On crystal versus fiber formation in dipeptide hydrogelator systems

Scrolling in Supramolecular Gels: A Designer's Guide

Gelation by small molecules is a topic of enormous importance in catalysis, nanomaterials, drug delivery, and pharmaceutical crystallization. The mechanism by which gelators self-organize into a fibrous gel network is poorly understood. Herein, we describe the crystal structures and gelation properties of a library of bis(urea) compounds and show, via molecular dynamics simulations, how gelator aggregation progresses from a continuous pattern of supramolecular motifs to a homogeneous fiber network. Our model suggests that lamellae with asymmetric surfaces scroll into uniform unbranched fibrils, while sheets with symmetric surfaces undergo stacking to form crystals. The self-assembly of asymmetric lamellae is associated with specific molecular features, such as the presence of narrow and flexible end groups with high packing densities, and likely represents a general mechanism for the formation of small-molecule gels.

Creating an Amyloid 'Kaleidoscope' Using Short Iodinated Peptides

Amyloid fibrils formed by peptides with different sequences exhibit diversified morphologies, material properties and activities, making them valuable for developing functional bionanomaterials. However, the molecular understanding underlying the structural diversity of peptide fibrillar assembly at atomic level is still lacking. In this study, by using cryogenic electron microscopy, we first revealed the structural basis underlying the highly reversible assembly of 1 GFGGNDNFG9 (referred to as hnRAC1) peptide fibril. Furthermore, by installing iodine at different sites of hnRAC1, we generated a collection of peptide fibrils with distinct thermostability. By determining the atomic structures of the iodinated fibrils, we discovered that iodination at different sites of the peptide facilitates the formation of diverse halogen bonds and triggers the assembly of entirely different structures of iodinated fibrils. Finally, based on this structural knowledge, we designed an iodinated peptide that assembles into new atomic structures of fibrils, exhibiting superior thermostability, that aligned with our design. Our work provides an in-depth understanding of the atomic-level processes underlying the formation of diverse peptide fibril structures, and paves the way for creating an amyloid "kaleidoscope" by employing various modifications and peptide sequences to fine-tune the atomic structure and properties of fibrillar nanostructures.

π-System Functionalization Transforms Amyloidogenic Peptide Fragment of Human Islet Amyloid Polypeptide into a Super Hydrogelator

While a considerable number of ultra-short/short amyloid peptides have been reported to form 3D supramolecular hydrogels, they all possess high minimum gelation concentration (MGC) (≥1 wt%), which preclude their applications. In this context, we demonstrate that functionalisation of a well-known amyloidogenic ultra-short peptide fragment NFGAIL (IAPf) of human Islet amyloid polypeptide with a π-system (Fluorenyl, Fm) at the N-terminus of the peptide (Fm-IAPf) yield not only highly thermostable hydrogel at physiological pH but also exhibited super gelator nature as the MGC (0.08 wt%) falls below 0.1 wt%. Various experimental results confirmed that aromatic π-π interactions from fluorenyl moieties and hydrogen bonding interactions between the IAPf drive the self-assembly/fibril formation. Fm-IAPf is the first super hydrogelator derived from amyloid-based ultra-short peptides, to the best of our knowledge. We strongly believe that this report, i. e., functionalization of an amyloid peptide with π-system, provides a lead to develop super hydrogelators from other amyloid-forming peptide fragments for their potential applications.

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

Side-chain halogen effects on self-assembly and hydrogelation of cationic phenylalanine derivatives

Low molecular weight (LMW) supramolecular hydrogels have great potential as next-generation biomaterials for drug delivery, tissue engineering, and regenerative medicine. The design of LMW gelators is complicated by the lack of understanding regarding how the chemical structure of the gelator correlates to self-assembly potential and emergent hydrogel material properties. The fluorenylmethyloxycarbonyl-phenylalanine (Fmoc-Phe) motif is a privileged scaffold that is prone to undergo self-assembly into self-supporting hydrogel networks. Cationic Fmoc-Phe-DAP derivatives modified with diaminopropane (DAP) at the C-terminus have been developed that self-assemble into hydrogel networks in aqueous solutions of sufficient ionic strength. We report herein the impact of side-chain halogenation on the self-assembly and hydrogelation properties of Fmoc-Phe-DAP derivatives. A systematic study of the self-assembly and hydrogelation of monohalogenated Fmoc-Phe-DAP derivatives with F, Cl, or Br atoms in the ortho, meta, or para positions of the phenyl side chain reveal significant differences in self-assembly and gelation potential, nanoscale assembly morphology, and hydrogel viscoelastic properties as a function of halogen identity and substitution position. These results demonstrate the profound impact that subtle changes to the chemical scaffold can have on the behavior of LMW supramolecular gelators and illustrate the ongoing difficulty of predicting the emergent self-assembly and hydrogelation behavior of LMW gelators that differ even modestly in chemical structure.

