Control of Peptide Amphiphile Supramolecular Nanostructures by Isosteric Replacements

Enhanced Hydrogen Bonding by Urea Functionalization Tunes the Stability and Biological Properties of Peptide Amphiphiles

Self-assembled nanostructures such as those formed by peptide amphiphiles (PAs) are of great interest in biological and pharmacological applications. Herein, a simple and widely applicable chemical modification, a urea motif, was included in the PA's molecular structure to stabilize the nanostructures by virtue of intermolecular hydrogen bonds. Since the amino acid residue nearest to the lipid tail is the most relevant for stability, we decided to include the urea modification at that position. We prepared four groups of molecules (13 PAs in all), with varying levels of intermolecular cohesion, using amino acids with distinct β-sheet promoting potential and/or containing hydrophobic tails of distinct lengths. Each subset contained one urea-modified PA and nonmodified PAs, all with the same peptide sequence. The varied responses of these PAs to variations in pH, temperature, counterions, and biologically related proteins were examined using microscopic, X-ray, spectrometric techniques, and molecular simulations. We found that the urea group contributes to the stabilization of the morphology and internal arrangement of the assemblies against environmental stimuli for all peptide sequences. In addition, microbiological and biological studies were performed with the cationic PAs. These assays reveal that the addition of urea linkages affects the PA-cell membrane interaction, showing the potential to increase the selectivity toward bacteria. Our data indicate that the urea motif can be used to tune the stability of a wide range of PA nanostructures, allowing flexibility on the biomaterial's design and opening a myriad of options for clinical therapies.

Structure-Activity Relationship Study to Develop Peptide Amphiphiles as Species-Specific Antimicrobials

Antimicrobial peptide amphiphiles (PAs) are a promising class of molecules that can disrupt the bacterial membrane or act as drug nanocarriers. In this study, we prepared 33 PAs to establish supramolecular structure-activity relationships. We studied the morphology and activity of the nanostructures against different Gram-positive and Gram-negative bacterial strains (such as Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii). Next, we used principal component analysis (PCA) to determine the key contributors to activity. We found that for S. aureus, the zeta potential was the major contributor to the activity while Gram-negative bacteria were more influenced by the partition coefficient (LogP) with the following order P. aeruginosa>E. coli>A. baumannii. We also performed a study of the mechanism of action of selected PAs on the bacterial membrane assessing the membrane permeability and depolarization, changes in zeta potential and overall integrity. We studied the toxicity of the nanostructures against mammalian cells. Finally, we performed an in vivo study using the wax moth larvae to determine the therapeutic efficacy of the active PAs. This study shows cationic PA nanostructures can be an intriguing platform for the development of nanoantibacterials.

Assessment of the Enzymatic Dephosphorylation Kinetics in the Assemblies of a Phosphopentapeptide that Forms Intranuclear Nanoribbons

Although the formation of peptide assemblies catalyzed by alkaline phosphatase (ALP) has received increasing attention in inhibiting cancer cells, the detailed enzyme kinetics of the dephosphorylation of the corresponding phosphopeptide assemblies have yet to be determined. We recently discovered that assemblies from a phosphopentapeptide can form intracellular nanoribbons that kill induced pluripotent stem cells or osteosarcoma cells, but the kinetics of enzymatic dephosphorylation remain unknown. Thus, we chose to examine the enzyme kinetics of the dephosphorylation of the phosphopentapeptide [NBD-LLLLpY (1)] from concentrations below to above its critical micelle concentration (CMC). Our results show that the phosphopeptide exhibits a CMC of 75 μM in phosphate saline buffer, and the apparent Vmax and Km values of alkaline phosphatase catalyzed dephosphorylation are approximately 0.24 μM/s and 5.67 mM, respectively. Despite dephosphorylation remaining incomplete at 60 min in all the concentrations tested, dephosphorylation of the phosphopeptide at concentrations of 200 μM or above mainly results in nanoribbons, dephosphorylation at concentrations of CMC largely produces nanofibers, and dephosphorylation below the CMC largely generates nanoparticles. Moreover, the formation of nanoribbons correlates with the intranuclear accumulation of the pentapeptide. By providing the first examination of the enzymatic kinetics of phosphopeptide assemblies, this work further supports the notion that the assemblies of phosphopentapeptides can act as a new functional entity for controlling cell fates.

Cell Adhesion Motif-Functionalized Lipopeptides: Nanostructure and Selective Myoblast Cytocompatibility

