Supramolecular Gelation Based on Native Amino Acid Tyrosine and Its Charge-Transfer Complex Formation

Self-assembly of amino acids and short-peptide derivatives attracted significant curiosity worldwide due to their unique self-assembly process and wide variety of applications. Amino acid is considered one of the important synthons in supramolecular chemistry. Self-assembly processes and applications of unfunctionalized native amino acids have been less reported in the literature. In this article, we are first-time reporting the self-assembly process of tyrosine (Tyr), an aromatic amino acid, in dimethyl sulfoxide (DMSO) solvent. Most of the studies related to Tyr self-assembly were reported in different aqueous solutions. In our work, we studied the self-assembly in several common organic solvents and found that Tyr could self-assemble into a supramolecular gel in dimethyl sulfoxide (DMSO) solvent. The self-assembly process was investigated by several techniques, such as UV-vis, fluorescence, FTIR, and NMR spectroscopy. Morphological features on the nanoscale were investigated through scanning electron microscopy (SEM). SEM images indicated the formation of nanofibrils with high aspect ratios. The supramolecular gel property was investigated by different rheological experiments. Computational study on the self-assembly process of Tyr in DMSO medium suggested that noncovalent interactions like hydrogen bonding and π-π stacking among the Tyr molecules played a prominent role. Finally, the charge-transfer complex formation ability of electron-rich Tyr with electron-deficient 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was studied. In the presence of DDQ due to the charge-transfer complex formation, the supramolecular gel converted into a reddish color solution, and their fibrillar nanoscale morphologies collapsed.

Characterization of sub-micrometre-sized voids in fixed human brain tissue using scanning X-ray microdiffraction

Using a 5 µm-diameter X-ray beam, we collected scanning X-ray microdiffraction in both the small-angle (SAXS) and the wide-angle (WAXS) regimes from thin sections of fixed human brain tissue from Alzheimer's subjects. The intensity of scattering in the SAXS regime of these patterns exhibits essentially no correlation with the observed intensity in the WAXS regime, indicating that the structures responsible for these two portions of the diffraction patterns, which reflect different length scales, are distinct. SAXS scattering exhibits a power-law behavior in which the log of intensity decreases linearly with the log of the scattering angle. The slope of the log-log curve is roughly proportional to the intensity in the SAXS regime and, surprisingly, inversely proportional to the intensity in the WAXS regime. We interpret these observations as being due to the presence of sub-micrometre-sized voids formed during dehydration of the fixed tissue. The SAXS intensity is due largely to scattering from these voids, while the WAXS intensity derives from the secondary structures of macromolecular material surrounding the voids. The ability to detect and map the presence of voids within thin sections of fixed tissue has the potential to provide novel information on the degradation of human brain tissue in neurodegenerative diseases.

Viscoelastic Supramolecular Hyaluronan-Peptide Cross-Linked Hydrogels

Viscoelasticity plays a key role in hydrogel design. We designed a physically cross-linked hydrogel with tunable viscoelasticity, comprising supramolecular-assembled peptides coupled to hyaluronan (HA), a native extracellular matrix component. We then explored the structural and molecular mechanisms underlying the mechanical properties of a series of these HA-peptide hydrogels. By modifying the peptide sequence, we modulated both long- and short-time stress relaxation rates as a way to target viscoelasticity with limited impact on stiffness, leading to gels that relax up to 60% of stress in 10 min. Gels with the highest viscoelasticity exhibited large mesh sizes and β-sheet secondary structures. The stiffness of the gel correlated with hydrogen bonding between the peptide chains. These gels are cytocompatible: highly viscoelastic gels that mimic the native skin microenvironment promote dermal fibroblast cell spreading. Moreover, HA-peptide gels enabled cell encapsulation, as shown with primary human T cells. Overall, these physically-cross-linked hydrogels enable tunable viscoelasticity that can be used to modulate cell morphology.

Effect of Peptide-Polymer Host-Guest Electrostatic Interactions on Self-Assembling Peptide Hydrogels Structural and Mechanical Properties and Polymer Diffusivity

Peptide-based supramolecular hydrogels are an attractive class of soft materials for biomedical applications when biocompatibility is a key requirement as they exploit the physical self-assembly of short self-assembling peptides avoiding the need for chemical cross-linking. Based on the knowledge developed through our previous work, we designed two novel peptides, E(FKFE)2 and K(FEFK)2, that form transparent hydrogels at pH 7. We characterized the phase behavior of these peptides and showed the clear link that exists between the charge carried by the peptides and the physical state of the samples. We subsequently demonstrate the cytocompatibility of the hydrogel and its suitability for 3D cell culture using 3T3 fibroblasts and human mesenchymal stem cells. We then loaded the hydrogels with two polymers, poly-l-lysine and dextran. When polymer and peptide fibers carry opposite charges, the size of the elemental fibril formed decreases, while the overall level of fiber aggregation and fiber bundle formation increases. This overall network topology change, and increase in cross-link stability and density, leads to an overall increase in the hydrogel mechanical properties and stability, i.e., resistance to swelling when placed in excess media. Finally, we investigate the diffusion of the polymers out of the hydrogels and show how electrostatic interactions can be used to control the release of large molecules. The work clearly shows how polymers can be used to tailor the properties of peptide hydrogels through guided intermolecular interactions and demonstrates the potential of these new soft hydrogels for use in the biomedical field in particular for delivery or large molecular payloads and cells as well as scaffolds for 3D cell culture.