Exploiting Minimalistic Backbone Engineered γ-Phenylalanine for the Formation of Supramolecular Co-Polymer

Ordered supramolecular hydrogels assembled by modified aromatic amino acids often exhibit low mechanical rigidity. Aiming to stabilize the hydrogel and understand the impact of conformational freedom and hydrophobicity on the self-assembly process, we designed two building blocks based on 9-fluorenyl-methoxycarbonyl-phenylalanine (Fmoc-Phe) gelator which contain two extra methylene units in the backbone, generating Fmoc-γPhe and Fmoc-(3-hydroxy)-γPhe. Fmoc-γPhe spontaneously assembled in aqueous media forming a hydrogel with exceptional mechanical and thermal stability. Moreover, Fmoc-(3-hydroxy)-γPhe, with an extra backbone hydroxyl group decreasing its hydrophobicity while maintaining some molecular flexibility, self-assembled into a transient fibrillar hydrogel, that later formed microcrystalline aggregates through phase transition. Molecular dynamics simulations and single crystal X-ray analyses revealed the mechanism underlying the two residues' distinct self-assembly behaviors. Finally, we demonstrated Fmoc-γPhe and Fmoc-(3-OH)-γPhe co-assembly to form a supramolecular hydrogel with notable mechanical properties. We believe that the understanding of the structure-assembly relationship will enable the design of new functional amino acid-based hydrogels. This article is protected by copyright. All rights reserved.

Supramolecular Shear-Thinning Glycopeptide Hydrogels for Injectable Enzyme Prodrug Therapy Applications

Transplantable catalytic reactors have attracted considerable attention as therapeutic biomedical materials. However, existing transplantable reactors such as biocatalytic films are limited by their invasiveness. Here, we report the fabrication of biocatalytic supramolecular hydrogels via self-assembly of amphiphilic glycopeptides. We show that the hydrogels have shear-thinning properties, demonstrating their potential to be administered using a syringe. Enzymes can be loaded into the hydrogels by simply adding enzyme solution, and the enzyme-loaded hydrogels can transform a prodrug into an anticancer drug that inhibits tumor cell growth. This study demonstrates the potential of these biocatalytic hydrogels as injectable therapeutic reactors for enzyme prodrug therapy.

Layered supramolecular hydrogels from thioglycosides

Low molecular weight hydrogels are made of small molecules that aggregate via noncovalent interactions. Here, comprehensive characterization of the physical and chemical properties of hydrogels made from thioglycolipids of the disaccharides lactose and cellobiose with simple alkyl chains is reported. While thiolactoside hydrogels are robust, thiocellobioside gels are metastable, precipitating over time into fibrous crystals that can be entangled to create pseudo-hydrogels. Rheology confirms the viscoelastic solid nature of these hydrogels with storage moduli ranging from 10-600 kPa. Additionally, thiolactoside hydrogels are thixotropic which is a desirable property for many potential applications. Freeze-fracture electron microscopy of xerogels shows layers of stacked sheets that are entangled into networks. These structures are unique compared to the fibers or ribbons typically reported for hydrogels. Differential scanning calorimetry provides gel-to-liquid phase transition temperatures ranging from 30 to 80 °C. Prodan fluorescence spectroscopy allows assignment of phase transitions in the gels and other lyotropic phases of high concentration samples. Phase diagrams are estimated for all hydrogels at 1-10 wt% from 5 to ≥ 80 °C. These hydrogels represent a series of interesting materials with unique properties that make them attractive for numerous potential applications.

