Modulating Supramolecular Peptide Hydrogel Viscoelasticity Using Biomolecular Recognition

Access to Advanced Functional Materials through Postmodification of Biomimetic Assemblies via Click Chemistry

The design, synthesis, and fabrication of functional nanomaterials with specific properties remain a long-standing goal for many scientific fields. The self-assembly of sequence-defined biomimetic synthetic polymers presents a fundamental strategy to explore the chemical space beyond biological systems to create advanced nanomaterials. Moreover, subsequent chemical modification of existing nanostructures is a unique approach for accessing increasingly complex nanostructures and introducing functionalities. Of these modifications, covalent conjugation chemistries, such as the click reactions, have been the cornerstone for chemists and materials scientists. Herein, we highlight some recent advances that have successfully employed click chemistries for the postmodification of assembled one-dimensional (1D) and two-dimensional (2D) nanostructures to achieve applications in molecular recognition, mineralization, and optoelectronics. Specifically, biomimetic nanomaterials assembled from sequence-defined macromolecules such as peptides and peptoids are described.

Recent advancements in bionanomaterial applications of peptide nucleic acid assemblies

Peptide nucleic acid (PNA) is a unique combination of peptides and nucleic acids. PNA can exhibit hydrogen bonding interactions with complementary nucleobases like DNA/RNA. Also, its polyamide backbone allows easy incorporation of biomolecules like peptides and proteins to build hybrid molecular constructs. Because of chimeric structural properties, PNA has lots of potential to build diverse nanostructures. However, progress in the PNA material field is still immature compared with its massive applications in antisense oligonucleotide research. Examples of well-defined molecular assemblies have been reported with PNA amphiphiles, self-assembling guanine-PNA monomers/dimers, and PNA-decorated nucleic acids/ polymers/ peptides. All these works indicate the great potential of PNA to be used as bionanomaterials. The review summarizes the recent reports on PNA-based nanostructures and their versatile applications. Additionally, this review shares a perspective to promote a better understanding of controlling molecular assembly by the systematic structural modifications of PNA monomers.

The Design of a Participatory Peptide Nucleic Acid Duplex Crosslinker to Enhance the Stiffness of Self-Assembled Peptide Gels

Herein, peptide nucleic acids (PNAs) are employed in the design of a participatory duplex PNA-peptide crosslinking agent. Biophysical and mechanical studies show that crosslinkers present during peptide assembly leading to hydrogelation participate in the formation of fibrils while simultaneously installing crosslinks into the higher-order network that constitutes the peptide gel. The addition of 2 mol % crosslinker into the assembling system results in a ~100 % increase in mechanical stiffness without affecting the rate of peptide assembly or the local morphology of fibrils within the gel network. Stiffness enhancement is realized by only affecting change in the elastic component of the viscoelastic gel. A synthesis of the PNA-peptide duplex crosslinkers is provided that allows facile variation in peptide composition and addresses the notorious hydrophobic content of PNAs. This crosslinking system represents a new tool for modulating the mechanical properties of peptide-based hydrogels.

Hyaluronic Acid Hydrogels with Phototunable Supramolecular Cross-Linking for Spatially Controlled Lymphatic Tube Formation

The dynamics of the extracellular matrix (ECM) influences stem cell differentiation and morphogenesis into complex lymphatic networks. While dynamic hydrogels with stress relaxation properties have been developed, many require detailed chemical processing to tune viscoelasticity, offering a limited opportunity for in situ and spatiotemporal control. Here, a hyaluronic acid (HA) hydrogel is reported with viscoelasticity that is controlled and spatially tunable using UV light to direct the extent of supramolecular and covalent cross-linking interactions. This is achieved using UV-mediated photodimerization of a supramolecular ternary complex of pendant trans-Brooker's Merocyanine (BM) guests and a cucurbit[8]uril (CB[8]) macrocycle. The UV-mediated conversion of this supramolecular complex to its covalent photodimerized form is catalyzed by CB[8], offering a user-directed route to spatially control hydrogel dynamics in combination with orthogonal photopatterning by UV irradiation through photomasks. This material thus achieves spatial heterogeneity of substrate dynamics, recreating features of native ECM without the need for additional chemical reagents. Moreover, these dynamic hydrogels afford spatial control of substrate mechanics to direct human lymphatic endothelial cells (LECs) to form lymphatic cord-like structures (CLS). Specifically, cells cultured on viscoelastic supramolecular hydrogels have enhanced formation of CLS, arising from increased expression of key lymphatic markers, such as LYVE-1, Podoplanin, and Prox1, compared to static elastic hydrogels prepared from fully covalent cross-linking. Viscoelastic hydrogels promote lymphatic CLS formation through the expression of Nrp2, VEGFR2, and VEGFR3 to enhance the VEGF-C stimulation. Overall, viscoelastic supramolecular hydrogels offer a facile route to spatially control lymphatic CLS formation, providing a tool for future studies of basic lymphatic biology and tissue engineering applications.

