A Review on the Rheological Properties of Single Amino Acids and Short Dipeptide Gels

Self-assembled peptide-based hydrogels have attracted considerable interest from the research community. Particularly, low molecular weight gelators (LMWGs) consisting of amino acids and short peptides are highly suitable for biological applications owing to their facile synthesis and scalability, as well as their biocompatibility, biodegradability, and stability in physiological conditions. However, challenges in understanding the structure-property relationship and lack of design rules hinder the development of new gelators with the required properties for several applications. Hereby, in the plethora of peptide-based gelators, this review discusses the mechanical properties of single amino acid and dipeptide-based hydrogels. A mutual analysis of these systems allows us to highlight the relationship between the gel mechanical properties and amino acid sequence, preparation methods, or N capping groups. Additionally, recent advancements in the tuning of the gels' rheological properties are reviewed. In this way, the present review aims to help bridge the knowledge gap between structure and mechanical properties, easing the selection or design of peptides with the required properties for biological applications.

Methods of changing low molecular weight gel properties through gelation kinetics

Low molecular weight gels continue to attract notable interest, with many potential applications. However, there are still significant gaps in our understanding of these systems and the correlation between the pre-gel and final gel states. The kinetics of the gelation process plays a crucial role in the bulk properties of the hydrogel and presents an opportunity to fine-tune these systems to meet the requirements of the chosen application. Therefore, it is possible to use a single gelator for multiple applications. This review discusses four ways to modify the pre-gelled structures before triggering gelation. Such modifications can enhance the material's intended performance, which may result in significant advancements in high-tech areas, such as drug delivery, cell culturing, electronics, and tissue engineering.

Applications of peptide-based nanomaterials in targeting cancer therapy

To meet the demand for precision medicine, researchers are committed to developing novel strategies to reduce systemic toxicity and side effects in cancer treatment. Targeting peptides are widely applied due to their affinity and specificity, and their ability to be high-throughput screened, chemically synthesized and modified. More importantly, peptides can form ordered self-assembled structures through non-covalent supramolecular interactions, which can form nanostructures with different morphologies and functions, playing crucial roles in targeted diagnosis and treatment. Among them, in targeted immunotherapy, utilizing targeting peptides to block the binding between immune checkpoints and ligands, thereby activating the immune system to eliminate cancer cells, is an advanced therapeutic strategy. In this mini-review, we summarize the screening, self-assembly, and biomedical applications of targeting peptide-based nanomaterials. Furthermore, this mini-review summarizes the potential and optimization strategies of targeting peptides.

Tuning conductivity while maintaining mechanical properties in perylene bisimide hydrogels at physiological pH

By using different salts as a method to achieve gelation of two different amino-acid-functionalised perylene bisimides, we were able to tune reduction potentials while maintaining the mechanical and optical properties of the system all at pH 7.4.

Short-Peptide Supramolecular Hydrogels for In Situ Growth of Metal-Organic Framework-Peptide Biocomposites

The development of bio-MOFs or MOF biocomposites through the combination of MOFs with biopolymers offers the possibility of expanding the potential applications of MOFs, making use of more environmentally benign processes and reagents and giving rise to a new generation of greener and more bio-oriented composite materials. Now, with the increasing use of MOFs for biotechnological applications, the development of new protocols and materials to obtain novel bio-MOFs compatible with biomedical or biotechnological uses is needed. Herein, and as a proof of concept, we have explored the possibility of using short-peptide supramolecular hydrogels as media to promote the growth of MOF particles, giving rise to a new family of bio-MOFs. Short-peptide supramolecular hydrogels are very versatile materials that have shown excellent in vitro and in vivo biomedical applications such as tissue engineering and drug delivery vehicles, among others. These peptides self-assemble by noncovalent interactions, and, as such, these hydrogels are easily reversible, being more biocompatible and biodegradable. These peptides can self-assemble by a multitude of stimuli, such as changes in pH, temperature, solvent, adding salts, enzymatic activity, and so forth. In this work, we have taken advantage of this ability to promote peptide self-assembly with some of the components required to form MOF particles, giving rise to more homogeneous and well-integrated composite materials. Hydrogel formation has been triggered using Zn²⁺ salts, required to form ZIF-8, and formic acid, required to form MOF-808. Two different protocols for the in situ MOF growth have been developed. Finally, the MOF-808 composite hydrogel has been tested for the decontamination of water polluted with phosphate ions as well as for the catalytic degradation of toxic organophosphate methyl paraoxon in an unbuffered solution.