Nanogel delivery systems for cationic peptides: More than a 'One Size Fits All' solution

Self-assembled hyaluronic acid-based nanogels are versatile drug carriers due to their biodegradable nature and gentle preparation conditions, making them particularly interesting for delivery of peptide therapeutics. This study aims to elucidate the relation between peptide structure and encapsulation in a nanogel. Key peptide properties that affect encapsulation in octenyl succinic anhydride-modified hyaluronic acid nanogels were identified as we explored the effect on nanogel characteristics using 12 peptides with varying charge and hydrophobicity. The size and surface properties of the microfluidics-assembled peptide-loaded nanogels were evaluated using dynamic light scattering, laser Doppler electrophoresis, and small angle neutron scattering. Additionally, the change in peptide secondary structure upon encapsulation in nanogels, their release from the nanogels, and the in vitro antimicrobial activity were assessed. In conclusion, the more hydrophobic peptides showed stronger binding to the nanogel carrier and localized internally rather than on the surface of the nanogel, resulting in more spherical nanogels with smoother surfaces and slower release profiles. In contrast, cationic and hydrophilic peptides localized at the nanogel surface resulting in fluffier nanogel structures and quick and more complete release in biorelevant medium. These findings emphasize that the advantages of nanogel delivery systems for different applications depend on the therapeutic peptide properties.

Unlocking the Mitochondria for Nanomedicine-based Treatments: Overcoming Biological Barriers, Improving Designs, and Selecting Verification Techniques

Enhanced targeting approaches will support the treatment of diseases associated with dysfunctional mitochondria, which play critical roles in energy generation and cell survival. Obstacles to mitochondria-specific targeting include the presence of distinct biological barriers and the need to pass through (or avoid) various cell internalization mechanisms. A range of studies have reported the design of mitochondrially-targeted nanomedicines that navigate the complex routes required to influence mitochondrial function; nonetheless, a significant journey lies ahead before mitochondrially-targeted nanomedicines become suitable for clinical use. Moving swiftly forward will require safety studies, in vivo assays confirming effectiveness, and methodologies to validate mitochondria-targeted nanomedicines' subcellular location/activity. From a nanomedicine standpoint, we describe the biological routes involved (from administration to arrival within the mitochondria), the features influencing rational design, and the techniques used to identify/validate successful targeting. Overall, rationally-designed mitochondria-targeted-based nanomedicines hold great promise for precise subcellular therapeutic delivery.

Self-assembly and hydrogelation of a potential bioactive peptide derived from quinoa proteins

In this work the identification of peptides derived from quinoa proteins which could potentially self-assemble, and form hydrogels was carried out with TANGO, a statistical mechanical based algorithm that predicts β-aggregate propensity of peptides. Peptides with the highest aggregate propensity were subjected to gelling screening experiments from which the most promising bioactive peptide with sequence KIVLDSDDPLFGGF was selected. The self-assembling and hydrogelation properties of the C-terminal amidated peptide (KIVLDSDDPLFGGF-NH2) were studied. The effect of concentration, pH, and temperature on the secondary structure of the peptide were probed by circular dichroism (CD), while its nanostructure was studied by transmission electron microscopy (TEM) and small-angle neutron scattering (SANS). Results revealed the existence of random coil, α-helix, twisted β-sheet, and well-defined β-sheet secondary structures, with a range of nanostructures including elongated fibrils and bundles, whose proportion was dependant on the peptide concentration, pH, or temperature. The self-assembly of the peptide is demonstrated to follow established models of amyloid formation, which describe the unfolded peptide transiting from an α-helix-containing intermediate into β-sheet-rich protofibrils. The self-assembly is promoted at high concentrations, elevated temperatures, and pH values close to the peptide isoelectric point, and presumably mediated by hydrogen bond, hydrophobic and electrostatic interactions, and π-π interactions (from the F residue). At 15 mg/mL and pH 3.5, the peptide self-assembled and formed a self-supporting hydrogel exhibiting viscoelastic behaviour with G' (1 Hz) ~2300 Pa as determined by oscillatory rheology measurements. The study describes a straightforward method to monitor the self-assembly of plant protein derived peptides; further studies are needed to demonstrate the potential application of the formed hydrogels in food and biomedicine.

Neutron reflection and scattering in characterising peptide assemblies

Self-assemblies of de novo designed short peptides at interface and in bulk solution provide potential platforms for developing applications in many medical and technological areas. However, characterising how bioinspired supramolecular nanostructures evolve with dynamic self-assembling processes and respond to different stimuli remains challenging. Neutron scattering technologies including small angle neutron scattering (SANS) and neutron reflection (NR) can be advantageous and complementary to other state-of-the-art techniques in tracing structural changes under different conditions. With more neutron sources now available, SANS and NR are becoming increasingly popular in studying self-assembling processes of diverse peptide and protein systems, but the difficulty in experimental manipulation and data analysis can deter beginners. This review will introduce the basic theory, general experimental setup and data analysis of SANS and NR, followed by provision of their applications in characterising interfacial and solution self-assemblies of representative peptides and proteins. SANS and NR are remarkably effective in determining the morphological features self-assembled short peptides, especially size and shape transitions as a result of either sequence changes or in response to environmental stimuli, demonstrating the unique capability of NR and SANS in unravelling the interactive processes. These examples highlight the potential of NR and SANS in supporting the development of novel short peptides and proteins as biopharmaceutical candidates in the fight against many diseases and infections that share common features of membrane interactive processes.