The conformation and self-assembly of four lipopeptides, peptide amphiphiles comprising peptides conjugated to lipid chains, in aqueous solution have been examined. The peptide sequence in all four lipopeptides contains the integrin cell adhesion RGDS motif, and the cytocompatibility of the lipopeptides is also analyzed. Lipopeptides have either tetradecyl (C₁₄, myristyl) or hexadecyl (C₁₆, palmitoyl) lipid chains and peptide sequence WGGRGDS or GGGRGDS, that is, with either a tryptophan-containing WGG or triglycine GGG tripeptide spacer between the bioactive peptide motif and the alkyl chain. All four lipopeptides self-assemble above a critical aggregation concentration (CAC), determined through several comparative methods using circular dichroism (CD) and fluorescence. Spectroscopic methods [CD and Fourier transform infrared (FTIR) spectroscopy] show the presence of β-sheet structures, consistent with the extended nanotape, helical ribbon, and nanotube structures observed by cryogenic transmission electron microscopy (cryo-TEM). The high-quality cryo-TEM images clearly show the coexistence of helically twisted ribbon and nanotube structures for C₁₄-WGGRGDS, which highlight the mechanism of nanotube formation by the closure of the ribbons. Small-angle X-ray scattering shows that the nanotapes comprise highly interdigitated peptide bilayers, which are also present in the walls of the nanotubes. Hydrogel formation was observed at sufficiently high concentrations or could be induced by a heat/cool protocol at lower concentrations. Birefringence due to nematic phase formation was observed for several of the lipopeptides, along with spontaneous flow alignment of the lyotropic liquid crystal structure in capillaries. Cell viability assays were performed using both L929 fibroblasts and C2C12 myoblasts to examine the potential uses of the lipopeptides in tissue engineering, with a specific focus on application to cultured (lab-grown) meat, based on myoblast cytocompatibility. Indeed, significantly higher cytocompatibility of myoblasts was observed for all four lipopeptides compared to that for fibroblasts, in particular at a lipopeptide concentration below the CAC. Cytocompatibility could also be improved using hydrogels as cell supports for fibroblasts or myoblasts. Our work highlights that precision control of peptide sequences using bulky aromatic residues within "linker sequences" along with alkyl chain selection can be used to tune the self-assembled nanostructure. In addition, the RGDS-based lipopeptides show promise as materials for tissue engineering, especially those of muscle precursor cells.

Self-Assembly of Short Amphiphilic Peptides and Their Biomedical Applications

A series of functional biomaterials with different sizes and morphologies can be constructed through self-assembly, among which amphiphilic peptide-based materials have received intense attention. One main possible reason is that the short amphiphilic peptides can facilitate the formation of versatile materials and promote their further applications in different fields. Another reason is that the simple structure of amphiphilic peptides can help establish the structure-function relationship. This review highlights the recent advances in the self-assembly of two typical peptide species, surfactant-like peptides (SLPs) and peptides amphiphiles (PAs). These peptides can self-assemble into diverse nanostructures. The formation of these different nanostructures resulted from the delicate balance of varied non-covalent interactions. This review embraced each non-covalent interaction and then listed the typical routes for regulating these non-covalent interactions, then realized the morphologies modulation of the self-assemblies. Finally, their applications in some biomedical fields, such as the stabilization of membrane proteins, templating for nanofabrication and biomineralization, acting as the antibacterial and antitumor agents, hemostasis, and synthesis of melanin have been summarized. Further advances in the self-assembly of SLPs and PAs may focus on the design of functional materials with targeted properties and exploring their improved properties.

Spatiotemporal Self-Assembly of Peptide Amphiphiles by Carbonic Anhydrase IX-Targeting Induces Cancer-Lysosomal Membrane Disruption

To achieve spatiotemporal control, an enzyme-instructed self-assembly system is widely used, but this approach typically has a small effect on cellular fate. In this study, we show that the intralysosomal assembly by a carbonic anhydrase IX (CAIX)-targeting peptide amphiphile (Pep-AT) can control cellular fate with a low therapeutic dose by tuning the surface charge based on pH change. Pep-AT self-assembles into a fibrous aggregate with a negative surface charge in an extracellular environment near CAIX. During endocytosis, it changes into a nanofiber with a positive surface charge at the lysosome. Then, it can disrupt the lysosomal membrane and induce cellular apoptosis. This study demonstrates that a spatiotemporal assembly induced by a cancer enzyme and specific organelle can control the cellular fate of cancer.

Urea-Modified Self-Assembling Peptide Amphiphiles That Form Well-Defined Nanostructures and Hydrogels for Biomedical Applications

Hydrogen bonding plays a critical role in the self-assembly of peptide amphiphiles (PAs). Herein, we studied the effect of replacing the amide linkage between the peptide and lipid portions of the PA with a urea group, which possesses an additional hydrogen bond donor. We prepared three PAs with the peptide sequence Phe-Phe-Glu-Glu (FFEE): two are amide-linked with hydrophobic tails of different lengths and the other possesses an alkylated urea group. The differences in the self-assembled structures formed by these PAs were assessed using diverse microscopies, nuclear magnetic resonance (NMR), and dichroism techniques. We found that the urea group influences the morphology and internal arrangement of the assemblies. Molecular dynamics simulations suggest that there are about 50% more hydrogen bonds in nanostructures assembled from the urea-PA than those assembled from the other PAs. Furthermore, in silico studies suggest the presence of urea-π stacking interactions with the phenyl group of Phe, which results in distinct peptide conformations in comparison to the amide-linked PAs. We then studied the effect of the urea modification on the mechanical properties of PA hydrogels. We found that the hydrogel made of the urea-PA exhibits increased stability and self-healing ability. In addition, it allows cell adhesion, spreading, and growth as a matrix. This study reveals that the inclusion of urea bonds might be useful in controlling the morphology, mechanical, and biological properties of self-assembled nanostructures and hydrogels formed by the PAs.