Molecular dynamics study of low molecular weight gel forming salt-triggered dipeptide

Molecular dynamics simulation method was used to study the aggregation of Na and Ca salts in different concentrations of Naphthalene-dipeptide (2NapFF) solutions. The results show that high-valence Ca2+ triggers the formation of a gel at a certain dipeptide concentration, and the low-valence Na+ system follows the aggregation law of general surfactants. The results also show that hydrophobic and electrostatic forces are the main driving forces for the formation of dipeptide aggregates, and that hydrogen bonds do not play a major role in the formation of dipeptide solution aggregates. Hydrophobic and electrostatic effects are the main driving forces for the formation of gels in dipeptide solutions triggered by Ca2+. Electrostatic attraction drives Ca2+ to form a weak coordination with four oxygen atoms on two carboxyl groups, which causes the dipeptide molecules to form a gel with a branched network structure.

Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into "nano-fishnet" fibrillar hydrogels with unusual β-sheet-like raft domains

Low-molecular weight gelators (LMWGs) are small molecules (M_w < ∼1 kDa), which form self-assembled fibrillar network (SAFiN) hydrogels in water when triggered by an external stimulus. A great majority of SAFiN gels involve an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. In some rare cases, a combination of attractive van der Waals and repulsive electrostatic forces drives the formation of bundles with a suprafibrillar hexagonal order. In this work, an unexpected micelle-to-fiber transition is triggered by Ca²⁺ or Ag⁺ ions added to a micellar solution of a novel glycolipid surfactant, whereas salt-induced fibrillation is not common for surfactants. The resulting SAFiN, which forms a hydrogel above 0.5 wt%, has a "nano-fishnet" structure, characterized by a fibrous network of both entangled fibers and β-sheet-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but not known for SAFiNs. The β-sheet-like raft domains are characterized by a combination of cryo-TEM and SAXS and seem to contribute to the stability of glycolipid gels. Furthermore, glycolipid is obtained by fermentation from natural resources (glucose, rapeseed oil), thus showing that naturally engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles.

Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual β-sheet-like domains

Low-molecular weight gelators (LMWGs) are small molecules (M_w < ∼1 kDa), which form self-assembled fibrillar network (SAFiN) hydrogels in water. A great majority of SAFiN gels are described by an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. Here, fibrillation of a biobased glycolipid bolaamphiphile is triggered by Ca²⁺ or Ag⁺ ions which are added to its diluted micellar phase. The resulting SAFiN, which forms a hydrogel above 0.5 wt%, has a "nano-fishnet" structure, characterized by a fibrous network of both entangled fibers and β-sheet-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but generally not known for SAFiN. This work focuses on the strength of the SAFIN gels, their fast recovery after applying a mechanical stimulus (strain) and their unusual resistance to temperature, studied by coupling rheology to small angle X-ray scattering (rheo-SAXS) using synchrotron radiation. The Ca²⁺-based hydrogel maintains its properties up to 55 °C, while the Ag⁺-based gel shows a constant elastic modulus up to 70 °C, without the appearance of any gel-to-sol transition temperature. Furthermore, the glycolipid is obtained by fermentation from natural resources (glucose and rapeseed oil), thus showing that naturally engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles.

Cation-Induced Fibrillation of Microbial Glycolipid Biosurfactant Probed by Ion-Resolved In Situ SAXS

Biological amphiphiles are molecules with a rich phase behavior. Micellar, vesicular, and even fibrillar phases can be found for the same molecule by applying a change in pH or by selecting the appropriate metal ion. The rich phase behavior paves the way toward a broad class of soft materials, from carriers to hydrogels. The present work contributes to understanding the fibrillation of a microbial glycolipid, glucolipid G-C18:1, produced by Starmerella bombicola ΔugtB1 and characterized by a micellar phase at alkaline pH and a vesicular phase at acidic pH. Fibrillation and prompt hydrogelation is triggered by adding either alkaline earth, Ca²⁺, or transition metal, Ag⁺, Fe²⁺, Al³⁺, ions to a G-C18:1 micellar solution. A specifically designed apparatus coupled to a synchrotron SAXS beamline allows the performing of simultaneous cation- and pH-resolved in situ monitoring of the morphological evolution from spheroidal micelles to crystalline fibers, when Ca²⁺ is employed, or to wormlike aggregates, when Fe²⁺ or Al³⁺ solutions are employed. The fast reactivity of Ag⁺ and the crystallinity of Ca²⁺-induced fibers suggest that fibrillation is driven by direct metal-ligand interactions, while the shape transition from spheroidal to elongated micelles with Fe²⁺ or Al³⁺ rather suggest charge screening between the lipid and the hydroxylated cation species.

