Ultra-Low Molecular Weight Photoswitchable Hydrogelators.

Two photoswitchable arylazopyrozoles form hydrogels at a concentration of 1.2 % (w/v). With a molecular weight of 258.28 g mol-1 , these are the lowest known molecular weight hydrogelators that respond reversibly to light. Photoswitching of the E- to the Z-form by exposure to 365 nm light results in a macroscopic gel→sol transition; nearly an order of magnitude reduction in the measured elastic and loss moduli. In the case of the meta-arylazopyrozole, cryogenic transmission electron microscopy suggests that the 29±7 nm wide sheets in the E-gel state narrow to 13±2 nm upon photoswitching to the predominantly Z-solution state. Photoswitching for meta-arylazopyrozole is reversible through cycles of 365 nm and 520 nm excitation with little fatigue. The release of a rhodamine B dye encapsulated in gels formed by the arylazopyrozoles is accelerated more than 20-fold upon photoswitching with 365 nm light, demonstrating these materials are suitable for light-controlled cargo release.

Effect of polar amino acid incorporation on Fmoc-diphenylalanine-based tetrapeptides.

Peptide hydrogels show great promise as extracellular matrix mimics due to their tuneable, fibrous nature. Through incorporation of polar cationic, polar anionic or polar neutral amino acids into the Fmoc-diphenylalanine motif, we show that electrostatic charge plays a key role in the properties of the subsequent gelators. Specifically, we show that an inverse relationship exists for biocompatibility in the solution state versus the gel state for cationic and anionic peptides. Finally, we use tethered bilayer lipid membrane (tBLM) experiments to suggest a likely mode of cytotoxicity for tetrapeptides which exhibit cytotoxicity in the solution state.

Gel- and Solid-State-Structure of Dialanine and Diphenylalanine Amphiphiles: Importance of C⋠⋠⋠H Interactions in Gelation.

To investigate the role of the capping group in the solution and solid-state self-assembly of short peptide amphiphiles, dialanine and diphenylalanine have been linked via the N-terminus to a benzene (phenyl) and 3-naphthyl capping groups using three different methylene linkers; (CH2 )n , n=0-4 for the benezene and 0, 1 and 2 for the naphthalene capping group. Atomic force microscopy (AFM), oscillatory rheology, circular dichroism (CD), and IR analysis have been employed to understand the properties of these peptide-based hydrogels. Several X-ray structures of these short peptide gelators give useful conformational information regarding packing. A comparison of these solid state structures with their gel state properties yielded greater insights into the process of self-assembly in short peptide gelators, particularly in terms of the important role of C⋅⋅⋅H interactions appear to play in determining if a short aromatic peptide does form a gel or not.

Kinetically Controlled Lifetimes in Redox-Responsive Transient Supramolecular Hydrogels.

It remains challenging to program soft materials to show dynamic, tunable time-dependent properties. In this work, we report a strategy to design transient supramolecular hydrogels based on kinetic control of competing reactions. Specifically, the pH-triggered self-assembly of a redox-active supramolecular gelator, N,N'-dibenzoyl-l-cystine (DBC) in the presence of a reducing agent, which acts to disassemble the system. The lifetimes of the transient hydrogels can be tuned simply by pH or reducing agent concentration. We find through kinetic analysis that gel formation hinders the ability of the reducing agent and enables longer transient hydrogel lifetimes than would be predicted. The transient hydrogels undergo clean cycles, with no kinetically trapped aggregates observed. As a result, multiple transient hydrogel cycles are demonstrated and can be predicted. This work contributes to our understanding of designing transient assemblies with tunable temporal control.

Enhanced Mechanical and Thermal Strength in Mixed-Enantiomers-Based Supramolecular Gel.

