Reduction-triggered Disassembly of Organic Cationic Nanorods Produces a Cysteine Protease Mimic (Miravet et†al.).

The emergence of catalytic activity associated with a disassembly process is reported, reminiscent of complex biological systems. A cystine derivative with pendant imidazole groups self-assembles into cationic nanorods in the presence of the cationic surfactants cetylpyridinium chloride (CPC) or cetyltrimethylammonium bromide (CTAB). Disulfide reduction triggers nanorod disassembly and the generation of a simple cysteine protease mimic, which shows a dramatically improved catalytic efficiency in the hydrolysis of p-nitrophenyl acetate (PNPA).

Reduction-Responsive Cationic Vesicles from Bolaamphiphiles with Ionizable Amino Acid or Dipeptide Polar Heads.

This paper presents a study of the aggregation of cationic bolaamphiphilic molecules into vesicles. These molecules are based on a cystamine core with protonated terminal dipeptide groups. The study found that vesicles can be formed at pH 4 for all of the dipeptide-terminated bolaamphiphiles containing different combinations of l-valine, l-phenylalanine, and l-tryptophan. The concentration for aggregation onset was determined by using pyrene as a fluorescent probe or light dispersion for compounds with tryptophan. Dynamic light scattering (DLS) studies and transmission electron microscopy (TEM) reveal that the vesicles have diameters ranging from 140 to 500 nm and show the capability of loading hydrophobic cargos, such as Nile red, and their liberation in reductive environments. Furthermore, the bolaamphiphiles are only fully protonated and prone to vesicle formation at acidic pH, making them a promising alternative for gastrointestinal delivery.

Liposome-Enveloped Molecular Nanogels.

Novel hydrogel@liposome particles were prepared by pH-triggered molecular gel formation inside of liposomes loaded with a low-molecular weight gelator derived from l-valine (1). Liposome formation was carried out using l-α-phosphatidylcholine (PC) and cholesterol as components of the lipid bilayer. Molecular hydrogelator 1 and pyranine, a ratiometric fluorescent pH probe, were entrapped in the liposomes at pH 9 and posterior acidification with d-glucono-1,5-lactone to pH 5-6 provoked intraliposomal gel formation. Removal of the lipid bilayer with sodium dodecyl sulfate yielded naked nanogel particles. The systems were characterized by transmission electron microscopy and dynamic light scattering. The hydrogel@liposomes were loaded with doxorubicin, showing a similar release than that observed for liposomes. The hybrid particles described here are the first case of nonpolymeric hydrogel@liposome systems reported. This type of nanocarriers merges the benefits of liposomal vehicles with the inherent stimuli responsiveness and enhanced biocompatibility of hydrogels formed by low-molecular weight molecules, foretelling a potential use in environmentally sensitive drug release.

In between molecules and self-assembled fibrillar networks: highly stable nanogel particles from a low molecular weight hydrogelator.

The preparation of molecular, non-polymeric nanogels from a low molecular weight hydrogelator is reported. The molecular nanogels are expected to overcome issues associated with the use of polymeric nanogels in biomedicine such as biodegradability, stimuli responsiveness, polydispersity, and batch-to-batch reproducibility. Nanogels formed by compound 1 were reproducibly prepared by sonication of a xerogel in PBS, with a total concentration of ca. 2 mM. The intensity averaged diameter of ca. 200 nm was determined by DLS. Electron microscopy (TEM and cryo-TEM) showed spherical particles. Light scattering (SALS) indicates that water is the main component of the nanoparticles, and the concentration of 1 in the nanogels is ca. 3 mg mL-1. These particles can be considered to constitute an intermediate state between free molecules and self-assembled fibrillar networks. The nanogels present excellent temporal and thermal stability and accessible hydrophobic domains, as demonstrated by the incorporation of the fluorescent dye Nile Red.

Photoactive Hexanuclear Molybdenum Nanoclusters Embedded in Molecular Organogels.

