Synthesis and characterization of pH responsive D-glucosamine based molecular gelators

Para-Methoxybenzylidene Acetal-Protected D-Glucosamine Derivatives as pH-Responsive Gelators and Their Applications for Drug Delivery

Carbohydrate-based low molecular weight gelators (LMWGs) are compounds with the capability to self-assemble into complex molecular networks within a solvent, leading to solvent immobilization. This process of gel formation depends on noncovalent interactions, including Van der Waals, hydrogen bonding, and π-π stacking. Due to their potential applications in environmental remediation, drug delivery, and tissue engineering, these molecules have emerged as an important area of research. In particular, various 4,6-O-benzylidene acetal-protected D-glucosamine derivatives have shown promising gelation abilities. In this study, a series of C-2-carbamate derivatives containing a para-methoxy benzylidene acetal functional group were synthesized and characterized. These compounds exhibited good gelation properties in several organic solvents and aqueous mixtures. Upon removal of the acetal functional group under acidic conditions, a number of deprotected free sugar derivatives were also synthesized. Analysis of these free sugar derivatives revealed two compounds were hydrogelators while their precursors did not form hydrogels. For those protected carbamates that are hydrogelators, removal of the 4,6-protection will result in a more water-soluble compound that produces a transition from gel to solution. Given the ability of these compounds to form gels from solution or solution from gels in situ in response to acidic environments, these compounds may have practical applications as stimuli-responsive gelators in an aqueous medium. In turn, one hydrogelator was studied for the encapsulation and release of naproxen and chloroquine. The hydrogel exhibited sustained drug release over a period of several days, with the release of chloroquine being faster at lower pH due to the acid lability of the gelator molecule. The synthesis, characterization, gelation properties, and studies on drug diffusion are discussed.

Hybrid hydrogels derived from renewable resources as a smart stimuli responsive soft material for drug delivery applications

The design and synthesis of amphiphilic molecules play a crucial role in fabricating smart functional materials via self-assembly. Especially, biologically significant natural molecules and their structural analogues have inspired chemists and made a major contribution to the development of advanced smart materials. In this report, a series of amphiphilic N-acyl amides were synthesized from natural precursors using a simple synthetic protocol. Interestingly, the self-assembly of amphiphiles 6a and 7a furnished a hydrogel and oleogel in vegetable oils. Morphological analysis of gels revealed the existence of a 3-dimensional fibrous network. Thermoresponsive and thixotropic behavior of these gels were evaluated using rheological analysis. A composite gel prepared by the encapsulation of curcumin in the hydrogel formed from 7a displayed a gel–sol transition in response to pH and could act as a dual channel responsive drug carrier.

The design and synthesis of amphiphilic molecules play a crucial role in fabricating smart functional materials via self-assembly.

Fine-Tuning of Molecular Structures to Generate Carbohydrate Based Super Gelators and Their Applications for Drug Delivery and Dye Absorption

Carbohydrate-based low molecular weight gelators (LMWGs) exhibit many desirable properties making them useful in various fields including applications as drug delivery carriers. In order to further understand the structural connection to gelation properties, especially the influence of halide substitutions, we have designed and synthesized a series of para-chlorobenzylidene acetal protected D-glucosamine amide derivatives. Fifteen different amides were synthesized, and their self-assembling properties were assessed in multiple organic solvents, as well as mixtures of organic solvents with water. All derivatives were found to be gelators for at least one solvent and majority formed gels in multiple solvents at concentrations lower than 2 wt%. A few derivatives rendered remarkably stable gels in aqueous solutions at concentrations below 0.1 wt%. The benzamide 13 formed gels in water and in EtOH/H2O (v/v 1:2) at 0.36 mg/mL. The gels were characterized using optical microscopy and atomic force microscopy, and the self-assembly mechanism was probed using variable temperature 1H-NMR spectroscopy. Gel extrusion studies using H2O/DMSO gels successfully printed lines of gels on glass slides, which retained viscoelasticity based on rheology. Gels formed by the benzamide 13 were used for encapsulation and the controlled release of chloramphenicol and naproxen, as well as for dye removal for toluidine blue aqueous solutions.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Recently Developed Carbohydrate Based Gelators and Their Applications

Carbohydrate based low molecular weight gelators have been an intense subject of study over the past decade. The self-assembling systems built from natural products have high significance as biocompatible materials and renewable resources. The versatile structures available from naturally existing monosaccharides have enriched the molecular libraries that can be used for the construction of gelators. The bottom-up strategy in designing low molecular weight gelators (LMWGs) for a variety of applications has been adopted by many researchers. Rational design, along with some serendipitous discoveries, has resulted in multiple classes of molecular gelators. This review covers the literature from 2017-2020 on monosaccharide based gelators, including common hexoses, pentoses, along with some disaccharides and their derivatives. The structure-based design and structure to gelation property relationships are reviewed first, followed by stimuli-responsive gelators. The last section focuses on the applications of the sugar based gelators, including their utilization in environmental remediation, ion sensing, catalysis, drug delivery and 3D-printing. We will also review the available LMWGs and their structure correlations to the desired properties for different applications. This review aims at elucidating the design principles and structural features that are pertinent to various applications and hope to provide certain guidelines for researchers that are working at the interface of chemistry, biochemistry, and materials science.

