Functional Supramolecular Polymers^*

Signaling cells with bioactive filaments

Signaling of cells with biocompatible artificial structures will provide many opportunities to cure disease and improve quality of life for the world’s population through regenerative medicine. If these artificial structures are to be placed in the human body, they should signal and then biodegrade into nutrients or be safely eliminated. A supramolecular polymer could be a rigid assembly of molecules displaying multiple signals to receptors in high density and highly directed geometry, but composed of water soluble monomers. Furthermore, the supramolecular polymer could also be reconstructed by the cell to fit the necessary geometry by easily rupturing and reforming in situ its noncovalent bonds, an opportunity obviously not accessible with covalent chains (see Fig. 3A and 3B).

Fig. 3A depicts schematically a rigid filamentous supramolecular polymer that displays signals spatially to receptors over long distances on the cell surface. These rigid assemblies of small molecules could also recruit the multiple receptors necessary in the raft structures of cell membranes. Fig. 3B depicts schematically how the rupture of non-covalent bonds in the supramolecular polymer could adapt its architecture to the high binding affinities of receptors for the signals displayed on the artificial nanostructure. In order to create a vast number of such supramolecular polymers, we need self-assembly codes that lead to rigid filaments. The functional form of peptide-based supramolecular polymers has been extensively developed by Stupp and co-workers in peptide amphiphiles, molecules containing peptide segments covalently bonded to lipid-like hydrophobic segments (see Figs. 3C–E) (20, 21). These supramolecular polymers have been shown to be highly bioactive in vivo when designed to promote biological events such as regeneration of axons in the injured spinal cord (22), the growth of blood vessels (23), the regeneration of bone and cartilage (24–26). In other work by Hartgerink and co-workers (27) peptide amphiphile supramolecular polymers have been engineered as cell scaffolds that integrate substrates for enzymes that can degrade the matrix with biological adhesion signals in order to promote cell viability, migration, and proliferation.

An example of complex function in ordered supramolecular polymers was their use to promote the rapid and selective differentiation of neural stem cells into neurons without the development of astrocytes (28). This selective differentiation was achieved in a supramolecular polymer capable of displaying up to 10¹⁵ signals per cm² of the pentapeptide laminin signal IKVAV. The actual signal density is of course dependent on the internal level of hydration and bundling of the filamentous assemblies. The observed differentiation required assemblies in which nearly 50% of the peptide amphiphile monomers used to create a filamentous network contained the biological signal (see Fig. 3D). In another example the supramolecular filaments have been used to promote blood vessel formation utilized heparin displaying nanofibers that bind the proteins vascular endothelial growth factor and fibroblast growth factor (23). The regeneration of cartilage was triggered by nanofibers that displayed peptides discovered by phage display that can bind the protein transforming growth factor β1 (25). In approaches using proteins bound to the supramolecular polymer the large real state on the rigid assembly can accommodate the nanoscale footprint of a growth factor or antibody to trigger a specific signal transduction pathway or target a specific cell for therapeutic purposes. It is also important that supramolecular polymers put together through noncovalent interactions will not only be likely to biodegrade at faster rates than covalent polymers but monomer design will allow fine tuning of their half life.

Biomimetic Dynamic Structures

The potential in supramolecular systems to achieve dynamic functions derives inspiration from the cell cytoskeleton. The reversible association of protein monomers to dynamically create and dissolve a skeleton of stiff filaments inside microscopic gels suggests many ideas for synthetic systems. In cells this fascinating process in which supramolecular polymers form and depolymerize rapidly not only controls cell migration, attachment, and division, but also serves as a highway for internal transport of molecules to specific compartments (29). If suitable chemistry could be developed to create biomimetic analogues, one could imagine many possibilities for novel functions. One example would be soft materials that stiffen or deliver chemicals reversibly with chemical or photonic stimuli.