Hexanuclear molybdenum clusters are attractive species because of their outstanding photonic properties, and in the past they have been attached to a variety of supports such as organic polymers and inorganic nanoparticles, as described in the recent literature. Here, a cluster of the formula TBA2[Mo6I8Ac6] (TBA = tetrabutylammonium; Ac = acetate) has been supported on molecular organogels for the first time, resulting in a new soft material with remarkable photoactivity. Electron and confocal microscopic analyses showed the alignment of the nanoclusters to 1D self-assembled fibers formed by the organic gelator, and emission spectroscopy corroborated the interaction of the emissive clusters with such fibrillary structures. The new hybrid system is a deep-red emissive material (phosphorescence maximum at ca. 680 nm), with chromatic coordinates x = 0.725 and y = 0.274, capable of efficiently generating singlet oxygen (1O2) upon illumination with white light, as demonstrated by the photooxygenation of 9,10-dimethylanthracene and 1,5-dihydroxynaphthalene. The organogels can been made in dichloromethane and toluene and in both solvents display phosphorescence emission and photocatalytic properties.

Synthesis, spectroscopic studies and biological evaluation of acridine derivatives: The role of aggregation on the photodynamic efficiency.

Two new photoactive compounds (1 and 2) derived from the 9-amidoacridine chromophore have been synthesized and fully characterized. Their abilities to produce singlet oxygen upon irradiation have been compared. The synthesized compounds show very different self-aggregating properties since only 1 present a strong tendency to aggregate in water. Biological assays were conducted with two cell types: hepatoma cells (Hep3B) and human umbilical vein endothelial cells (HUVEC). Photodynamic therapy (PDT) studies carried out with Hep3B cells showed that non-aggregating compound 2 showed photoxicity, ascribed to the production of singlet oxygen, being aggregating compound 1 photochemically inactive. On the other hand suspensions of 1, characterized as nano-sized aggregates, have notable antiproliferative activity towards this cell line in the dark.

Peptide-Based Molecular Hydrogels as Supramolecular Protein Mimics.

This Minireview concerns recent advances in the design, synthesis, and application of low molecular-weight peptidic hydrogelators. The sequence-specific combinations of amino acid side chain functionalities combined with hydrogen bonding of amide backbones and hydrophobic (aromatic) capping groups give these peptidic molecules the intrinsic tendency to self-assemble. The most prevalent designs include N-capped amino acid residues, bolamphiphilic peptides, and amphipathic peptides. Factors such as hydrophobic effects, the Hofmeister effect, and tunable ionization influence their aggregation properties. The self-assembly of simple bio-inspired building blocks into higher organized structures allows comparisons to be drawn with proteins and their complex functionalities, providing preliminary insights into complex biological functions and also enabling their application in a wide range of fields including catalysis, biomedical applications, and mimicry of natural dissipative systems. The Minireview is concluded by a short summary and outlook, highlighting the advances and steps required to bridge the gaps in the understanding of such systems.

Sizing Down a Supramolecular Gel into Micro- and Nanoparticles.

A low molecular weight gelator with a fluorescent 1,8-naphthalimide unit forms micro- and nanoparticles in aqueous media. Slow addition of a DMSO solution of the gelator into water affords either a self-assembled fibrillar network, sheaf-like microparticles, or nanoparticles depending to the concentration used in the experiment. The micro- and nanoparticles were characterized by dynamic light scattering (DLS), electron microscopy, and fluorescence measurements. In an initial assay of particle loading, Rose Bengal and Rhodamine 123 were shown to be incorporated in the particles. Light-promoted singlet oxygen generation capabilities of Rose Bengal were modulated by its incorporation in the particles. Additionally, the particles were found to promote the transport of Rhodamine 123 into human lung carcinoma live cells. These results indicate that nanoparticles arising from low molecular weight gelators may represent a new type of nanocarriers, being a potential alternative to polymeric nanogels used in nanomedicine.

Improved Efficiency of Molecular-Gel Formation by Adjusting Preorganization of Amino-Acid-Derived Flexible Molecules: A NMR and Thermodynamic study.

