polymers can give straightforward access to multifunctional supramolecular copolymers.

Rapid Self-Integrating, Injectable Hydrogel for Tissue Complex Regeneration

A novel rapid self-integrating, injectable, and bioerodible hydrogel is developed for bone-cartilage tissue complex regeneration. The hydrogels are able to self-integrate to form various structures, as can be seen after dying some hydrogel disks pink with rodamine. This hydrogel is demonstrated to engineer cartilage-bone complex.

New self-assembling multifunctional templates for the biofabrication and controlled self-release of cultured tissue

The need to source live human tissues for research and clinical applications has been a major driving force for the development of new biomaterials. Ideally, these should elicit the formation of scaffold-free tissues with native-like structure and composition. In this study, we describe a biologically interactive coating that combines the fabrication and subsequent self-release of live purposeful tissues using template-cell-environment feedback. This smart coating was formed from a self-assembling peptide amphiphile comprising a protease-cleavable sequence contiguous with a cell attachment and signaling motif. This multifunctional material was subsequently used not only to instruct human corneal or skin fibroblasts to adhere and deposit discreet multiple layers of native extracellular matrix but also to govern their own self-directed release from the template solely through the action of endogenous metalloproteases. Tissues recovered through this physiologically relevant process were carrier-free and structurally and phenotypically equivalent to their natural counterparts. This technology contributes to a new paradigm in regenerative medicine, whereby materials are able to actively direct and respond to cell behavior. The novel application of such materials as a coating capable of directing the formation and detachment of complex tissues solely under physiological conditions can have broad use for fundamental research and in future cell and tissue therapies.

Drug delivery with nanospherical supramolecular cell penetrating peptide-taxol conjugates containing a high drug loading

HYPOTHESIS

Supramolecular nanostructures via small molecule self-assembly hold great promise for controlled delivery of hydrophobic anticancer drugs. Particularly, taxol has recently been discovered to possess excellent self-assembly property, which may provide new opportunities to develop a new class of functional supramolecular nanomaterials for drug delivery application.

EXPERIMENTS

A cell penetrating peptide (CPP)-taxol conjugate (Taxol-CPP) was designed and synthesized. The self-assembling property of Taxol-CPP was investigated and the resultant nanomaterials were well characterized. Subsequently, the cytotoxicity of the Taxol-CPP after self-assembly against HepG2 cancer cells was evaluated.

FINDINGS

It is found that the Taxol-CPP possesses a high drug loading of 26.4% in each molecule, which is able to self-assemble into supramolecular nanospheres. By taking advantages of the self-assembly ability of taxol, Taxol-CPP supramolecular nanospheres with a mean size of around 130 nm can be obtained, composed of only the functional peptide (CPP) and the drug (taxol). Furthermore, we have demonstrated that the Taxol-CPP nanospheres do not compromise the taxol's potency, which can also be utilized as the carriers for co-delivery of another anticancer drug (doxorubicin).

Self-assembly of a dual functional bioactive peptide amphiphile incorporating both matrix metalloprotease substrate and cell adhesion motifs

We describe a bioactive lipopeptide that combines the capacity to promote the adhesion and subsequent self-detachment of live cells, using template-cell-environment feedback interactions. This self-assembling peptide amphiphile comprises a diene-containing hexadecyl lipid chain (C16e) linked to a matrix metalloprotease-cleavable sequence, Thr-Pro-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln, and contiguous with a cell-attachment and signalling motif, Arg-Gly-Asp-Ser. Biophysical characterisation revealed that the PA self-assembles into 3 nm diameter spherical micelles above a critical aggregation concentration (cac). In addition, when used in solution at 5-150 nM (well below the cac), the PA is capable of forming film coatings that provide a stable surface for human corneal fibroblasts to attach and grow. Furthermore, these coatings were demonstrated to be sensitive to metalloproteases expressed endogenously by the attached cells, and consequently to elicit the controlled detachment of cells without compromising their viability. As such, this material constitutes a novel class of multi-functional coating for both fundamental and clinical applications in tissue engineering.

From 2D to 3D: novel nanostructured scaffolds to investigate signalling in reconstructed neuronal networks

To recreate in vitro 3D neuronal circuits will ultimately increase the relevance of results from cultured to whole-brain networks and will promote enabling technologies for neuro-engineering applications. Here we fabricate novel elastomeric scaffolds able to instruct 3D growth of living primary neurons. Such systems allow investigating the emerging activity, in terms of calcium signals, of small clusters of neurons as a function of the interplay between the 2D or 3D architectures and network dynamics. We report the ability of 3D geometry to improve functional organization and synchronization in small neuronal assemblies. We propose a mathematical modelling of network dynamics that supports such a result. Entrapping carbon nanotubes in the scaffolds remarkably boosted synaptic activity, thus allowing for the first time to exploit nanomaterial/cell interfacing in 3D growth support. Our 3D system represents a simple and reliable construct, able to improve the complexity of current tissue culture models.