Single-Walled Carbon Nanotubes as Fluorescent Probes for Monitoring the Self-Assembly and Morphology of Peptide/Polymer Hybrid Hydrogels

Hydrogels formed via supramolecular self-assembly of fluorenylmethyloxycarbonyl (Fmoc)-conjugated amino acids provide excellent scaffolds for 3D cell culture, tissue engineering, and tissue recovery matrices. Such hydrogels are usually characterized by rheology or electron microscopy, which are invasive and cannot provide real-time information. Here, we incorporate near-infrared fluorescent single-walled carbon nanotubes (SWCNTs) into Fmoc-diphenylalanine hydrogels as fluorescent probes, reporting in real-time on the morphology and time-dependent structural changes of the self-assembled hydrogels in the transparency window of biological tissue. We further demonstrate that the gelation process and structural changes upon the addition of cross-linking ions are transduced into spectral modulations of the SWCNT-fluorescence. Moreover, morphological differences of the hydrogels induced by polymer additives are manifested in unique features in fluorescence images of the incorporated SWCNTs. SWCNTs can thus serve as optical probes for noninvasive, long-term monitoring of the self-assembly gelation process and the fate of the resulting peptide hydrogel during long-term usage.

Cooperative Metal Ion Coordination to the Short Self-Assembling Peptide Promotes Hydrogelation and Cellular Proliferation

Non-covalent interactions among short peptides and proteins led to their molecular self-assembly into supramolecular packaging, which provides the fundamental basis of life. These biomolecular assemblies are highly susceptible to the environmental conditions, including temperature, light, pH, and ionic concentration, thus inspiring the fabrication of a new class of stimuli-responsive biomaterials. Here, we report for the first time the cooperative effect of the divalent metal ions to promote hydrogelation in the short collagen inspired self-assembling peptide for developing advanced biomaterials. Introduction of the biologically relevant metal ions (Ca²⁺ /Mg²⁺ ) to the peptide surpasses its limitation to self-assemble into a multi-scale structure at physiological pH. In particular, in presence of metal ions, the negatively charged peptide showed a distinct shift in its equilibrium point of gelation and demonstrated conversion from sol to gel and thus enabling the scope of fabricating an advanced biomaterial for controlling cellular behaviour. Interestingly, tunable mechanical strength and improved cellular response were observed within ion-coordinated peptide hydrogels compared to the peptide gelator. Microscopic analyses, rheological assessment, and biological studies established the importance of utilizing a novel strategy by simply using metal ions to modulate the physical and biological attributes of CIPs to construct next-generation biomaterials. This article is protected by copyright. All rights reserved.

Organic/inorganic hydrogels by simultaneous self-assembly and mineralization of aromatic short-peptides

Hybrid hydrogels with a tunable structure–function relationship were prepared by simultaneous self-assembly and mineralization of aromatic short-peptides. Sub-stoichiometric Ca concentrations resulted in nanoapatite oriented along the peptide fiber.

Molecular Functionalization and Emergence of Long-Range Spin-Dependent Phenomena in Two-Dimensional Carbon Nanotube Networks

Molecular functionalization of CNTs is a routine procedure in the field of nanotechnology. However, whether and how these molecules affect the spin polarization of the charge carriers in CNTs are largely unknown. In this work we demonstrate that spin polarization can indeed be induced in two-dimensional (2D) CNT networks by "certain" molecules and the spin signal routinely survives length scales significantly exceeding 1 μm. This result effectively connects the area of molecular spintronics with that of carbon-based 2D nanoelectronics. By using the versatility of peptide chemistry, we further demonstrate how spin polarization depends on molecular structural features such as chirality as well as molecule-nanotube interactions. A chirality-independent effect was detected in addition to the more common chirality-dependent effect, and the overall spin signal was found to be a combination of both. Finally, the magnetic field dependence of the spin signals has been explored, and the "chirality-dependent" signal has been found to exist only in certain field angles.

