New Application of Proton Nuclear Spin Relaxation Unraveling the Intermolecular Structural Features of Low-Molecular-Weight Organogel Fibers

Probing assembly/disassembly of ordered molecular hydrogels

Supramolecular hydrogels have a wide range of applications in the biomedical field, acting as scaffolds for cell culture, matrices for tissue engineering and vehicles for drug delivery. L-Phenylalanine (Phe) is a natural amino acid that plays a significant role in several physiological and pathophysiological processes (phenylketonuria and assembly of fibrils linked to tissue damage). Since Myerson et al. [Chem. Eng. Commun., 2002, 189(8), 1079-1090] reported that Phe forms a fibrous network in vitro, Phe's self-assembly processes in water have been thoroughly investigated. We have reported structural control over gelation by introduction of a halogen atom in the aromatic ring of Phe, driving changes in the packing motifs, and therefore, dictating gelation functionality. The additional level of control gained over supramolecular gelation via the preparation of multi-component gel systems offers significant advantages in tuning functional properties of such materials. Gaining molecular-level information on the distribution of gelators between the inherent structural and dynamic heterogeneities of these materials remains a considerable challenge. Using multicomponent gels based on Phe and amino-L-phenylalanine (NH2-Phe), we will explore the patterns of ordered/disordered domains in the gel fibres and will attempt to come up with general trends of interactions in the gel fibres and at the fibre/solution interfaces. Phe and NH2-Phe were found to self-assemble in water into crystalline hydrogels. The determined faster dynamics of exchange between the gel and solution states of NH2-Phe in comparison with Phe were correlated with weaker intermolecular interactions, highlighting the role of head groups in dictating the strength of intermolecular interactions. In the mixed Phe/NH2-Phe systems, at a low concentration of NH2-Phe, disruption of the network was promoted by interference of the aliphatics of NH2-Phe with the electrostatic interactions between Phe molecules. At high concentrations of NH2-Phe, multiple-gelator hydrogels were formed with crystal habits different from those of the pure gel fibres. NMR crystallography approaches combining the strengths of solid- and solution-state NMR proved particularly suitable to obtain structural and dynamic insights into the "ordered" fibres, solution phase and fibre/solution interfaces in these gels. These findings are supported by a plethora of experimental (diffraction, rheology, microscopy and thermal analysis) and computational methods.

(N-Alkyloxalamido)-Amino Acid Amides as the Superior Thixotropic Phase Selective Gelators of Petrol and Diesel Fuels

(N-Alkyloxalamido)-amino acid amides 9-12 exhibit excellent gelation capacities toward some lipophilic solvents as well as toward the commercial fuels, petrol and diesel. Gelator 10 exhibits an excellent phase-selective gelation (PSG) ability and also possesses the highest gelation capacity toward petrol and diesel known to date, with minimum gelation concentration (MGC) values (%, w/v) as low as 0.012 and 0.015, respectively. The self-assembly motif of 10 in petrol and toluene gel fibres is determined from xerogel X-ray powder diffraction (XRPD) data via the simulated annealing procedure (SA) implemented in the EXPO2014 program and refined using the Rietveld method. The elucidated motif is strongly supported by the NMR (NOE and variable temperature) study of 10 toluene-d8 gel. It is shown that the triple unidirectional hydrogen bonding between gelator molecules involving oxalamide and carboxamide groups, together with their very low solubility, results in the formation of gel fibres of a very high aspect ratio (d = 10-30 nm, l = 0.6-1.3 μm), resulting in the as-yet unprecedented capacity of gelling commercial fuels. Rheological measurements performed at low concentrations of 10 confirmed the strength of the self-assembled network with the desired thixotropic properties that are advantageous for multiple applications. Instantaneous phase-selective gelation was obtained at room temperature through the addition of the 10 solution to the biphasic mixture of diesel and water in which the carrier solvent was congealed along with the diesel phase. The superior gelling properties and PSG ability of 10 may be used for the development of more efficient marine and surface oil spill recovery and waste water treatment technologies as well as the development of safer fuel storage and transport technologies.

Insights into the aggregation-induced emission of 1,8-naphthalimide-based supramolecular hydrogels

The study of aggregation-induced emission (AIE) of a series of 1,8-naphthalimide derivatives in aqueous media is reported herein. Some of these molecules constitute the first examples of 1,8-naphthalimide-containing amino acid derivatives that form hydrogels with excellent photophysical and mechanical properties. The present study provides further insights for the rational design of water-compatible stimuli-responsive photonic materials presenting AIE. AIE was quantitatively evaluated by measuring the fluorescence quantum yields of the molecules. Gelators 1 and 2 exhibit self-assembled fibrillar morphologies and present the best performance regarding the AIE effect, showing a remarkable enhancement in fluorescence intensity of 4700% and reaching a notable fluorescence quantum yield (Φf) of 30%. Non-gelator molecules 6 and 7 form nanoparticles, which also present AIE, but with emissions corresponding to their excimers. Therefore, the AIE intensity and wavelength are regulated by the type of aggregate morphology: fibers, nanoparticles or soluble species.

