Pyromellitamide gelators: exponential rate of aggregation, hierarchical assembly, and their viscoelastic response to anions

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.

Ion polarisation-assisted hydrogen-bonded ferroelectrics in liquid crystalline domains

An alkylamide-substituted (−NHCOC₁₀H₂₁) hydrogen-bonded dibenzo[18]crown-6 derivative (1) was prepared to stabilise the ionic channel structure in a discotic hexagonal columnar (Col_h) liquid crystal. The introduction of simple M⁺X⁻ salts such as Na⁺PF₆⁻ and K⁺I⁻ into the ionic channel of 1 enhanced the ionic conductivity of the Col_h phase of the M⁺·(1)·X⁻ salts, with the highest ionic conductivity reaching ∼10⁻⁶ S cm⁻¹ for K⁺·(1)·I⁻ and Na⁺·(1)·PF₆⁻ at 460 K, which was approximately 5 orders of magnitude higher than that of 1. The introduction of non-ferroelectric 1 into the ferroelectric N,N′,N′′-tri(tetradecyl)-1,3,5-benzenetricarboxamide (3BC) elicited a ferroelectric response from the mixed Col_h phase of (3BC)_x(1)_1−x with x = 0.9 and 0.8. The further doping of M⁺X⁻ into the ferroelectric Col_h phase of (3BC)_0.9(1)_0.1 enhanced the ferroelectric polarisation assisted by ion displacement in the half-filled ionic channel for the vacant dibenzo[18]crown-6 of (3BC)_0.9[(M⁺)_0.5·(1)·(X⁻)_0.5]_0.1.

An alkylamide-substituted (−NHCOC₁₀H₂₁) hydrogen-bonded dibenzo[18]crown-6 derivative (1) was prepared to stabilise the ionic channel structure in a discotic hexagonal columnar (Col_h) liquid crystal.

Pharmaceutical crystallization with nanocellulose organogels

A carboxylated nanocellulose organogel facilitates crystallization of pharmaceutical solid forms including a new sulfapyridine solvate.

Macromolecular crowding and hydrophobic effects on Fmoc-diphenylalanine hydrogel formation in PEG : water mixtures

Addition of water triggers gelation in (poly)ethylene glycol (PEG) solutions of peptide gelators, with or without drugs added. The gels are formed by a combination of macromolecular crowding and hydrophobic effects as evident by significant secondary structure changes when compared to gel formed from pure water.

Small angle neutron scattering (SANS) studies on the structural evolution of pyromellitamide self-assembled gels

The kinetics of aggregation of two pyromellitamide gelators, tetrabutyl- (C4) and tetrahexyl-pyromellitamide (C6), in deuterated cyclohexane has been investigated by small angle neutron scattering (SANS) for up to 6 days. The purpose of this study was to improve our understanding of how self-assembled gels are formed. Short-term (< 3 h) time scales revealed multiple phases with the data for the tetrabutylpyromellitamide C4, indicating one-dimensional stacking and aggregation corresponding to a multifiber braided cluster arrangement that is about 35 Å in diameter. The corresponding tetrahexylpyromellitamide C6 data suggest that the C6 also forms one-dimensional stacks but that these aggregate to a thicker multifiber braided cluster that has a diameter of about 62 Å. Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling. These data suggest that structural changes in self-assembled gels occur over a period exceeding several days and that fairly subtle changes in the structure (e.g., tail-length) can influence the packing of molecules in self-assembled gels on the single-to-few fiber bundle stage.

Dissolution and degradation of Fmoc-diphenylalanine self-assembled gels results in necrosis at high concentrations in vitro

Herein we report an approach to assess in vitro cellular responses to the dissolution or degradation products from Fmoc-diphenylalanine (Fmoc-FF) self-assembled hydrogels. Three cell lines were used in these studies and two-way ANOVA was used to assess (i) the age of gel dissolution and degradation products and (ii) exposure time on cell fate and state, using viability assays in conjunction with time-lapse fluorescence and high-resolution scanning electron microscopy investigation. The studies show that leaching time but not the exposure time affects the overall cell viability. The cytotoxic effect was only observed once the gel is completely dissolved. Further analysis revealed that the principal mechanism of cell death is necrosis. In addition, the effect of chemotherapeutics (5-fluorouracil and paclitaxel) released from the Fmoc-FF gel (with addition before and after gelation) on colorectal cancer cells were investigated using this methodology, demonstrating enhanced activity of these drugs compared to bulk control. This enhanced activity, however, appears to be a combination of the apoptosis caused by the cancer drugs and necrosis caused by gel dissolution and degradation products. Given that in vivo studies by others on Fmoc-peptides that this material is not harmful to animals, our work highlights that conventional in vitro cellular assays may yield conflicting messages when used for the evaluation of cytotoxicity and drug release from self-assembled gels such as Fmoc-FF and that better in vitro models, (e.g. 3D cell culture systems) need to be developed to evaluate these materials for biomedical applications.

Blending gelators to tune gel structure and probe anion-induced disassembly

Blending different low molecular weight gelators (LMWGs) provides a convenient route to tune the properties of a gel and incorporate functionalities such as fluorescence. Blending a series of gelators having a common bis-urea motif, and functionalised with different amino acid-derived end-groups and differing length alkylene spacers is reported. Fluorescent gelators incorporating 1- and 2-pyrenyl moieties provide a probe of the mixed systems alongside structural and morphological data from powder diffraction and electron microscopy. Characterisation of the individual gelators reveals that although the expected α-urea tape motif is preserved, there is considerable variation in the gelation properties, molecular packing, fibre morphology and rheological behaviour. Mixing of the gelators revealed examples in which: 1) the gels formed separate, orthogonal networks maintaining their own packing and morphology, 2) the gels blended together into a single network, either adopting the packing and morphology of one gelator, or 3) a new structure not seen for either of the gelators individually was created. The strong binding of the urea functionalities to anions was exploited as a means of breaking down the gel structure, and the use of fluorescent gel blends provides new insights into anion-mediated gel dissolution.

Calcium ions to cross-link supramolecular nanofibers to tune the elasticity of hydrogels over orders of magnitude

Learning from the stabilization of protein structures, we combine aromatic-aromatic and electrostatic interactions to tailor the elasticity of supramolecular hydrogels. Specifically, calcium ions allow interfiber cross-links among the supramolecular nanofibers of small peptides that consist of multiple carboxylic acid and aromatic groups. A small change in the concentration of calcium increases the elasticity of hydrogels over several orders of magnitude. This simple approach not only demonstrates a biomimetic design of materials but also confirms that the combination of multiple aromatic-aromatic interactions and multiple calcium salt bridges is a feasible way to control the mechanical properties of soft nanomaterials.