Impact of polymorphism in oleogels of N-palmitoyl-L-phenylalanine.

Gels of edible oils, also called oleogels, are developed as alternative products of solid fats to limit the uptake of saturated and trans-unsaturated fats and lower the associated risk of coronary disease. The gelation of oils can be achieved with a low molecular weight organogelator (LMWO), a compound that self-assembles at low concentrations in a solid 3D network and provides the mixture its solid-like behavior. We have studied N-palmitoyl-L-phenylalanine (Palm-Phe), an endogenous compound (i.e. naturally present in the human body) as a model LMWO of rapeseed oil. Palm-Phe forms gels at a concentration of 1 wt% in rapeseed oil. We have studied the thermodynamic and mechanical behavior of the corresponding gels. As evidenced by DSC and rheology, this system exhibits two transitions upon heating, in addition to the sol-gel transition, a gel-gel transition between two polymorphic gels. The structural differences between both polymorphs were revealed using cryo-SEM, X-rays scattering, and FTIR experiments. The metastability of one of the polymorphs was proven by ageing and annealing experiments.

Beyond Sol-Gel: Molecular Gels with Different Transitions.

The existence of sol-gel transitions is one of the most manifest properties of molecular gels. These transitions reflect their nature since they correspond to the association or dissociation of low weight molecules through non-covalent interactions to form the network constitutive of the gel. Most described molecular gels undergo only one gel-to-sol transition upon heating, and the reverse sol-to-gel transition upon cooling. It has been long observed that different conditions of formation could lead to gels with different morphologies, and that gels can undergo a transition from gel to crystals. However, more recent publications report molecular gels which exhibit additional transitions, for instance gel-to-gel transitions. This review surveys the molecular gels for which, in addition to sol-gel transitions, transitions of different nature have been reported: gel-to-gel transitions, gel-to-crystal transition, liquid-liquid phase separations, eutectic transformations, and synereses.

Palmitoylethanolamide gels edible oils.

Palmitoylethanolamide (PEA) is an endogenous compound with no adverse effect for oral intakes of a gram per day. We show that PEA gels edible oils at concentrations as low as 0.5 wt%. The elastic moduli values of the formed gels are 1400 Pa at 1 wt% and 9000 Pa at 2 wt%. The study of the gels by cryo-SEM, optical microscopy and WAXS show that PEA forms lamellar solid aggregates with widths of several tens of micrometers. Upon heating, the sample shows two transitions. The first one is the gel-to-sol transition, observed by rheology and defined by the switch from a solid to a liquid behavior. During this transition, the solid particles remain but do no longer form a network. The second transition, observed at higher temperature by DSC corresponds to the melting of the solid particles.

Investigation of an Organogel by Micro-Differential Scanning Calorimetry: Quantitative Relationship between the Shapes of the Thermograms and the Phase Diagram.

The phase diagrams of organogels are necessary for applications and fundamental aspects, for instance to understand their thermodynamics. Differential scanning calorimetry is one of the techniques implemented to map these diagrams. The thermograms of organogels upon heating show broad endotherms, increasing gradually to a maximum, at a temperature Tmax, and decreasing back to the baseline, sometimes 10 °C above. This broadening can lead to uncertainty in determining the molar enthalpies and the melting temperatures Tm of the gels. Herein, we have measured the thermograms of the 12-hydroxystearic acid/nitrobenzene gels for weight fractions ranging from 0.0015 to 0.04. Compared with transition temperatures measured by other techniques, the inflection points of the thermograms provide a measurement of Tm with less bias than Tmax. The phase diagram explains why the molar melting enthalpies derived from the thermograms for samples of low concentration are lower than expected. The shapes of the heat flows below the peak correlate quantitatively with the diagrams: after suitable correction and normalization, the integral curves superimpose with the phase diagram in their ascending branch and reach a plateau when the gel is fully melted. The shape of the thermograms upon cooling is also qualitatively explained within the frame of the diagrams.

