The role of amino acids on supramolecular co-assembly of naphthalenediimide–pyrene based hydrogelators

Process Dependent Complexity in Multicomponent Gels

Mixing low molecular weight gelators (LMWGs) can be used to combine favorable properties of the individual components within a multifunctional gel. Such multicomponent systems are complex enough in themselves but the method of combining components is not commonly considered something to influence self-assembly. Herein, two multicomponent systems comprising of a naphthalene-based dipeptide hydrogelator and one of two modified naphthalene diimides (NDIs), one of which forms gels, and the other does not, are investigated. These systems are probed, examining the structures formed and their gel properties (when preparing a solution from either a mixed powder of both components or by mixing pre-formed solutions of each component) using rheology, small angle neutron scattering (SANS), and absorbance spectroscopy. It is found that by altering the method of preparation, it is can either induce self-sorting or co-assembly within the fibers formed that underpin the gel network.

Tuning Rheological Behaviors of Supramolecular Aqueous Gels via Charge Transfer Interactions

Rheological properties are critical for determining real applications of supramolecular gels in various fields. Correspondingly, the modulation of gel rheology will be very important for meeting real requirements. In this aspect, a few strategies were applied to tune the rheological behaviors of supramolecular gels, but some specific interactions like charge transfer (CT) interactions were less explored at the molecular level. Herein, we report a pyrene-containing derivative of diphenylalanine as a donor gelator and naphthalenediimide or 3,5-dinitrobenzene as matching acceptor molecules. It was found that the viscoelastic properties and strength of the original gel could be tuned through addition of different acceptor molecules to the original gel with changing the ratios of the selected acceptor molecules. As a result, storage modulus was continuously adjusted over a wide range from 190,000 to 50,000 Pa by CT interactions. Furthermore, the mechanism of the CT-induced change in rheological properties was understood and clarified through relevant techniques (e.g., UV-Vis, fluorescence, and FT-IR spectroscopy and TEM). The findings in this work would provide a novel strategy to modulate the rheological properties of supramolecular gels for adaption to broader fields of real applications.

Enzyme-Induced Supramolecular Order in Pyrene Dipeptide Hydrogels for the Development of an Efficient Energy-Transfer Template

Peptide self-assembly is gathering much attention due to the precise control it provides for the arrangement of functional moieties for the fabrication of advanced functional materials. It is desirable to use a physical, chemical, or biological trigger that can control the self-assembly process. In the current article, we have applied an enzyme to induce the peptide self-assembly of an aromatic peptide amphiphile, which modulates the supramolecular order in the final gel phase material. We accessed diverse peptide hydrogels from identical gelator concentrations by simply changing the enzyme concentration, which controlled the reaction kinetics and influenced the dynamics of self-assembly. Depending upon the concentration of the enzyme, a bell-shaped relationship was observed in terms of intermolecular interactions, morphology, and properties of the final gel phase material. The access of non-equilibrium structures was further demonstrated by fluorescence emission spectroscopy, circular dichroism spectroscopy, atomic force microscopy, transmission electron microscopy, and rheology. This strategy is applied to construct a charge-transfer hydrogel by doping the donor hydrogel with an acceptor moiety, which exhibits efficient energy transfer. Interestingly, such structural control at the nanoscopic level can further tune the energy-transfer efficiency by simply modulating the enzyme concentration.

A two-component charge transfer hydrogel with excellent sensitivity towards the microenvironment: a responsive platform for biogenic thiols

A two-component charge transfer (CT) hydrogel has been derived from a supramolecular heteroassembly of a pyrene amino acid conjugate (PyHisOH, donor) with a 4-chloro-7-nitrobenzofurazan (NBD-Ox, acceptor) derivative in aqueous medium. The mechanical stiffness, as well as the thermal stability of the CT hydrogels largely depend on the relative ratios of donor and acceptor units as well as on their overall concentration. Moreover, the gel-to-sol transition is found to be susceptible to various external stimuli such as heat, pH, metal ions, etc. Circular dichroism and morphological investigation reveal the formation of left-handed helical fibers in the CT gel network. XRD studies show the lamellar packing of the interactive units in the 3D network of the CT hydrogel. The determination of different rheological parameters confirms the viscoelastic as well as the thixotropic nature of the CT gel. Furthermore, the CT gel is employed for turn-on sensing of biogenic thiols, cyan fluorescence was observed with cysteine/homocysteine, while blue fluorescence with glutathione. Nucleophilic attack at the NBD moiety leads to the formation of thermodynamically stable amino-linked derivatives for cysteine or homocysteine and kinetically controlled thiol-linked adduct for glutathione. Thus, the current system presents a unique opportunity, where a CT hydrogel sample is involved for discriminating biogenic thiols via specific chemodosimetric interactions.

The aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence properties of histidine-appended naphthalenediimide based two-component hydrogels

A histidine attached naphthalenediimide (NDI)-containing amphiphilic molecule (NDIP) self-assembles into nanotubes in aqueous solution at pH 6.6 as revealed by high-resolution transmission electron microscopy studies. This histidine-appended NDI forms a two-component hydrogel in the presence of tartaric acid at a molar ratio of 1 : 2. A morphological transformation was observed from a nanotube structure in the non-gel aggregated state of histidine appended NDI to interconnected cross-linked nanofibers of the two-component hydrogel in the presence of tartaric acid. Interestingly, the gel exhibits an unusual behavior upon aging compared to the fresh gel. It is found that the thermal stability and gel stiffness increase very significantly upon aging. Another important feature noted is that the very weak fluorescence of the fresh gel is transformed into bright greenish fluorescence upon aging. These results suggest that intermolecular interactions among the gelator molecules and tartaric acid in the gel phase slowly increase with time to form a mechanically very stiff and thermally robust gel.

A review on recent advances in polymer and peptide hydrogels

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Synthesis of amino acid derivatives of 5-alkoxy-3,4-dihalo-2(5H)-furanones and their preliminary bioactivity investigation as linkers

A series of amino acid derivatives are successfully synthesized via a metal-free C-N coupling reaction of 5-alkoxy-3,4-dihalo-2(5H)-furanones and amino acids. Their structures are well characterized with ¹H NMR, ¹³C NMR, ESI-MS and elemental analysis. As potential linkers of the 2(5H)-furanone unit with other drug moieties containing a hydroxyl or amino group, the effect of amino acids is investigated by comparison with other 2(5H)-furanone compounds by constructing C-O/C-S bonds. The preliminary results of the biological activity assay by the MTT method on a series of cancer cell lines in vitro reveal that the introduction of amino acids basically has no toxic effect. This can lead to these 2(5H)-furanone derivatives being further well-linked with other bioactive moieties with amino or hydroxy groups as expected. Thus, the biological activity assay gives a direction for the design of bioactive 2(5H)-furanones based on these amino acid linkers.

The Supramolecular Organogel Formed by Self-Assembly of Ursolic Acid Appended with Aromatic Rings

Ursolic acid (UA) as a natural ursane-triterpenoid has rich pharmacological activities. We have found that it possesses aggregation properties and could self-assemble into organogels. Based on the aggregation property of ursolic acid in suitable solvents, its derivative appended with aromatic rings by amide groups was synthesized. The property of self-assembly into organogel was studied in this paper. The results revealed that this derivative could form supramolecular gel in halogenated benzene and also gelate chloroform in the presence of toluene or p-xylene. By Fourier-transform infrared spectra (FT-IR) and variable temperature proton nuclear magnetic resonance (¹H NMR), it was proved that intermolecular hydrogen bonding and π⁻π stacking interaction were the primary driving forces for the aggregation to form organogel.