Mixing supramolecular gels based on enantiomers leads to re-arrangement of gel fibers at the molecular level, which results in more favorable packing and tunable properties. Bis(urea) compounds tagged with a phenylalanine methyl ester in racemic and enantiopure forms were synthesized. Both enantiopure and racemate compounds formed gels in a wide range of solvents and the racemate (1-rac) formed a stronger gel network compared with the enantiomers. The gel (1R+1S) obtained by mixing equimolar amount of enantiomers (1R and 1S) showed enhanced mechanical and thermal stability compared to enantiomers and racemate gels. The preservation of chirality in these compounds was analyzed by circular dichroism and optical rotation measurements. Analysis of the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed that the network in the mixed gel is a combination of enantiomers and racemate fibers, which was further supported by solid-state NMR. The analysis of the packing in xerogels by solid-state NMR spectra and the existence of twisted-tape morphology in SEM and AFM images confirmed the presence of both self-sorted and co-assembled fibers in mixed gel. The enhanced thermal and mechanical strength may be attributed to the enhanced intermolecular forces between the racemate and the enantiomer and the combination of both self-sorted and co-assembled enantiomers in the mixed gel.

Thermal annealing behaviour and gel to crystal transition of a low molecular weight hydrogelator.

The thermal annealing behaviour of an electrolyte-triggered calixarene hydrogelator is found to depend strongly on the specific metal chloride used. While the lithium chloride gel showed typical gel-sol transitions as a function of temperature, the magnesium chloride gel was found to repeatedly strengthen with heat-cool cycles. Structural investigations using small-angle neutron scattering, and scanning probe microscopy, suggest that the annealing behaviour is associated with a change in morphology of the fibrous structures supporting the gel. On prolonged standing at room temperature, the magnesium chloride gel underwent a gel-crystal transition, with the collapsing gel accompanied by the deposition of crystals of a magnesium complex of the proline-functionalised calix[4]arene gelator.

A Capped Dipeptide Which Simultaneously Exhibits Gelation and Crystallization Behavior.

Short peptides capped at their N-terminus are often highly efficient gelators, yet notoriously difficult to crystallize. This is due to strong unidirectional interactions within fibers, resulting in structure propagation only along one direction. Here, we synthesize the N-capped dipeptide, benzimidazole-diphenylalanine, which forms both hydrogels and single crystals. Even more remarkably, we show using atomic force microscopy the coexistence of these two distinct phases. We then use powder X-ray diffraction to investigate whether the single crystal structure can be extrapolated to the molecular arrangement within the hydrogel. The results suggest parallel ß-sheet arrangement as the dominant structural motif, challenging existing models for gelation of short peptides, and providing new directions for the future rational design of short peptide gelators.

Effect of heterocyclic capping groups on the self-assembly of a dipeptide hydrogel.

The mechanism and design rules associated with the self-assembly of short peptides into hydrogels is currently not well understood. In this work, four diphenylalanine-based peptides have been synthesised, bearing heterocyclic capping groups which have different degrees of hydrogen bonding potential and nitrogen substitution. For these four peptides, zeta potential and electrical impedance spectroscopy measurements were undertaken to monitor gelation, with the impedance data showing different gelation times for each peptide hydrogel. Through a combination of atomic force microscopy and rheological measurmeents, including dynamic strain and frequency sweeps, and thixotropic tests, the relationship between the mechanism of self-assembly in these hydrogels and their macroscopic behaviour can be established. It is observed that the degree of nitrogen substitution affects the self-assembly mechanisms of the hydrogels and as such, that there is an interplay between branching and bundling self-assembly pathways that are responsible for the final properties of each hydrogel.

Recent progress in synthetic self-adjuvanting vaccine development.

Vaccination is a proven way to protect individuals against many infectious diseases, as currently highlighted in the global COVID-19 pandemic. Peptides- or small molecule antigen-based vaccination offer advantages over the classical vaccine approaches. However, peptides or small molecules by themselves are generally not sufficiently immunogenic, and thus require an adjuvant to boost an immune response. Several conjugated systems have been developed in recent years to overcome this obstacle. This review summarises different moieties which, when conjugated to peptide antigens, facilitate a specific immune response. Different classes of self-adjuvant moieties are reviewed, including self-assembly peptides, lipids, glycolipids, and polymers.

Hydrogels with intrinsic antibacterial activity prepared from naphthyl anthranilamide (NaA) capped peptide mimics.

