Liam C. Palmer

Hybrid bonding polymers of donor–acceptor molecules and functionalized polymers exhibit noncentrosymmetric structures and ferroelectricity. The ferroelectricity arises from symmetry-breaking defects caused by electrostatic repulsions in the material.

Supramolecular polymers with precise micrometer-scale segments of opposite charge are challenging to synthesize as discrete assemblies in water. These systems could be used to pattern charged surfaces, interact with the membranes of cells, or form hierarchical structures. We report a living polymerization of fibrillar peptide assemblies with a central positively charged segment flanked by negative ones that can have a collective molar mass on the order of billions of daltons. The self-capping supramolecular polymerization required an untwisted β sheet core persisting along the entire length of the assembly as well as dormant ends in the absence of a monomer source. The positive domains of the segmented structures were anchored to the membranes of neurons, leaving the connected negative segments in a highly dynamic state near the cells through charge repulsion. The segmented architecture generated extensive neurite growth and synaptic activity.

The blood–brain barrier (BBB) prevents diffusion of most molecules from the vasculature into the brain tissue, and therefore is of great interest for therapeutic delivery into the central nervous system. We investigated the potential of supramolecular assemblies to cross the BBB using peptide amphiphiles (PAs) in which the lipid tail length was varied to modify supramolecular cohesion but maintaining the same tetrapeptide sequence VVEE. Alkyl tails containing 16, 14, or 12 carbon atoms in the PAs studied were found to contain decreasing amounts of β-sheet secondary structure by wide-angle X-ray scattering and Fourier transform infrared spectroscopy. We also found diminishing pKa values as the lipid tails were shortened, and therefore concluded that longer tails resulted in the most cohesive assemblies. Using confocal microscopy and flow cytometry, we demonstrated that PAs with longer tails had significantly higher accumulation in brain endothelial cells, whereas an in vitro BBB transwell assay showed that C12 assemblies were endocytosed but were also able to exocytose most efficiently from confluent layers of the cells. Live cell imaging demonstrated that all assemblies entered lysosomes, but transcytosis occurred to the greatest extent in C12 assemblies and to a lesser degree in the C14 PA. We conclude that strong hydrophobic collapse, combined with high pKa values, creates stable nanostructures that remain sequestered within brain endothelial cells without the possibility of permeation. Our work also indicates that PA supramolecular nanostructures retain the capacity to reassemble after transcytosis, thus offering potential for their development into therapeutic delivery platforms to the brain.

Dynamics in supramolecular biomaterials have been recently identified as a critical feature in their in vivo performance based on cell signaling. This was originally observed in the context of peptide amphiphile (PA) assemblies that promote neural regeneration in the central nervous system, known to be an enormous challenge. Specifically, enhanced supramolecular motion controlled by the peptide sequence was found to reverse paralysis after trauma to the spinal cord, whereas slower dynamics failed to attain a similar outcome. Here, we report that the most bioactive large filamentous assemblies─bearing the laminin-derived IKVAV signal─exhibit the highest supramolecular dynamics, characterized for the first time by high-resolution magic-angle spinning NMR spectroscopy. For dynamic systems, we also report on ¹H and ¹³C chemical shifts, ¹H-¹H dipolar couplings, and ¹H relaxation data, which suggest pronounced β-sheet structuring within the IKVAV domain. Furthermore, dipolar coupling measurements suggest the possible formation of salt bridges between the E and K residues, folding of PA molecules, and the potential coexistence of parallel and antiparallel molecular arrangements within the dynamic assemblies. X-ray scattering and electron microscopy also revealed a significant population of small PA clusters in equilibrium with micron-length nanofilaments in the more dynamic, bioactive systems, consistent with our previous observations on a related supramolecular system. Based on DOSY NMR, these clusters have a hydrodynamic diameter of 6.6 nm, closely matching the 6.2 nm effective diameter from solution X-ray scattering. Finally, multiphase NMR unambiguously demonstrated the coassembly of PAs bearing the IKVAV moiety with those displaying a peptide that mimics fibroblast growth factor 2, which was necessary to reverse paralysis.