The efficiency of the formation of molecular gels of simple derivatives of l-valine and l-isoleucine is greatly improved in different organic solvents when a hexyl fragment is replaced by a bulkier cyclohexyl one. A study using NMR and IR spectroscopy provides information on the preferred conformations of the molecules, indicating that the cyclohexyl moiety precludes intramolecular H bonding and preorganises the system for intermolecular interactions, which are responsible for fiber formation. NMR data of the gels provides thermodynamic data on fibrillization, revealing that the origin of this effect is mainly entropic. Electron microscopy (SEM and TEM) images show fibrillar and tape-like objects, which are observed commonly in molecular gels. Rheological measurements reveal significant differences between cyclohexyl and hexyl appended gelators. These findings could contribute to the rational design of small, flexible, building blocks for self-assembly.

Selective Interaction of Dopamine with the Self-Assembled Fibrillar Network of a Molecular Hydrogel Revealed by STD-NMR.

A molecular hydrogel formed by a derivative of L-valine with pendant isonicotinoyl moieties interacts selectively with protonated dopamine in the presence of related compounds such as 3-methylcatechol, and protonated or neutral phenethylamine. A two-point interaction with the gel fibers is postulated to explain the results. The conclusions are obtained from nuclear magnetic resonance saturation transfer experiments (STD-NMR), illustrating how this technique is perfectly suited to monitor the interaction of substrates with the fibrillar network of a molecular gel.

Structural insight into the aggregation of L-prolyl dipeptides and its effect on organocatalytic performance.

NMR and organocatalytic studies of four dipeptides derived from L-proline are described. Results indicate that important conformational changes around the catalytic L-proline moiety are observed for free dipeptides upon changing the adjacent amino acid. Also, an aggregation process is detected as the concentration increases. The self-association of the dipeptides has been fitted to a cooperative binding model. All the compounds have been assayed as catalysts for the conjugated addition of cyclohexanone to trans-ß-nitrostyrene in toluene. In agreement with the structural studies, noticeable changes in the catalytic performance are detected upon changing the catalyst concentration, as the catalyst is activated by self-aggregation.

Tetrapeptidic molecular hydrogels: self-assembly and co-aggregation with amyloid fragment Aß1-40.

A new family of isomeric tetrapeptides containing aromatic and polar amino acid residues that are able to form molecular hydrogels following a smooth change in pH is described. The hydrogels have been studied by spectroscopic and microscopic techniques showing that the peptide primary sequence has an enormous influence on the self-assembly process. In particular, the formation of extended hydrophobic regions and the appearance of π-stacking interactions have been revealed as the driving forces for aggregation. Moreover, the interaction of these compounds with amyloid peptidic fragment Aß1-40 has been studied and some of them have been shown to act as templates for the aggregation of this peptide into non-ß-sheet fibrillar structures. These compounds could potentially be used for the capture of toxic, soluble amyloid oligomers.

Rational design of heat-set and specific-ion-responsive supramolecular hydrogels based on the Hofmeister effect.

Smart supramolecular hydrogels have been prepared from a bolaamphiphilic L-valine derivative in aqueous solutions of different salts. The hydrogels respond selectively to different ions and are either reinforced or weakened. In one case, in contrast to conventional systems, the hydrogels are formed upon heating of the system. The use of the hydrogels in the controlled release of an entrapped dye is described as a proof of the potential applications of these systems. The responsive hydrogels were rationally designed by taking into account the noticeable effect of different ions from the Hofmeister series in the solubility of the hydrogelator, which was assessed by using NMR experiments. On the one hand, kosmotropic anions such as sulfate produce a remarkable solubility decrease in the gelator, which is associated with gel reinforcement, as measured by rheological experiments. On the other hand, chaotropic species such as perchlorate weaken the gel. A dramatic effect was observed in the presence of guanidinium chloride, which boosted the solubility of the gelator, in accordance with its chaotropic behaviour reported in protein science. In this case, a direct interaction of the guanidinium species with the carbonyl groups of the hydrogelator is detected by (13) C NMR spectroscopy. The weakening of this interaction upon a temperature increase allows for the preparation of heat-set hydrogelating systems.

