Laboratoire Léon Brillouin

UMR12 CEA-CNRS, Bât. 563 CEA Saclay

91191 Gif sur Yvette Cedex, France

+33-169085241 llb-sec@cea.fr

BD diffusons les neutrons

Faits marquants 2018

12 décembre 2018

Souha Ben Mahmoud, Wafa Essafi, Annie Brûlet and François Boué

The chain conformation in sulfonated polystyrene PSSNa of a degree of sulfonation 0.34 ≤ f ≤ 1, i.e., of various hydrophobicity, is followed in mixtures of water and increasing content of tetrahydrofuran (THF), a good solvent of the hydrophobic polystyrene moieties (which improves the solvent quality of the mixture). This is achieved by measuring the chain form factor by small-angle neutron scattering using the zero average contrast method (ZAC). Polymer concentrations 0.17 and 0.34 M correspond in our case to the semidilute regime or its limit with dilute regime depending on the chain conformation. The main result is the monitoring with added THF of the pearl necklace conformation. This heterogeneous structure, made of wormlike chain parts (strings) and pearls, was observed formerly in water: when f decreases, the string contribution decreases, and the pearls size, characterized by a maximum in Kratky q2S1(q) representation, slightly increases. Here we see that in the presence of increasing content of added THF (i) the pearls contribution decreases, as expected, and (ii) their size does not change by more than 10% in most cases (30% at the most). Among different modeling, the most complete has been done following the pearl necklace models of Schweins, Huber et al. and Lages, Huber et al.; beyond the size and distribution of pearls, it addresses the radius of gyration, the correlation distance between spheres, weakly visible, and, importantly their number N. The values of Rg, as well as the modeling, suggest that while the sphere size varies a few, N decreases clearly with added good solvent. A link with the simulation of Liao et al. can be found. A second result, important in practice, is that all modeling of the scattering establishes that THF addition makes vanish an excess of low q scattering due to large compact spheres present in water. A third result is obtained after a “THF treatment” procedure, consisting of adding THF and then removing it by evaporation: (i) the spherical aggregates are washed out, and (ii) the pearl necklace conformation of the chain returns to the one in water solution before treatment. Therefore, the pearl necklace conformation of the hydrophobic polyelectrolyte in aqueous solution appears to be in an annealed equilibrium state resulting from hydrophobic attraction and electrostatic repulsion.

DOI: 10.1021/acs.macromol.8b00990

20 novembre 2018

Jyotsana Lal

This article is dedicated to the memory of Loic Auvray

The results on Winsor phases, droplet and bicontinous microemulsions phases with polymer-grafted lipids studied by Small Angle Neutron Scattering (SANS) are reported below, together with the contrast variation techniques used to characterize the average curvature in the system. We have clearly shown that polymer-grafted lipids change the interaction between microemulsion droplets --it need not be just repulsive but could also be attractive. They induce structural changes or bring about complete phase changes as observed visually in the Winsor phases when added in sufficient amounts. In the bicontinous microemulsion phases, the polymer-grafted lipids decrease the persistence length, hence the bending rigidity, increase the apparent average thickness of the film, and cause a complex deformation of the film which brings about a negative curvature change at a semi-local scale. Contrary to the naive prediction that the polymer-grafted lipids should increase membrane rigidity our experiments show a decrease. This is a subtle effect caused by perhaps an indirect coupling between film curvature and concentration fluctuations.

DOI : https://dx.doi.org/10.1140/epje/i2018-11734-4

15 octobre 2018

Raphael Dos Santos Morais, Olivier Delalande, Javier Perez, Dominique Mias-Lucquin, Melanie Lagarrigue, Anne Martel, Anne-Elisabeth Molza, Angelique Cheron, Celine Raguenes-Nicol, Thomas Chenuel, Arnaud Bondon, Marie-Sousai Appavou, Elisabeth Le Rumeur, Sophie Combet, and Jean-Francois Hubert

