Assemblies Of Peptide Amphiphiles Research Articles

Development of Nanoscale Hybrids from Ionic Liquid-Peptide Amphiphile Assemblies as New Functional Materials.

Over the years, ionic liquids (ILs) have gained tremendous importance because of their unique properties and plethora of applications. In this work, we have developed a new nanoscale hybrid gel consisting of 1-ethyl-3-methylimidazolium dimethyl phosphate, [C2mim][dmp], and self-assembled peptide nanoassemblies. The peptide nanoassemblies were formed by self-assembly of a newly synthesized peptide bolaamphiphile bis(N-α-amido-threonine) 1,7 heptane dicarboxylate (ThrC7). Upon mild heating and sonication of the IL and ThrC7 nanoassemblies, ThrC7-IL nanocomposites were formed. We explored the formation of nanohybrids by varying the ratio of IL to ThrC7 assemblies. While at lower IL ratios, a gelatinous matrix was formed, at higher IL ratios, highly ordered multilayered structures were observed by atomic force microscopy (AFM) imaging. The interactions between the ThrC7 nanofibers and [C2mim][dmp] IL were probed by Fourier transform infrared spectroscopy, transmission electron microscopy, and AFM imaging. Differential scanning calorimetry and thermogravimetric analysis showed that the nanohybrids illustrated distinct thermal phase changes due to changes in hydrogen bonding interactions and unfolding of the nanoassemblies. The viscoelastic behavior of the nanohybrids indicated that the materials displayed higher storage modulus upon incorporation of the ThrC7 nanoassemblies when compared to the IL. Furthermore, the nanohybrids were found to adhere to and promote proliferation of human dermal fibroblasts, while cytotoxicity was observed toward MCF-7 breast cancer cells. Thus, for the first time, we have developed peptide-based nanohybrids with an imidazolium-based IL with unique structural properties that may open new avenues for exploring potential biological applications.

Chiral Recognition of Lipid Bilayer Membranes by Supramolecular Assemblies of Peptide Amphiphiles.

On the basis of the exclusive existence of homochirality in biomolecules and the well-known phenomenon of chiral recognition, it is obvious that chirality is a crucial factor in biological events. We report here that supramolecular assemblies of peptide amphiphiles interact with lipid bilayer membranes in a stereospecific manner. When negatively charged chiral phospholipid bilayer vesicles were subjected to the assemblies, we found that peptide amphiphiles with l-amino acids show stronger affinity for the liposomes compared to the ones with d-amino acids. To examine their biological functions, we tested the cytotoxicity of nanofibers against mammalian primary cells using human bone marrow mesenchymal stem cells and murine astroglial cells. We demonstrated that cell viability increased when d-amino acids were incorporated in the structure of peptide amphiphiles, which is consistent with our finding of their weaker interactions with lipid bilayer membranes.

Functional Control of Peptide Amphiphile Assemblies via Modulation of Internal Cohesion and Surface Chemistry Switch.

Understanding the impacts of the internal cohesion and surface chemistry of supramolecular systems on the collective behaviors in the contacts between the systems and biomolecules can greatly expand the functional diversity and adaptivity of supramolecular nanostructures. Here we show how the tuned molecular interactions modulate the morphologies and internal cohesion of peptide amphiphile (PA) self-assemblies and their resultant functions. Circular dichroism spectroscopy, fluorescence probing, atomic force and electron microscopy, along with molecular dynamics simulations, revealed that the PA self-assembly formed compact long fibers when surface charge repulsion was screened, but formed loose short fibers or micelle-like assemblies when hydrogen bonding was disrupted or hydrophobic core was enhanced. More importantly, depending on the strength of the phospholipid affinity for the cationic segment of the PA, the same internal cohesion of PA nanostructures can lead to either cell death or cell survival, providing unique insights into the design of supramolecular materials.