Gel-to-gel non-variant transition of an organogel caused by polymorphism from nanotubes to crystallites.

An amide based gelator forms gels in trans-decalin. Below concentrations of 1 wt% the gels melt at temperatures varying with concentration. Above a concentration of 1 wt%, upon heating, the gel transforms into an opaque gel at an invariant temperature, and melts at higher temperature. The gel-to-gel transition is evidenced by several techniques: DSC, rheology, NMR, OM and turbidimetry. The phase diagram with the domain of the existence of both morphs was mapped by these techniques. Optical and electronic microscopy studies show that the first gel corresponds to the self-assembled nanotubes while the second gel is formed by crystalline fibers. The fibers are crystalline, as shown by the presence of Bragg peaks in the scattering curves. Both morphs correspond to a different H-bonding pattern as shown by FTIR. The first gel forms at a higher cooling rate, is metastable and transforms slowly into the second one. The second gel is stable. It forms at a low cooling rate, or by thermal annealing or aging of the first gel.

Structure of Nanotubes Self-Assembled from a Monoamide Organogelator.

Some organic compounds are known to self-assemble into nanotubes in solutions, but the packing of the molecules into the walls of the tubes is known only in a very few cases. Herein, we study two compounds forming nanotubes in alkanes. They bear a secondary alkanamide chain linked to a benzoic acid propyl ester (HUB-3) or to a butyl ester (HUB-4). They gel alkanes for concentrations above 0.2 wt.%. The structures of these gels, studied by freeze fracture electron microscopy, exhibit nanotubes: for HUB-3 their external diameters are polydisperse with a mean value of 33.3 nm; for HUB-4, they are less disperse with a mean value of 25.6 nm. The structure of the gel was investigated by small- and wide-angle X-ray scattering. The evolution of the intensities show that the tubes are metastable and transit slowly toward crystals. The intensities of the tubes of HUB-4 feature up to six oscillations. The shape of the intensities proves the tubular structure of the aggregates, and gives a measurement of 20.6 nm for the outer diameters and 11.0 nm for the inner diameters. It also shows that the electron density in the wall of the tubes is heterogeneous and is well described by a model with three layers.

Impact of Chirality on Hydrogen-Bonded Supramolecular Assemblies and Photoconductivity of Diketopyrrolopyrrole Derivatives.

Hydrogen bonds can efficiently guide the self-assembly of organic materials, enabling to tune the properties of the aggregation processes. In the case of π-conjugated materials, several parameters such as temperature, concentration and solvent can be used to modify the aggregation state while tuning the optoelectronic properties. Chirality can be included within the impacting parameters due to the differences in molecular packing. Here, chiral and achiral thiophene-capped diketopyrrolopyrrole derivatives were designed and synthesized containing amide bonds, with the aim to study the interplay between chiral assemblies and their stabilization through hydrogen-bonding. Differences in aggregation properties were observed with spectroscopy and microscopy, and a contactless microwave-based technique was used to study their intrinsic charge carrier mobility. The positive role of hydrogen-bonding has been highlighted and the differences between chiral and achiral compounds have been elucidated.

Variable temperature NMR of organogelators: the intensities of a single sample describe the full phase diagram.

Organogelators constitute a numerous class of compounds, able to form gels in organic solvents. Their phase diagrams are useful to understand their mechanisms of formation and their stability, but their mapping is often a tedious task. We show that liquid NMR can simplify and quicken the acquisition of phase diagrams. In liquid NMR spectra of organogels, the visible signals of the gelator represent only its soluble fraction. The intensities increase with temperature, until the gel melts. Suitable normalization of these intensities yields the solubility as a function of temperature, which is sufficient to map the phase diagram. We verified it experimentally with three organogelators, chosen because independent authors have previously mapped out their phase diagram by other techniques including DSC and rheology. We show that the curves obtained by NMR superimpose with these diagrams. A variable temperature NMR experiment with a single sample can yield the phase diagram with sensitivity of the order of 0.01 wt%.