Self-Assembly Dipeptide Hydrogel: The Structures and Properties

Self-assembly peptide-based hydrogels are well known and popular in biomedical applications due to the fact that they are readily controllable and have biocompatibility properties. A dipeptide is the shortest self-assembling motif of peptides. Due to its small size and simple synthesis method, dipeptide can provide a simple and easy-to-use method to study the mechanism of peptides' self-assembly. This review describes the design and structures of self-assembly linear dipeptide hydrogels. The strategies for preparing the new generation of linear dipeptide hydrogels can be divided into three categories based on the modification site of dipeptide: 1) COOH-terminal and N-terminal modified dipeptide, 2) C-terminal modified dipeptide, and 3) uncapped dipeptide. With a deeper understanding of the relationship between the structures and properties of dipeptides, we believe that dipeptide hydrogels have great potential application in preparing minimal biocompatible materials.

Metal Cation Triggered Peptide Hydrogels and Their Application in Food Freshness Monitoring and Dye Adsorption

Metal-ligand interactions have emerged as an important tool to trigger and modulate self-assembly, and to tune the properties of the final supramolecular materials. Herein, we report the metal-cation induced self-assembly of a pyrene-peptide conjugate to form hydrogels. The peptide has been rationally designed to favor the formation of β-sheet 1D assemblies and metal coordination through the Glu side chains. We studied in detail the self-assembly process in the presence of H+, Li+, Na+, K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Co2+, Fe3+, and Cr3+ and found that the morphology and mechanical properties of the hydrogels are ion-dependent. Moreover, thanks to the presence of the metal, new applications could be explored. Cu2+ metallogels could be used for amine sensing and meat freshness monitoring, while Zn2+ metallogels showed good selectivity for cationic dye adsorption and separation.

Controlling Nucleopeptide Hydrogel Self-Assembly and Formation for Cell-Culture Scaffold Applications

Hydrogels made from self-assembling peptides have significant advantages in tissue engineering, namely a biocompatible nature and large molecular repertoire. Short peptides in particular allow for straightforward synthesis, self-assembly, and reproducibility. Applications are currently limited, however, due to potential toxicity of the chemical modifications that drive self-assembly and harsh gelation conditions. Peptides conjugated to nucleobases present one opportunity for a naturally derived species to minimize cytotoxicity. We have developed a hydrogel-formation environment for nucleopeptide gelation modulated entirely by biological buffers and salts. Self-assembly in this system is dependent on buffer and ion identity mediated by pK_a and formulation in the former and by valency and ionicity in the latter. Solutions at physiological pH and osmolarity, and in turn compatible with cell culture, initiate hydrogel formation and analytical and computational methods are used to explore pH and salt effects at the molecular and structural level. The mechanism of nucleopeptide self-assembly enables tuning of mechanical properties through the addition of divalent cations and one order of magnitude increase in hydrogel storage modulus. The stability of these constructs therefore provides an opportunity for long-term cell culture, and we demonstrate survival and proliferation of fibroblasts on hydrogel surfaces. This novel, biological buffer-mediated gelation methodology expands opportunities for tissue engineering applications of short peptides and their derivatives.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

The pH- and salt-controlled self-assembly of [1]benzothieno[3,2-b][1]-benzothiophene–peptide conjugates in supramolecular hydrogels

Salt- and pH-triggered supramolecular hydrogels were obtained from a novel [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-peptide hybrid.

Insights into the co-assemblies formed by different aromatic short-peptide amphiphiles

The mechanism of co-assembly of different aromatic dipeptides has been studied using a combination of microscopy and spectroscopy techniques. At an equimolar ratio, the kinetics of the process is favored giving rise to alternate copolymers.

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.