Force and time-dependent self-assembly, disruption and recovery of supramolecular peptide amphiphile nanofibers

Biological feedback mechanisms exert precise control over the initiation and termination of molecular self-assembly in response to environmental stimuli, while minimizing the formation and propagation of defects through self-repair processes. Peptide amphiphile (PA) molecules can self-assemble at physiological conditions to form supramolecular nanostructures that structurally and functionally resemble the nanofibrous proteins of the extracellular matrix, and their ability to reconfigure themselves in response to external stimuli is crucial for the design of intelligent biomaterials systems. Here, we investigated real-time self-assembly, deformation, and recovery of PA nanofibers in aqueous solution by using a force-stabilizing double-pass scanning atomic force microscopy imaging method to disrupt the self-assembled peptide nanofibers in a force-dependent manner. We demonstrate that nanofiber damage occurs at tip-sample interaction forces exceeding 1 nN, and the damaged fibers subsequently recover when the tip pressure is reduced. Nanofiber ends occasionally fail to reconnect following breakage and continue to grow as two individual nanofibers. Energy minimization calculations of nanofibers with increasing cross-sectional ellipticity (corresponding to varying levels of tip-induced fiber deformation) support our observations, with high-ellipticity nanofibers exhibiting lower stability compared to their non-deformed counterparts. Consequently, tip-mediated mechanical forces can provide an effective means of altering nanofiber integrity and visualizing the self-recovery of PA assemblies.

Interactions of Glycopolymers with Assemblies of Peptide Amphiphiles via Dynamic Covalent Bonding.

In this work, peptide amphiphile (PA) with benzoboroxole (BOB) group at the hydrophilic end was prepared and assembled into fibers (PAA) with BOB group on the fiber surface. Then glycopolymer with mannopyranoside as pendent group interacted with the PAA via dynamic covalent bond between sugar and BOB. By combining the results from 2D 1H NMR spectroscopy, the exact binding mode of mannopyranoside pendent group and BOB, i.e., mannopyranoside participated by its diol on 2,3-position instead of that on 4,6-position, which was clearly observed on the fiber surface. The success in determining this binding mode in macroscopic material was due to the high density of BOB on PAA and the multivalent effect between the multiple BOB moieties on fiber surface and repeating mannopyranoside groups of the glycopolymer.

Control of Heme Coordination and Catalytic Activity by Conformational Changes in Peptide-Amphiphile Assemblies.

Self-assembling peptide materials have gained significant attention, due to well-demonstrated applications, but they are functionally underutilized. To advance their utility, we use noncovalent interactions to incorporate the biological cofactor heme-B for catalysis. Heme-proteins achieve differing functions through structural and coordinative variations. Here, we replicate this phenomenon by highlighting changes in heme reactivity as a function of coordination, sequence, and morphology (micelles versus fibers) in a series of simple peptide amphiphiles with the sequence c16-xyL3K3-CO2H where c16 is a palmitoyl moiety and xy represents the heme binding region: AA, AH, HH, and MH. The morphology of this peptide series is characterized using transmission electron and atomic force microscopies as well as dynamic light scattering. Within this small library of peptide constructs, we show that three spectroscopically (UV/visible and electron paramagnetic resonance) distinct heme environments were generated: noncoordinated/embedded high-spin, five-coordinate high-spin, and six-coordinate low-spin. The resulting material's functional dependence on sequence and supramolecular morphology is highlighted 2-fold. First, the heme active site binds carbon monoxide in both micelles and fibers, demonstrating that the heme active site in both morphologies is accessible to small molecules for catalysis. Second, peroxidase activity was observed in heme-containing micelles yet was significantly reduced in heme-containing fibers. We briefly discuss the implications these findings have in the production of functional, self-assembling peptide materials.

All-Atom Molecular Dynamics Simulations of Peptide Amphiphile Assemblies That Spontaneously Form Twisted and Helical Ribbon Structures.

Self-assembly of peptide amphiphiles (PAs) has been an active research area as the assemblies can be programmed into variously shaped nanostructures. Although cylindrical micelles are common structures, gold-binding peptide conjugates can self-assemble into chiral nanofibers with single or double helices. When gold nanoparticles bind to the helices, the resulting chiral nanoparticle assemblies have a collective plasmonic circular dichroism signal that can serve as nanoscale circular polarizers or chiroptical sensors. A better atomic-level understanding of the factors which lead to helical PA assemblies is therefore of significant importance. In this study we show that all-atom molecular dynamics simulations can describe the spontaneous structural transformation from a planar assembly of PAs to a twisted assembly or to a helical ribbon. The twist angle and the helical diameter calculated from the simulations closely match the experimental results, with the oxidation of a single Met residue in each PA leading to a change from bilayer to monolayer assemblies with significantly different ribbon properties. A secondary structure analysis shows how a combination of β-sheet formation near the hydrophobic core of the micelle and PPII structures from proline-rich C-terminus regions favors helix formation. The simulations presented here demonstrate the capability of predicting self-assembly in chiral structures, protocols that can easily be applied to the assembly of other amphiphilic molecules.