Origin of Invariant Gel Melting Temperatures in the c-T Phase Diagram of an Organogel.

Binary c-T phase diagrams of organogelators in solvent are frequently simplified to two domains, gel and sol, even when the melting temperatures display two distinct regimes, an increase with T and a plateau. Herein, the c-T phase diagram of an organogelator in solvent is elucidated by rheology, DSC, optical microscopy, and transmitted light intensity measurements. We evidence a miscibility gap between the organogelator and the solvent above a threshold concentration, cL. In this domain the melting or the formation of the gel becomes a monotectic transformation, which explains why the corresponding temperatures are nonvariant above cL. As shown by further studies by variable temperature FTIR and NMR, different types of H-bonds drive both the liquid-liquid phase separation and the gelation.

Design of Nanohybrid Systems from a Partially Fluorinated Organogelator and Syndiotactic Polystyrene Thermoreversible Gel.

Nanohybrid systems are prepared from organogels of a partially fluorinated molecule and from thermoreversible gels of syndiotactic polystyrene. The thermodynamic behavior, morphology, and structure are investigated by using differential scanning calorimetry, atomic force microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The outcomes of these investigations suggest that the fibrils of the organogel coil around the sPS fibrils, probably through a heterogeneous nucleation process. These systems therefore differ from previously investigated sPS/OPV systems (oligo vinylene phenylene) where OPV fibrils pervade the sPS network.

Investigation of the interactions involved in the formation of nanotubes from organogelators.

Investigations into the formation of nanosized structures, particularly nanotubes, by a diamide ester compound are reported. Two aspects are concurrently examined: the role of the solvent and the role of the alkyl chain. The former is addressed by using a benzene derivative (o-xylene) and a totally saturated double ring (trans-decahydronaphthalene) whereas the latter is achieved by replacing the hydrogenous alkyl chain with its fluorinated counterpart while keeping the overall architecture the same. The thermodynamic behavior by differential scanning calorimetry, the morphology by transmission electron microscopy, and the structure by X-ray scattering and small-angle neutron scattering are studied. Despite the identical architecture, the fluorinated molecule does not produce any nanotubes, unlike its totally hydrogenous counterpart. Also, o-xylene prevents the hydrogenous molecule from forming nanotubes, while nanotapes are produced instead. Conversely, the fluorinated molecule produces regularly twisted protostructures in either solvent. Neutron scattering experiments show that the fluorinated alky chain is located within the core of this structure. This suggests that the prerequisite for forming nanotubes relies on the necessity of the alkyl group to point outward.

Bioaffinity sensor based on nanoarchitectonic films: control of the specific adsorption of proteins through the dual role of an ethylene oxide spacer.

The identification and quantification of biomarkers or proteins is a real challenge in allowing the early detection of diseases. The functionalization of the biosensor surface has to be properly designed to prevent nonspecific interactions and to detect the biomolecule of interest specifically. A multilayered nanoarchitecture, based on polyelectrolyte multilayers (PEM) and the sequential immobilization of streptavidin and a biotinylated antibody, was elaborated as a promising platform for the label-free sensing of targeted proteins. We choose ovalbumin as an example. Thanks to the versatility of PEM films, the platform was built on two types of sensor surface and was evaluated using both optical- and viscoelastic-based techniques, namely, optical waveguide lightmode spectroscopy and the quartz crystal microbalance, respectively. A library of biotinylated poly(acrylic acids) (PAAs) was synthesized by grafting biotin moieties at different grafting ratios (GR). The biotin moieties were linked to the PAA chains through ethylene oxide (EO) spacers of different lengths. The adsorption of the PAA-EOn-biotin (GR) layer on a PEM precursor film allows tuning the surface density in biotin and thus the streptavidin adsorption mainly through the grafting ratio. The nonspecific adsorption of serum was reduced and even suppressed depending on the length of the EO arms. We showed that to obtain an antifouling polyelectrolyte the grafting of EO9 or EO19 chains at 25% in GR is sufficient. Thus, the spacer has a dual role: ensuring the antifouling property and allowing the accessibility of biotin moieties. Finally, an optimized platform based on the PAA-EO9-biotin (25%)/streptavidin/biotinylated-antibody architecture was built and demonstrated promising performance as interface architecture for bioaffinity sensing of a targeted protein, in our case, ovalbumin.