Scaffolding proteins play important roles in supporting the plasma membrane (sarcolemma) of muscle cells. Among them, dystrophin strengthens the sarcolemma through protein-lipid interactions, and its absence due to gene mutations leads to the severe Duchenne muscular dystrophy. Most of the dystrophin protein consists of a central domain made of 24 spectrin-like coiled-coil repeats (R). Using small angle neutron scattering (SANS) and the contrast variation technique, we specifically probed the structure of the three first consecutive repeats 1–3 (R1–3), a part of dystrophin known to physiologically interact with membrane lipids. R1–3 free in solution was compared to its structure adopted in the presence of phospholipid-based bicelles. SANS data for the protein/lipid complexes were obtained with contrast-matched bicelles under various phospholipid compositions to probe the role of electrostatic interactions. When bound to anionic bicelles, large modifications of the protein threedimensional structure were detected, as revealed by a significant increase of the protein gyration radius from 42 5 1 to 60 5 4 A . R1–3/anionic bicelle complexes were further analyzed by coarse-grained molecular dynamics simulations. From these studies, we report an all-atom model of R1–3 that highlights the opening of the R1 coiled-coil repeat when bound to the membrane lipids. This model is totally in agreement with SANS and click chemistry/mass spectrometry data. We conclude that the sarcolemma membrane anchoring that occurs during the contraction/elongation process of muscles could be ensured by this coiled-coil opening. Therefore, understanding these structural changes may help in the design of rationalized shortened dystrophins for gene therapy. Finally, our strategy opens up new possibilities for structure determination of peripheral and integral membrane proteins not compatible with different high-resolution structural methods.

Dos Santos Morais R et al. Biophys J 115: 1231–1239, 2018.

https://doi.org/10.1016/j.bpj.2018.07.039

 

24 septembre 2018

Fengjiao Qian, Lars J. Bannenberg, Heribert Wilhelm, Grégory Chaboussant, Lisa M. Debeer-Schmitt, Marcus P. Schmidt, Aisha Aqeel, Thomas T. M. Palstra, Ekkes Brück, Anton J. E. Lefering, Catherine Pappas, Maxim Mostovoy, Andrey O. Leonov

The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetizationmeasurements, we observe a newmultidomain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiralwave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify the physical origin of the new state and discuss its multiferroic properties.
 

http://dx.doi.org/10.1126/sciadv.aat7323

08 septembre 2018
Lien avec leurs propriétés photovoltaïques et maitrise de la séparation e--trou

Les cellules solaires à base de pérovskites présentent de fort rendement car elles permettent d'élargir la fraction du spectre solaire qui peut être converti en électricité. Ces performances sont directement liées à leur structure cristalline bien particulière, dont les propriétés de vibrations ont été étudiées par diffusion de neutrons.

Cette étude permet d'atteindre l'ensemble des propriétés élastiques de ces matériaux, données indispensables pour progresser dans l'utilisation des pérovskites hybrides halogénées pour une utilisation en cellule photovoltaïque ou comme émetteur de lumière.

29 août 2018

I. Mirebeau, N. Martin, M. Deutsch, L. J. Bannenberg, C. Pappas, G. Chaboussant, R. Cubitt, C. Decorse, and A. O. Leonov

Reentrant spin glasses are frustrated disordered ferromagnets developing vortexlike textures under an applied magnetic field. Our study of a Ni0.81Mn0.19 single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3–15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for “frustrated” skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small noninteger topological charge. The vortices keep an anisotropic shape on a three-dimensional lattice, recalling “croutons” in a “ferromagnetic soup.” Their size and number can be tuned independently by the magnetic field and concentration x (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting nontrivial spin textures.

http://dx.doi.org/10.1103/PhysRevB.98.014420

09 août 2018

Maelenn Chevreuil, Didier Law-Hine, Jingzhi Chen, Stéphane Bressanelli, Sophie Combet, Doru Constantin, Jéril Degrouard, Johannes Möller, Mehdi Zeghal and Guillaume Tresset

The survival of viruses partly relies on their ability to self-assemble inside host cells. Although coarse-grained simulations have identified different pathways leading to assembled virions from their components, experimental evidence is severely lacking. Here, we use timeresolved small-angle X-ray scattering to uncover the nonequilibrium self-assembly dynamics of icosahedral viral capsids packaging their full RNA genome. We reveal the formation of amorphous complexes via an en masse pathway and their relaxation into virions via a synchronous pathway. The binding energy of capsid subunits on the genome is moderate (~7kBT0, with kB the Boltzmann constant and T0 = 298 K, the room temperature), while the energy barrier separating the complexes and the virions is high (~ 20kBT0). A synthetic polyelectrolyte can lower this barrier so that filled capsids are formed in conditions where virions cannot build up. We propose a representation of the dynamics on a free energy landscape.

DOI : https://dx.doi.org/10.1038/s41467-018-05426-8

20 juin 2018

Unravelling a mechanism of action for a cecropin a‑melittin hybrid antimicrobial peptide: the induced formation of multilamellar lipid stacks
T. Silva, B. Claro, B. F. B. Silva, N. Vale, P. Gomes, M.-S. Gomes, S. S. Funari, J. Teixeira, D. Uhríková, and M. Bastos, Langmuir, 2018, 34 (5), pp 2158–2170.