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.

Supramolecular Nanofibers of Peptide Amphiphiles for Medicine.

Peptide nanostructures are an exciting class of supramolecular systems that can be designed for novel therapies with great potential in advanced medicine. This paper reviews progress on nanostructures based on peptide amphiphiles capable of forming one-dimensional assemblies that emulate in structure the nanofibers present in extracellular matrices. These systems are highly tunable using supramolecular chemistry, and can be designed to signal cells directly with bioactive peptides. Peptide amphiphile nanofibers can also be used to multiplex functions through co-assembly and designed to deliver proteins, nucleic acids, drugs, or cells. We illustrate here the functionality of these systems describing their use in regenerative medicine of bone, cartilage, the nervous system, the cardiovascular system, and other tissues. In addition, we highlight recent work on the use of peptide amphiphile assemblies to create hierarchical biomimetic structures with order beyond the nanoscale, and also discuss the future prospects of these supramolecular systems.

Biomimetic Mineralization: The Role of Nanoscale Architecture in Supramolecular Templating of Biomimetic Hydroxyapatite Mineralization (Small 14/2012)

When 3D peptide amphiphile assemblies with flat and cylindrical morphologies are used as templates for mineralization, the nanoscale architecture is found to play a significant role in orienting hydroxyapatite in a bone-like fashion. Cylindrical nanostructures facilitate mineral orientation parallel to the long axis of the 1D assembly while flatter, ribbonlike architectures nucleate randomly oriented mineral. Macroscopically aligned constructs of the cylindrical nanostructures further demonstrate the ability to orient hydroxyapatite across length scales. This work by S. I. Stupp and co-workers emphasizes the importance of template morphology.

Direct observation of morphological transformation from twisted ribbons into helical ribbons.

We report on the direct observation of a nanostructural transformation from a twisted ribbon to a helical ribbon in supramolecular assemblies of peptide amphiphiles. Using cryogenic electron microscopy, a peptide amphiphile molecule containing aromatic residues was found to first assemble into short twisted ribbons in the time range of seconds, which then elongate in the time scale of minutes, and finally transform into helical ribbons over the course of weeks. By synthesizing an analogous molecule without the aromatic side groups, it was found that a cylindrical nanostructure is formed that does not undergo any transitions during the same time period. The study of metastable states in peptide aggregation can contribute to our understanding of amyloid-related diseases, such as Alzheimer's disease.

Peptide amphiphile nanofibers with conjugated polydiacetylene backbones in their core.

The coupling of electronic and biological functionality through self-assembly is an interesting target in supramolecular chemistry. We report here on a set of diacetylene-derivatized peptide amphiphiles (PAs) that react to form conjugated polydiacetylene backbones following self-assembly into cylindrical nanofibers. The polymerization reaction yields highly conjugated backbones when the peptidic segment of the PAs has a linear, as opposed to a branched, architecture. Given the topotactic nature of the polymerization, these results suggest that a high degree of internal order exists in the supramolecular nanofibers formed by the linear PA. On the basis of microscopy, the formation of a polydiacetylene backbone to covalently connect the beta-sheets that help form the fibers does not disrupt the fiber shape. Interestingly, we observe the appearance of a polydiacetylene (PDA) circular dichroism band at 547 nm in linear PA nanofibers suggesting the conjugated backbone in the core of the nanostructures is twisted. We believe this CD signal is due to chiral induction by the beta-sheets, which are normally twisted in helical fashion. Heating and cooling shows simultaneous changes in beta-sheet and conjugated backbone structure, indicating they are both correlated. At the same time, poor polymerization in nanofibers formed by branched PAs indicates that less internal order exists in these nanostructures and, as expected, then a circular dichroism signal is not observed for the conjugated backbone. The general variety of materials investigated here has the obvious potential to couple electronic properties and in vitro bioactivity. Furthermore, the polymerization of monomers in peptide amphiphile assemblies by a rigid conjugated backbone also leads to mechanical robustness and insolubility, two properties that may be important for the patterning of these materials at the cellular scale.