Robustness of heteroaggregates involving hydrophobic cholesterol and its mimetics

Exploring the self and cross aggregation affinity of cholesterol (CHL) and some of its lookalikes, e.g., cholesteryl hemisuccinate (CHM), campesterol (CAM) and arjunic acid (ARJ), provides crucial understanding towards the influence of weak forces in inducing mixed micellization through heteroaggregation. Strongly hydrophobic CHL, with a benchmark inclination towards aggregation, often forms detrimental plaques in crucial human organs that are fairly difficult to disintegrate. Traditionally known anti-dyslipidemic agents like CAM and ARJ are known to interact strongly with CHL in the gut when ingested. They further form mixed micelles along with the bile components and interfere with the CHL absorption across the epithelial cell layer of the intestine. Some invariant questions like how robust are the heteroaggregates formed between these mimetics and CHL are very important to appreciate the efficacy of such anti-dyslipidemic agents. In this work using molecular dynamics simulations and varied structural analysis, we characterize the heteroaggregates. Simulations indicate that CHL-CHM mixed assemblies are comparatively bigger and significantly stabilized by strong electrostatic and favourable vdW forces. Small and diffused CHL-ARJ aggregates are observed in our simulations with a not so favourable energetics, indicating a possible attenuation pathway of CHL aggregation in the presence of ARJ.

A Light Scattering Investigation of Enzymatic Gelation in Self-Assembling Peptides

Self-assembling peptides (SAPs) have been increasingly studied as hydrogel-former gelators because they can create biocompatible environments. A common strategy to trigger gelation, is to use a pH variation, but most methods result in a change in pH that is too rapid, leading to gels with hardly reproducible properties. Here, we use the urea-urease reaction to tune gel properties, by a slow and uniform pH increase. We were able to produce very homogeneous and transparent gels at several SAP concentrations, ranging from c=1g/L to c=10g/L. In addition, by exploiting such a pH control strategy, and combining photon correlation imaging with dynamic light scattering measurements, we managed to unravel the mechanism by which gelation occurs in solutions of (LDLK)3-based SAPs. We found that, in diluted and concentrated solutions, gelation follows different pathways. This leads to gels with different microscopic dynamics and capability of trapping nanoparticles. At high concentrations, a strong gel is formed, made of relatively thick and rigid branches that firmly entrap nanoparticles. By contrast, the gel formed in dilute conditions is weaker, characterized by entanglements and crosslinks of very thin and flexible filaments. The gel is still able to entrap nanoparticles, but their motion is not completely arrested. These different gel morphologies can potentially be exploited for controlled multiple drug release.

Crowder-directed interactions and conformational dynamics in multistimuli-responsive intrinsically disordered protein

The consequences of crowding on the dynamic conformational ensembles of intrinsically disordered proteins (IDPs) remain unresolved because of their ultrafast motion. Here, we report crowder-induced interactions and conformational dynamics of a prototypical multistimuli-responsive IDP, Rec1-resilin. The effects of a range of crowders of varying sizes, forms, topologies, and concentrations were examined using spectroscopic, spectrofluorimetric, and contrast-matching small- and ultrasmall-angle neutron scattering investigation. To achieve sufficient neutron contrast against the crowders, deuterium-labeled Rec1-resilin was biosynthesized successfully. Moreover, the ab initio "shape reconstruction" approach was used to obtain three-dimensional models of the conformational assemblies. The IDP revealed crowder-specific systematic extension and compaction with the level of macromolecular crowding. Last, a robust extension-contraction model has been postulated to capture the fundamental phenomena governing the observed behavior of IDPs. The study provides insights and fresh perspectives for understanding the interactions and structural dynamics of IDPs in crowded states.

Personal Perspective on Understanding Low Molecular Weight Gels

Low molecular weight gels are formed by the self-assembly of small molecules into anisotropic structures that form a network capable of immobilizing the solvent. Such gels are common, with a huge number of different examples existing, and they have many applications. However, there are still significant gaps in our understanding of these systems and challenges that need to be addressed if we are to be able to fully design such systems. Here, a number of these challenges are discussed.

Systems chemistry of peptide-assemblies for biochemical transformations

During the billions of years of the evolutionary journey, primitive polymers, involved in proto metabolic pathways with low catalytic activity, played critical roles in the emergence of modern enzymes with remarkable substrate specificity. The precise positioning of amino acid residues and the complex orchestrated interplay in the binding pockets of evolved enzymes promote covalent and non-covalent interactions to foster a diverse set of complex catalytic transformations. Recent efforts to emulate the structural and functional information of extant enzymes by minimal peptide based assemblies have attempted to provide a holistic approach that could help in discerning the prebiotic origins of catalytically active binding pockets of advanced proteins. In addition to the impressive sets of advanced biochemical transformations, catalytic promiscuity and cascade catalysis by such small molecule based dynamic systems can foreshadow the ancestral catalytic processes required for the onset of protometabolism. Looking beyond minimal systems that work close to equilibrium, catalytic systems and compartments under non-equilibrium conditions utilizing simple prebiotically relevant precursors have attempted to shed light on how bioenergetics played an essential role in chemical emergence of complex behaviour. Herein, we map out these recent works and progress where diverse sets of complex enzymatic transformations were demonstrated by utilizing minimal peptide based self-assembled systems. Further, we have attempted to cover the examples of peptide assemblies that could feature promiscuous activity and promote complex multistep cascade reaction networks. The review also covers a few recent examples of minimal transient catalytic assemblies under non-equilibrium conditions. This review attempts to provide a broad perspective for potentially programming functionality via rational selection of amino acid sequences leading towards minimal catalytic systems that resemble the traits of contemporary enzymes.

Structures of synthetic helical filaments and tubes based on peptide and peptido-mimetic polymers

Synthetic peptide and peptido-mimetic filaments and tubes represent a diverse class of nanomaterials with a broad range of potential applications, such as drug delivery, vaccine development, synthetic catalyst design, encapsulation, and energy transduction. The structures of these filaments comprise supramolecular polymers based on helical arrangements of subunits that can be derived from self-assembly of monomers based on diverse structural motifs. In recent years, structural analyses of these materials at near-atomic resolution (NAR) have yielded critical insights into the relationship between sequence, local conformation, and higher-order structure and morphology. This structural information offers the opportunity for development of new tools to facilitate the predictable and reproduciblede novodesign of synthetic helical filaments. However, these studies have also revealed several significant impediments to the latter process – most notably, the common occurrence of structural polymorphism due to the lability of helical symmetry in structural space. This article summarizes the current state of knowledge on the structures of designed peptide and peptido-mimetic filamentous assemblies, with a focus on structures that have been solved to NAR for which reliable atomic models are available.