An understanding of the mechanism of action of antimicrobial peptides is fundamental to the development of new and more active antibiotics. In the present work, we use a wide range of techniques (SANS, SAXD, DSC, ITC, CD, and confocal and electron microscopy) in order to fully characterize the interaction of a cecropin A-melittin hybrid antimicrobial peptide, CA(1-7)M(2-9), of known antimicrobial activity, with a bacterial model membrane of POPE/POPG in an effort to unravel its mechanism of action.

We found that CA(1-7)M(2-9) disrupts the vesicles, inducing membrane condensation and forming an onionlike structure of multilamellar stacks, held together by the intercalated peptides. SANS and SAXD revealed changes induced by the peptide in the lipid bilayer thickness and the bilayer stiffening in a tightly packed liquid-crystalline lamellar phase. The analysis of the observed abrupt changes in the repeat distance upon the phase transition to the gel state suggests the formation of an Lγ phase. To the extent of our knowledge, this is the first time that the Lγ phase is identified as part of the mechanism of action of antimicrobial peptides. The energetics of interaction depends on temperature, and ITC results indicate that CA(1-7)M(2-9) interacts with the outer leaflet. This further supports the idea of a surface interaction that leads to membrane condensation and not to pore formation.

As a result, we propose that this peptide exerts its antimicrobial action against bacteria through extensive membrane disruption that leads to cell death.

http://dx.doi.org/10.1021/acs.langmuir.7b03639

28 mai 2018

Béatrice Gillon, Albert Hammerschmied, Arsen Gukasov, Alain Cousson, Thomas Cauchy, Eliseo Ruiz, John A. Schlueter, Jamie L. Manson

We report neutron‐diffraction investigations of the quasi‐2D MnII(dca)2(pym)(H2O) (pym = N2C4H4) compound, where high‐spin MnII ions are bridged by dicyanamide anions, [N(CN)2] (herein abbreviated dca). Inside the layers, Mn2+ ions are connected by single or double dca bridges. The magnetic phase diagram was established by neutron diffraction on a single crystal. In the low‐field phase, the MnII ions are antiferromagnetically ordered in the layers, with moments nearly parallel to the c axis, and the layers are antiferromagnetically coupled. The spin‐flop phase corresponds to ferromagnetic coupling between the antiferromagnetic layers, in which the MnII moments are nearly perpendicular to the c axis. The induced spin‐density distribution in the paramagnetic phase, determined by polarized neutron diffraction, visualizes the superexchange pathways through the dca ligands within the layers and through H bonding between neighboring layers. The theoretical spin density obtained by bidimensional periodic DFT calculations is compared with the experimental results. Furthermore, quantum Monte Carlo simulations have been performed to compare the DFT results with experimental susceptibility measurements.

https://doi.org/10.1002/ejic.201700971

22 mai 2018

Les transitions de phase sont des phénomènes physiques bien connus, qui font partie de notre vie quotidienne : l’eau liquide gèle à 0°c, bout à 100 °C ; certains métaux comme l’étain (étain blanc et étain gris) changent de structure cristalline en fonction de la température... Mais certaines de ces transitions peuvent être de nature bien différente, en lien avec l'existence de défauts "topologiques", qui ne peuvent apparaitre ou disparaitre qu'accompagnés de leurs anti-défauts

Une collaboration de théoriciens et d'expérimentateurs, dont des spécialistes de diffusion de neutrons, s’est intéressée à un matériau quantique unidimensionnel : BaCo2V2O8 (BACOVO). L'étude montre qu’il s’agit d’un système modèle pour l’étude d’une nouvelle transition de phase topologique, gouvernée non pas par un seul, mais par deux types d’excitations topologiques, qui sont de plus quantiquement conjuguées : l'application d'un champ magnétique externe permet de piloter, le système que l'on peut placer dans chacune des phases où une des deux excitations est dominante.