Supramolecular Hydrogels with Properties Tunable by Calcium Ions: A Bio-Inspired Chemical System

Boc-L-DOPA(OBn)₂-OH is a simple synthetic molecule that promotes hydrogelation through electrostatic and π-π stacking interactions. Hydrogelation can occur in alkaline conditions by the use of triggers. Four hydrogels were prepared varying the base, NaOH or Na₂CO₃, and the trigger, GdL or CaCl₂. When the hydrogel formed in the presence of Na₂CO₃ and CaCl₂, the concomitant production of CaCO₃ crystals occurred, generating an organic/inorganic composite material. It was observed that the hydrogel once self-assembled preserved its status even if the trigger, the calcium ions, was removed. The viscoelastic behavior of the hydrogels was analyzed through rheological experiments, which showed a solid-like behavior of the hydrogels. The corresponding xerogels were analyzed mainly by scanning electron microscopy (SEM) and synchrotron X-ray diffraction analysis (XRD). They showed differences in structure, morphology, and fiber organization according to their source. This research presents a hydrogel system that can be applied as a soft biomaterial for tissue engineering, cosmetics, food, and environmental science. Moreover, it represents a model for biomineralization studies in which the hydrogel structure can act as an analogue of the insoluble matrix that confines the calcification site, provides Ca²⁺, and preserves its structure.

A Comprehensive Study on Self-Assembly and Gelation of C13-Dipeptides-From Design Strategies to Functionalities

Computational and experimental methods were applied to investigate the self-assembly and gelation of C₁₃-dipeptides. A modified aggregation propensity (AP_S) was introduced to correlate the effects of side chains of amino acids on the tendency to aggregate. From the experimental results, the ranges of 0.156 < AP_S < 0.250 seemed to be a proper region for the C₁₃-dipeptides to form hydrogels, while other molecules with higher or lower AP_S were insoluble or dissociated. As observed from molecular dynamics simulations, the C₁₃-dipeptides first formed small aggregates through hydrophobic interactions and then rearranged through electrostatic attractions and hydrogen bonds for self-assembly. The C₁₃-dipeptides tended to be antiparallel packed, as shown by hydrogen bonding analyses. Experimental observations and analyses on the structures of C₁₃-dipeptide hydrogels matched the computational conclusions very well. From the five selected gelators, i.e., C₁₃-GW, C₁₃-VY, and C₁₃-WT, strong π-π stacking was observed. For C₁₃-WS, strong hydrogen bonding was found, and in C₁₃-WY, both strong π-π interactions and hydrogen bonds were found. It takes around 90 min or longer for C₁₃-dipeptides to form hydrogels, and those formed by C₁₃-WY and C₁₃-WS had weak water holding capacities, which might be due to strong intermolecular hydrogen bonding. From rheological studies, the C₁₃-dipeptides formed strong chemical gels that were stabilized by strong interactions between the molecular aggregates. These gelators exhibit the potentials to be environmentally friendly substitutes for the common functionalized peptide gelators.

Tuning Mechanical Properties of Pseudopeptide Supramolecular Hydrogels by Graphene Doping

Supramolecular hydrogels, obtained from small organic molecules, may be advantageous over polymeric ones for several applications, because these materials have some peculiar properties that differentiate them from the traditional polymeric hydrogels, such as elasticity, thixotropy, self-healing propensity, and biocompatibility. We report here the preparation of strong supramolecular pseudopeptide-based hydrogels that owe their strength to the introduction of graphene in the gelling mixture. These materials proved to be strong, stable, thermoreversible and elastic. The concentration of the gelator, the degree of graphene doping, and the nature of the trigger are crucial to get hydrogels with the desired properties, where a high storage modulus coexists with a good thixotropic behavior. Finally, NIH-3T3 cells were used to evaluate the cell response to the presence of the most promising hydrogels. The hydrogels biocompatibility remains good, if a small degree of graphene doping is introduced.

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.

Probing the self-assembled structures and pKa of hydrogels using electrochemical methods

The surface chemistry of the aggregated structures that form the scaffold in self-assembled hydrogels - their charge, hydrophobicity and ion-binding dynamics - plays an important role in determining the gel properties and the gel's suitability for specific applications. However, there are limited methods available for the study of this surface chemistry. Here, we show that electrochemical techniques can be used to measure the surface chemical properties of the self-assembled aggregate structures and also to determine the pKa of the gelators. We also provide a method to quickly determine whether a functionalised-dipeptide will form a gel or not. This method has scope for use in high-throughput screening and further complex pH-triggered self-assembled gelation systems.

Self-sorted Oligophenylvinylene and Perylene Bisimide Hydrogels

We describe two component hydrogels with networks composed of self-sorted fibres. The component gelators are based on 1,4-distyrylbenzene (OPV3) and perylene bisimide (PBI) units. Self-sorted gels can be formed by a slow decrease in pH, which leads to sequential assembly. We demonstrate self-sorting by NMR, rheology and small angle X-ray scattering (SAXS). Photoconductive xerogels can be prepared by drying these gels. The wavelength response of the xerogel is different to that of the PBI alone.