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Using small angle scattering to understand low molecular weight gels

The material properties of a gel are determined by the underpinning network that immobilises the solvent. When gels are formed by the self-assembly of small molecules into a so-called low molecular weight gel, the network is the result of the molecules forming one-dimensional objects such as fibres or nanotubes which entangle or otherwise cross-link to form a three-dimensional network. Characterising the one-dimensional objects and the network is difficult. Many conventional techniques rely on drying to probe the network, which often leads to artefacts. An effective tool to probe the gel in the solvated state is small angle scattering. Both small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) can be used. Here, we discuss these approaches and provide a tutorial review to describe how these approaches work, what opportunities there are and how the data treatment should be approached. We aim to show the power of this approach and provide enabling information to make them accessible to the non-specialist.

Manipulating Assemblies in Metallosupramolecular Gels, Driven by Isomeric Ligands, Metal Coordination, and Adaptive Binary Gelator Systems

Metallosupramolecular gel (MSG) is a unique combination of metal-ligand coordination chemistry and supramolecular gel chemistry with extraordinary adaptivity and softness. Such materials find broad uses in industry, pharmaceutical and biomedical sectors, and in technology generation among many others. Pyridyl-appended bis(urea) gelator systems have been extensively studied as potential MSG-forming materials in the presence of various metal ions. The previous molecular engineering approaches depicted competitive intermolecular and intramolecular binding modes involving urea and pyridyl groups and further fine-tuned by the presence of various molecular spacers. In those studies, formation of intermolecular hydrogen bonding among urea moieties to form urea tape was found to be the key factor in one-dimensional assembly and gel formation. In the present study, we show how two isomeric pyridyl-appended bis(urea) ligands can be designed appropriately to essentially eliminate the interference of competitive factors, leaving the intermolecular urea assembly practically unaffected even in the presence of metal ions. We found that one of the two ligands (L2) and the mixed ligand (L1 + L2) assemblies formed gel in the presence and absence of various metal ions. A metal ion with a linear coordination geometry significantly strengthened the gels. Moreover, an inherently weak L1 + L2 assembly appears to be more adaptive in accommodating larger metal ions especially with nonlinear coordination geometry preferences. Small-angle neutron scattering and rheological, spectroscopic, and morphological characterizations, collectively, capture a detailed interplay among ligand assembly, metal-ligand coordination, and adaptivity, driven by the pure versus mixed ligand assemblies. The knowledge gathered from the present study would be highly beneficial in engineering the metallosupramolecular polymeric assemblies toward their functional applications.

Peptide-Based Supramolecular Systems Chemistry

Peptide-based supramolecular systems chemistry seeks to mimic the ability of life forms to use conserved sets of building blocks and chemical reactions to achieve a bewildering array of functions. Building on the design principles for short peptide-based nanomaterials with properties, such as self-assembly, recognition, catalysis, and actuation, are increasingly available. Peptide-based supramolecular systems chemistry is starting to address the far greater challenge of systems-level design to access complex functions that emerge when multiple reactions and interactions are coordinated and integrated. We discuss key features relevant to systems-level design, including regulating supramolecular order and disorder, development of active and adaptive systems by considering kinetic and thermodynamic design aspects and combinatorial dynamic covalent and noncovalent interactions. Finally, we discuss how structural and dynamic design concepts, including preorganization and induced fit, are critical to the ability to develop adaptive materials with adaptive and tunable photonic, electronic, and catalytic properties. Finally, we highlight examples where multiple features are combined, resulting in chemical systems and materials that display adaptive properties that cannot be achieved without this level of integration.

Elongation rate and average length of amyloid fibrils in solution using isotope-labelled small-angle neutron scattering

We demonstrate a solution method that allows both elongation rate and average fibril length of assembling amyloid fibrils to be estimated. The approach involves acquisition of real-time neutron scattering data during the initial stages of seeded growth, using contrast matched buffer to make the seeds effectively invisible to neutrons. As deuterated monomers add on to the seeds, the labelled growing ends give rise to scattering patterns that we model as cylinders whose increase in length with time gives an elongation rate. In addition, the absolute intensity of the signal can be used to determine the number of growing ends per unit volume, which in turn provides an estimate of seed length. The number of ends did not change significantly during elongation, demonstrating that any spontaneous or secondary nucleation was not significant compared with growth on the ends of pre-existing fibrils, and in addition providing a method of internal validation for the technique. Our experiments on initial growth of alpha synuclein fibrils using 1.2 mg ml-1 seeds in 2.5 mg ml-1 deuterated monomer at room temperature gave an elongation rate of 6.3 ± 0.5 Å min-1, and an average seed length estimate of 4.2 ± 1.3 μm.

Protein Hydrogels: The Swiss Army Knife for Enhanced Mechanical and Bioactive Properties of Biomaterials

Biomaterials are dynamic tools with many applications: from the primitive use of bone and wood in the replacement of lost limbs and body parts, to the refined involvement of smart and responsive biomaterials in modern medicine and biomedical sciences. Hydrogels constitute a subtype of biomaterials built from water-swollen polymer networks. Their large water content and soft mechanical properties are highly similar to most biological tissues, making them ideal for tissue engineering and biomedical applications. The mechanical properties of hydrogels and their modulation have attracted a lot of attention from the field of mechanobiology. Protein-based hydrogels are becoming increasingly attractive due to their endless design options and array of functionalities, as well as their responsiveness to stimuli. Furthermore, just like the extracellular matrix, they are inherently viscoelastic in part due to mechanical unfolding/refolding transitions of folded protein domains. This review summarizes different natural and engineered protein hydrogels focusing on different strategies followed to modulate their mechanical properties. Applications of mechanically tunable protein-based hydrogels in drug delivery, tissue engineering and mechanobiology are discussed.