Emerging low-molecular weight nucleopeptide-based hydrogels: state of the art, applications, challenges and perspectives

Over the last twenty years, low-molecular weight gelators and, in particular, peptide-based hydrogels, have drawn great attention from scientists thanks to both their inherent advantages in terms of properties and their high modularity (e.g., number and nature of the amino acids). These supramolecular hydrogels originate from specific peptide self-assembly processes that can be driven, modulated and optimized via specific chemical modifications brought to the peptide sequence. Among them, the incorporation of nucleobases, another class of biomolecules well-known for their abilities to self-assemble, has recently appeared as a new promising and burgeoning approach to finely design supramolecular hydrogels. In this minireview, we would like to highlight the interest, high potential, applications and perspectives of these innovative and emerging low-molecular weight nucleopeptide-based hydrogels.

Dopamine-Substituted Multidomain Peptide Hydrogel with Inherent Antimicrobial Activity and Antioxidant Capability for Infected Wound Healing

Wound infection can cause a delay in wound healing or even wound deterioration, threatening patients' lives. The excessive accumulation of reactive oxygen species (ROS) in infected wounds activates a strong inflammatory response to delay wound healing. Therefore, it is highly desired to develop hydrogels with inherent antimicrobial activity and antioxidant capability for infected wound healing. Herein, a dopamine-substituted multidomain peptide (DAP) with inherent antimicrobial activity, strong skin adhesion, and ROS scavenging has been developed. DAP can form bilayer β-sheets with dopamine residues on the surface of nanofibers. The enhanced rheological properties of DAP-based hydrogel can be achieved not only through UV irradiation but also by incorporation of multivalent ions (e.g., PO₄^3-). Furthermore, the DAP hydrogel shows a broad spectrum of antimicrobial activity due to the high positive charges of lysine residues and the β-sheet formation. When applied to full-thickness dermal wounds in mice, the DAP hydrogel results in a significantly shortened inflammatory stage of the healing process because of its remarkable antimicrobial activity and antioxidant capability. Accelerated wound closure with thick granulation tissue, uniform collagen arrangement, and dense vascularization can be achieved. This work suggests that the DAP hydrogel can serve as antimicrobial coating and ROS-scavenging wound dressing for bacterial-infected wound treatment.

Co-assembly and multicomponent hydrogel formation upon mixing nucleobase-containing peptides

Peptide-based hydrogels are physical gels formed through specific supramolecular self-assembling processes, leading to ordered nanostructures which constitute the water entrapping scaffold of the soft material. Thanks to the inherent properties of peptides, these hydrogels are highly considered in the biomedical domain and open new horizons in terms of application in advanced therapies and biotechnologies. The use of one, and only one, native peptide to formulate a gel is by far the most reported approach to design such materials, but suffers from several limitations, including in terms of mechanical properties. To improve peptide-based hydrogels interest and give rise to innovative properties, several strategies have been proposed in the recent years, and the development of multicomponent peptide-based hydrogels appears as a promising and relevant strategy. Indeed, mixing two or more compounds to develop new materials is a much-used approach that has proven its effectiveness in a wide variety of domains, including polymers, composites and alloys. While still limited to a handful of examples, we would like to report herein on the formulation and the comprehensive study of multicomponent hybrid DNA-nucleobase/peptide-based hydrogels using a multiscale approach based on a large panel of analytical techniques (i.e., rheometry, proton relaxometry, SAXS, electronic microscopy, infrared, circular dichroism, fluorescence, Thioflavin T assays). Among the six multicomponent systems studied, the results highlight the synergistic role of the presence of the two complementary DNA-nucleobases (i.e., adenine/thymine and guanine/cytosine) on the co-assembling process from structural (e.g., morphology of the nanoobjects) to physicochemical (e.g., kinetics of formation, fluorescence properties) and mechanical (e.g., stiffness, resistance to external stress) properties. All the data confirm the relevance of the multicomponent peptide-based approach in the design of innovative hydrogels and bring another brick in the wall of the understanding of these complex and promising systems.