Configuration-Controlled Crystal and/or Gel Formation of Protected d-Glucosamines Supported by Promiscuous Interaction Surfaces and a Conformationally Heterogeneous Solution State

The configuration-dependent self-association mode of the two anomers of O-Ac,N-Fmoc-d-glucosamine, a foldamer building block, leading to gel and/or single crystal formation is described. The β-anomer of the sugar amino acid (2) forms a gel from various solvents (confirmed by SEM, rheology measurements, NMR, and ECD spectroscopy), whereas the α-anomer (1) does not form a gel with any solvent tested. Transition from the solution state to a gel is coupled to a concurrent shift of the Fmoc-groups: from a freely rotating (almost symmetrical) to a specific, asymmetric orientation. Whereas the crystal structure of the α-anomer is built as an evenly packed 3D system, the β-anomer forms a looser superstructure of well-packed 2D layers. Modeling indicates that in the lowest energy, but scarcely sampled conformer of the β-anomer, the Fmoc-group bends above the sugar moiety, stabilized by intramolecular CH↔π interactions between the aromatic rings. It is concluded that possessing an extended and promiscuous interaction surface and a conformationally heterogeneous solution state are among the basic requirements of gel formation for a candidate molecule.

Ultrashort Peptide Self-Assembly: Front-Runners to Transport Drug and Gene Cargos

The translational therapies to promote interaction between cell and signal come with stringent eligibility criteria. The chemically defined, hierarchically organized, and simpler yet blessed with robust intermolecular association, the peptides, are privileged to make the cut-off for sensing the cell-signal for biologics delivery and tissue engineering. The signature service and insoluble network formation of the peptide self-assemblies as hydrogels have drawn a spell of research activity among the scientists all around the globe in the past decades. The therapeutic peptide market players are anticipating promising growth opportunities due to the ample technological advancements in this field. The presence of the other organic moieties, enzyme substrates and well-established protecting groups like Fmoc and Boc etc., bring the best of both worlds. Since the large sequences of peptides severely limit the purification and their isolation, this article reviews the account of last 5 years' efforts on novel approaches for formulation and development of single molecule amino acids, ultra-short peptide self-assemblies (di- and tri- peptides only) and their derivatives as drug/gene carriers and tissue-engineering systems.

Insights into a novel class of azobenzenes incorporating 4,6-O-protected sugars as photo-responsive organogelators

A novel class of 4,6-O-butylidene/ethylidene/benzylidene β-d-glucopyranose gelator functionalized with photo-responsive azobenzene moieties were designed and synthesized and also characterized using different spectral techniques. These azobenzene-based organogelators can gel even at lower concentrations (critical gelation concentration – 0.5% and 1%). A morphological study of the gels shows one-dimensional aggregated bundles and helical fibres. The main driving force for the self-assembly is through cooperative interactions exhibited by the different groups viz., sugar hydroxyl (hydrogen bonding interaction), azobenzene (aromatic π–π interaction) and alkyl chain of the protecting group (van der Waals interaction).

A novel class of 4,6-O-butylidene/ethylidene/benzylidene β-d-glucopyranose gelator functionalized with photo-responsive azobenzene moieties were designed and synthesized and also characterized using different spectral techniques.

Synthesis and characterization of 3-O-esters of N-acetyl-d-glucosamine derivatives as organogelators

Fourteen glycolipids were synthesized; all alkyl esters were organogelators. The hexanoate was a phase-selective gelator for oil in water.

Release of small bioactive molecules from physical gels

Pharmaceutical drugs with low water solubility have always received great attention within the scientific community. The reduced bioavailability and the need of frequent administrations have motivated the investigation of new drug delivery systems. Within this context, drug carriers that release their payload in a sustained way and hence reduce the administration rate are highly demanded. One interesting strategy to meet these requirements is the entrapment of the drugs into gels. So far, the most investigated materials for such drug-loaded gels are derived from polymers and based on covalent linkages. However, over the last decade the use of physical (or supramolecular) gels derived from low molecular weight compounds has experienced strong growth in this field, mainly due to important properties such as injectability, stimuli responsiveness and ease of synthesis. This review summarizes the use of supramolecular gels for the encapsulation and controlled release of small therapeutic molecules.

Spatially-resolved soft materials for controlled release - hybrid hydrogels combining a robust photo-activated polymer gel with an interactive supramolecular gel

Self-supporting photo-patterned hybrid gels achieve controlled directional release depending on their surrounding environment.