Opening a Can of Worm(-like Micelle)s: The Effect of Temperature of Solutions of Functionalized Dipeptides

A simple heat/cool cycle can be used to significantly affect the properties of a solution of a low‐molecular‐weight gelator at high pH. The viscosity and extensional viscosity are increased markedly, leading to materials with very different properties than when the native solution is used.

Supramolecular biofunctional materials

This review discusses supramolecular biofunctional materials, a novel class of biomaterials formed by small molecules that are held together via noncovalent interactions. The complexity of biology and relevant biomedical problems not only inspire, but also demand effective molecular design for functional materials. Supramolecular biofunctional materials offer (almost) unlimited possibilities and opportunities to address challenging biomedical problems. Rational molecular design of supramolecular biofunctional materials exploit powerful and versatile noncovalent interactions, which offer many advantages, such as responsiveness, reversibility, tunability, biomimicry, modularity, predictability, and, most importantly, adaptiveness. In this review, besides elaborating on the merits of supramolecular biofunctional materials (mainly in the form of hydrogels and/or nanoscale assemblies) resulting from noncovalent interactions, we also discuss the advantages of small peptides as a prevalent molecular platform to generate a wide range of supramolecular biofunctional materials for the applications in drug delivery, tissue engineering, immunology, cancer therapy, fluorescent imaging, and stem cell regulation. This review aims to provide a brief synopsis of recent achievements at the intersection of supramolecular chemistry and biomedical science in hope of contributing to the multidisciplinary research on supramolecular biofunctional materials for a wide range of applications. We envision that supramolecular biofunctional materials will contribute to the development of new therapies that will ultimately lead to a paradigm shift for developing next generation biomaterials for medicine.

Thixotropic Peptide-Based Physical Hydrogels Applied to Three-Dimensional Cell Culture

Pseudopeptides containing the d-Oxd or the d-pGlu [Oxd = (4R,5S)-4-methyl-5-carboxyl-oxazolidin-2-one, pGlu = pyroglutamic acid] moiety and selected amino acids were used as low-molecular-weight gelators to prepare strong and thixotropic hydrogels at physiological pH. The addition of calcium chloride to the gelator solutions induces the formation of insoluble salts that get organized in fibers at a pH close to the physiological one. Physical characterization of hydrogels was carried out by morphologic evaluation and rheological measurements and demonstrated that the analyzed hydrogels are thixotropic, as they have the capability to recover their gel-like behavior. As these hydrogels are easily injectable and may be used for regenerative medicine, they were biologically assessed by cell seeding and viability tests. Human gingival fibroblasts were embedded in 2% hydrogels; all of the hydrogels allow the growth of encapsulated cells with a very good viability. The gelator toxicity may be correlated with their tendency to self-assemble and is totally absent when the hydrogel is formed.

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.

Nonlinear Effects in Multicomponent Supramolecular Hydrogels

Multicomponent low molecular weight gels are useful for a range of applications. However, when mixing two components, both of which can independently form a gel, there are many potential scenarios. There is a limited understanding as to how to control and direct the assembly. Here, we focus on a pH-triggered two-component system. At high pH, colloidal structures are formed, and there is a degree of mixing of the two gelators. As the pH is decreased, there is a complex situation, where one gelator directs the assembly in a "sergeants and soldiers" manner. The second gelator is not fully incorporated, and the remainder forms an independent network. The result is that there is a nonlinear dependence on the final mechanical properties of the gels, with the storage or loss modulus being very dependent on the absolute ratio of the two components in the system.

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.

Probing the surface chemistry of self-assembled peptide hydrogels using solution-state NMR spectroscopy