Hierarchical Structures

The use of noncovalent interactions to design functional materials will be greatly enhanced if the supramolecular structure codes can be extended beyond a single assembly of molecules defined as the polymer. A number of examples of supramolecular hierarchical structures have been reported recently. Cylindrical assemblies of peptide amphiphiles were found to crystallize spontaneously or triggered to do so reversibly by x-ray photons that create charge on surfaces of the supramolecular filaments (30). In this case the hexagonal crystal formed as a result of repulsive electrostatic forces among the supramolecular polymers trapped mechanically by the networks they formed. In another example, bundles of dozens of peptide amphiphile cylindrical nanofibers were observed to form through a mechanism analogous to a two-dimensional Rayleigh instability and to consequently self-order into an aqueous liquid crystal at exceedingly low concentrations in the range of 0.5% by weight (31). These solutions of bundled supramolecular polymers can be sheared by a human hand into orientational monodomains of arbitrary macroscopic length. Their potential functions in aligning large populations of cells or promoting electrical connections among cardiac cells has been demonstrated. These hierarchical systems may serve as scaffolds for wires to repair parts of the brain, the heart, and the spinal cord. As recently shown, hierarchical supramolecular polymers could also incorporate in their building blocks the multimeric association of molecules observed in biological systems such as the collagen triple helix (32).

In a final example of hierarchical structure, positively charged supramolecular peptide filaments in dilute aqueous solution come into contact with dilute solutions of oppositely charged biopolymers with an average molar mass in the range of millions of Daltons, and assemble into a membrane with speeds in the range of milliseconds (33). The supramolecular rigid filaments screened by polymer chains bearing opposite charge assemble into a dense layer which serves as a diffusion barrier to prevent the chaotic mixing of both solutions. This remarkably ordered structure formed by a hierarchical structure of supramolecular polymers is permeable to large molecules such as proteins. The miniaturization of this process to scales of microns is a potential platform demonstrated recently to create the scaffold of synthetic cells with filamentous surfaces (34).

Semiconducting Nanofibers

π-Conjugated molecules that self-assemble by stacking have the potential for developing semiconducting nanowires that transport charge carriers unidirectionally over long distances (35, 36). Nearly a decade ago, several pioneering examples of such electroactive nanowires (37–41) were reported, many of which utilized π-conjugated aromatic units in conjunction with hydrogen-bonding motifs. For example, mono- and bithiophene derivatives carrying urea functionalities self-assemble into nanofibers, which display highly mobile charge carriers on pulse radiolysis (37). The self-complementary quadruple hydrogen-bonding motifs (13) when coupled with oligo(p-phenylenevinylene) derivatives (Fig. 1) are useful for supramolecular electronics (38). Meanwhile, a different strategy to create electroactive one-dimensional nanostructures has been reported by making use of molecules with architectures that include rod-like, coil-like, and dendritic segments (dendron rodcoils) (39, 40). These molecules polymerize into ribbons through hydrogen bonding and π-stacking interactions.

More recent studies have focused on ordered supramolecular polymers based on very large monomeric units with extended π-conjugated motifs in order to attain high charge mobilities (42, 43). In this context, Müllen et al. initiated work on coronene derivatives which have attracted attention as monomers for functional supramolecular polymers (44). Nuckolls and coworkers reported that contorted hexabenzocoronene (HBC) (molecular graphene) derivatives with eight long paraffinic side chains self-assemble into one-dimensional, single-crystalline nanofibers that can be used to create field effect transistors (42). Because of strong π-stacking interactions among the molecular units these one-dimensional nanostructures form without the involvement of any other interactions.