In this study, we prepared antibacterial hydrogels through the self-assembly of naphthyl anthranilamide (NaA) capped amino acid based cationic peptide mimics. These ultra-short cationic peptide mimics were rationally designed with NaA as a capping group, L-phenylalanine, a short aliphatic linker, and a cationic group. The synthesized peptide mimics efficiently formed hydrogels with minimum gel concentrations between 0.1 and 0.3%w/v. The resulting hydrogels exhibited desirable viscoelastic properties which can be tuned by varying the cationic group, electronegative substituent, or counter anion. Importantly, nanofibers from the NaA-capped cationic hydrogels were found to be the source of hydrogels' potent bacteriacidal actvity against both Gram-positive and Gram-negative bacteria while remaining non-cytotoxic. These intrinsically antibacterial hydrogels are ideal candidates for further development in applications where bacterial contamination is problematic.

A dendronised polymer architecture breaks the conventional inverse relationship between porosity and mechanical properties of hydrogels.

We present a series of synthetic polymer hydrogels which break the traditional correlation between pore size and mechanical properties. The hydrogels are prepared from a dendronised polymer architecture based on a methacrylate copolymer to which poly(amido amine) dendrons are attached. Our approach will be useful in tailoring hydrogels for tissue engineering, controlled drug release, and flexible electronics.

Beyond Fmoc: a review of aromatic peptide capping groups.

Self-assembling short peptides have attracted widespread interest due to their tuneable, biocompatible nature and have potential applications in energy materials, tissue engineering, sensing and drug delivery. The hierarchical self-assembly of these peptides is highly dependent on the selection of not only amino acid sequence, but also the capping group which is often employed at the N-terminus of the peptide to drive self-assembly. Although the Fmoc (9H-fluorenylmethyloxycarbonyl) group is commonly used due to its utility in solid phase peptide synthesis, many other aromatic capping groups have been reported which yield functional, responsive materials. This review explores recent developments in the utilisation of functional, aromatic capping groups beyond the Fmoc group for the creation of redox-responsive, fluorescent and drug delivering hydrogel scaffolds.

Correction: Non-reversible heat-induced gelation of a biocompatible Fmoc-hexapeptide in water.

Correction for 'Non-reversible heat-induced gelation of a biocompatible Fmoc-hexapeptide in water' by Jonathan P. Wojciechowski et al., Nanoscale, 2020, 12, 8262-8267, DOI: .

Non-reversible heat-induced gelation of a biocompatible Fmoc-hexapeptide in water.

Hydrogel materials which respond to changes in temperature are widely applicable for injectable drug delivery or tissue engineering applications. Here, we report the unsual heat-induced gelation behaviour of a low molecular weight gelator based on an Fmoc-hexapeptide, Fmoc-GFFRGD. We show that Fmoc-GFFRGD forms kinetically stable fibres when mixed with divalent cations (e.g. Ca2+). Gelation of the mixture occurs upon heating of the mixture which enables electrostatic screening by the divalent cations and hydrophobic collapse of the fibres to give a self-supporting hydrogel network that shows good biocompatibility with L929 fibroblast cells. This work highlights a unique mechanism to initiate heat-induced gelation which should find opportunities as a gelation trigger for injectable hydrogels or fundamental self-assembly applications.

Anthranilamide-based Short Peptides Self-Assembled Hydrogels as Antibacterial Agents.

In this study, we describe the synthesis and molecular properties of anthranilamide-based short peptides which were synthesised via ring opening of isatoic anhydride in excellent yields. These short peptides were incorporated as low molecular weight gelators (LMWG), bola amphiphile, and C3-symmetric molecules to form hydrogels in low concentrations (0.07-0.30% (w/v)). The critical gel concentration (CGC), viscoelastic properties, secondary structure, and fibre morphology of these short peptides were influenced by the aromaticity of the capping group or by the presence of electronegative substituent (namely fluoro) and hydrophobic substituent (such as methyl) in the short peptides. In addition, the hydrogels showed antibacterial activity against S. aureus 38 and moderate toxicity against HEK cells in vitro.

Programmable enzymatic oxidation of tyrosine-lysine tetrapeptides.