Generic concept to program the time domain of self-assemblies with a self-regulation mechanism

Nature regulates complex structures in space and time via feedback loops, kinetically controlled transformations, and under energy dissipation to allow non-equilibrium processes. Although man-made static self-assemblies realize excellent control over hierarchical structures via molecular programming, managing their temporal destiny by self-regulation is a largely unsolved challenge. Herein, we introduce a generic concept to control the time domain by programming the lifetimes of switchable self-assemblies in closed systems. We conceive dormant deactivators that, in combination with fast promoters, enable a unique kinetic balance to establish an autonomously self-regulating, transient pH-state, whose duration can be programmed over orders of magnitude-from minutes to days. Coupling this non-equilibrium state to pH-switchable self-assemblies allows predicting their assembly/disassembly fate in time, similar to a precise self-destruction mechanism. We demonstrate a platform approach by programming self-assembly lifetimes of block copolymers, nanoparticles, and peptides, enabling dynamic materials with a self-regulation functionality.

Nanospheres of doxorubicin as cross-linkers for a supramolecular hydrogelation

In this study, we synthesized a peptide of Nap-GFFYGRGD, which could self-assemble into supramolecular nanofibers. The peptide itself could only form nanofibers but not hydrogels due to the relative weak inter-fiber interactions. The resulting nanofibers were then utilized as the vehicles for anticancer drug doxorubicin. It was found that the nanofibers of Nap-GFFYGRGD could not encapsulate doxorubicin, whereas the drug formed nanospheres, which were located at the surface of the nanofibers. Due to the electrostatic interactions between the negatively charged nanofibers and the positively charged doxorubicin nanospheres, the doxorubicin nanospheres were able to serve as a cross-linker to increase the inter-fiber interactions, leading to the formation of stable three-dimentional fiber networks and hydrogels. The resulting doxorubicin-peptide hydrogels were capable of releasing the drug in a sustained manner, which also showed comparable cytotoxicity as compared to free doxorubicin against a variety of cancer cell lines including HeLa and MCF-7 cancer cells. Therefore, this successful example using drug as the peptide nanofiber cross-linkers provided a new strategy for fabricating supramolecular hydrogelation for controlled delivery of anticancer drugs.

The powerful functions of peptide-based bioactive matrices for regenerative medicine

In an effort to develop bioactive matrices for regenerative medicine, peptides have been used widely to promote interactions with cells and elicit desired behaviors in vivo. This paper describes strategies that utilize peptide-based molecules as building blocks to create supramolecular nanostructures that emulate not only the architecture but also the chemistry of the extracellular matrix in mammalian biology. After initiating a desired regenerative response in vivo, the innate biodegradability of these systems allow for the natural biological processes to take over in order to promote formation of a new tissue without leaving a trace of the nonnatural components. These bioactive matrices can either bind or mimic growth factors or other protein ligands to elicit a cellular response, promote specific mechano-biological responses, and also guide the migration of cells with programmed directionality. In vivo applications discussed in this review using peptide-based matrices include the regeneration of axons after spinal cord injury, regeneration of bone, and the formation of blood vessels in ischemic muscle as a therapy in peripheral arterial disease and cardiovascular diseases.

Consequences of chirality on the dynamics of a water-soluble supramolecular polymer

The rational design of supramolecular polymers in water is imperative for their widespread use, but the design principles for these systems are not well understood. Herein, we employ a multi-scale (spatial and temporal) approach to differentiate two analogous water-soluble supramolecular polymers: one with and one without a stereogenic methyl. Initially aiming simply to understand the molecular behaviour of these systems in water, we find that while the fibres may look identical, the introduction of homochirality imparts a higher level of internal order to the supramolecular polymer. Although this increased order does not seem to affect the basic dimensions of the supramolecular fibres, the equilibrium dynamics of the polymers differ by almost an order of magnitude. This report represents the first observation of a structure/property relationship with regard to equilibrium dynamics in water-soluble supramolecular polymers.