Understanding gel-to-crystal transitions in supramolecular gels

Most supramolecular gels are stable or assumed to be stable over time, and aging effects are often not studied. However, some gels do show clear changes on aging, and a small number of systems exhibit gel-to-crystal transitions. In these cases, crystals form over time, typically at the expense of the network underpinning the gel; this leads to the gel falling apart. These systems are rare, and little is known about how these gel-to-crystal transitions occur. Here, we use a range of techniques to understand in detail a gel-to-crystal transition for a specific functionalised dipeptide based gelator. We show that the gel-to-crystal transition depends on the final pH of the medium which we control by varying the amount of glucon-δ-lactone (GdL) added. In the gel phase, at low concentrations of GdL, and at early time points with high concentrations of GdL, we are able to show the nanometre scale dimensions of the self-assembled fibre using SAXS; however there is no evidence of molecular ordering of the gel fibres in the WAXS. At low concentrations of GdL, these self-assembled fibres stiffen with time but do not crystallise over the timescale of the SAXS experiment. At high concentrations of GdL, the fibres are already stiffened, and then, as the pH drops further, give way to the presence of crystals which appear to grow preferentially along the direction of the fibre axis. We definitively show therefore that the gel and crystal phase are not the same. Our work shows that many assumptions in the literature are incorrect. Finally, we also show that the sample holder geometry is an important parameter for these experiments, with the rate of crystallisation depending on the holder in which the experiment is carried out.

Exploiting and controlling gel-to-crystal transitions in multicomponent supramolecular gels

Multicomponent supramolecular gels provide opportunities to form materials that are not accessible when using the single components alone. Different scenarios are possible when mixing multiple components, from complete co-assembly (mixing of the components within the self-assembled structures formed) to complete self-sorting such that each structure contains only one of the components. Most examples of multicomponent gels that currently exist form stable gels. Here, we show that this can be used to control the mechanical properties of the gels, but what is probably most exciting is that we show that we can use a magnetic field to control the shape of the crystals. The gelling component aligns in a magnetic field and so results in anisotropic crystals being formed.

Computational Tools to Rationalize and Predict the Self-Assembly Behavior of Supramolecular Gels

Supramolecular gels form a class of soft materials that has been heavily explored by the chemical community in the past 20 years. While a multitude of experimental techniques has demonstrated its usefulness when characterizing these materials, the potential value of computational techniques has received much less attention. This review aims to provide a complete overview of studies that employ computational tools to obtain a better fundamental understanding of the self-assembly behavior of supramolecular gels or to accelerate their development by means of prediction. As such, we hope to stimulate researchers to consider using computational tools when investigating these intriguing materials. In the concluding remarks, we address future challenges faced by the field and formulate our vision on how computational methods could help overcoming them.

Self-Templated, Enantioselective Assembly of an Amyloid-like Dipeptide into Multifunctional Hierarchical Helical Arrays

Chiral self-assembly of peptides has attracted great interest owing to their promising applications in biomedicine, chemistry, and materials science. However, compared with the rich knowledge about their chiral self-assembly at the molecular or nanoscale, the formation of long-range-ordered hierarchical helical arrays (HHAs) from simple peptides remains a formidable challenge. Herein, we report the self-templated assembly of an amyloid-like dipeptide into long-range-ordered HHAs by their spontaneous fibrillization and hierarchical helical assembly within a confined film. The chiral interactions between the peptide and diamines result in geometry frustration and the phase transition of self-assembling peptide films from achiral spherulite structures into chiral HHAs. By changing the chirality and enantioselective interactions, we can control the phase behavior, handedness, and chiroptics of the self-assembled HHAs precisely. Moreover, the redox activity of the HHAs allows the in situ decoration of nanoparticles with high catalytic activity. These results provide insights into the chiral self-assembly of peptides and the fabrication of highly ordered materials with complex architectures and promising applications in chiroptics and catalysis.

L-Dopa in small peptides: an amazing functionality to form supramolecular materials

l-Dopa (3,4-dihydroxyphenylalanine) is a chiral amino acid generated via biosynthesis from l-tyrosine in plants and some animals. The presence of multiple interacting sites makes l-Dopa a multifunctional building block for the preparation of supramolecular materials. The possibility to form hydrogen bonds and the presence of the aromatic ring allow l-Dopa molecules to interact through a series of non-covalent interactions. The additional presence of the catechol moiety really makes this compound unique: not only does it have implications in the self-assembly process of Dopa itself and with other substrates, but also it highly increases the number of applications of the final material, since it works as an antioxidant, radical trapper, metal chelator, reducing agent and adhesive. l-Dopa and catechol containing derivatives have been extensively introduced inside both synthetic and natural polymers to obtain amazing functional materials. In this review we report the preparation of small peptides containing l-Dopa, focusing on the supramolecular materials that can be obtained with them, ranging from fibrils to fibres, gels, films and coatings, all having the different applications mentioned above and many others.