The ability to control the response of self-assembled systems upon exposure to external stimuli has been a long-standing goal of supramolecular chemistry. Short peptides are an attractive platform to realise this objective due to their chemical diversity and modular nature. Here, we synthesise a library of Fmoc-capped tetrapeptides, each containing two tyrosine and two lysine residues and varying in their amino acid sequence. Despite having similar secondary structure, these tetrapeptides form structures which are highly sequence dependent, yielding aggregates, nanofibres or monomers. This in turn highly affects the rate and degree of oxidative polymerisation by the enzyme tyrosinase, with self-assembled nanofibres exhibiting a greater degree of polymerisation. We monitor the formation of tyrosine oxidation products over time, finding that the precipitation of polymers is driven by quinone-based species. This affects the electrochemical properties of the oxidised peptide polymers, as determined through electrical impedance spectroscopy. Finally, intrinsic fluorescence microscale thermophoresis studies confirm that the degree of oxidative polymerisation is highly dependent on tyrosine solvent accessibility and the presence of peptide monomers. The ability to tune the kinetics of enzymatically active substrates and understand their polymerisation pathways on a molecular level is important for the creation of programmable, enzyme responsive biomaterials.

Optically robust, highly permeable and elastic protein films that support dual cornea cell types.

Damaged corneas can lead to blindness. Due to the worldwide shortage of donor corneas there is a tremendous unmet demand for a robust corneal replacement that supports growth of the major corneal cell types. Commercial artificial corneas comprise plastic polymers that do not adequately support diverse cell growth. We present a new class of protein elastomer-dominated synthetic corneas with attractive performance that intimately couple biologically active tropoelastin to mechanically robust and durable protein silk. Fabricated films substantially replicate the natural cornea physically and by interacting with both key cells types used in cornea repair. Performance encompasses optical clarity at high transmittance, compatible refractive index, substantial glucose permeability, compliant mechanical properties, and support of both growth and function of corneal epithelial and endothelial cells.

Faceted polymersomes: a sphere-to-polyhedron shape transformation.

The creation of "soft" deformable hollow polymeric nanoparticles with complex non-spherical shapes via block copolymer self-assembly remains a challenge. In this work, we show that a perylene-bearing block copolymer can self-assemble into polymeric membrane sacs (polymersomes) that not only possess uncommonly faceted polyhedral shapes but are also intrinsically fluorescent. Here, we further reveal for the first time an experimental visualization of the entire polymersome faceting process. We uncover how our polymersomes facet through a sphere-to-polyhedron shape transformation pathway that is driven by perylene aggregation confined within a topologically spherical polymersome shell. Finally, we illustrate the importance in understanding this shape transformation process by demonstrating our ability to controllably isolate different intermediate polymersome morphologies. The findings presented herein should provide opportunities for those who utilize non-spherical polymersomes for drug delivery, nanoreactor or templating applications, and those who are interested in the fundamental aspects of polymersome self-assembly.

Decoupling the effects of hydrophilic and hydrophobic moieties at the neuron-nanofibre interface.

Peptide-based nanofibres are a versatile class of tunable materials with applications in optoelectronics, sensing and tissue engineering. However, the understanding of the nanofibre surface at the molecular level is limited. Here, a series of homologous dilysine-diphenylalnine tetrapeptides were synthesised and shown to self-assemble into water-soluble nanofibres. Despite the peptide nanofibres displaying similar morphologies, as evaluated through atomic force microscopy and neutron scattering, significant differences were observed in their ability to support sensitive primary neurons. Contact angle and labelling experiments revealed that differential presentation of lysine moieties at the fibre surface did not affect neuronal viability; however the mobility of phenylalanine residues at the nanofibre surface, elucidated through solid- and gel-state NMR studies and confirmed through tethered bilayer lipid membrane experiments, was found to be the determining factor in governing the suitability of a given peptide as a scaffold for primary neurons. This work offers new insights into characterising and controlling the nanofibre surface at the molecular level.

Engineering Biocompatible Scaffolds through the Design of Elastin-Based Short Peptides.

A small library of short peptides has been synthesized based upon the repeat tetrapeptide sequence Gly-Val-Ala-Pro (GVAP) which is found in the hydrophobic domain of tropoelastin. Of the five peptides synthesized, four formed self-supporting hydrogels with similar secondary structures. The ability to tune the mechanical properties of the resultant hydrogels was demonstrated, and this is understood in relation to fiber bundling. Finally, the cytotoxicity of these elastin-based short peptide hydrogels towards HeLa cells was assessed, with clear evidence that increased aromaticity of the peptide is detrimental towards cell viability.