Co-assembly of Peptide Amphiphiles and Lipids into Supramolecular Nanostructures Driven by Anion-π Interactions.

Co-assembly of binary systems driven by specific non-covalent interactions can greatly expand the structural and functional space of supramolecular nanostructures. We report here on the self-assembly of peptide amphiphiles and fatty acids driven primarily by anion-π interactions. The peptide sequences investigated were functionalized with a perfluorinated phenylalanine residue to promote anion-π interactions with carboxylate headgroups in fatty acids. These interactions were verified here by NMR and circular dichroism experiments as well as investigated using atomistic simulations. Positioning the aromatic units close to the N-terminus of the peptide backbone near the hydrophobic core of cylindrical nanofibers leads to strong anion-π interactions between both components. With a low content of dodecanoic acid in this position, the cylindrical morphology is preserved. However, as the aromatic units are moved along the peptide backbone away from the hydrophobic core, the interactions with dodecanoic acid transform the cylindrical supramolecular morphology into ribbon-like structures. Increasing the ratio of dodecanoic acid to PA leads to either the formation of large vesicles in the binary systems where the anion-π interactions are strong, or a heterogeneous mixture of assemblies when the peptide amphiphiles associate weakly with dodecanoic acid. Our findings reveal how co-assembly involving designed specific interactions can drastically change supramolecular morphology and even cross from nano to micro scales.

Energy landscapes and functions of supramolecular systems.

By means of two supramolecular systems--peptide amphiphiles engaged in hydrogen-bonded ß-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.

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.

Nucleation and Growth of Ordered Arrays of Silver Nanoparticles on Peptide Nanofibers: Hybrid Nanostructures with Antimicrobial Properties.

Silver nanoparticles have been of great interest as plasmonic substrates for sensing and imaging, catalysts, or antimicrobial systems. Their physical properties are strongly dependent on parameters that remain challenging to control such as size, chemical composition, and spatial distribution. We report here on supramolecular assemblies of a novel peptide amphiphile containing aldehyde functionality in order to reduce silver ions and subsequently nucleate silver metal nanoparticles in water. This system spontaneously generates monodisperse silver particles at fairly regular distances along the length of the filamentous organic assemblies. The metal-organic hybrid structures exhibited antimicrobial activity and significantly less toxicity toward eukaryotic cells. Metallized organic nanofibers of the type described here offer the possibility to create hydrogels, which integrate the useful functions of silver nanoparticles with controllable metallic content.

Bioactive DNA-peptide nanotubes enhance the differentiation of neural stem cells into neurons.

We report the construction of DNA nanotubes covalently functionalized with the cell adhesion peptide RGDS as a bioactive substrate for neural stem cell differentiation. Alteration of the Watson-Crick base pairing program that builds the nanostructures allowed us to probe independently the effect of nanotube architecture and peptide bioactivity on stem cell differentiation. We found that both factors instruct synergistically the preferential differentiation of the cells into neurons rather than astrocytes.

Shape-Dependent Targeting of Injured Blood Vessels by Peptide Amphiphile Supramolecular Nanostructures.

Targeting of vascular intervention by systemically delivered supramolecular nanofibers after balloon angioplasty is described. Tracking of self-assembling peptide amphiphiles using fluorescence shows selective binding to the site of vascular intervention. Cylindrical nanostructures are observed to target the site of arterial injury, while spherical nanostructures with an equivalent diameter display no binding.

Nanoscale clustering of carbohydrate thiols in mixed self-assembled monolayers on gold.

Self-assembled monolayers (SAMs) bearing pendant carbohydrate functionality are frequently employed to tailor glycan-specific bioactivity onto gold substrates. The resulting glycoSAMs are valuable for interrogating glycan-mediated biological interactions via surface analytical techniques, microarrays, and label-free biosensors. GlycoSAM composition can be readily modified during assembly by using mixed solutions containing thiolated species, including carbohydrates, oligo(ethylene glycol) (OEG), and other inert moieties. This intrinsic tunability of the self-assembled system is frequently used to optimize bioavailability and antibiofouling properties of the resulting SAM. However, until now, our nanoscale understanding of the behavior of these mixed glycoSAMs has lacked detail. In this study, we examined the time-dependent clustering of mixed sugar + OEG glycoSAMs on ultraflat gold substrates. Composition and surface morphologic changes in the monolayers were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. We provide evidence that the observed clustering is consistent with a phase separation process in which surface-bound glycans self-associate to form dense glycoclusters within the monolayer. These observations have significant implications for the construction of mixed glycoSAMs for use in biosensing and glycomics applications.

Universal Minimal Model for Glucose-Insulin Relationship with the Influence of Food Dynamic.

We propose a minimal model defining the relationship between glucose and insulin with the added influence of food intakes. The constructed model consists of a system of 3 nonlinear ordinary differential equations (ODEs). The solutions of our model for both normal and diabetic subjects are compared with a minimal model and a maximal model representing the same relationship. We found that the outputs of our model are similar to those from the minimal and maximal models for both normal and diabetic subjects; the R 2 are 0.9997 and 0.9922, respectively, when compared with the minimal model, and are 0.9995 and 0.9940, respectively, when compared with the maximal model. Moreover, the relative errors between solutions are at most 0.9035% and as low as 1.488 × 10-2% on average when compared with the minimal model for normal subjects and at most 1.331% and as low as 0.1159% on average for diabetic subjects. The discrepancy between our model and the maximal model are at most 1.590% and 5.453% for normal and diabetic subjects, respectively, with a relative error averaging 0.2138% and 0.9002% for normal and diabetic subjects, respectively.