Dopamine Self-Polymerization as a Simple and Powerful Tool to Modulate the Viscoelastic Mechanical Properties of Peptide-Based Gels

Dopamine is a small versatile molecule used for various biotechnological and biomedical applications. This neurotransmitter, in addition to its biological role, can undergo oxidative self-polymerization to yield polydopamine, a robust universal coating material. Herein, we harness dopamine self-polymerization to modulate the viscoelastic mechanical properties of peptide-based gels, expanding their ever-growing application potential. By combining rapid peptide assembly with slower dopamine auto-polymerization, a double network gel is formed, where the fibrillar peptide gel network serves as a scaffold for polydopamine deposition, allowing polydopamine to interpenetrate the gel network as well as establishing crosslinks within the matrix. We have shown that triggering the assembly of a lysine-rich peptide gelator in the presence of dopamine can increase the mechanical rigidity of the resultant gel by a factor of 90 in some cases, while retaining the gel's shear thin-recovery behavior. We further investigate how factors such as polymerization time, dopamine concentration and peptide concentration alter the mechanical properties of the resultant gel. The hybrid peptide-dopamine gel systems were characterized using rheological measurements, circular dichroism spectroscopy and transmission electron microscopy. Overall, triggering peptide gelation in the presence of dopamine represents a simple yet powerful approach to modulate the viscoelastic mechanical properties of peptide-based gels.

Utilizing Frémy's Salt to Increase the Mechanical Rigidity of Supramolecular Peptide-Based Gel Networks

Peptide-based supramolecular gels are an important class of biomaterials that can be used for biomedical applications ranging from drug delivery to tissue engineering. Methodology that allows one to readily modulate the mechanical properties of these gels will allow yet even a broader range of applications. Frémy's salt is an inorganic salt and long-lived free radical that is known to oxidize phenols. Herein, we show that Frémy's salt can be used to dramatically increase the mechanical rigidity of hydrogels formed by tyrosine-containing self-assembling β-hairpin peptides. When Frémy's salt is added to pre-formed gels, it converts tyrosine residues to o-quinones that can subsequently react with amines present within the lysine side chains of the assembled peptide. This results in the installation of chemical crosslinks that reinforce the gel matrix. We characterized the unoxidized and oxidized gel systems using UV-Vis, transmission electron microscopy and rheological measurements and show that Frémy's salt increases the gel rigidity by nearly one order of magnitude, while retaining the gel's shear-thin/recovery behavior. Thus, Frémy's salt represents an on-demand method to modulate the mechanical rigidity of peptide-based self-assembled gels.

Recent Progress in the Design and Application of Supramolecular Peptide Hydrogels in Cancer Therapy

Supramolecular peptide hydrogel (SPH) is a class of biomaterials self-assembled from peptide-based gelators through non-covalent interactions. Among many of its biomedical applications, the potential of SPH in cancer therapy has been vastly explored in the past decade, taking advantage of its good biocompatibility, multifunctionality, and injectability. SPHs can exert localized cancer therapy and induce systemic anticancer immunity to prevent tumor recurrence, depending on the design of SPH. This review first gives a brief introduction to SPH and then outlines the major types of peptide-based gelators that have been developed so far. The methodologies to tune the physicochemical properties and biological activities are summarized. The recent advances of SPH in cancer therapy as carriers, prodrugs, or drugs are highlighted. Finally, the clinical translation potential and main challenges in this field are also discussed.

Improving and fine-tuning the properties of peptide-based hydrogels via incorporation of peptide nucleic acids

Peptide self-assemblies have attracted intense research interest over the last few decades thanks to their implications in key biological processes (e.g., amyloid formation) and their use in biotechnological and (bio)material fields. In particular, peptide-based hydrogels have been highly considered as high potential supramolecular materials in the biomedical domain and open new horizons in terms of applications. To further understand their self-assembly mechanisms and to optimize their properties, several strategies have been proposed with the modification of the constituting amino acid chains via, per se, the introduction of d-amino acids, halogenated amino acids, pseudopeptide bonds, or other chemical moieties. In this context, we report herein on the incorporation of DNA-nucleobases into their peptide nucleic acid (PNA) forms to develop a new series of hybrid nucleopeptides. Thus, depending on the nature of the nucleobase (i.e., thymine, cytosine, adenine or guanine), the physicochemical and mechanical properties of the resulting hydrogels can be significantly improved and fine-tuned with, for instance, drastic enhancements of both the gel stiffness (up to 70-fold) and the gel resistance to external stress (up to 40-fold), and the generation of both thermo-reversible and uncommon red-edge excitation shift (REES) properties. To decipher the actual role of each PNA moiety in the self-assembly processes, the induced modifications from the molecular to the macroscopic scales are studied thanks to the multiscale approach based on a large panel of analytical techniques (i.e., rheology, NMR relaxometry, TEM, thioflavin T assays, FTIR, CD, fluorescence, NMR chemical shift index). Thus, such a strategy provides new opportunities to adapt and fit hydrogel properties to the intended ones and pushes back the limits of supramolecular materials.

