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.

Design Rules for Binary Bisamide Gelators: toward Gels with Tailor-Made Structures and Properties

This study intends to develop design rules for binary mixture of gelators that govern their assembly behavior and subsequently explore the impact of their supramolecular assembly patterns on the gels' rheological properties. To achieve these goals, nBA gelators with odd and even parities [n-methylene spacers between the amide groups (n = 5-10) and 17 carbons at each end] were blended at different ratios. Such bisamides with simple structures were selected to study because their different spacer lengths offer the possibility to have matching or non-matching hydrogen bonds. The results show that the assembly behavior of binary mixtures of bisamide gelators is the same in the solid and gel states. Binary mixtures of gelators, which only differ two methylene moieties in the spacer length, form compounds and co-assemble into fibers and sheets observed for (5BA)1(7BA)1 and (6BA)1(8BA)1 mixtures, respectively. Binary gelator mixtures of the same parity and a larger spacer length difference still lead to mixing for the odd parity couple (5BA)1(9BA)1), but to partial phase separation for the even parity mixture (6BA)1(10BA)1. Binary mixtures of gelators of different parities gave complete phase separation in the solid state, and self-sorted gels consisting of discrete fibers and sheets in the gels of (5BA)3(6BA)1 and (5BA)3(10BA)1. The even-even binary gels (20 wt %) consisting of co-assembled sheets show higher G' than odd-odd binary gels (20 wt %) consisting of co-assembled fibers. In general, the self-sorting of odd and even molecules into the separate primary structures results in a dramatic decrease of G' compared to the co-assembled gels (20 wt %), except for (5BA)1(9BA)1 gel (20 wt %). It might be due to larger woven spheres in (5BA)1(9BA)1 gel (20 wt %), which probably have a less entangled gel network.

The Role of Functional Groups in Tuning the Self-Assembly Modes and Physical Properties of Multicomponent Gels

We have analyzed the nature and role of functional groups on the self-assembly modes and the physical properties of multicomponent gels with structurally similar individual components. The gelation properties of individual and mixed enantiomeric compounds of biphenyl bis-(amides) of alanine (BPA) or phenylalanine (BPP) methyl ester were analyzed in various solvent/solvent mixtures. Multicomponent gels were formed by mixing the enantiomeric BPP compounds at a lower concentration, but a higher concentration was required for mixed alanine-based BPA gels. The comparison of the mechanical strength of the individual and mixed BPP compounds indicated that the mixed BPP gels displayed enhanced mechanical strength (∼2-fold increase) in p-xylene, but a weaker gel was observed in DMSO/water. However, a reverse trend was observed for BPA gels, indicating the role of functional groups in the gel network formation. X-ray diffraction analysis of the gelator and the xerogels in the solid state confirmed the formation of co-assembled networks in mixed enantiomeric gels. The stability of the gels towards anions was evaluated by analyzing the anion induced stimuli-responsive properties. These results indicate the effective modeling of the functional groups of the individual components could lead to multicomponent gels with tunable properties.

Self-assembly hydrogels of therapeutic agents for local drug delivery

Advanced drug delivery systems are of vital importance to enhance therapeutic efficacy. Among various recently developed formulations, self-assembling hydrogels composed of therapeutic agents have shown promising potential for local drug delivery owing to their excellent biocompatibility, high drug-loading efficiency, low systemic toxicity, and sustained drug release behavior. In particular, therapeutic agents self-assembling hydrogels with well-defined nanostructures are beneficial for direct delivery to the target site via injection, not only improving drug availability, but also extending their retention time and promoting cellular uptake. In brief, the self-assembly approach offers better opportunities to improve the precision of pharmaceutical treatment and achieve superior treatment efficacies. In this review, we intend to cover the recent developments in therapeutic agent self-assembling hydrogels. First, the molecular structures, self-assembly mechanisms, and application of self-assembling hydrogels are systematically outlined. Then, we summarize the various self-assembly strategies, including the single therapeutic agent, metal-coordination, enzyme-instruction, and co-assembly of multiple therapeutic agents. Finally, the potential challenges and future perspectives are discussed. We hope that this review will provide useful insights into the design and preparation of therapeutic agent self-assembling hydrogels.