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.

On the Mechanism of Self-Assembly by a Hydrogel-Forming Peptide

Self-assembling peptide-based hydrogels are a class of tunable soft materials that have been shown to be highly useful for a number of biomedical applications. The dynamic formation of the supramolecular fibrils that compose these materials has heretofore remained poorly characterized. A better understanding of this process would provide important insights into the behavior of these systems and could aid in the rational design of new peptide hydrogels. Here, we report the determination of the microscopic steps that underpin the self-assembly of a hydrogel-forming peptide, SgI_37-49. Using theoretical models of linear polymerization to analyze the kinetic self-assembly data, we show that SgI_37-49 fibril formation is driven by fibril-catalyzed secondary nucleation and that all the microscopic processes involved in SgI_37-49 self-assembly display an enzyme-like saturation behavior. Moreover, this analysis allows us to quantify the rates of the underlying processes at different peptide concentrations and to calculate the time evolution of these reaction rates over the time course of self-assembly. We demonstrate here a new mechanistic approach for the study of self-assembling hydrogel-forming peptides, which is complementary to commonly used materials science characterization techniques.

The Aggregation of αB-Crystallin under Crowding Conditions Is Prevented by αA-Crystallin: Implications for α-Crystallin Stability and Lens Transparency

One of the most crowded biological environments is the eye lens which contains a high concentration of crystallin proteins. The molecular chaperones αB-crystallin (αBc) with its lens partner αA-crystallin (αAc) prevent deleterious crystallin aggregation and cataract formation. However, some forms of cataract are associated with structural alteration and dysfunction of αBc. While many studies have investigated the structure and function of αBc under dilute in vitro conditions, the effect of crowding on these aspects is not well understood despite its in vivo relevance. The structure and chaperone ability of αBc under conditions that mimic the crowded lens environment were investigated using the polysaccharide Ficoll 400 and bovine γ-crystallin as crowding agents and a variety of biophysical methods, principally contrast variation small-angle neutron scattering. Under crowding conditions, αBc unfolds, increases its size/oligomeric state, decreases its thermal stability and chaperone ability, and forms kinetically distinct amorphous and fibrillar aggregates. However, the presence of αAc stabilizes αBc against aggregation. These observations provide a rationale, at the molecular level, for the aggregation of αBc in the crowded lens, a process that exhibits structural and functional similarities to the aggregation of cataract-associated αBc mutants R120G and D109A under dilute conditions. Strategies that maintain or restore αBc stability, as αAc does, may provide therapeutic avenues for the treatment of cataract.

How Do Surface Properties of Nanoparticles Influence Their Diffusion in the Extracellular Matrix? A Model Study in Matrigel Using Polymer-Grafted Nanoparticles

Diffusion of nanomedicines inside the extracellular matrix (ECM) has been identified as a key factor to achieve homogeneous distribution and therefore therapeutic efficacy. Here, we sought to determine the impact of nanoparticles' (NPs) surface properties on their ability to diffuse in the ECM. As model nano-objects, we used a library of gold nanoparticles grafted with a versatile polymethacrylate corona, which enabled the surface properties to be modified. To accurately recreate the features of the native ECM, diffusion studies were carried out in a tumor-derived gel (Matrigel). We developed two methods to evaluate the diffusion ability of NPs inside this model gel: an easy-to-implement one based on optical monitoring and another one using small-angle X-ray scattering (SAXS) measurements. Both enabled the determination of the diffusion coefficients of NPs and comparison of the influence of their various surface properties, while the SAXS technique also allowed to monitor the NPs' structure as they diffused inside the gel. Positive charges and hydrophobicity were found to particularly hinder diffusion, and the different results suggested on the whole the presence of NPs-matrix interactions, therefore underlying the importance of the ECM model. The accuracy of the tumor-derived gels used in this study was evidenced by in vivo experiments involving intratumoral injections of NPs on mice, which showed that diffusion patterns in the peripheral tumor tissues were quite similar to the ones obtained within the chosen ECM model.

PAINT-ing Fluorenylmethoxycarbonyl (Fmoc)-Diphenylalanine Hydrogels

Self-assembly of fluorenylmethoxycarbonyl-protected diphenylalanine (FmocFF) in water is widely known to produce hydrogels. Typically, confocal microscopy is used to visualize such hydrogels under wet conditions, that is, without freezing or drying. However, key aspects of hydrogels like fiber diameter, network morphology and mesh size are sub-diffraction limited features and cannot be visualized effectively using this approach. In this work, we show that it is possible to image FmocFF hydrogels by Points Accumulation for Imaging in Nanoscale Topography (PAINT) in native conditions and without direct gel labelling. We demonstrate that the fiber network can be visualized with improved resolution (≈50 nm) both in 2D and 3D. Quantitative information is extracted such as mesh size and fiber diameter. This method can complement the existing characterization tools for hydrogels and provide useful information supporting the design of new materials.

Isotopic Control over Self-Assembly in Supramolecular Gels

It is common to switch between H₂O and D₂O when examining peptide-based systems, with the assumption being that there are no effects from this change. Here, we describe the effect of changing from H₂O to D₂O in a number of low-molecular-weight dipeptide-based gels. Gels are formed by decreasing the pH. In most cases, there is little difference in the structures formed at high pH, but this is not universally true. On lowering the pH, the kinetics of gelation are affected and, in some cases, the structures underpinning the gel network are different. Where there are differences in the self-assembled structures, the resulting gel properties are different. We, therefore, show that isotopic control over gel properties is possible.