Size-selective 2D ordering of gold nanoparticles using surface topography of self-assembled diamide template.

Size-selective organization of ~2 nm dodecanethiol stabilized gold nanoparticles (AuNPs) into periodic 1D arrays by using the surface topographical features of a soft template is described. The template consists of micrometer length nanotapes organized into nanosheets with periodic valleys running along their length and is generated by the hierarchical self-assembly of a diamide molecule (BHPB) in cyclohexane. The AuNP ordering achieved simply by mixing the preformed template with the readily available ~2 nm dodecanethiol stabilized AuNPs is comparable to those obtained using programmable DNA and functional block copolymers. The observed periodicity of the AuNP arrays provided valuable structural clues about the organization of nanotapes into nanosheets. Self-assembling BHPB molecules in the presence of AuNPs by heating and cooling the two components led to a comparatively disordered organization because the template structure was changed under these conditions. Moreover, the template could not order larger AuNPs (~5 nm) into a similar 1D array, owing to the steric restriction imposed by the dimension of the valleys on the template. Interestingly, this geometric constraint led to AuNP size sorting when a polydisperse sample (2.5 ± 0.9 nm) was used for organization, with AuNPs attached to the template edges being larger (≥2.2 ± 0.9 nm) than those associated to the inner valleys (1.6 ± 0.8 nm). This is a unique example of size-sorting induced by the surface topographical features of a soft template.

Cyto-mechanoresponsive polyelectrolyte multilayer films.

Cell adhesion processes take place through mechanotransduction mechanisms where stretching of proteins results in biological responses. In this work, we present the first cyto-mechanoresponsive surface that mimics such behavior by becoming cell-adhesive through exhibition of arginine-glycine-aspartic acid (RGD) adhesion peptides under stretching. This mechanoresponsive surface is based on polyelectrolyte multilayer films built on a silicone sheet and where RGD-grafted polyelectrolytes are embedded under antifouling phosphorylcholine-grafted polyelectrolytes. The stretching of this film induces an increase in fibroblast cell viability and adhesion.

Self-assembled nanotubes and helical tapes from diacetylene nonionic amphiphiles. Structural studies before and after polymerization.

We synthesized new amphiphiles comprised of a single diacetylenic chain and an oligoethylenoxide polar chain linked by an amide bond. In aqueous medium, they are not soluble at room temperature but form weak gels. Electron microscopy studies have shown that they self-assemble into helical tapes or nanotubes with lengths of several micrometers, and inner and outer diameters of 50 ± 1 and 59 ± 1 nm, respectively. The wall has a thickness of 10 ± 1 nm for both kinds of objects and has an amphiphile bilayer structure. The hydrophobic chains are ordered, and the amide groups are linked with each other by H-bonds. The dissociation of the tubes is a first-order transition with an enthalpy of ca. 40 kJ mol(-1). The nanotubes were photopolymerized to yield purple solutions consisting of helical tapes and almost flat ribbons. The polymers exhibit irreversible thermochromism upon heating.

Terbium, a fluorescent probe for investigation of siderophore pyochelin interactions with its outer membrane transporter FptA.