Organogelators derived from the bisphenol A scaffold

Bisphenol A, a common precursor molecule used in the preparation of some polymers, was investigated as a possible scaffold for the design and synthesis of small-molecule gelators.

Self-assembling behaviour of a modified aromatic amino acid in competitive medium

Aromatic amino acid, specifically phenylalanine (Phe), is one of the most studied building blocks in peptide synthesis due to its importance in biology. It is reported in the literature that Phe-containing peptides have a high tendency to form different self-assembled materials due to efficient aromatic-aromatic interactions. In this article, we have tuned the supramolecular interactions of phenylalanine by making it electron-deficient upon introduction of the nitro group in the ring. The presence of the nitro group has a profound influence on the self-assembly process. It has been observed that 4-nitrophenylalanine (4NP) is a highly efficient gelator compared with the native phenylalanine in DMSO solvent in terms of minimum gelation concentration and it forms hydrogen bonding mediated crystals in water. The change of self-assembling patterns of 4NP in these solvents was studied using X-ray diffraction, UV-Vis spectroscopy, FE-SEM and other techniques. With the help of different experimental data and density functional theory (DFT), we have simulated the theoretical structure of 4NP in DMSO. The theoretical structure of 4NP in DMSO is different compared with that of crystals in water. We then studied the self-assembly process of 4NP in the mixed solvent of DMSO (polar aprotic) and water (polar protic). Different competitive non-covalent interactions of solvents as well as the ratio of the solvent mixture guide the final self-assembly state of 4NP.

Nanomechanical Properties and Phase Behavior of Phenylalanine Amyloid Ribbon Assemblies and Amorphous Self-Healing Hydrogels

Phenylalanine was the minimalistic and first of numerous nonproteinaceous building blocks to be demonstrated to form amyloid-like fibrils. This unexpected organization of such a simple building block into canonical architecture, which was previously observed only with proteins and peptides, has numerous implications for medicine and supramolecular chemistry. However, the morphology of phenylalanine fibrils and their mechanical properties was never characterized in solutions. Here, using electron and atomic force microscopy, we analyze the morphological and mechanical properties of phenylalanine fibrils in both air and fluids. The fibrils demonstrate an exceptionally high Young's modulus (up to 30 GPa) and are found to be composed of intertwined protofilaments in a helical or twisted ribbon morphology. In addition, X-ray scattering experiments provide convincing evidence of an amyloidal cross-β-like secondary structure within the nanoassemblies. Furthermore, increasing the phenylalanine concentration results in the formation of highly homogenous, noncrystalline, self-healing hydrogels that display storage and loss moduli significantly higher than similar noncovalently cross-linked biomolecular nanofibrillar scaffolds. These remarkably stiff nanofibrillar hydrogels can be harnessed for various technological and biomedical applications, such as self-healing, printable, structural, load-bearing 3D scaffolds. The properties of this simple but quite remarkable hydrogel open a possibility to utilize it in the biomaterial industry.

Topochemical synthesis of different polymorphs of polymers as a paradigm for tuning properties of polymers

Different packing is a mechanism through which nature can produce materials of different properties from the same basic units. There is great interest in constructing different forms of the same polymer by utilising different packing. Common solution-synthesized polymers are amorphous and their post-synthesis crystallization into different topologies is almost impossible. Here we show solid-state polymerization of different reactive polymorphs of a monomer pre-organized in different topologies. Trimorphs of a dipeptide monomer pack in a head-to-tail fashion, placing the azide and alkyne of adjacent monomers in proximity. On heating, these crystals undergo a topochemical azide-alkyne cycloaddition reaction yielding triazole-linked polymer in three different crystalline states; one with antiparallel arrangement of polymer chains, another with parallelly oriented chains, and a third form containing a 1:1 blend of two different conformers aligned in parallel. This approach of exploiting different polymorphs of a monomer for topochemical polymerization to yield polymorphs of polymers is promising for future research.