Exploring the Tunable Optical and Mechanical Properties of Multicomponent Low-Molecular-Weight Gelators

A class of amino acid-based low-molecular-weight gelators (LMWGs) was used for single and multicomponent gel studies to investigate their tunable optical properties and their self-assembly process. The optical properties of multicomponent gels were found to be easily tuned by changing the proportion of the components, varying from opaque to highly transparent gels as analyzed using ultraviolet-visible spectroscopy. This phenomenon allows tunability without introducing another variable into the system. Scanning electron microscopy, differential scanning calorimetry, and small-angle X-ray scattering (SAXS) were used to investigate the structures of the gels. It was found that because of the structural similarities of the molecules, the gelators favor coassembly packing over self-sorting. The emergence of transparency was ascribed to changes in the fiber diameters. Moreover, analysis of the SAXS data allowed us to compare the molecular order present in the gel phase with single-crystal X-ray diffraction (SCXRD) data. Our analysis suggests that the packing of molecules seen in the crystalline phase is translated into the gel network. This reveals that the structure of the crystalline phase seen through SCXRD is a useful tool to aid in understanding the molecular packing in the gel phase.

Coassembly of C13-Dipeptides: Gelations from Solutions and Precipitations

C₁₃-dipeptides that did not gel on their own were found to form hydrogels when combined with mixtures (coassembly). At pH = 4.6, by mixing negatively charged C₁₃-WD (C₁₃-WD^2- and/or C₁₃-WD^-) with C₁₃-KW or C₁₃-YK, where the side chain of K carried positive charge, two composite hydrogels with different mechanical properties were formed. The gels exhibited various fiber structures that would account for their individual functionalities. According to molecular dynamics computer simulations, the composite systems formed spherical micelles through hydrophobic interactions that further aggregate to form gels through electrostatic interactions. The electrostatic repulsions between C₁₃-WD molecules were interfered by insertions of C₁₃-KW or C₁₃-YK molecules, which result in gel formation in the composite systems. The results of computer simulations well explained the experimental observations, which provided new insights into the design and selection strategies for peptide gelators.

Multi-stimuli responsive heterotypic hydrogels based on nucleolipids show selective dye adsorption

Analogous to nucleic acids, the building blocks of nucleic acids and their derivatives are widely used to create supramolecular architectures for application mainly in the field of biomedicine. Here, we describe the construction of a multi-stimuli responsive and toxic dye adsorbing heterotypic hydrogel system formed using simple nucleoside-fatty acid conjugates. The nucleolipids are derived by coupling fatty acid chains of different lengths at the 5' position of ribothymidine and uridine. The nucleolipids in the presence of a strong base (e.g. NaOH) undergo partial hydrolysis, which triggers the self-assembly of the hydrolysed components resulting in the formation of heterotypic hydrogels. Notably, the gels are formed specifically in the presence of Na+ ions as other ions such as Li+ and K+ did not support the hydrogelation process. Systematic analysis by microscopy, NMR, single crystal and powder X-ray diffraction and rheology indicated that the deprotonated nucleolipid and fatty acid salt interdigitate and provide necessary electrostatic interactions supported by Na+ ions to set the path for the hierarchical assembly process. Notably, the hydrogels are highly sensitive to external stimuli, wherein gel-sol transition can be reversibly controlled by using temperature, pH and host-guest interaction. One of the hydrogels made of 5'-O-myristate-conjugated ribothymidine was found to selectively adsorb cationic dyes such as methylene blue and rhodamine 6G in a recyclable fashion. Taken together, the easily scalable assembly, multi-stimuli responsiveness and ability to capture and release dyes highlight the potential of our nucleolipid hydrogel system in material applications and in the treatment of dye industry wastes.

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

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

Chirality on dendrimers: “roll booster” of the molecule-level self-sorting assembly in two-component supramolecular gel system

Chirality-induced recognition-promoted thermodynamic phase separation and different rate-kinetically controlled assembly helps molecule-level self-sorting emerge in a dendritic multicomponent gel.

