Cristelle Mériadec

Control of peptide nanotube diameter by chemical modifications of an aromatic residue involved in a single close contact

Supramolecular self-assembly is an attractive pathway for bottom-up synthesis of novel nanomaterials. In particular, this approach allows the spontaneous formation of structures of well-defined shapes and monodisperse characteristic sizes. Because nanotechnology mainly relies on size-dependent physical phenomena, the control of monodispersity is required, but the possibility of tuning the size is also essential. For self-assembling systems, shape, size, and monodispersity are mainly settled by the chemical structure of the building block. Attempts to change the size notably by chemical modification usually end up with the loss of self-assembly. Here, we generated a library of 17 peptides forming nanotubes of monodisperse diameter ranging from 10 to 36 nm. A structural model taking into account close contacts explains how a modification of a few Å of a single aromatic residue induces a fourfold increase in nanotube diameter. The application of such a strategy is demonstrated by the formation of silica nanotubes of various diameters.

Hexacyano octahedral metallic clusters as versatile building blocks in the design of extended polymeric framework and clustomesogens

Using self-assembling processes to generate hybrid organic inorganic materials allows the control of their structuration at the nanometric scale. We describe in this work the synthesis, liquid crystal and photo-physical properties of [M6Qi8(CN)a6]n− (M = Mo, Re; Qi = Br, Se; n = 2, 3 or 4) cluster anionic units containing clustomesogens. A new and efficient synthetic route was developed to synthesize the [Mo6Bri8(CN)a6]2− building block that is stable in water solution and that could be crystallized as the porous [trans-Cd(H2O)2][Mo6Bri8(CN)a6]. Hybrids were obtained by a metathesis reaction with a specifically designed organic cation. Their self-assembling abilities can be tailored by playing with the charge of the inorganic building blocks going from a nematogenic behaviour, which is particularly rare for ionic mesomorphic material, to the formation of layered structures. The intrinsic properties (luminescence or magnetism) of transition metal clusters are well retained in the hybrid matrices. The nanostructuration of the material influences its ability to emit light despite the isotropy of the emissive nanocluster. Finally, we demonstrate that the magnetic [Re6Se8CN6]3− can be reduced into the luminescent [Re6Se8CN6]4− upon heating at 150 °C. These hybrid materials show promising prospects in the field of luminescent material.

χ (2) semiconductor photonic crystals

Playing a central role in microelectronics and optoelectronics, semiconductors almost stand apart from applications involving second-order nonlinear effects such as frequency converters, tunable sources, parametric amplifiers, and switches. The reasons are twofold: their strong chromatic dispersion, which prevents the interacting waves from propagating with the same phase velocity (phase mismatch), and the shortness of the semiconductor devices, which adds more difficulty to achieving reasonable nonlinear efficiencies. By exploiting the unique properties of photonic crystals, we demonstrate simultaneous phase matching and enhancement of the fields under nonlinear interaction. We demonstrate a second-harmonic efficiency growth faster than the fifth power of the structure length.

Thermotropic luminescent clustomesogen showing a nematic phase: a combination of experimental and molecular simulation studies.

Octahedral Mo6 nanoclusters are functionalized with two organic ligands containing cyanobiphenyl (CB) units, giving luminescent hybrid liquid crystals (LC). Although the mesogenic density around the bulky inorganic core is constant, the two hybrids show different LC properties. Interestingly, one of them shows a nematic phase, which is particularly rare for this kind of supermolecular system. This surprising result is explained by using large-scale molecular dynamic simulations.

Structural role of counterions adsorbed on self-assembled peptide nanotubes.

Among noncovalent forces, electrostatic ones are the strongest and possess a rather long-range action. For these reasons, charges and counterions play a prominent role in self-assembly processes in water and therefore in many biological systems. However, the complexity of the biological media often hinders a detailed understanding of all the electrostatic-related events. In this context, we have studied the role of charges and counterions in the self-assembly of lanreotide, a cationic octapeptide. This peptide spontaneously forms monodisperse nanotubes (NTs) above a critical concentration when solubilized in pure water. Free from any screening buffer, we assessed the interactions between the different peptide oligomers and counterions in solutions, above and below the critical assembly concentration. Our results provide explanations for the selection of a dimeric building block instead of a monomeric one. Indeed, the apparent charge of the dimers is lower than that of the monomers because of strong chemisorption. This phenomenon has two consequences: (i) the dimer-dimer interaction is less repulsive than the monomer-monomer one and (ii) the lowered charge of the dimeric building block weakens the electrostatic repulsion from the positively charged NT walls. Moreover, additional counterion condensation (physisorption) occurs on the NT wall. We furthermore show that the counterions interacting with the NTs play a structural role as they tune the NTs diameter. We demonstrate by a simple model that counterions adsorption sites located on the inner face of the NT walls are responsible for this size control.

Stereochemical effect revealed in self-assemblies based on archaeal lipid analogues bearing a central five-membered carbocycle: a SAXS study.