Dynamic biostability, biodistribution, and toxicity of L/D-peptide-based supramolecular nanofibers

Self-assembling peptide nanofibers (including naturally L-amino acid-based and unnaturally D-amino acid-based ones) have been widely utilized in biomedical research. However, there has been no systematic study on their in vivo stability, distribution, and toxicity. Herein we systematically study the in vivo dynamic biostability, biodistribution, and toxicity of supramolecular nanofibers formed by Nap-GFFYGRGD (L-amino acid-based, L-fibers) and Nap-G(D)F(D)F(D)YGRGD (D-amino acid-based, D-fibers), respectively. The D-fibers have better in vitro and in vivo biostabilities than L-fibers. It is found that D-fibers keep a good integrity in plasma during 24 h, while half of l-fibers are digested upon incubation in plasma for 6 h. The biodistributions of L- and D-fibers are also studied using the iodine-125 radiolabeling technique. The results reveal that L-fibers mainly accumulate in stomach, whereas d-fibers preferentially distribute in liver. Successive administrations of both L- and D-fibers with the dose of 30 mg/kg/dose cause no significant inflammation, liver and kidney function damages, immune reaction, and dysfunction of hematopoietic system. This study will provide fundamental guidelines for utilization of self-assembling peptide-based supramolecular nanomaterials in biomedical applications, such as drug delivery, bioimaging, and regenerative medicine.

Energy transfer within responsive pi-conjugated coassembled peptide-based nanostructures in aqueous environments

Steady-state and time-resolved photophysical measurements demonstrate energy transfer within π-conjugated peptide nanostructures composed of oligo-(p-phenylenevinylene)-based donor units and quaterthiophene-based acceptor units in completely aqueous environments. These peptide-based assemblies encourage energy migration along the stacking axis, thus resulting in the quenching of donor emission peaks along with the development of new spectral features reminiscent of acceptor emission. These spectral changes were observed even at minute amounts of the acceptor (starting at 1 mol%), suggesting that exciton migration is involved in energy transport and supporting a funnel-like energy transduction mechanism. The reversibility of nanostructure formation and the associated photophysical responses under different conditions (pH, temperature) were also studied. This unique material design incorporates two different semiconducting units coassembled within peptide nanostructures and offers a new platform for the engineering of energy migration through bioelectronic materials in aqueous environments.

Alginate-peptide amphiphile core-shell microparticles as a targeted drug delivery system

We describe in this work the synthesis of microparticles with a doxorubicin drug conjugated alginate core and a shell of peptide amphiphile nanofibres functionalized for targeting the folate receptor. The spherical geometry of the particle core allows high drug loading per surface area, whereas the nanoscale fibrous shell formed by self-assembly of peptide amphiphiles offers a high surface to volume ratio that is ideal for targeting. The synthesised microparticles have a 60-fold higher cytotoxicity against MDA-MB-231 breast cancer cells compared to non-targeting particles.

Qualitative/chiral sensing of amino acids by naked-eye fluorescence change based on morphological transformation and hierarchizing in supramolecular assemblies of pyrene-conjugated glycolipids

Supramolecular assemblies exhibited fluorescence-color changes in response to not only a specific amino acid but also the chirality of the recognized amino acid.

A new water-soluble aromatic polyamide hydrogelator with thixotropic properties

The water-soluble aromatic polyamide poly(3-sodium sulfo-p-phenylene terephthalamide) forms a hydrogel with anisotropy, which exhibits good thixotropic behaviour, even at the critical gel concentration of the gelator (1.0 wt%).

Supramolecular nanofibers of self-assembling peptides and proteins for protein delivery

Supramolecular nanofibers of proteins and peptides could be used for intracellular protein delivery.

Improved mechanical properties of polyacrylamide hydrogels created in the presence of low-molecular-weight hydrogelators

It was found that the crushing stress of the obtained polyacrylamide hydrogels was enhanced by using the molecular gel as removable templates.

Long-chain alkylamide-derived oil gels: mixing induced onset of thixotropy and application in sustained drug release

Oil gels composed of long-chain alkylamides exhibited thixotropic properties, although the same property was absent in each alkylamide.

Magnetically aligned supramolecular hydrogels

The magnetic-field-induced alignment of the fibrillar structures present in an aqueous solution of a dipeptide gelator, and the subsequent retention of this alignment upon transformation to a hydrogel upon the addition of CaCl2 or upon a reduction in solution pH is reported. Utilising the switchable nature of the magnetic field coupled with the slow diffusion of CaCl2 , it is possible to precisely control the extent of anisotropy across a hydrogel, something that is generally very difficult to do using alternative methods. The approach is readily extended to other compounds that form viscous solutions at high pH. It is expected that this work will greatly expand the utility of such low-molecular-weight gelators (LMWG) in areas where alignment is key.