Beyond Fmoc: a review of aromatic peptide capping groups

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

Strategy to Identify Improved N-Terminal Modifications for Supramolecular Phenylalanine-Derived Hydrogelators

Supramolecular hydrogels formed by self-assembly of low molecular weight (LMW) compounds have been identified as promising materials for applications in tissue engineering and regenerative medicine. In many cases, the relationship between the chemical structure of the gelator and the emergent hydrogel properties is poorly understood. As a result, empirical screening strategies instead of rational design approaches are often relied upon to tune the emergent properties of the gels. Herein, we describe a novel strategy to identify improved phenylalanine (Phe) derived gelators using a focused empirical approach. Fluorenylmethoxycarbonyl (Fmoc) protected Phe derivatives are a privileged class of gelators that spontaneously self-assemble into fibrils that entangle to form a hydrogel network upon dissolution into water. However, the Fmoc group has been shown to have toxicity drawbacks for potential biological applications, requiring the identification of new N-terminal modifications that promote efficient self-assembly but lack the shortcomings of the Fmoc group. We previously discovered that fibrils in Fmoc-p-nitrophenylalanine (Fmoc-4-NO2-Phe) hydrogels transition to crystalline microtubes after several hours by a mechanism that involves the hierarchical assembly and fusion of the hydrogel fibrils. We hypothesized that this hierarchical crystallization behavior could form the basis of a screening approach to identify alternative N-terminal functional groups to replace Fmoc in Phe-derived LMW gelators. Specifically, screening N-terminal modifying groups for 4-NO2-Phe that stabilize the hydrogel state by preventing subsequent hierarchical crystallization would facilitate empirical identification of functional Fmoc replacements. To test this approach, we screened a small series of 4-NO2-Phe derivatives with various N-terminal modifying groups to determine if any provided stable LMW supramolecular hydrogels. All but one of the 4-NO2-Phe derivatives assembled into crystalline forms. Only the 1-naphthaleneacetic acid (1-Nap) 4-NO2-Phe derivative self-assembled into a stable hydrogel network. Additional Phe derivatives were modified by N-terminal 1-Nap groups to confirm the general potential of 1-Nap as a suitable replacement for Fmoc, and all derivatives formed stable hydrogels under similar conditions to their Fmoc-Phe counterparts. These results illustrate the potential of this approach to identify next-generation Phe-derived LMW gelators with improved emergent properties.

Wet spinning and radial self-assembly of a carbohydrate low molecular weight gelator into well organized hydrogel filaments

In this work, we describe how a simple single low molecular weight gelator (LMWG) molecule - N-heptyl-d-galactonamide, which is easy to produce at the gram scale - is spun into gel filaments by a wet spinning process based on solvent exchange. A solution of the gelator in DMSO is injected into water and the solvent diffusion triggers the supramolecular self-assembly of the N-heptyl-d-galactonamide molecules into nanometric fibers. These fibers entrap around 97% of water, thus forming a highly hydrated hydrogel filament, deposited in a well organized coil and locally aligned. This self-assembly mechanism also leads to a very narrow distribution of the supramolecular fiber width, around 150 nm. In addition, the self-assembled fibers are oriented radially inside the wet-spun filaments and at a high flow rate, fibers are organized in spirals. As a result, this process gives rise to a high control of the gelator self-assembly compared with the usual thermal sol-gel transition. This method also opens the way to the controlled extrusion at room temperature of these very simple, soft, biocompatible but delicate hydrogels. The gelator concentration and the flow rates leading to the formation of the gel filaments have been screened. The filament diameter, its internal morphology, the solvent exchange and the velocity of the jet have been investigated by video image analysis and electron microscopy. The stability of these delicate hydrogel ropes has been studied, revealing a polymorphic transformation into macroscopic crystals with time under some storage conditions. The cell viability of a neuronal cell line on the filaments has also been estimated.

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.

Conformation Dependence of Diphenylalanine Self-Assembly Structures and Dynamics: Insights from Hybrid-Resolution Simulations

The molecular design of peptide-assembled nanostructures relies on extensive knowledge pertaining to the relationship between conformational features of peptide constituents and their behavior regarding self-assembly, and characterizing the conformational details of peptides during their self-assembly is experimentally challenging. Here, we demonstrate that a hybrid-resolution modeling method can be employed to investigate the role that conformation plays during the assembly of terminally capped diphenylalanines (FF) through microsecond simulations of hundreds or thousands of peptides. Our simulations discovered tubular or vesicular nanostructures that were consistent with experimental observation while reproducing critical self-assembly concentration and secondary structure contents in the assemblies that were measured in our experiments. The atomic details provided by our method allowed us to uncover diverse FF conformations and conformation dependence of assembled nanostructures. We found that the assembled morphologies and the molecular packing of FFs in the observed assemblies are linked closely with side-chain angle and peptide bond orientation, respectively. Of various conformations accessible to soluble FFs, only a select few are compatible with the assembled morphologies in water. A conformation resembling a FF crystal, in particular, became predominant due to its ability to permit highly ordered and energetically favorable FF packing in aqueous assemblies. Strikingly, several conformations incompatible with the assemblies arose transiently as intermediates, facilitating key steps of the assembly process. The molecular rationale behind the role of these intermediate conformations were further explained. Collectively, the structural details reported here advance the understanding of the FF self-assembly mechanism, and our method shows promise for studying peptide-assembled nanostructures and their rational design.