Role of Sheet-Edge Interactions in β-sheet Self-Assembling Peptide Hydrogels

Hydrogels' hydrated fibrillar nature makes them the material of choice for the design and engineering of 3D scaffolds for cell culture, tissue engineering, and drug-delivery applications. One particular class of hydrogels which has been the focus of significant research is self-assembling peptide hydrogels. In the present work, we were interested in exploring how fiber-fiber edge interactions affect the self-assembly and gelation properties of amphipathic peptides. For this purpose, we investigated two β-sheet-forming peptides, FEFKFEFK (F8) and KFEFKFEFKK (KF8K), the latter one having the fiber edges covered by lysine residues. Our results showed that the addition of the two lysine residues did not affect the ability of the peptides to form β-sheet-rich fibers, provided that the overall charge carried by the two peptides was kept constant. However, it did significantly reduce edge-driven hydrophobic fiber-fiber associative interactions, resulting in reduced tendency for KF8K fibers to associate/aggregate laterally and form large fiber bundles and consequently network cross-links. This effect resulted in the formation of hydrogels with lower moduli but faster dynamics. As a result, KF8K fibers could be aligned only under high shear and at high concentration while F8 hydrogel fibers were found to align readily at low shear and low concentration. In addition, F8 hydrogels were found to fragment at high concentration because of the high aggregation state stabilizing the fiber bundles, resulting in fiber breakage rather than disentanglement and alignment.

Therapeutic potential of polypeptide-based conjugates: Rational design and analytical tools that can boost clinical translation

The clinical success of polypeptides as polymeric drugs, covered by the umbrella term "polymer therapeutics," combined with related scientific and technological breakthroughs, explain their exponential growth in the development of polypeptide-drug conjugates as therapeutic agents. A deeper understanding of the biology at relevant pathological sites and the critical biological barriers faced, combined with advances regarding controlled polymerization techniques, material bioresponsiveness, analytical methods, and scale up-manufacture processes, have fostered the development of these nature-mimicking entities. Now, engineered polypeptides have the potential to combat current challenges in the advanced drug delivery field. In this review, we will discuss examples of polypeptide-drug conjugates as single or combination therapies in both preclinical and clinical studies as therapeutics and molecular imaging tools. Importantly, we will critically discuss relevant examples to highlight those parameters relevant to their rational design, such as linking chemistry, the analytical strategies employed, and their physicochemical and biological characterization, that will foster their rapid clinical translation.

Mechanistic investigations into the encapsulation and release of small molecules and proteins from a supramolecular nucleoside gel in vitro and in vivo

Supramolecular gels have recently emerged as promising biomaterials for the delivery of a wide range of bioactive molecules, from small hydrophobic drugs to large biomolecules such as proteins. Although it has been demonstrated that each encapsulated molecule has a different release profile from the hydrogel, so far diffusion and steric impediment have been identified as the only mechanisms for the release of molecules from supramolecular gels. Erosion of a supramolecular gel has not yet been reported to contribute to the release profiles of encapsulated molecules. Here, we use a novel nucleoside-based supramolecular gel as a drug delivery system for proteins with different properties and a hydrophobic dye and describe for the first time how these materials interact, encapsulate and eventually release bioactive molecules through an erosion-based process. Through fluorescence microscopy and spectroscopy as well as small angle X-ray scattering, we show that the encapsulated molecules directly interact with the hydrogel fibres - rather than being physically entrapped in the gel network. The ability of these materials to protect proteins against enzymatic degradation is also demonstrated here for the first time. In addition, the released proteins were proven to be functional in vitro. Real-time fluorescence microscopy together with macroscopic release studies confirm that erosion is the key release mechanism. In vivo, the gel completely degrades after two weeks and no signs of inflammation are detected, demonstrating its in vivo safety. By establishing the contribution of erosion as a key driving force behind the release of bioactive molecules from supramolecular gels, this work provides mechanistic insight into the way molecules with different properties are encapsulated and released from a nucleoside-based supramolecular gel and sets the basis for the design of more tailored supramolecular gels for drug delivery applications.

On the Mechanical Properties of N-Functionalised Dipeptide Gels

The properties of a hydrogel are controlled by the underlying network that immobilizes the solvent. For gels formed by the self-assembly of a small molecule, it is common to show the primary fibres that entangle to form the network by microscopy, but it is difficult to access information about the network. One approach to understand the network is to examine the effect of the concentration on the rheological properties, such that G'∝ cx, where G' is the storage modulus and c is the concentration. A number of reports link the exponent x to a specific type of network. Here, we discuss a small library of gels formed using functionalized dipeptides, and describe the underlying networks of these gels, using microscopy, small angle scattering and rheology. We show that apparently different networks can give very similar values of x.

Controlling the Assembly and Properties of Low-Molecular-Weight Hydrogelators

Low-molecular-weight gels are formed by the self-assembly of small molecules into fibrous networks that can immobilize a significant amount of solvent. Here, we focus on our work with a specific class of gelator, the functionalized dipeptide. We discuss the current state of the art in the area, focusing on how these materials can be controlled. We also highlight interesting and unusual observations and unanswered questions in the field.