Semiconducting Nanotubes

The design strategy to create supramolecular nanotubes can be very different from that used for nanofibers since they derive from the polymerization of units into sheet-like objects. Aida and coworkers reported that an amphiphilic HBC, carrying triethylene glycol (TEG)-appended phenyl groups on one side and dodecyl side chains on the other, self-assembles into several tens-to-hundreds of micrometer-long graphite-like nanotubes with a wall thickness of 3 nm (45) (Fig. 1). These supramolecular nanotubes are thus characterized by a very large aspect ratio and have a uniform, 14-nm-wide, open-ended hollow space. The oxidized nanotube across a 180-nm-gap electrode displays an electrical conductivity of 2.5 MΩ at 285 K (45). From a suspension of long nanotubes, it is possible to obtain a macroscopic fiber a few centimeters in length, consisting of unidirectionally oriented nanotubes. Because of this alignment, the doped macroscopic fiber displays a clear anisotropy in electrical conductivity (46). Graphite-like supramolecular nanotubes with a variety of surface functional groups in their TEG termini can be fabricated through this strategy (e.g., chemically reactive groups, metal-binding functions, and chromophores) (47). When an asymmetric center is incorporated into the ether side chains, nanotubes (48) and even nanocoils with one-handed helical chirality are formed. Knoester and coworkers reported the polymerization of a supramolecular nanotube with a bilayer wall based on a J-aggregated amphiphilic cyanine dye (49). By means of polarization-resolved near-field scanning optical microscopy, such a red-shifted exciton emission can be detected directly from a single piece of the supramolecular nanotube (50).

Photoconductive Nanowires and Device Applications

One of the interesting functions for ordered electronically active supramolecular polymers would be photovoltaic activity. Although examples of such functional systems are still limited, work reported so far is creating a foundation and vision for this field (Fig. 4A). The nanowire polymers required must contain electron-donating (D) and accepting (A) components, either properly located in a single one-dimensional object or in different ones within a suitable electron transfer distance for exciton splitting after light absorption. The D- and A-components must assemble into individual p- and n-type semiconducting arrays (p/n-heterojunction) in such a way that the photogenerated holes and electrons can efficiently move toward opposite electrodes. Ordered supramolecular polymers would offer the possibility to more precisely design the heterojunctions in contrast to the common approach that relies on phase separation. One of the challenges is to avoid unfavorable charge-transfer (CT) D/A interactions at distances on the scale of molecular dimensions. In pioneering work, a D-A-D supramolecular triad, consisting of oligo(p-phenylenevinylene) (D) and perylene bisimide (A), was found to self-assemble into nanofibers that presumably consist of segregated D- and A-arrays (51, 52). Although radical anion and cation could be generated photochemically in the nanofiber, they recombined within 60 ps (51).

Recently, a design strategy for photoconductive nanotubes with a coaxial p/n-heterojunction based on HBC units was successfully elaborated (53). In this supramolecular structure, a molecular layer of the electron acceptor trinitrofluorenone laminates a graphite-like tubular wall of HBC units which are electron donors (Fig. 4B). This coaxial p/n-heterojunction structure enables photocurrent generation with a large on/off ratio greater than 10⁴ (Fig. 4C). It is interesting to note that at high concentrations of the building blocks, the anticipated charge transfer interactions prevail and only ill-defined microfibers form that hardly display a photocurrent (53). When a fullerene is utilized in these structures as the acceptor, the resultant carbon-rich coaxial nanotube shows upon light illumination an ambipolar charge carrier transport profile, where the intratubular mobility is as large as the intersheet mobility in graphite. Furthermore, the nanotube shows a photovoltaic response (54) although the overall device performance is not satisfactory for practical use. More recently, this assembling strategy was extended to the formation of a graphite-like nanotube with a linear D/A-heterojunction using a surface-crosslinked HBC nanotube as the seed and a fluorinated HBC as the monomer (55). Matile and coworkers proposed a zipper-type supramolecular copolymerization of positively and negatively charged D-A dyads from initiating groups immobilized on a gold substrate (56, 57). This approach has the potential to yield aligned systems on electrode surfaces with many tunable properties. Generally speaking, a key issue to consider is the essential difficulty in aligning ordered supramolecular polymers unidirectionally over macroscopic length scales to create devices, and here is where approaches to hierarchical supramolecular structures described in the previous section could become useful.