Is Gelation Behavior Predictable through a Crystal Engineering Approach? A Case Study in Four Similar Coordination Compounds

In this paper, a detailed study on the gelation properties of a series of terpyridine and dipyrazine-pyridine ligands in the presence of metal salts is reported. To reveal the driving forces for the self-assembly of the metallogelators, their crystal structure is scrutinized. Inspired by the gelation of CuCl₂[Terpy- nCN], where "Terpy- nCN" is 4'-( n-cyanophenyl)-2,2',6',2″-terpyridine, to look into the aggregation behavior of the related analogues, synthesis of CuCl₂[Dipyz-py- nCN] derivatives, where "Dipyz-py- nCN" is 4-( n-cyanophenyl)-2,6-di-pyrazin-2-yl-pyridine, with the same cyano groups is performed. We then find that the Dipyz-py counterpart forms crystals when the molecules are stacked in an alternating way, instead of the unidirectional one required for gel formation. A crystal engineering approach is applied to determine the interactions that are favorable for fabricating a fiber network that is likely to be present in both crystalline and gel states and to find the interaction that disturbs this delicate balance between gelation and crystallization in coordination compounds; then, we conclude that the subtle balance between the molecular shape and intermolecular interactions is the origin of the gelation and crystallization of the current molecular system. This enables us to find the mutual connection among the structure of molecules, assembly behavior, and intermolecular interactions. With our experiments, a deep understanding of the balance among solution, gelation, and crystallization with subtle molecular diversions is provided.

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

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

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.

The Kinetics, Thermodynamics and Mechanisms of Short Aromatic Peptide Self-Assembly

The self-assembly of short aromatic peptides and peptide derivatives into a variety of different nano- and microstructures (fibrillar gels, crystals, spheres, plates) is a promising route toward the creation of bio-compatible materials with often unexpected and useful properties. Furthermore, such simple self-assembling systems have been proposed as model systems for the self-assembly of longer peptides, a process that can be linked to biological function and malfunction. Much effort has been made in the last 15 years to explore the space of peptide sequences, chemical modifications and solvent conditions in order to maximise the diversity of assembly morphologies and properties. However, quantitative studies of the corresponding mechanisms of, and driving forces for, peptide self-assembly have remained relatively scarce until recently. In this chapter we review the current state of understanding of the thermodynamic driving forces and self-assembly mechanisms of short aromatic peptides into supramolecular structures. We will focus on experimental studies of the assembly process and our perspective will be centered around diphenylalanine (FF), a key motif of the amyloid β sequence and a paradigmatic self-assembly building block. Our main focus is the basic physical chemistry and key structural aspects of such systems, and we will also compare the mechanism of dipeptide aggregation with that of longer peptide sequences into amyloid fibrils, with discussion on how these mechanisms may be revealed through detailed analysis of growth kinetics, thermodynamics and other fundamental properties of the aggregation process.

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.

Molecular simulations of self-assembling bio-inspired supramolecular systems and their connection to experiments

In bionanotechnology, the field of creating functional materials consisting of bio-inspired molecules, the function and shape of a nanostructure only appear through the assembly of many small molecules together. The large number of building blocks required to define a nanostructure combined with the many degrees of freedom in packing small molecules has long precluded molecular simulations, but recent advances in computational hardware as well as software have made classical simulations available to this strongly expanding field. Here, we review the state of the art in simulations of self-assembling bio-inspired supramolecular systems. We will first discuss progress in force fields, simulation protocols and enhanced sampling techniques using recent examples. Secondly, we will focus on efforts to enable the comparison of experimentally accessible observables and computational results. Experimental quantities that can be measured by microscopy, spectroscopy and scattering can be linked to simulation output either directly or indirectly, via quantum mechanical or semi-empirical techniques. Overall, we aim to provide an overview of the various computational approaches to understand not only the molecular architecture of nanostructures, but also the mechanism of their formation.