Investigating the effect of supramolecular gel phase crystallization on gel nucleation

Supramolecular gel phase crystallization offers a new strategy for drug polymorph screening and discovery. In this method, the crystallization outcome depends on the interaction between solute and gel fibre. While supramolecular gels have shown success in producing new polymorphs and crystals with novel morphologies, role of the gel and nature of gel-solute interaction remains largely unexplored. The present study aims to provide a comprehensive picture of the structural evolution of a supramolecular gel produced from a bis(urea) based gelator (G) in the presence of a polymorphic drug carbamazepine (CBZ). The structural aspects of the gel have been assessed by single crystal X-ray analysis, X-ray powder diffraction (XRPD) and solid state NMR spectroscopy. Small Angle Neutron Scattering (SANS) has been used to follow the changes in gel structure in the presence of CBZ. Visual evidence from morphological study and structural evolution observed at a macroscopic level from rheological measurements, shows good agreement with the SANS results. The concentration of the gelator and the relative proportion of G to CBZ were found to be crucial factors in determining the competitive nucleation events involving gelation and crystallization. At a critical G to CBZ ratio the effect of CBZ on gel structure was maximum and fiber bundling in the gel was found to be critically affected. This study offers important information about how the interplay of gelator assembly and gel-solute interactions can fine-tune the nucleation events in a supramolecular gel phase crystallization.

Tough Photocrosslinked Silk Fibroin/Graphene Oxide Nanocomposite Hydrogels

The development of protein-based hydrogels for tissue engineering applications is often limited by their mechanical properties. Herein, we present the facile fabrication of tough regenerated silk fibroin (RSF)/graphene oxide (GO) nanocomposite hydrogels by a photochemical cross-linking method. The RSF/GO composite hydrogels demonstrated soft and adhesive properties during initial stages of photocrosslinking (<2 min), which is not observed for the pristine RSF hydrogel, and rendered a tough and nonadhesive hydrogel upon complete cross-linking (10 min). The composite hydrogels exhibited superior tensile mechanical properties, increased β-sheet content, and decreased chain mobility compared to that of the pristine RSF hydrogels. The composite hydrogels demonstrated Young's modulus as high as ∼8 MPa, which is significantly higher than native cartilage (∼1.5 MPa), and tensile toughness as high as ∼2.4 MJ/m³, which is greater than that of electroactive polymer muscles and at par with RSF/GO composite membranes fabricated by layer-by-layer assembly. Small-angle scattering study reveals the hierarchical structure of photocrosslinked RSF hydrogels to comprise randomly distributed water-poor (hydrophobic) and water-rich (hydrophilic) regions at the nanoscale, whereas water pores and channels exhibiting fractal-like characteristics at the microscale. The size of hydrophobic domain (containing β-sheets) was observed to increase slightly with GO incorporation and/or alcohol post-treatment, whereas the size of the hydrophilic domain (intersheet distance containing random coils) was observed to increase significantly, which influences/affects water uptake capacity, cross-link density, and mechanical properties of hydrogels. The presented results have implications for both fundamental understanding of the structure-property relationship of RSF-based hydrogels and their technological applications.

Targetable Mechanical Properties by Switching between Self-Sorting and Co-assembly with In Situ Formed Tripodal Ketoenamine Supramolecular Hydrogels

A new family of supramolecular hydrogelators are introduced in which self-sorting and co-assembly can be utilised in the tuneability of the mechanical properties of the materials, a property closely tied to the nanostructure of the gel network. The in situ reactivity of the components of the gelators allows for system chemistry concepts to be applied to the formation of the gels and shows that molecular properties, and not necessarily the chemical identity, determines some gel properties in these family of gels.

Designing Peptide/Graphene Hybrid Hydrogels through Fine-Tuning of Molecular Interactions

A recent strategy that has emerged for the design of increasingly functional hydrogels is the incorporation of nanofillers in order to exploit their specific properties to either modify the performance of the hydrogel or add functionality. The emergence of carbon nanomaterials in particular has provided great opportunity for the use of graphene derivatives (GDs) in biomedical applications. The key challenge when designing hybrid materials is the understanding of the molecular interactions between the matrix (peptide nanofibers) and the nanofiller (here GDs) and how these affect the final properties of the bulk material. For the purpose of this work, three gelling β-sheet-forming, self-assembling peptides with varying physiochemical properties and five GDs with varying surface chemistries were chosen to formulate novel hybrid hydrogels. First the peptide hydrogels and the GDs were characterized; subsequently, the molecular interaction between peptides nanofibers and GDs were probed before formulating and mechanically characterizing the hybrid hydrogels. We show how the interplay between electrostatic interactions, which can be attractive or repulsive, and hydrophobic (and π-π in the case of peptide containing phenylalanine) interactions, which are always attractive, play a key role on the final properties of the hybrid hydrogels. The shear modulus of the hydrid hydrogels is shown to be related to the strength of fiber adhesion to the flakes, the overall hydrophobicity of the peptides, as well as the type of fibrillar network formed. Finally, the cytotoxicity of the hybrid hydrogel formed at pH 6 was also investigated by encapsulating and culturing human mesemchymal stem cells (hMSC) over 14 days. This work clearly shows how interactions between peptides and GDs can be used to tailor the mechanical properties of the resulting hydrogels, allowing the incorporation of GD nanofillers in a controlled way and opening the possibility to exploit their intrinsic properties to design novel hybrid peptide hydrogels for biomedical applications.

Does Drying Affect Gel Networks?

The properties of low molecular weight gels are determined by the underlying, self-assembled network. To access information on the network, it is common for techniques to be used that require the gel to be dried, such as transmission electron microscopy or scanning electron microscopy. The implicit assumption is that this drying has no bearing on the data collected. Here, we discuss the validity of this assumption.

How should multicomponent supramolecular gels be characterised?

We discuss the current state of characterising multicomponent low molecular weight gels across all length scales, and the effectiveness of the different techniques that have been used.

Drying Affects the Fiber Network in Low Molecular Weight Hydrogels

Low molecular weight gels are formed by the self-assembly of a suitable small molecule gelator into a three-dimensional network of fibrous structures. The gel properties are determined by the fiber structures, the number and type of cross-links and the distribution of the fibers and cross-links in space. Probing these structures and cross-links is difficult. Many reports rely on microscopy of dried gels (xerogels), where the solvent is removed prior to imaging. The assumption is made that this has little effect on the structures, but it is not clear that this assumption is always (or ever) valid. Here, we use small angle neutron scattering (SANS) to probe low molecular weight hydrogels formed by the self-assembly of dipeptides. We compare scattering data for wet and dried gels, as well as following the drying process. We show that the assumption that drying does not affect the network is not always correct.