Freezing capture of polymorphic aggregates of bolaamphiphilic (L)-valine-based molecular hydrogelators.

Nanostructured xerogels have been prepared by the freeze-drying of hydrogels and aggregates formed by bolaamphiphilic L-valine derivatives after aging under different environmental conditions. A wide variety of shapes and sizes has been achieved by a simple methodology. These nanostructures have been studied by SEM and WAXD and a dramatic influence of structural flexibility on the kinetics of aggregation has been observed. Such flexibility and a modulation of the hydrophobic effect have shown a profound influence in the packing of these compounds and revealed a high degree of polymorphism.

Control of molecular gelation by chemical stimuli.

Molecular gels are formed by the self-assembly of low-molecular weight compounds by weak non-covalent interactions and thus, they may be easily disassembled in response to external stimuli. Chemically sensitive gels can be prepared by introducing in the molecular design functional groups that may interact either by covalent or non-covalent forces with other molecules present in the medium. Functional molecular gels have been reported that are sensitive to acids, bases, ions, redox-active compounds, neutral species, reactive compounds and enzymes. Here we present a broad revision of the different chemical inputs that can be used to tune gel properties through some appealing application-based selected examples.

pH-Induced conversion of bolaamphiphilic vesicles to reduction-responsive nanogels for enhanced Nile Red and Rose Bengal delivery.

This study details the preparation and investigation of molecular nanogels formed by the self-assembly of bolaamphiphilic dipeptide derivatives containing a reduction-sensitive disulfide unit. The described bolaamphiphiles, featuring amino acid terminal groups, generate cationic vesicles at pH 4, which evolve into gel-like nanoparticles at pH 7. The critical aggregation concentration has been determined, and the nanogels' size and morphology have been characterized through Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). Circular Dichroism (CD) spectroscopy reveals substantial molecular reconfigurations accompanying the pH shift. These nanogels enhance the in vitro cellular uptake of the lipophilic dye Nile Red and the ionic photosensitizer Rose Bengal into Human colon adenocarcinoma (HT-29) cells, eliminating the need for organic co-solvents in the former case. Fluorescence measurements with Nile Red as a probe indicate the reduction-sensitive disassembly of the nanogels. In photodynamic therapy (PDT) applications, Rose Bengal-loaded nanogels demonstrate notable improvements, with flow cytometry analysis evidencing increased apoptotic activity in the study with HT-29 cells.

Interplay of molecular hydrogelators and SDS affords responsive soft matter systems with tunable properties.

The gelation efficiency of low molecular weight bolaamphiphilic hydrogelators 1 and 2 is influenced by the presence of SDS micelles. Similarly, the critical micellar concentration value of SDS is reduced in the presence of the studied molecular hydrogelators. Rheological measurements indicate that the strength of the hydrogels can be modulated with SDS, the gels becoming weaker in the presence of micelles. This behavior has been rationalized with the help of NMR studies using diffusion measurements and NOE correlations. The results obtained clearly point to the formation of mixed micelles composed of SDS and the hydrogelators. In the case of 1, the gelator:SDS ratio in the mixed micelles has been estimated from solubility studies to be ca. 1:2.5. Electron microscopy reveals that when SDS is present, the morphology of the xerogels is modified in its appearance at the micrometer scale but fibers with diameter in the nanometer range are observed in all the cases. The interplay between the surfactant and the gelators provides with new possibilities for the modulation of both gel and micelle formation. Examples are shown to highlight the potential usefulness of this type of interconnected system. In one case the release of a gel entrapped dye is modulated by the presence of SDS and sodium chloride. In another example, an intricate system that responds to a temperature excursion by irreversible micelle disassembly is shown.

Alkaline cations dramatically control molecular hydrogelation by an amino acid-derived anionic amphiphile.