Peptide-siRNA Supramolecular Particles for Neural Cell Transfection.

Small interfering ribonucleic acid (siRNA)-based gene knockdown is an effective tool for gene screening and therapeutics. However, the use of nonviral methods has remained an enormous challenge in neural cells. A strategy is reported to design artificial noncationic modular peptides with amplified affinity for siRNA via supramolecular assembly that shows efficient protein knockdown in neural cells. By solid phase synthesis, a sequence that binds specifically double-stranded ribonucleic acid (dsRNA) with a self-assembling peptide for particle formation is integrated. These supramolecular particles can be further functionalized with bioactive sequences without affecting their biophysical properties. The peptide carrier is found to silence efficiently up to 83% of protein expression in primary astroglial and neuronal cell cultures without cytotoxicity. In the case of neurons, a reduction in electrical activity is observed once the presynaptic protein synaptophysin is downregulated by the siRNA-peptide particles. The results demonstrate that the supramolecular particles offer an siRNA delivery platform for efficient nonviral gene screening and discovery of novel therapies for neural cells.

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.

(19)F Magnetic Resonance Imaging Signals from Peptide Amphiphile Nanostructures Are Strongly Affected by Their Shape.

Magnetic resonance imaging (MRI) is a noninvasive imaging modality that provides excellent spatial and temporal resolution. The most commonly used MR probes face significant challenges originating from the endogenous (1)H background signal of water. In contrast, fluorine MRI ((19)F MRI) allows quantitative probe imaging with zero background signal. Probes with high fluorine content are required for high sensitivity, suggesting nanoscale supramolecular assemblies containing (19)F probes offer a potentially useful strategy for optimum imaging as a result of improved payload. We report here on supramolecular nanostructures formed by fluorinated peptide amphiphiles containing either glutamic acid or lysine residues in their sequence. We identified molecules that form aggregates in water which transition from cylindrical to ribbon-like shape as pH increased from 4.5 to 8.0. Interestingly, we found that ribbon-like nanostructures had reduced magnetic resonance signal, whereas their cylindrical counterparts exhibited strong signals. We attribute this drastic difference to the greater mobility of fluorinated tails in the hydrophobic compartment of cylindrical nanostructures compared to lower mobility in ribbon-like assemblies. This discovery identifies a strategy to design supramolecular, self-assembling contrast agents for (19)F MRI that can spatially map physiologically relevant changes in pH using changes in morphology.

Epitope topography controls bioactivity in supramolecular nanofibers.

Incorporating bioactivity into artificial scaffolds using peptide epitopes present in the extracellular matrix (ECM) is a well-known approach. A common strategy has involved epitopes that provide cells with attachment points and external cues through interaction with integrin receptors. Although a variety of bioactive sequences have been identified so far, less is known about their optimal display in a scaffold. We report here on the use of self-assembled peptide amphiphile (PA) nanofiber matrices to investigate the impact of spatial presentation of the fibronectin derived epitope RGDS on cell response. Using one, three, or five glycine residues, RGDS epitopes were systematically spaced out from the surface of the rigid nanofibers. We found that cell morphology was strongly affected by the separation of the epitope from the nanofiber surface, with the longest distance yielding the most cell-spreading, bundling of actin filaments, and a round-to-polygonal transformation of cell shape. Cell response to this type of epitope display was also accompanied with activated integrin-mediated signaling and formation of stronger adhesions between cells and substrate. Interestingly, unlike length, changing the molecular flexibility of the linker had minimal influence on cell behavior on the substrate for reasons that remain poorly understood. The use in this study of high persistence length nanofibers rather than common flexible polymers allows us to conclude that epitope topography at the nanoscale structure of a scaffold influences its bioactive properties independent of epitope density and mechanical properties.

Epitope topography controls bioactivity in supramolecular nanofibers.

Incorporating bioactivity into artificial scaffolds using peptide epitopes present in the extracellular matrix (ECM) is a well-known approach. A common strategy has involved epitopes that provide cells with attachment points and external cues through interaction with integrin receptors. Although a variety of bioactive sequences have been identified so far, less is known about their optimal display in a scaffold. We report here on the use of self-assembled peptide amphiphile (PA) nanofiber matrices to investigate the impact of spatial presentation of the fibronectin derived epitope RGDS on cell response. Using one, three, or five glycine residues, RGDS epitopes were systematically spaced out from the surface of the rigid nanofibers. We found that cell morphology was strongly affected by the separation of the epitope from the nanofiber surface, with the longest distance yielding the most cell-spreading, bundling of actin filaments, and a round-to-polygonal transformation of cell shape. Cell response to this type of epitope display was also accompanied with activated integrin-mediated signaling and formation of stronger adhesions between cells and substrate. Interestingly, unlike length, changing the molecular flexibility of the linker had minimal influence on cell behavior on the substrate for reasons that remain poorly understood. The use in this study of high persistence length nanofibers rather than common flexible polymers allows us to conclude that epitope topography at the nanoscale structure of a scaffold influences its bioactive properties independent of epitope density and mechanical properties.

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.