Sub-zero temperature mechanically stable low molecular weight hydrogels

We show here a low molecular weight hydrogelator based on a functionalised-dipeptide which is stable down to temperatures of -12 °C despite being made from >99% water. This stabilty at low temperature can be extended to ∼-40 °C by gelling water : glycerol mixtures. The temperature range is wider than that of the glycerol : water mixtures alone. The rheological properties of the gels do not change at this low temperature compared to that of gels at 25 °C. This freezing point depression offers a potentially new method of transporting gels and offers the prospect of hydrogels being used at much lower working temperatures whilst retaining the desired rheological properties, this is useful for cryopreservation.

Synthesis of a Bis-Urea Dimer and Its Effects on the Physical Properties of an Amphiphilic Tris-Urea Supramolecular Hydrogel

The successful development of stiff supramolecular gels is an important goal toward their practical application. One approach to stiffen supramolecular gels is to introduce covalent cross-links. The bis-urea dimer 2, having a structure similar to that of the low-molecular-weight gelator 1, was synthesized. Supramolecular hydrogels were formed from mixtures of 1 and 2 in appropriate ratios, with 2 acting as a covalent cross-linker to connect the fibrous aggregates formed by the self-assembly of 1. The introduction of these covalent cross-links greatly influenced the dynamic viscoelasticity of the supramolecular hydrogels. In the supramolecular hydrogel of 1 mixed with 5 % 2, the storage modulus was 1.35 times higher than that of the supramolecular hydrogel of 1 alone, and the crossover strain was extended from 5 % to over 20 %. The supramolecular hydrogel of 1 and 2 was free-standing and supported 13 times its own weight.

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.

Competition versus Cooperation in Catalytic Hydrogelators for anti-Selective Mannich Reaction

Chemical systems find similarities in different sociological and biological processes, in which the entities compete or cooperate for a favorable outcome. The structural and functional adaptations leading to emergent properties, especially in catalysis, are based on factors such as abundance of substrates, stability of the transition state, and structural/functional attributes of catalysts. Proline and acid groups appended to catalytic fibers of two self-sorting hydrogelators compete for the Mannich reaction between aniline, benzaldehyde, and cyclohexanone to give low overall selectivity (anti/syn 77:23). In a sol-gel system of the same molecules, on the other hand, the soluble acid appended molecules tend to cooperate with the fibers of proline-appended catalyst to give improved selectivity (anti/syn 95:5). The available options for the catalytic molecules are to carry out the reaction independently or in cooperation. However, these options are chosen based on the efficiency, selectivity, and mobility of catalysts as a result of their abilities to self-assemble.

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.

Tandem reactions in self-sorted catalytic molecular hydrogels

By equipping mutually incompatible carboxylic acid and proline catalytic groups with different self-assembling motives we have achieved self-sorting of the resulting catalytic gelators, namely SucVal8 and ProValDoc, into different supramolecular fibers, thus preventing the acidic and basic catalytic groups from interfering with each other. The resulting spatial separation of the incompatible catalytic functions is found to be essential to achieve one-pot deacetalization-aldol tandem reactions with up to 85% efficiency and 90% enantioselectivity. On the contrary, when SucVal8 was co-assembled with a structurally similar catalytically active hydrogelator (ProVal8), self-sorting was precluded and no tandem catalysis was observed.

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.

Multicomponent low molecular weight gelators

We discuss the potential and challenges of multicomponent low molecular weight gels.

Modulation of physical properties of supramolecular hydrogels based on a hydrophobic core

We demonstrate herein the variation in viscoelastic properties of supramolecular hydrogels (SMG) composed of two amphiphiles, N-Palmitoyl-Gly-His (PalGH) and sodium palmitate (PalNa).

Synergetic functional properties of two-component single amino acid-based hydrogels

Hybrid hydrogels composed of the Fmoc-Tyr and Fmoc-DOPA building blocks present mechanical rigidity and redox activity.

The effect of self-sorting and co-assembly on the mechanical properties of low molecular weight hydrogels

Self-sorting in low molecular weight hydrogels can be achieved using a pH triggered approach. We show here that this method can be used to prepare gels with different types of mechanical properties. Cooperative, disruptive or orthogonal assembled systems can be produced. Gels with interesting behaviour can be also prepared, for example self-sorted gels where delayed switch-on of gelation occurs. By careful choice of gelator, co-assembled structures can also be generated, which leads to synergistic strengthening of the mechanical properties.