Understanding the factors that control the formation of (supra)molecular hydrogels permits a rational tuning of their properties and represents a primary challenge for developing smart biocompatible soft materials. Hydrogel formation by molecular amphiphilic anions at millimolar concentrations is counterintuitive, considering the solubility of these species in water. Here we report hydrogel formation by a simple anionic molecular amphiphile and a rationale for the fibrillisation process observed. The studied molecule, DodValSuc, consists of a 12C alkyl chain, an l-valine unit and a terminal succinic acid moiety. Hydrogelation depends to a large degree on the nature and concentration of the alkaline cations present in the medium (Li+, Na+ or K+). As a result, gelation efficiency and properties like thermal stability or rheology are highly tunable using the alkaline cation present or its concentration as variables. A detailed study is reported, which includes the determination of minimum gelation concentration (MGC) by tabletop rheology, critical micelle concentration (CMC) using pyrene as a fluorescent probe, thermal stability (solubility) by 1H NMR, the morphology of the fibres by transmission electron microscopy (TEM), crystallinity by X-ray diffraction (XRD) and gel strength by oscillatory rheology. Additionally, dynamic light scattering (DLS) was used to evaluate the size of the micelles and permitted monitoring of the fibrillisation process. Altogether, the results are consistent with the formation of micelles that experience head crystallisation and subsequent aggregation into crystalline fibres. The alkaline cations play a crucial role in providing the cement that glues together the gelator molecules, making their concentration a critical parameter for gelation efficiency and properties. Furthermore, the gelation-promoting effects are inversely correlated with the size of the cations so that the highest thermal stability and rheological strength were found for the hydrogels formed in the presence of Li+.

Anion-responsive diguanidinium-based chiral organogelators.

The chiral bicyclic diguanidinium chloride 1 forms gels in aromatic apolar solvents. The gels were characterized at different levels of organization, from the macroscopic to the molecular level by using microscopy, spectroscopy, and powder X-ray diffraction. The dependency on chirality has been highlighted by circular dichroism and electron microscopy. Furthermore, the gel has been shown to be effectively responsive to anionic stimuli, thus allowing the reversible control of the organic-phase gelation in contact with different salted aqueous solutions.

Molecular hydrogels from bolaform amino acid derivatives: a structure-properties study based on the thermodynamics of gel solubilization.

Insight is provided into the aggregation thermodynamics associated to hydrogel formation by molecular gelators derived from L-valine and L-isoleucine. Solubility data from NMR measurements are used to extract thermodynamic parameters for the aggregation in water. It is concluded that at room temperature and up to 55 °C, these systems form self-assembled fibrillar networks in water with quite low or zero enthalpic component, whereas the entropy of the aggregation is favorable. These results are explained by considering that the hydrophobic effect is dominant in the self-assembly. However, studies by NMR and IR spectroscopy reveal that intermolecular hydrogen bonding is also a key issue in the aggregation process of these molecules in water. The low enthalpy values measured for the self-assembly process are ascribed to the result of a compensation of the favorable intermolecular hydrogen-bond formation and the unfavorable enthalpy component of the hydrophobic effect. Additionally, it is shown that by using the hydrophobic character as a design parameter, enthalpy-controlled hydrogel formation, as opposed to entropy-controlled hydrogel formation, can be achieved in water if the gelator is polar enough. It is noteworthy that these two types of hydrogels, enthalpy-versus entropy-driven hydrogels, present quite different response to temperature changes in properties such as the minimum gelator concentration (mgc) or the rheological moduli. Finally, the presence of a polymorphic transition in a hydrogel upon heating above 70 °C is reported and ascribed to the weakening of the hydrophobic effect upon heating. The new soft polymorphic materials present dramatically different solubility and rheological properties. Altogether these results are aimed to contribute to the rational design of molecular hydrogelators, which could be used for the tailored preparation of this type of soft materials. The reported results could also provide ground for the rationale of different self-assembly processes in aqueous media.