Substitution of Percutaneous Ethanol Injection with a Low Molecular Weight Peptide Gel Mimicking Chemoembolization for Cancer Therapy

In order to avoid the instability and quick separation between emulsifier and drug in the interventional chemoembolization, an injectable low molecular weight peptide gel (LMWG) was prepared to localize ethanol and chemotherapeutic for in situ synergistic therapy. The formation mechanism, rheological property and morphology of the LMWG were investigated by NMR, UV-vis, MS and SEM. The interaction between gelator and anticancer drug doxorubicin hydrochloride (DOX) was evaluated by fluorescence spectroscopy and its contribution on drug loading properties was demonstrated. The gel was non-toxic to both 3T3 fibroblasts and 4T1 breast cancer cells. DOX as well as ethanol were encapsulated in the gel and injected in breast cancer bearing mice with low drug dose (2.5 mg/kg body weight). The LMWG surrounded tumors act as a depot for ethanol release and release DOX to induce the apoptosis of cancer cells. With the combination of percutaneous ethanol injection (PEI) and chemotherapy, the DOX loaded LMWG exhibited great significance in necrosis of tumor tissue and exciting tumor inhibition efficiency.

Supramolecular Amino Acid Based Hydrogels: Probing the Contribution of Additive Molecules using NMR Spectroscopy

Supramolecular hydrogels are composed of self-assembled solid networks that restrict the flow of water. l-Phenylalanine is the smallest molecule reported to date to form gel networks in water, and it is of particular interest due to its crystalline gel state. Single and multi-component hydrogels of l-phenylalanine are used herein as model materials to develop an NMR-based analytical approach to gain insight into the mechanisms of supramolecular gelation. Structure and composition of the gel fibres were probed using PXRD, solid-state NMR experiments and microscopic techniques. Solution-state NMR studies probed the properties of free gelator molecules in an equilibrium with bound molecules. The dynamics of exchange at the gel/solution interfaces was investigated further using high-resolution magic angle spinning (HR-MAS) and saturation transfer difference (STD) NMR experiments. This approach allowed the identification of which additive molecules contributed in modifying the material properties.

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.

Halogen effects on the solid-state packing of phenylalanine derivatives and the resultant gelation properties

Phenylalanine is an important amino acid both biologically, essential to human health, and industrially, as a building block of artificial sweeteners. Our interest in this particular amino acid and its derivatives lies with its ability to form gels in a number of solvents. We present here the studies of the influence of halogen addition to the aromatic ring on the gelation properties and we analyse the crystal structures of a number of these materials to elucidate the trends in their behaviour based on the halogen addition to the aromatic group and the interactions that result.

Functional Supramolecular Polymers: A Fluorescent Microfibrous Network in a Supramolecular Hydrogel for High-Contrast Live Cell-Material Imaging in 3D Environments

A new bottom-up strategy based on aromatic peptide amphiphile is developed for a high-contrast visualization of 3D live cell-material imaging-something that has been difficult to achieve previously because of the problems associated with the diffraction of light by the nanosized peptide materials and the aggregation-caused quenching of aggregated π-conjugated fluorophores in the nanostructures. This study reports an example of a novel supramolecular hydrogelator, naphthaleneimide-phenylalanine (NI-Phe), which forms a self-supporting hydrogel displaying a unique microfibrous network and promising aggregation-induced emission characteristics at pH 7.4. The storage modulus of the NI-Phe gel supports the mass of a cell for 3D cell culturing. This work illustrates a new dopant-free supramolecular approach, complementary to well-established doping procedures that should facilitate the development of live cell imaging in 3D scaffolding materials.

Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-assembled gel-phase materials

We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel-sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools-as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.

A novel glucose/pH responsive low-molecular-weight organogel of easy recycling

A new phenylboronic acid based gelator was developed to prepare low-molecular-weight organogel (LMOG), which could interact with several solvents to assemble into a three-dimensional nanofiber network. (1)H NMR spectroscopy study suggests that the driving force for the gelation includes hydrogen bonding and π-π stacking. Evaluated by UV-spectroscopy, the gel showed a prompt initial response to glucose at low concentration of 0.012 mmol/mL, which is a critical concentration of venous plasma glucose for diabetes. Significantly, this organogel exhibits excellent sensitivity to glucose among seven sugars tested (i.e., mannitol, galactose, lactose, maltose, sucrose, and fructose). The proposed formation of hydrogen-bonded complexes during the glucose sensing was supported by our energy calculation. Meanwhile, this organogel exhibits pH-response. Importantly, this LMOG could be conveniently recycled and thus be reused.