Polypeptide-based self-healing hydrogels: Design and biomedical applications

Self-healing hydrogels can heal themselves on the damaged sites, which opens up a fascinating way for enhancing lifetimes of materials. Polypeptide/poly(amino acid) is a class of polymers in which natural amino acid monomers or derivatives are linked by amide bonds with a stable and similar secondary structure as natural proteins (α-helix or β-fold). They have the advantages of nontoxicity, biodegradability, and low immunogenicity as well as easy modification. All these properties make polypeptides extremely suitable for the preparation of self-healing hydrogels for biomedical applications. In this review, we mainly focus on the progress in the fabrication strategies of polypeptide-based self-healing hydrogels and their biomedical applications in the recent 5 years. Various crosslinking methods for the preparation of polypeptide-based self-healing hydrogels are first introduced, including host-guest interactions, hydrogen bonding, electrostatic interactions, supramolecular self-assembly of β-sheets, and reversible covalent bonds of imine and hydrazone as well as molecular multi-interactions. Some representative biomedical applications of these self-healing hydrogels such as delivery system, tissue engineering, 3D-bioprinting, antibacterial and wound healing as well as bioadhesion and hemostasis are also summarized. Current challenges and perspectives in future for these "smart" hydrogels are proposed at the end . STATEMENT OF SIGNIFICANCE: Polypeptides with the advantages of nontoxicity, biodegradability, hydrophilicity and low immunogenicity, are extremely suitable for the preparation of self-healing hydrogels in biomedical applications. Recently, the researches of polypeptide-based self-healing hydrogel have drawn the great attentions for scientists and engineers. A review to summarize the recent progress in design and biomedical applications of these polypeptide-based self-healing hydrogels is highly needed. In this review, we mainly focus on the progress in fabrication strategies of polypeptide-based self-healing hydrogels and biomedical applications in recent five years and aim to draw the increased attention to the importance of these "smart" hydrogels, facilitating the advances in biomedical applications. We believe this work would draw interest from readers of Acta Biomaterialia.

Active Modulation of States of Prestress in Self-Assembled Short Peptide Gels

Peptide hydrogels are excellent candidates for medical therapeutics due to their tuneable viscoelastic properties, however, in vivo they will be subject to various osmotic pressures, temperature changes, and biological co-solutes, which could alter their performance. Peptide hydrogels formed from the synthetic peptide I3K have a temperature-induced hardening of their shear modulus by a factor of 2. We show that the addition of uncross-linked poly( N-isopropylacrylamide) chains to the peptide gels increases the gels' temperature sensitivity by 3 orders of magnitude through the control of osmotic swelling and cross-linking. Using machine learning combined with single-molecule fluorescence microscopy, we measured the modulation of states of prestress in the gels on the level of single peptide fibers. A new self-consistent mixture model was developed to simultaneously quantify the energy and the length distributions of the states of prestress. Switching the temperature from 20 to 40 °C causes 6-fold increases in the number of states of prestress. At the higher temperature, many of the fibers experience constrained buckling with characteristic small wavelength oscillations in their curvature.

Synthesis and Characterization of 1,2-Dithiolane Modified Self-Assembling Peptides

This report focuses on the synthesis of an N-terminus 1,2-dithiolane modified self-assembling peptide and the characterization of the resulting self-assembled supramolecular structures. The synthetic route takes advantage of solid-phase peptide synthesis with the on-resin coupling of the dithiolane precursor molecule, 3-(acetylthio)-2-(acetylthiomethyl)propanoic acid, and the microwave-assisted thioacetate deprotection of the peptide N-terminus before final cleavage from the resin to yield the 1,2-dithiolane modified peptide. After the high-performance liquid chromatography (HPLC) purification of the 1,2-dithiolane peptide, derived from the nucleating core of the Aβ peptide associated with Alzheimer's disease, the peptide is shown to self-assemble into cross-β amyloid fibers. Protocols to characterize the amyloid fibers by Fourier-transform infrared spectroscopy (FT-IR), circular dichroism spectroscopy (CD) and transmission electron microscopy (TEM) are presented. The methods of N-terminal modification with a 1,2-dithiolane moiety to well-characterized self-assembling peptides can now be explored as model systems to develop post-assembly modification strategies and explore dynamic covalent chemistry on supramolecular peptide nanofiber surfaces.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.

Multicomponent self-assembly as a tool to harness new properties from peptides and proteins in material design

Nature is enriched with a wide variety of complex, synergistic and highly functional protein-based multicomponent assemblies.