Modulating the binding of polycyclic aromatic hydrocarbons inside a hexacationic cage by anion-π interactions

We report the template-directed synthesis of BlueCage(6+), a macrobicyclic cyclophane composed of six pyridinium rings fused with two central triazines and bridged by three paraxylylene units. These moieties endow the cage with a remarkably electron-poor cavity, which makes it a powerful receptor for polycyclic aromatic hydrocarbons (PAHs). Upon forming a 1:1 complex with pyrene in acetonitrile, however, BlueCage⋅6 PF6 exhibits a lower association constant Ka than its progenitor ExCage⋅6 PF6. A close inspection reveals that the six PF6(-) counterions of BlueCage(6+) occupy the cavity in a fleeting manner as a consequence of anion-π interactions and, as a result, compete with the PAH guests. This conclusion is supported by a one order of magnitude increase in the Ka value for pyrene in BlueCage(6+) when the PF6(-) counterions are replaced by much bulkier anions. The presence of anion-π interactions is supported by X-ray crystallography, and confirms the presence of a PF6(-) counterion inside its cavity.

Amino acid sequence in constitutionally isomeric tetrapeptide amphiphiles dictates architecture of one-dimensional nanostructures

The switching of two adjacent amino acids can lead to differences in how proteins fold thus affecting their function. This effect has not been extensively explored in synthetic peptides in the context of supramolecular self-assembly. Toward this end, we report here the use of isomeric peptide amphiphiles as molecular building blocks to create one-dimensional (1D) nanostructures. We show that four peptide amphiphile isomers, with identical composition but a different sequence of their four amino acids, can form drastically different types of 1D nanostructures under the same conditions. We found that molecules with a peptide sequence of alternating hydrophobic and hydrophilic amino acids such as VEVE and EVEV self-assemble into flat nanostructures that can be either helical or twisted. On the other hand, nonalternating isomers such as VVEE and EEVV result in the formation of cylindrical nanofibers. Furthermore, we also found that when the glutamic acid is adjacent to the alkyl tail the supramolecular assemblies appear to be internally flexible compared to those with valine as the first amino acid. These results clearly demonstrate the significance of peptide side chain interactions in determining the architectures of supramolecular assemblies.

Exploiting Hydrophobic Interactions at the Nanoscale

Hydrophobic effects are ubiquitous and manifest themselves in everyday processes such as solubilizing oil, precipitating molecules, and formation of particles or foam. Although this phenomenon is often intuitively recognized, it is not straightforward to predict it and, in particular, to control it experimentally. Hydrophobic effects are however progressively gaining recognition as an important tool providing control at the nanoscale, which may ultimately lead to the design of responsive metamaterials with unprecedented functionalities under nonequilibrium conditions.

Electrospinning bioactive supramolecular polymers from water

Electrospinning is a high-throughput, low-cost technique for manufacturing long fibers from solution. Conventionally, this technique is used with covalent polymers with large molecular weights. We report here the electrospinning of functional peptide-based supramolecular polymers from water at very low concentrations (<4 wt %). Molecules with low molecular weights (<1 kDa) could be electrospun because they self-assembled into one-dimensional supramolecular polymers upon solvation and the critical parameters of viscosity, solution conductivity, and surface tension were optimized for this technique. The supramolecular structure of the electrospun fibers could ensure that certain residues, like bioepitopes, are displayed on the surface even after processing. This system provides an opportunity to electrospin bioactive supramolecular materials from water for biomedical applications.

Post-assembly functionalization of supramolecular nanostructures with bioactive peptides and fluorescent proteins by native chemical ligation

Post-assembly functionalization of supramolecular nanostructures has the potential to expand the range of their applications. We report here the use of the chemoselective native chemical ligation (NCL) reaction to functionalize self-assembled peptide amphiphile (PA) nanofibers. This strategy can be used to incorporate specific bioactivity on the nanofibers, and as a model, we demonstrate functionalization with the RGDS peptide following self-assembly. Incorporation of bioactivity is verified by the observation of characteristic changes in fibroblast morphology following NCL-mediated attachment of the signal to PA nanofibers. The NCL reaction does not alter the PA nanofiber morphology, and biotinylated RGDS peptide was found to be accessible on the nanofiber surface after ligation for binding with streptavidin-conjugated gold nanoparticles. In order to show that this strategy is not limited to short peptides, we utilized NCL to conjugate yellow fluorescent protein and/or cyan fluorescent protein to self-assembled PA nanofibers. Förster resonance energy transfer and fluorescence anisotropy measurements are consistent with the immobilization of the protein on the PA nanofibers. The change in electrophoretic mobility of the protein upon conjugation with PA molecules confirmed the formation of a covalent linkage. NCL-mediated attachment of bioactive peptides and proteins to self-assembled PA nanofibers allows the independent control of self-assembly and bioactivity while retaining the biodegradable peptide structure of the PA molecule and thus can be useful in tailoring design of biomaterials.

Visualized detection of melamine in milk by supramolecular hydrogelations

We reported on a simple assay for visualized detection of melamine in milk by supramolecular hydrogelations.

Redox-controllable self-assembly and anti-bacterial activity of a vancomycin derivative

We report a selenium containing vancomycin derivative with redox-controllable self-assembly property and anti-bacterial activity.