Self-assembly pathways and polymorphism in peptide-based nanostructures

Dipeptide derivative molecules can self-assemble into space-filling nanofiber networks at low volume fractions (<1%), allowing the formation of molecular gels with tunable mechanical properties. The self-assembly of dipeptide-based molecules is reminiscent of pathological amyloid fibril formation by naturally occurring polypeptides. Fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) is the most widely studied such molecule, but the thermodynamic and kinetic phenomena giving rise to Fmoc-FF gel formation remain poorly understood. We have previously presented evidence that the gelation process is a first order phase transition characterized by low energy barriers to nucleation, short induction times, and rapid quasi-one-dimensional crystal growth, stemming from solvent-solute interactions and highly specific molecular packing. Here, we discuss the phase behavior of Fmoc-FF in different solvents. We find that Fmoc-FF gel formation can be induced in apolar solvents, in addition to previously established pathways in aqueous systems. We further show that in certain solvent systems anisotropic crystals (nanofibers) are an initial metastable state, after which macroscopic crystal aggregates with no preferred axis of growth are formed. The molecular conformation is sensitive to solvent composition during assembly, indicating that Fmoc-FF may be a simple model system to study complex thermodynamic and kinetic phenomena involved in peptide self-assembly.

Gels with sense: supramolecular materials that respond to heat, light and sound

Advances in the field of supramolecular chemistry have made it possible, in many situations, to reliably engineer soft materials to address a specific technological problem. Particularly exciting are "smart" gels that undergo reversible physical changes on exposure to remote, non-invasive environmental stimuli. This review explores the development of gels which are transformed by heat, light and ultrasound, as well as other mechanical inputs, applied voltages and magnetic fields. Focusing on small-molecule gelators, but with reference to organic polymers and metal-organic systems, we examine how the structures of gelator assemblies influence the physical and chemical mechanisms leading to thermo-, photo- and mechano-switchable behaviour. In addition, we evaluate how the unique and versatile properties of smart materials may be exploited in a wide range of applications, including catalysis, crystal growth, ion sensing, drug delivery, data storage and biomaterial replacement.

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.

Systematic Moiety Variations of Ultrashort Peptides Produce Profound Effects on Self-Assembly, Nanostructure Formation, Hydrogelation, and Phase Transition

Self-assembly of small biomolecules is a prevalent phenomenon that is increasingly being recognised to hold the key to building complex structures from simple monomeric units. Small peptides, in particular ultrashort peptides containing up to seven amino acids, for which our laboratory has found many biomedical applications, exhibit immense potential in this regard. For next-generation applications, more intricate control is required over the self-assembly processes. We seek to find out how subtle moiety variation of peptides can affect self-assembly and nanostructure formation. To this end, we have selected a library of 54 tripeptides, derived from systematic moiety variations from seven tripeptides. Our study reveals that subtle structural changes in the tripeptides can exert profound effects on self-assembly, nanostructure formation, hydrogelation, and even phase transition of peptide nanostructures. By comparing the X-ray crystal structures of two tripeptides, acetylated leucine-leucine-glutamic acid (Ac-LLE) and acetylated tyrosine-leucine-aspartic acid (Ac-YLD), we obtained valuable insights into the structural factors that can influence the formation of supramolecular peptide structures. We believe that our results have major implications on the understanding of the factors that affect peptide self-assembly. In addition, our findings can potentially assist current computational efforts to predict and design self-assembling peptide systems for diverse biomedical applications.

Scaleable two-component gelator from phthalic acid derivatives and primary alkyl amines: acid-base interaction in the cooperative assembly

A series of phthalic acid derivatives (P) with a carbon-chain tail was designed and synthesized as single-component gelators. A combination of the single-component gelator P and a non-gelling additive n-alkylamine A through acid-base interaction brought about a series of novel phase-selective two-component gelators PA. The gelation capabilities of P and PA, and the structural, morphological, thermo-dynamic and rheological properties of the corresponding gels were investigated. A molecular dynamics simulation showed that the H-bonding network in PA formed between the NH of A and the carbonyl oxygen of P altered the assembly process of gelator P. Crude PA could be synthesized through a one-step process without any purification and could selectively gel the oil phase without a typical heating-cooling process. Moreover, such a crude PA and its gelation process could be amplified to the kilogram scale with high efficiency, which offers a practical economically viable solution to marine oil-spill recovery.

