Self-assembly of trifunctional tripeptides to form neural scaffolds

Quest for New Generation Biocompatible Materials: Tailoring β-Peptide Structure and Interactions via Synergy of Experiments and Modelling

Peptide-based self-assembly has been used to produce a wide range of nanostructures. While most of these systems involve self-assembly of α-peptides, more recently β-peptides have also been shown to undergo supramolecular self-assembly, and have been used to produce materials for applications in tissue engineering, cell culture and drug delivery. In order to engineer new materials with specific structure and function, theoretical molecular modelling can provide significant insights into the collective balance of non-covalent interactions that drive the self-assembly and determine the structure of the resultant supramolecular materials under different conditions. However, this approach has only recently become feasible for peptide-based self-assembled nanomaterials, particularly those that incorporate non α-amino acids. This perspective provides an overview of the challenges associated with computational modelling of the self-assembly of β-peptides and the recent success using a combination of experimental and computational techniques to provide insights into the self-assembly mechanisms and fully atomistic models of these new biocompatible materials.

Atomic Scale Structure of Self-Assembled Lipidated Peptide Nanomaterials

β-Peptides have great potential as novel biomaterials and therapeutic agents, due to their unique ability to self-assemble into low dimensional nanostructures, and their resistance to enzymatic degradation in vivo. However, the self-assembly mechanisms of β-peptides, which possess increased flexibility due to the extra backbone methylene groups present within the constituent β-amino acids, are not well understood due to inherent difficulties of observing their bottom-up growth pathway experimentally. A computational approach is presented for the bottom-up modelling of the self-assembled lipidated β3-peptides, from monomers, to oligomers, to supramolecular low-dimensional nanostructures, in all-atom detail. The approach is applied to elucidate the self-assembly mechanisms of recently discovered, distinct structural morphologies of low dimensional nanomaterials, assembled from lipidated β3-peptide monomers. The resultant structures of the nanobelts and the twisted fibrils are stable throughout subsequent unrestrained all-atom molecular dynamics simulations, and these assemblies display good agreement with the structural features obtained from X-ray fiber diffraction and atomic force microscopy data. This is the first reported, fully-atomistic model of a lipidated β3-peptide-based nanomaterial, and the computational approach developed here, in combination with experimental fiber diffraction analysis and atomic force microscopy, will be useful in elucidating the atomic scale structure of self-assembled peptide-based and other supramolecular nanomaterials.

A switch in N-terminal capping of β-peptides creates novel self-assembled nanoparticles

Small tripeptides composed entirely of β3-amino acids have been shown to self-assemble into fibres following acylation of the N-terminus. Given the use of Fmoc as a strategy to initiate self-assembly in α-peptides, we hypothesized that the acyl cap can be replaced by an Fmoc without perturbation to the self-assembly and enable simpler synthetic protocols. We therefore replaced the N-acyl cap for an Fmoc group and herein we show that these Fmoc-protected β3-peptides produce regular spherical particles, rather than fibrous structures, that are stable and capable of encapsulating cargo. We then demonstrated that these particles were able to deliver cargo to cells without any obvious signs of cytotoxicity. This is the first description of such regular nanoparticles derived from Fmoc-protected β3-peptides.

Elucidating the cell penetrating properties of self-assembling β-peptides

Self-assembling lipopeptide hydrogels have been widely developed for the delivery of therapeutics due to their rapid gelation, injectability, and highly controlled physicochemical properties. Lipopeptides are also known for their membrane-associating and cell penetrating properties, which may impact on their application in cell-encapsulation. Self-assembling lipidated-β3-peptide materials developed in our laboratory have previously been used in cell culture as 2D substrates, thus as a continuation of this work we aimed to encapsulate cells in 3D by forming a hydrogel. We therefore assessed the self-assembling lipidated-β3-peptides for cell-penetrating properties in mesenchymal stems cells (MSC) using fluorescence microscopy and membrane association with surface plasmon resonance spectroscopy (SPR). The results demonstrated that lipidated β3-peptides penetrate the MSC plasma membrane and localise to the mitochondrial network. While self-assembling lipopeptide hydrogels have shown tremendous potential for delivery of therapeutics, further optimisation may be required to minimise the membrane uptake of the lipidated-β3-peptides for cell encapsulation applications.

