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This article describes self-assembly of supramolecularly engineered naphthalene-diimide (NDI)-derived amphiphiles NDI-1 and NDI-2. They have the same hydrophobic/hydrophilic balance but merely differ by a single functional group, amide or ester. They exhibit distinct self-assembly in water; NDI-1 forms hydrogel, which upon aging forms crystals, whereas NDI-2 forms micelles as revealed by in-depth structural analysis using cryo-TEM, dynamic light scattering, and small-angle X-ray scattering studies. These results suggest that the H-bonding among the amide groups fully regulates the self-assembly by overruling the packing parameters. Further, the present study elucidates sharp lower critical solution temperature exhibited by these π-amphiphiles, which has been extensively studied for many important applications of water-soluble polymers but hardly known in the literature of small-molecule surfactants. Control experiments with the same water-soluble hydrophilic wedge did not show such a property, confirming this to be a consequence of the supramolecular polymerization by extended amide-amide H-bonding and not inherent to the structure of the hydrophilic wedge containing oligo-oxyethylene chains.

We have demonstrated that solutions of 3,5-bis-(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10) can form metastable nanostructures on solid substrates and in the bulk. BHPB-10 is an achiral molecule involving several distinct, strongly interacting groups (SIGs), one aromatic-ester ring and two amide groups per molecule. Specific solvents affect the interactions between particular SIGs, thus promoting various nano-structures: lamellae, nanoribbons, helical ribbons, or nanotubes. In cyclohexane, a solvent allowing for both inter-amide hydrogen bonds and mutual attraction of rings, the formation of nanotubes with a diameter of 28 ± 5 nm was observed in the bulk and on surfaces. By contrast, in cyclohexanone, which suppresses inter-amide hydrogen bonds, flat nanoribbons with a specific width of 12 ± 4 nm were formed on solid substrates after drying. By annealing in cyclohexane vapor, we followed the process of switching surface structures from nanoribbons to nanotubes and observed helical ribbons as the precursor of nanotubes. We also turned nanotubes back into nanoribbons by adding cyclohexanone, thus demonstrating reversible switching along the route: tubes → lamellae → flat ribbons → helical ribbons → tubes. We propose models explaining the observed nanostructures and their transformations, including the origin of spontaneous chirality of the helical ribbons. Our findings on the self-assembly in the achiral BHPB-10 solutions provide insight into the influence of complementary inter-molecular specific SIG-based interactions and demonstrate an effective route for tailoring the shape and size of nanostructures derived from the same building unit.

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.

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.

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., (Copyright © 2011. Published by Elsevier Inc.)

In iron-deficient conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophore named pyoverdin (Pvd), which after chelating iron(III) is transported back into the cell via its outer membrane receptor FpvA. FpvA is a TonB-dependent transport protein and has the ability to bind Pvd in its apo- or iron-loaded form. The fluorescence properties of Pvd were used to determine the binding kinetics of metal-free and metal-loaded Pvd to FpvA and showed two major features. First, the kinetics of formation of the FpvA-Pvd complex, in vivo and in vitro, are markedly slower compared to those observed for FpvA-Pvd-metal. Second, apo-Pvd and Pvd-metal absorbed with biphasic kinetics to FpvA: the bimolecular step (association of the ligand with the receptor) is followed by a slower step (t(1/2) values of 5 and 34 min for Pvd-metal and Pvd, respectively) that presumably leads to a more stable complex. The most likely explanation for this second step is that the binding of the ligand to the receptor induces a conformational change on FpvA, which may be different, depending on the loading status of Pvd. Analysis of the dissociation of metal-free Pvd from FpvA revealed an energy and a TonB dependency. The dissociation of iron-free Pvd from FpvA in the absence of the TonB protein occurs with slow kinetics in the range of hours, but it can be highly activated by the protonmotive force and TonB to reach a kinetic with a t(1/2) of 1 min. Apparently, under iron-limited conditions, TonB activates the FpvA receptor, resulting in a fast release of iron-free Pvd and generating an unloaded FpvA receptor, competent for binding extracellular Pvd-Fe.