Nanoscale dynamics and aging of fibrous peptide-based gels

Fmoc-diphenylalanine gelating nanoarchitectonics: A simplistic peptide self-assembly to meet complex applications

9-fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF), has been has been extensively explored due to its ultrafast self-assembly kinetics, inherent biocompatibility, tunable physicochemical properties, and especially, the capability of forming self-sustained gels under physiological conditions. Consequently, various methodologies to develop Fmoc-FF gels and their corresponding applications in biomedical and industrial fields have been extensively studied. Herein, we systemically summarize the mechanisms underlying Fmoc-FF self-assembly, discuss the preparation methodologies of Fmoc-FF hydrogels, and then deliberate the properties as well as the diverse applications of Fmoc-FF self-assemblies. Finally, the contemporary shortcomings which limit the development of Fmoc-FF self-assembly are raised and the alternative solutions are proposed, along with future research perspectives.

Microscopic dynamics underlying the stress relaxation of arrested soft materials

Arrested soft materials such as gels and glasses exhibit a slow stress relaxation with a broad distribution of relaxation times in response to linear mechanical perturbations. Although this macroscopic stress relaxation is an essential feature in the application of arrested systems as structural materials, consumer products, foods, and biological materials, the microscopic origins of this relaxation remain poorly understood. Here, we elucidate the microscopic dynamics underlying the stress relaxation of such arrested soft materials under both quiescent and mechanically perturbed conditions through X-ray photon correlation spectroscopy. By studying the dynamics of a model associative gel system that undergoes dynamical arrest in the absence of aging effects, we show that the mean stress relaxation time measured from linear rheometry is directly correlated to the quiescent superdiffusive dynamics of the microscopic clusters, which are governed by a buildup of internal stresses during arrest. We also show that perturbing the system via small mechanical deformations can result in large intermittent fluctuations in the form of avalanches, which give rise to a broad non-Gaussian spectrum of relaxation modes at short times that is observed in stress relaxation measurements. These findings suggest that the linear viscoelastic stress relaxation in arrested soft materials may be governed by nonlinear phenomena involving an interplay of internal stress relaxations and perturbation-induced intermittent avalanches.

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.

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.

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.

Gelation of Fmoc-diphenylalanine is a first order phase transition

We explore the gel transition of the aromatic dipeptide derivative molecule fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF). The addition of water to a solution of Fmoc-FF in dimethyl sulfoxide (DMSO) results in increased attractions leading to self-assembly of Fmoc-FF molecules into a space-filling fibrous network. We provide evidence that gel formation is associated with a first order phase transition resulting in nucleation and growth of strongly anisotropic crystals with high aspect ratios. The strength of attraction between Fmoc-FF molecules as a function of water concentration is estimated from long-time self-diffusion measurements using (1)H NMR diffusion-ordered spectroscopy (DOSY). The resulting phase behavior follows that observed for a wide range of other crystallizing nanoparticles and small molecules - a result consistent with the short-range nature of the intermolecular attractions. Furthermore, we use NMR to measure the rate of increase in the fraction of bound Fmoc-FF molecules after water is suddenly mixed into the system. We observe a lag time in the formation of the new phase indicative of the existence of a free energy barrier to the formation of a crystal nucleus of critical size. The application of classical nucleation theory for a cylindrical nucleus indicates that one-dimensional crystal growth is driven by an imbalance of the surface energies of the ends and sides of the fiber.