Hybrid hydrogels based on self-assembling low-molecular-weight gelator (LMWG) DBS-CONHNH₂ (DBS = 1,3;2,4-dibenzylidene-d-sorbitol) and crosslinked polymer gelator (PG) PEGDM (poly(ethyleneglycol) dimethacrylate) are reported, and an active pharmaceutical ingredient (naproxen, NPX) is incorporated. The use of PEGDM as PG enhances the mechanical stiffness of the hybrid gel (G′ increases from 400 to 4500 Pa) – the LMWG enhances its stability to very high frequency. Use of DBS-CONHNH₂ as LMWG enables interactions with NPX and hence allows pH-mediated NPX release – the PG network is largely orthogonal and only interferes to a limited extent. Use of photo-activated PEGDM as PG enables spatially-resolved photo-patterning of robust hybrid gel domains within a preformed LMWG network – the presence of the LMWG enhances the spatial resolution. The photo-patterned multi-domain gel retains pH-mediated NPX release properties and directionally releases NPX into a compartment of higher pH. The two components within these hybrid PG/LMWG hydrogels therefore act largely independently of one another, although they do modify each others properties in subtle ways. Hybrid hydrogels capable of spatially controlled unidirectional release have potential applications in tissue engineering and drug-delivery.

Synthesis and Study of Molecular Assemblies Formed by 4,6-O-(2-Phenylethylidene)-Functionalized d-Glucosamine Derivatives

Low-molecular-weight gelators are interesting small molecules with potential applications as advanced materials. Carbohydrate-based small molecular gelators are especially useful because they are derived from renewable resources and are more likely to be biocompatible and biodegradable. Various 4,6-benzylidene acetal protected α-methyl 2-d-glucosamine derivatives have been found to be effective low-molecular-weight gelators. To understand the influence of the 4,6-benzylidene acetal functional group toward molecular self-assembly and to obtain effective molecular gelators, we synthesized and analyzed a new series of d-glucosamine derivatives in which the phenyl group of the acetal is replaced by a benzyl group. The homologation of the acetal protection from aromatic to aliphatic functional groups allows us to probe the effect of increasing structural flexibility on molecular self-assembly and gelation. In this study, nine representative amides and nine urea analogs were synthesized, and their gelation properties were analyzed in a series of organic solvents and aqueous solutions. The resulting amide and urea derivatives are versatile organogelators forming gels in toluene, ethanol, isopropanol, ethylene glycol, and aqueous mixtures of organic solvents. More interestingly, the amide analogs are also effective gelators for pump oil and engine oil. NMR spectroscopy at variable temperatures was used to analyze the molecular assemblies and intermolecular forces. The selected gelators with several drug and dye molecules in DMSO and water were studied for their effectiveness of encapsulation and release of these agents.

Ultrasound- and Temperature-Induced Gelation of Gluconosemicarbazide Gelator in DMSO and Water Mixtures

We have developed amphiphilic supramolecular gelators carrying glucose moiety that could gel a mixture of dimethyl sulfoxide (DMSO) and water upon heating as well as ultrasound treatment. When the suspension of gluconosemicarbazide was subjected to ultrasound treatment, gelation took place at much lower concentrations compared to thermal treatment, and the gels transformed into a solution state at higher temperatures compared to temperature-induced gels. The morphology was found to be influenced by the nature of the stimulus and presence of salts such as KCl, NaCl, CaCl₂ and surfactant (sodium dodecyl sulphate) at a concentration of 0.05 M. The gel exhibited impressive tolerance to these additives, revealing the stability and strength of the gels. Fourier transform infrared spectroscopy (FTIR) revealed the presence of the intermolecular hydrogen bonding interactions while differential scanning calorimetry (DSC) and rheological studies supported better mechanical strength of ultrasound-induced (UI) gels over thermally-induced (TI) gels.

A versatile carbohydrate based gelator for oil water separation, nanoparticle synthesis and dye removal

A versatile green gelator suitable for multiple applications is reported. Gelation of organic solvents in a significantly low gelation time (<5 s) is achieved.

Urethane tetrathiafulvalene derivatives: synthesis, self-assembly and electrochemical properities

This paper reports the self-assembly of two new tetrathiafulvalene (TTF) derivatives that contain one or two urethane groups. The formation of nanoribbons was evidenced by scanning electron microscopy (SEM) and X-ray diffraction (XRD), which showed that the self-assembly ability of T₁ was better than that of T₂. The results revealed that more urethane groups in a molecule did not necessarily instigate self-assembly. UV–vis and FTIR spectra were measured to explore noncovalent interactions. The driving forces for self-assembly of TTF derivatives were mainly hydrogen bond interactions and π–π stacking interactions. The electronic conductivity of the T₁ and T₂ films was tested by a four-probe method.

Carbohydrates in Supramolecular Chemistry

Carbohydrates are involved in a variety of biological processes. The ability of sugars to form a large number of hydrogen bonds has made them important components for supramolecular chemistry. We discuss recent advances in the use of carbohydrates in supramolecular chemistry and reveal that carbohydrates are useful building blocks for the stabilization of complex architectures. Systems are presented according to the scaffold that supports the glyco-conjugate: organic macrocycles, dendrimers, nanomaterials, and polymers are considered. Glyco-conjugates can form host-guest complexes, and can self-assemble by using carbohydrate-carbohydrate interactions and other weak interactions such as π-π interactions. Finally, complex supramolecular architectures based on carbohydrate-protein interactions are discussed.

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.