A comparison of fixation methods for SEM analysis of self-assembling peptide hydrogel nanoarchitecture

Determining the porosity of hydrogels is an important component of material characterisation. While scanning electron microscopy (SEM) is a widely used method to study hydrogel nanoarchitecture, it is well-established that SEM sample preparation methods can alter the structure of hydrogels. Herein we describe the impact of sample preparation on the SEM analysis of self-assembling β-peptide hydrogels. Three methods of hydrogel preparation for SEM were compared, and each method preserved distinctly different nanoarchitecture, specifically, different levels of fibre alignment and porosity. Comparison of conventional SEM preparation and our hybrid method, which comprises high pressure freezing, freeze substitution without fixative and critical point drying, showed a high degree of similarity at the nanometre scale and diverging architecture at the micron scale. This study quantified the impact of chemical fixation versus high pressure freezing on self-assembling β³-peptide hydrogels, demonstrated the effect of sample preparation on fibre alignment and porosity, and presents a novel hybrid preparation method where chemical fixation can be avoided when conventional SEM is desired.

Self-Assembling Peptide-Based Functional Biomaterials

Peptides can self-assemble into various hierarchical nanostructures through noncovalent interactions and form functional materials exhibiting excellent chemical and physical properties, which have broad applications in bio-/nanotechnology. The self-assembly mechanism, self-assembly morphology of peptide supramolecular architecture and their various applications, have been widely explored which have the merit of biocompatibility, easy preparation, and controllable functionality. Herein, we introduce the latest research progress of self-assembling peptide-based nanomaterials and review their applications in biomedicine and optoelectronics, including tissue engineering, anticancer therapy, biomimetic catalysis, energy harvesting. We believe that this review will inspire the rational design and development of novel peptide-based functional bio-inspired materials in the future.

Photothermal Sensitive 3D Printed Biodegradable Polyester Scaffolds with Polydopamine Coating for Bone Tissue Engineering

Biodegradable scaffolds with photothermal effects and customizable pore structures are a hot topic of research in the field of bone repair. In this study, we prepared porous scaffolds using poly(lactic acid) (PLA) as the raw material and customized the pore structure with 3D printing technology. First, we investigated the effect of pore structure on the mechanical properties of this 3D PLA scaffold. Subsequently, the optimally designed PLA scaffolds were coated with PDA to enhance their hydrophilicity and bioactivity. XRD (X-ray diffraction), FTIR (Fourier transform infrared spectroscopy) and EDS (Energy dispersive spectroscopy) results indicated that PDA was successfully coated on the surface of PLA scaffolds. SEM (Scanning electron microscopy) micrographs showed that the surface of the PDA/PLA scaffolds became rough. WCA (water contact angle) confirmed that the material has enhanced hydrophilic properties. PDA/PLA scaffolds exhibit a tunable photothermal effect under NIR (near infrared) irradiation. The 3D-printed PLA/PDA scaffolds have remarkable potential as an alternative material for repairing bone defects.

Esterase-Mediated Sustained Release of Peptide-Based Therapeutics from a Self-Assembled Injectable Hydrogel

A synthetic strategy for conjugating small molecules and peptide-based therapeutics, via a cleavable ester bond, to a lipidated β³-tripeptide is presented. The drug-loaded β³-peptide was successfully co-assembled with a functionally inert lipidated β³-tripeptide to form a hydrogel. Quantitative release of lactose from the hydrogel, by the action of serum esterases, is demonstrated over 28 days. The esterase-mediated sustained release of the bioactive brain-derived neurotrophic factor (BDNF) peptide mimics from the hydrogel resulted in increased neuronal survival and normal neuronal function of peripheral neurons. These studies define a versatile strategy for the facile synthesis and co-assembly of self-assembling β³-peptide-based hydrogels with the ability to control drug release using endogenous esterases with potential in vivo applications for sustained localized drug delivery.