The relative stereochemistry (cis or trans) of a 1,3-disubstituted cyclopentane unit in the middle of tetraether archaeal bipolar lipid analogues was found to have a dramatic influence on their supramolecular self-assembly properties. SAXS studies of two synthetic diastereomeric archaeal lipids bearing two lactosyl polar head groups at opposite ends revealed different lyotropic behaviors. The cis isomer led to L(c)-L(α)-Q(II) transitions whereas the trans isomer retained an L(α) phase from 20 to 100 °C. These main differences originate from the conformational equilibrium (pseudorotation) of 1,3-disubstituted cyclopentanes. Indeed, this pseudorotation exhibits quite similar orientations of the two substituents in a trans isomer whereas several orientations of the two alkyl chains are expected in a cis-1,3-dialkyl cyclopentane, thus authorizing more conformational flexibility in the lipid packing.

Magnetically Recoverable Palladium(0) Nanocomposite Catalyst for Hydrogenation Reactions in Water

An easy and straightforward strategy has been used for loading palladium(0) nanoparticles onto the magnetic surface of maghemite (γ‐Fe2O3), without the use of organic modifiers. The nanocomposite was fully characterized by TEM, XRD, 57Fe Mössbauer spectroscopy, X‐ray photoelectron spectroscopy, and superconducting quantum interference device measurements. The Pd0@γ‐Fe2O3 nanocatalyst has been investigated in various catalytic reactions, under mild conditions (100 kPa H2, RT), and in neat water. Relevant catalytic activities were achieved in the hydrogenation of olefinic substrates, as well as in hydrodehalogenation reactions of halogenoarenes, that constitutes a promising process for wastewater treatment. These nanocatalysts proved also pertinent for the reduction of nitroarene derivatives into the corresponding anilines, which are promising substrates for fine chemistry. Finally, these catalysts proved to be easily recoverable through the use of an external magnet, without significant loss of activity.

Effects of a novel archaeal tetraether-based colipid on the in vivo gene transfer activity of two cationic amphiphiles.

Gene therapy for treating inherited diseases like cystic fibrosis might be achieved using multimodular nonviral lipid-based systems. To date, most optimizations have concerned cationic lipids rather than colipids. In this study, an original archaeal tetraether derivative was used as a colipid in combination with one or the other of two monocationic amphiphiles. The liposomes obtained, termed archaeosomes, were characterized regarding lipid self-assembling properties, macroscopic/microscopic structures, DNA condensation/neutralization/relaxation abilities, and colloidal stability in the presence of serum. In addition, gene transfer experiments were conducted in mice with lipid/DNA complexes being administered via systemic or local delivery routes. Altogether, the results showed that the tetraether colipid can provide complexes with different in vivo transfection abilities depending on the lipid combination, the lipid/colipid molar ratio, and the administration route. This original colipid appears thus as an innovative modular platform endowed with properties possibly beneficial for fine-tuning of in vivo lipofection and other biomedical applications.

Atomic view of the histidine environment stabilizing higher-pH conformations of pH-dependent proteins.

External stimuli are powerful tools that naturally control protein assemblies and functions. For example, during viral entry and exit changes in pH are known to trigger large protein conformational changes. However, the molecular features stabilizing the higher pH structures remain unclear. Here we elucidate the conformational change of a self-assembling peptide that forms either small or large nanotubes dependent on the pH. The sub-angstrom high-pH peptide structure reveals a globular conformation stabilized through a strong histidine-serine H-bond and a tight histidine-aromatic packing. Lowering the pH induces histidine protonation, disrupts these interactions and triggers a large change to an extended β-sheet-based conformation. Re-visiting available structures of proteins with pH-dependent conformations reveals both histidine-containing aromatic pockets and histidine-serine proximity as key motifs in higher pH structures. The mechanism discovered in this study may thus be generally used by pH-dependent proteins and opens new prospects in the field of nanomaterials.

Experimental observation of double-walled peptide nanotubes and monodispersity modeling of the number of walls.

Self-assembled nanoarchitectures based on biological molecules are attractive because of the simplicity and versatility of the building blocks. However, size control is still a challenge. This control is only possible when a given system is deeply understood. Such is the case with the lanreotide acetate, an octapeptide salt that spontaneously forms monodisperse nanotubes when dissolved into pure water. Following a structural approach, we have in the past demonstrated the possibility to tune the diameter of these nanotubes while keeping a strict monodispersity, either by chemical modification of one precise amino acid on the peptide sequence or by changing the size of the counterions. On the basis of these previous studies, we replaced monovalent counterions by divalent ones to vary the number of walls. Indeed, in the present work, we show that lanreotide associated with a divalent counterion forms double-walled nanotubes while keeping the average diameter constant. However, the strict monodispersity of the number of walls was unexpected. We propose that the divalent counterions create an adhesion force that can drive the wall packing. This adhesion force is counterbalanced by a mechanical one that is related to the stiffness of the peptide wall. By taking into account these two opposite forces, we have built a general model that fully explains why the lanreotide nanotubes formed with divalent counterions possess two walls and not more.