The surface chemistry of self-assembled hydrogel fibres - their charge, hydrophobicity and ion-binding dynamics - is recognised to play an important role in determining how the gels develop as well as their suitability for different applications. However, to date there are no established methodologies for the study of this surface chemistry. Here, we demonstrate how solution-state NMR spectroscopy can be employed to measure the surface chemical properties of the fibres in a range of hydrogels formed from N-functionalised dipeptides, an effective and versatile class of gelator that has attracted much attention. By studying the interactions with the gel fibres of a diverse range of probe molecules and ions, we can simultaneously study a number of surface chemical properties of the NMR invisible fibres in an essentially non-invasive manner. Our results yield fresh insights into the materials. Most notably, gel fibres assembled using different tiggering methods bear differing amounts of negative charge as a result of a partial deprotonation of the carboxylic acid groups of the gelators. We also demonstrate how chemical shift imaging (CSI) techniques can be applied to follow the formation of hydrogels along chemical gradients. We apply CSI to study the binding of Ca²⁺ and subsequent gelation of peptide assemblies at alkaline pH. Using metal ion-binding molecules as probes, we are able to detect the presence of bound Ca²⁺ ions on the surface of the gel fibres. We briefly explore how knowledge of the surface chemical properties of hydrogels could be used to inform their practical application in fields such as drug delivery and environmental remediation.

Tuning hydrogels through metal-based gelation triggers

N-terminal capped tripeptides self-assemble into hydrogels with tuneable properties depending on gelation trigger, giving differences in structure, stiffness and biocompatibility.

Temperature and ion dual responsive biphenyl-dipeptide supramolecular hydrogels as extracellular matrix mimic-scaffolds for cell culture applications

Illustration of the gelation process of a new aromatic short peptide gelator based on biphenyl and its application in cell culture.

Stabilizing bubble and droplet interfaces using dipeptide hydrogels

Hydrophobic dipeptide molecules can be used to create interfacial films covering bubbles and droplets made from a range of oils.

Self-sorted photoconductive xerogels

Self-sorting between n-type and p-type gelators results in effective visible-active photoconductive xerogels.

We show that a perylene bisimide (PBI)-based gelator forms self-sorted mixtures with a stilbene-based gelator. To form the self-sorted gels, we use a slow pH change induced by the hydrolysis of glucono-δ-lactone (GdL) to gluconic acid. We prove that self-sorting occurs using NMR spectroscopy, UV-Vis spectroscopy, rheology, and viscometry. The corresponding xerogels are photoconductive. Importantly, the wavelength dependence of the photoconductive films is different to that of the films formed from the perylene bisimide alone. Transient absorption spectroscopy of the xerogels reveals changes in the spectrum of the PBI on the picosecond timescale in the presence of stilbene with a PBI radical anion being formed within 10 ps when the stilbene is present. The ability to form the PBI radical anion under visible light leads to the enhanced spectral response of the multicomponent gels. These systems therefore have potential as useful visible-active optoelectronics.

Will it gel? Successful computational prediction of peptide gelators using physicochemical properties and molecular fingerprints

The self-assembly of low molecular weight gelators to form gels has enormous potential for cell culturing, optoelectronics, sensing, and for the preparation of structured materials. There is an enormous "chemical space" of gelators. Even within one class, functionalised dipeptides, there are many structures based on both natural and unnatural amino acids that can be proposed and there is a need for methods that can successfully predict the gelation propensity of such molecules. We have successfully developed computational models, based on experimental data, which are robust and are able to identify in silico dipeptide structures that can form gels. A virtual computational screen of 2025 dipeptide candidates identified 9 dipeptides that were synthesised and tested. Every one of the 9 dipeptides synthesised and tested were correctly predicted for their gelation properties. This approach and set of tools enables the "dipeptide space" to be searched effectively and efficiently in order to deliver novel gelator molecules.

Remarkable Role of C-I···N Halogen Bonding in Thixotropic 'Halo'gel Formation

Halogen-bonding-induced self-assembly in the solution of equimolar mixtures of certain pyridyl-ended oligo p-phenylenevinylene (OPV)-derivatives with 1,4-diiodotetrafluorobenzene is reported. The mode of self-assembly, that is, cocrystallization, thixotropic gelation, or precipitation, depends strongly on the nature of chains (n-alkyl chains as a function of length or short oxyethylene chain) appended to the OPV-backbone as well as on the cooling rate of the corresponding hot solution. Single-crystal X-ray diffraction studies of the cocrystals reveal the "infinite" chain formation via C-I···N halogen-bonding interactions between the two components. In addition, multiple noncovalent interactions induce cross-links among these halogen-bonded "infinite" chains. Interestingly, the molecular packing in the "Cogel" bearing OPV-derivative with oxyethylene chains is found to be very similar to that of the cocrystal of the same.