Damage to the spinal cord can lead to irreversible paralysis and loss of sensory function, but translation of preclinical therapies remains elusive. We recently showed that bioactive supramolecular assemblies of peptide amphiphiles can reverse paralysis in an acute mouse model following severe spinal cord injury (SCI). Here we report the development of two human spinal cord organoid injury models to simulate SCI in vitro, a laceration of the organoid with a scalpel and a compressive contusion commonly used in preclinical models, both resulting in immediate neuronal death and the formation of glial scar-like tissue. Treatment of the injured organoids with the preclinical therapy suppressed the scar-like tissue and promoted significant axonal regeneration, as observed previously in vivo. With the inclusion of microglia into the spinal cord organoids, we demonstrate that the supramolecular nanomaterial reduced pro-inflammatory factors commonly associated with injury. The human spinal cord organoid models developed here could accelerate the discovery of therapies to treat SCI and possibly damage of other central nervous system tissues owing to trauma or disease.

Organic ferroelectrics are of great interest in sustainable energy conversion, information storage, flexible electronics, and potential biomedical applications as soft implants, among many other applications. Despite their broad potential, the development of organic ferroelectrics has remained limited, with only a few known examples in solid‐state systems, primarily due to the lack of well‐established design strategies compared to inorganic systems. Bio‐inspired supramolecular chemistry offers a path to create functional nanostructures that are water‐processable and biocompatible. We report here on supramolecular charge transfer (CT) systems in which peptides are covalently linked to dyads of electron‐donating and electron‐accepting moieties, creating amphiphiles that self‐assemble into nanoscale ribbons in water. The peptide chirality‐induced symmetry breaking in these crystalline nanostructures not only results in second harmonic activity but also generates ferroelectric behavior across multiple CT systems, demonstrating a versatile supramolecular approach to the design of new organic ferroelectrics. Furthermore, culturing primary neuron cells on coatings of the ferroelectric materials promoted axonal growth and enhanced action potentials, indicating improved neuronal maturity facilitated by the polar structure of the ferroelectric nanomaterials. The supramolecular strategy used here holds promise to create new water‐processable ferroelectric biomaterials, opening avenues for innovative applications in cell charge transfer, neuronal axon growth, peptide symmetry breaking, self‐assembling peptides, supramolecular ferroelectrics, proliferation, and bioelectronics.

The regeneration of human tissues is a great scientific challenge and a critical factor to achieve a long healthspan and prevent disabilities due to injury or disease. Materials chemistry can contribute to this goal with the development of bioactive supramolecular systems that can signal cells for regeneration. Recent work in our laboratory using in vivo models of spinal cord injury and cartilage regeneration has demonstrated that the motion of bioactive molecules in supramolecular scaffolds enhances receptor signaling. We report here on a novel molecular strategy to control supramolecular motion in filamentous assemblies using bone regeneration as a functional target. The supramolecular assemblies are composed of monomers that arrange, by design, with either parallel or antiparallel β-sheets, and some of them contain a terminal peptide sequence that binds BMP-2. We found that parallel β-sheet supramolecular assemblies promote greater osteogenic differentiation of progenitor cells in vitro relative to antiparallel assemblies, as well as superior quality of newly regenerated bone in a rat model of spinal fusion. Furthermore, these assemblies drastically reduce the dangerous supraphysiological dose of BMP-2 used clinically for spinal fusion. We attribute the enhanced bioactivity to the weaker nature of hydrogen bonds in parallel relative to antiparallel β-sheet assemblies, which in turn allows greater supramolecular motion and cell signaling of the growth factor-binding molecules.

Neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis are characterized by progressive neuronal loss and the accumulation of misfolded proteins including amyloid proteins. Current therapeutic options include the use of antibodies for these proteins, but novel chemical strategies need to be developed. The disaccharide trehalose has been widely reported to prevent misfolding and aggregation of proteins, and we therefore investigated the conjugation of this moiety to biocompatible peptide amphiphiles (TPAs) known to undergo supramolecular polymerization. Using X-ray scattering, circular dichroism, and infrared spectroscopy, we found that trehalose conjugation destabilized the internal β-sheet structures within the TPA supramolecular polymers as evidenced by a lower thermal transition. Thioflavin T fluorescence showed that these metastable TPA nanofibers suppressed A42 aggregation. Interestingly, we found that the suppression involved supramolecular copolymerization of TPA polymers with Aβ42, which effectively trapped the peptides within the filamentous structures. In vitro assays with human induced pluripotent stem cell-derived neurons demonstrated that these TPAs significantly improved neuron survival compared to other conditions. Our study highlights the potential of properly tuned supramolecular polymerizations of monomers to safely remove amyloidogenic proteins in neurodegeneration.