Pyochelin (Pch) is a siderophore and FptA is its outer membrane transporter produced by Pseudomonas aeruginosa to import iron. The fluorescence of the element terbium is affected by coordinated ligands and it can therefore be used as a probe to investigate the pyochelin-iron uptake pathway in P. aeruginosa. At pH 8.0, terbium fluorescence is greatly enhanced in the presence of pyochelin indicating chelation of the metal by the siderophore. Titration curves showed a 2:1 (Pch:Tb(3+)) stoichiometry and an affinity of K=(2±-1)×10(11)M(-2) was determined. Pch-Tb interaction with the transporter FptA could be followed in vitro and in vivo in P. aeruginosa cells, by Fluorescence Resonance Energy Transfer (FRET) between three partners: the tryptophans of FptA (donor), Pch (acceptor for the Trps and donor for Tb(3+)) and Tb(3+) (acceptor). Pch-Tb binds to the Pch-Fe outer membrane transporter FptA with a dissociation constant (K(d)) of 4.6µM. This three-partner FRET is a potentially valuable tool for investigation of the interactions between FptA and its siderophore Pch.

Polyelectrolyte multilayers capped with polyelectrolytes bearing phosphorylcholine and triethylene glycol groups: parameters influencing antifouling properties.

In this paper, we investigate the design of antifouling surfaces by the deposition of polyelectrolytes modified by grafting of antifouling groups onto a (PSS/PAH)n precursor multilayer film [PSS, poly(styrenesulfonate); PAH, poly(allylamine)]. Different polyelectrolytes and different antifouling moieties are investigated, in particular, (EO)3 and (EO)3PC moieties (EO, ethylene oxide; PC, phosphorylcholine group). We find that protein adsorption can strongly be reduced and even practically suppressed through the deposition of only one layer of polyelectrolyte modified with PC and/or (EO)3 groups. We discuss the influence of various parameters such as the nature of the polyelectrolyte backbone, the nature of the antifouling moiety, and the grafting ratio on the reduction of protein adsorption. We find in particular that (EO)3 and (EO)3PC moieties grafted on poly(acrylic acid) (PAA) totally prevent protein adsorption for grafting ratios of 25% or more, at least within the detection limits of the used quartz crystal microbalance and optical waveguide light mode spectroscopy devices. The mechanism that leads to the antifouling property is discussed and compared to that leading to the antifouling properties of ethylene oxide self-assembled monolayers. Finally, by incorporating biotin on top of the precursor film, we show that one layer of PAA-(EO)3PC is not sufficient to prevent interaction with streptavidin but a PAA-(EO)3PC/PAH/PAA-(EO)3PC multilayer largely protects the biotin from interacting with streptavidin.

Direct functionalization of self-assembled nanotubes overcomes unfavorable self-assembling processes.

Diamides containing alkyne and azido were self-assembled into nanotubes and were reacted under their self-assembled state with small molecules by "click chemistry"; the resulting compounds remain self-assembled into new nanotubes that cannot be formed by simple self-assembly of the constituting molecules.

Design, synthesis, and degradation studies of new enzymatically erodible poly(hydroxyethyl methacrylate)/poly(ethylene oxide) hydrogels.

This work describes the synthesis and the study of poly(hydroxyethyl methacrylate) PHEMA hydrogels, cross-linked by poly(ethylene oxide)(PEO) chains containing the Gly-Gly-Leu tripeptide. This sequence was selected for its ability to be cleaved by subtilisin, a bacterial protease. The cross-linker was synthesized by coupling the peptide with two amino-terminated PEO chains of M(w)=3400 g/mol. The resulting polymer was characterized by size exclusion chromatography, nuclear magnetic resonance, and mass spectroscopy, and was shown to be readily cleaved by subtilisin. Its esterification of the hydroxyl end groups into methacrylate afforded a macromonomer that was used as a degradable cross-linker and copolymerized with hydroxyethylmethacrylate to form hydrogels. The swelling ratio of the gels increases when the PEO cross-linker/polyHEMA ratio increases. Incubation of these gels with the enzyme led to the total degradation of the gels. These assays show that these gels can be used as drug-delivery systems where the release is triggered by the presence of proteases.