Linking micellar structures to hydrogelation for salt-triggered dipeptide gelators

Some functionalised dipeptides can form hydrogels when salts are added to solutions at high pH. We have used surface tension, conductivity, rheology, optical, confocal and scanning electron microscopy, (1)H NMR and UV-Vis spectroscopy measurements to characterise fully the phase behaviour of solutions of one specific gelator, 2NapFF, at 25 °C at pH 10.5. We show that this specific naphthalene-dipeptide undergoes structural transformations as the concentration is increased, initially forming spherical micelles, then worm-like micelles, followed by association of these worm-like micelles. On addition of a calcium salt, gels are generally formed as long as worm-like micelles are initially present in solution, although there are structural re-organisations that occur at lower concentrations, allowing gelation at lower than expected concentration. Using IR and SANS, we show the differences between the structures present in the solution and hydrogel phases.

Self-healing hydrogels triggered by amino acids

Nine amino acids with different chemical properties have been chosen to promote the formation of hydrogels based on the bolamphiphilic gelator A: three basic amino acids (arginine, histidine and lysine), one acidic amino acid (aspartic acid), two neutral aliphatic amino acids (alanine and serine) and three neutral aromatic amino acids (phenylalanine, tyrosine and tryptophan).

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

Using solution state NMR spectroscopy to probe NMR invisible gelators

Supramolecular hydrogels are formed via the self-assembly of gelator molecules upon application of a suitable trigger. The exact nature of this self-assembly process has been widely investigated as a practical understanding is vital for the informed design of these materials. Solution-state NMR spectroscopy is an excellent non-invasive tool to follow the self-assembly of supramolecular hydrogels. However, in most cases the self-assembled aggregates are silent by conventional (1)H NMR spectroscopy due to the low mobility of the constituent molecules, limiting NMR spectroscopy to following only the initial assembly step(s). Here, we present a new solution-state NMR spectroscopic method which allows the entire self-assembly process of a dipeptide gelator to be followed. This gelator forms transparent hydrogels by a multi-stage assembly process when the pH of an initially alkaline solution is lowered via the hydrolysis of glucono-δ-lactone (GdL). Changes in the charge, hydrophobicity and relative arrangement of the supramolecular aggregates can be followed throughout the assembly process by measuring the residual quadrupolar couplings (RQCs) of various molecular probes (here, (14)NH4(+) and isopropanol-d8), along with the NMR relaxation rates of (23)Na(+). The initially-formed aggregates comprise negatively charged fibrils which gradually lose their charge and become increasingly hydrophobic as the pH falls, eventually resulting in a macroscopic contraction of the hydrogel. We also demonstrate that the in situ measurement of pH by NMR spectroscopy is both convenient and accurate, representing a useful tool for the characterisation of self-assembly processes by NMR.

Probing gelation ability for a library of dipeptide gelators

Functionalised dipeptides are a class of interesting and useful low molecular weight hydrogelators. Here, we report a significantly expanded library of materials, including dipeptides conjugated to carbazole, phenanthracene, anthracene, pyrene and substituted naphthalenes. We assess the effect of using two different gelation methods; a pH-switch and a solvent switch on the gelation behaviour and properties of the dipeptides. Importantly, we investigate the relationship between the structure of these dipeptides and their ability to form gels. From an analysis of the gelation ability of all these dipeptides, it is clear that those containing a phenylalanine as either of the constituent amino acids are much more likely to lead to a gelator being formed as opposed to using non-aromatic amino acids only.

Structural determinants in a library of low molecular weight gelators

Low molecular weight hydrogels are formed by molecules that form a matrix that immobilises water to form a self-supporting gel. Such gels have uses as biomaterials such as molecular scaffolds and structures for tissue engineering. One class of low molecular weight gelator (LMWG), naphthalene-conjugated dipeptides, has been shown to form hydrogels via self-assembly following a controlled drop in pH. A library of naphthalene-dipeptides has been generated previously although the relationship between the precursor sequence and the resulting self-assembled structures remained unclear. Here, we have investigated the structural details of a set of dipeptide sequences containing alanine (A) and valine (V) conjugated to naphthalene groups substituted with a Br, CN or H at the 6-position. Electron microscopy, circular dichroism and X-ray fibre diffraction shows that these LMWG may be structurally classified by their composition: the molecular packing is determined by the class of conjugate, whilst the chirality of the self-assemblies can be attributed to the dipeptide sequence. This provides insights into the relationship between the precursor sequence and the macromolecular and molecular structures of the fibres that make up the resulting hydrogels.