Sulfamide-Lattice Restructuring To Form Dimensionally Controlled Molecular Arrays and Gel-Forming Systems

A design approach that incorporates structural requirements for the formation of a 1D assembly, fibril stability, and fibril-fibril interactions for gelation was attempted by using amino acid-based sulfamides with the general structure Aa-NH-SO₂ -NH-Aa (Aa=amino acid). A preference for 1D assembly alone was not a sufficient condition for gelation, which became evident from studies involving sulfamide esters 1-5. Reducing the crystallization tendency without hindering unidirectional growth was executed through diacids of the sulfamide precursors with various amines that form an envelope around the sulfamide core through salt bridges. This strategy was fruitful, and gels of a wide variety of solvents could be formed by varying the acid and amine components. The use of dodecylamine or benzylamine, which could stabilize the molecular layers through alkyl-chain segregation or π-π interactions improved the gelation tendency, whereas the nature of the amino acid side chain, especially the rotational freedom and hydrophobicity, had a direct role in dictating the solvent preference. Crystallographic studies of these two-component systems gave molecular-level insight into the assembly and showed the importance of anisotropy in the distribution of secondary interactions in gelation.

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.

Remarkable solvent isotope dependence on gelation strength in low molecular weight hydro-gelators

Hydrophobic/hydrophilic, π–π stacking, ionic and hydrogen bond formation interactions determine the strength of hydrogelators and here solvent deuteration and electrolyte strength impact gel melting heavily.

Using experimental and computational energy equilibration to understand hierarchical self-assembly of Fmoc-dipeptide amphiphiles

Despite progress, a fundamental understanding of the relationships between the molecular structure and self-assembly configuration of Fmoc-dipeptides is still in its infancy. In this work, we provide a combined experimental and computational approach that makes use of free energy equilibration of a number of related Fmoc-dipeptides to arrive at an atomistic model of Fmoc-threonine-phenylalanine-amide (Fmoc-TF-NH₂) which forms twisted fibres. By using dynamic peptide libraries where closely related dipeptide sequences are dynamically exchanged to eventually favour the formation of the thermodynamically most stable configuration, the relative importance of C-terminus modifications (amide versus methyl ester) and contributions of aliphatic versus aromatic amino acids (phenylalanine F vs. leucine L) is determined (F > L and NH₂ > OMe). The approach enables a comparative interpretation of spectroscopic data, which can then be used to aid the construction of the atomistic model of the most stable structure (Fmoc-TF-NH₂). The comparison of the relative stabilities of the models using molecular dynamic simulations and the correlation with experimental data using dynamic peptide libraries and a range of spectroscopy methods (FTIR, CD, fluorescence) allow for the determination of the nanostructure with atomistic resolution. The final model obtained through this process is able to reproduce the experimentally observed formation of intertwining fibres for Fmoc-TF-NH₂, providing information of the interactions involved in the hierarchical supramolecular self-assembly. The developed methodology and approach should be of general use for the characterization of supramolecular structures.

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.

Peptide self-assembly: thermodynamics and kinetics

Self-assembling systems play a significant role in physiological functions and have therefore attracted tremendous attention due to their great potential for applications in energy, biomedicine and nanotechnology. Peptides, consisting of amino acids, are among the most popular building blocks and programmable molecular motifs. Nanostructures and materials assembled using peptides exhibit important potential for green-life new technology and biomedical applications mostly because of their bio-friendliness and reversibility. The formation of these ordered nanostructures pertains to the synergistic effect of various intermolecular non-covalent interactions, including hydrogen-bonding, π-π stacking, electrostatic, hydrophobic, and van der Waals interactions. Therefore, the self-assembly process is mainly driven by thermodynamics; however, kinetics is also a critical factor in structural modulation and function integration. In this review, we focus on the influence of thermodynamic and kinetic factors on structural assembly and regulation based on different types of peptide building blocks, including aromatic dipeptides, amphiphilic peptides, polypeptides, and amyloid-relevant peptides.

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.

Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs

Supramolecular or molecular gels are attractive for various applications, including diagnostics, tissue scaffolding and targeted drug release. Gelators derived from natural products are of particular interest for biomedical purposes, as they are generally biocompatible and stimuli-responsive. The building blocks of nucleic acids (i.e. nucleobases, nucleosides, and nucleotides) are desirable candidates for supramolecular gelation as they readily engage in reversible, noncovalent interactions. In this review, we describe a number of organo- and hydrogels formed through the assembly of nucleosides, nucleotides, and their derivatives. While natural nucleosides and nucleotides generally require derivatization to induce gelation, guanosine and its corresponding nucleotides are well known gelators. This unique gelating ability is due to propensity of the guanine nucleobase to self-associate into stable higher-order assemblies, such as G-ribbons, G4-quartets, and G-quadruplexes.