Controlling self-assembly of diphenylalanine peptides at high pH using heterocyclic capping groups

Using small angle neutron scattering (SANS), it is shown that the existence of pre-assembled structures at high pH for a capped diphenylalanine hydrogel is controlled by the selection of N-terminal heterocyclic capping group, namely indole or carbazole. At high pH, changing from a somewhat hydrophilic indole capping group to a more hydrophobic carbazole capping group results in a shift from a high proportion of monomers to self-assembled fibers or wormlike micelles. The presence of these different self-assembled structures at high pH is confirmed through NMR and circular dichroism spectroscopy, scanning probe microscopy and cryogenic transmission electron microscopy.

Controlling the network type in self-assembled dipeptide hydrogels

We show that the same low molecular weight gelator can form gels using three different methods. Gels were formed from a high pH solution either by adding a salt or by adding an acid; gels were also formed by adding water to a solution of the gelator in an organic solvent. The mechanical properties for the gels formed by the different methods are different from one another. We link this to the network type that is formed, as well as the fibrous structures that are formed. The salt-triggered gels show a significant number of fibres that tend to align. The acid-triggered gels contain many thin fibres, which form an entangled network. The solvent-triggered gels show the presence of spherulitic domains. We show that it is tractable to vary the trigger mechanism for an established, robust gelator to prepare gels with targeted properties as opposed to synthesising new gelators.

Multifunctional thermoresponsive designer peptide hydrogels

We report the synthesis and characterization of multifunctional peptides comprised of a hydrogel forming β-sheet peptide segment and a matrix metalloproteinase 2 substrate containing a propargylglycinyl linker that is further derivatized with an RGD peptide sequence via "click" chemistry. In contrast to currently known systems, these multifunctional peptides formed gels that are stiffer than those formed by their respective precursors. All the peptides showed reversible thermoresponsive properties, which render them as suitable lead systems for a variety of possible biomedical applications.

STATEMENT OF SIGNIFICANCE

In general, it has been frequently observed that chemical biofunctionalization of peptide hydrogels adversely affects peptide assembly, hydrogel formation or mechanical properties, which severely compromises their application. A functionalization protocol that allows to generate peptide hydrogels that display significantly improved mechanical properties over their unfunctionalized counterparts is reported in this work. These peptides also showed thermoresponsive viscoelastic characteristics, including an example of a peptide hydrogel that displays lower critical solution temperature behaviour.

Self-assembled peptide nanostructures for functional materials

Nature is an important inspirational source for scientists, and presents complex and elegant examples of adaptive and intelligent systems created by self-assembly. Significant effort has been devoted to understanding these sophisticated systems. The self-assembly process enables us to create supramolecular nanostructures with high order and complexity, and peptide-based self-assembling building blocks can serve as suitable platforms to construct nanostructures showing diverse features and applications. In this review, peptide-based supramolecular assemblies will be discussed in terms of their synthesis, design, characterization and application. Peptide nanostructures are categorized based on their chemical and physical properties and will be examined by rationalizing the influence of peptide design on the resulting morphology and the methods employed to characterize these high order complex systems. Moreover, the application of self-assembled peptide nanomaterials as functional materials in information technologies and environmental sciences will be reviewed by providing examples from recently published high-impact studies.

Supramolecular Nanostructure Formation of Coassembled Amyloid Inspired Peptides

Characterization of amyloid-like aggregates through converging approaches can yield deeper understanding of their complex self-assembly mechanisms and the nature of their strong mechanical stability, which may in turn contribute to the design of novel supramolecular peptide nanostructures as functional materials. In this study, we investigated the coassembly kinetics of oppositely charged short amyloid-inspired peptides (AIPs) into supramolecular nanostructures by using confocal fluorescence imaging of thioflavin T binding, turbidity assay and in situ small-angle X-ray scattering (SAXS) analysis. We showed that coassembly kinetics of the AIP nanostructures were consistent with nucleation-dependent amyloid-like aggregation, and aggregation behavior of the AIPs was affected by the initial monomer concentration and sonication. Moreover, SAXS analysis was performed to gain structural information on the size, shape, electron density, and internal organization of the coassembled AIP nanostructures. The scattering data of the coassembled AIP nanostructures were best fitted into to a combination of polydisperse core-shell cylinder (PCSC) and decoupling flexible cylinder (FCPR) models, and the structural parameters were estimated based on the fitting results of the scattering data. The stability of the coassembled AIP nanostructures in both fiber organization and bulk viscoelastic properties was also revealed via temperature-dependent SAXS analysis and oscillatory rheology measurements, respectively.

Modification of β-Sheet Forming Peptide Hydrophobic Face: Effect on Self-Assembly and Gelation

β-Sheet forming peptides have attracted significant interest for the design of hydrogels for biomedical applications. One of the main challenges is the control and understanding of the correlations between peptide molecular structure, the morphology, and topology of the fiber and network formed as well as the macroscopic properties of the hydrogel obtained. In this work, we have investigated the effect that functionalizing these peptides through their hydrophobic face has on their self-assembly and gelation. Our results show that the modification of the hydrophobic face results in a partial loss of the extended β-sheet conformation of the peptide and a significant change in fiber morphology from straight to kinked. As a consequence, the ability of these fibers to associate along their length and form large bundles is reduced. These structural changes (fiber structure and network topology) significantly affect the mechanical properties of the hydrogels (shear modulus and elasticity).