 

04 mai 2018

Anne-Sophie Robbes, Fabrice Cousin , Florian Meneau, and Jacques Jestin

We probe by SANS the conformation of polymer chains of the matrix in various nanocomposites based on the same building blocks, namely spherical magnetic nanoparticles of maghemite (γ-Fe2O3) as fillers and polystyrene (PS) for the matrix. Given that the nanoparticles can be arranged in oriented chains during the processing by an external magnetic field and/or grafted by tethered PS chains with a grafting density of ∼0.15 chains/nm2, very different organizations of the nanofillers were tested according to different particle–polymer interactions: (i) homogeneous isotropic dispersion of aggregates of bare nanoparticles; (ii) chains of bare nanoparticles oriented along one direction over the whole sample; (iii) perfect dispersion of grafted nanoparticles; (iv) homogeneous isotropic dispersion of large aggregates of grafted nanoparticles; and (v) chains of large aggregates of grafted nanoparticles objects oriented along one direction over the whole sample. Measurements were performed by the extrapolation to the zero concentration method made possible by the fact that γ-Fe2O3 has the same neutron scattering length density (SLD) as a deuterated polystyrene, so that the nanoparticles scattering is matched in a deuterated PS matrix, whether they are grafted or not. This robust method enables to check that only the polymer chain form factor is effectively probed in a very accurate way. This allows us to show some deviations of the radius of gyration induced by the nanoparticles: (i) for the case of very weak interaction between the polymer and the bare particles, the radius of gyration is swollen by 16% whatever the filler dispersion and orientation; (ii) for the athermal interaction between grafted particles and polymer, the radius of gyration is either unchanged when particles are individually dispersed or compressed of almost 11% when particles are forming overlapped clusters. Despite the remaining relatively small deviations, this is to our best knowledge the first unambiguous experimental evidence on a single system of the influence of the well-known nanofiller dispersion onto the mean chain conformation in nanocomposites for different polymer–particles interactions ranging from attractive to repulsive.

https://pubs.acs.org/doi/10.1021/acs.macromol.7b02318

17 avril 2018

We report on the self-assembly behavior of poly(2-methyl-2-oxazoline)–block–poly(2-octyl-2-oxazoline) comprising different terminal perfluoroalkyl fragments in aqueous solutions. As reported previously [Kaberov et al. (2017)] such polyphiles can form a plethora of nanostructures depending of the composition and on the way of preparation. Here we report, for the first time, detailed information on the internal structure of the nanoparticles resulting from the self-assembly of these copolymers. Small-angle neutron and X-ray scattering (SANS/SAXS) experiments unambiguously prove the existence of polymersomes, wormlike micelles and their aggregates in aqueous solution. It is shown that increasing content of fluorine in the poly(2-oxazoline) copolymers results in a morphological transition from bilayered or multi-layered vesicles to wormlike micelles for solutions prepared by direct dissolution.

In contrast, nanoparticles prepared by dialysis of a polymer solution in a non-selective organic solvent against water are characterized by SAXS method. The internal structure of the nanoparticles could be assessed by fitting of the scattering data, revealing complex core-double shell architecture of spherical symmetry. Additionally, long range ordering is identified for all studied nanoparticles due to the crystallization of the poly(2-octyl-2-oxazoline) segments inside the nanoparticles.

DOI : https://doi.org/10.1016/j.eurpolymj.2018.01.007

24 mars 2018

Cationic thermoresponsive poly(n-vinylcaprolactam) microgels synthesized by emulsion polymerization using a reactive cationic macro-raft agent
L. Etchenausia, E. Deniau, A. Brûlet, J. Forcada and M. Save,  Macromolecules  (2018) sous presse

Abstract :

A series of reactive poly([2-(acryloyloxy)ethyl]trimethylammonium chloride) (P(AETAC)) cationic polymers with varying degrees of polymerization were synthesized by RAFT/MADIX polymerization and investigated as stabilizers for the emulsion polymerization of N-vinylcaprolactam (PVCL) in the presence of a cross-linker. It was demonstrated that the xanthate chain end of the cationic P(AETAC-X) polymers played a crucial role to produce stable cationic PVCL-based microgels at higher initial solids content (5–10 wt %) than usually reported for the synthesis of PVCL microgels. The thermoresponsive PVCL microgels with cationic shell undergo a reversible volume shrinkage upon heating in the absence of any hysteresis in accordance with the narrow particle size distribution.

The values of the volume phase transition temperature ranged between 28 and 30 °C for the microgels synthesized using 4 and 8 wt % of P(AETAC-X) based on VCL. The presence of a cationic outer shell onto the microgels was evidenced by the positive values of the electrophoretic mobility. The swelling behavior of the thermoresponsive microgel particles can be tuned by playing on two synthesis variables which are the initial solids content and the content of P(AETAC-X) macro-RAFT stabilizer. Furthermore, the inner structure of the synthesized microgels was probed by transverse relaxation nuclear magnetic resonance (T2 NMR) and small-angle neutron scattering (SANS) measurements. The fit of T2 NMR data confirmed a core–shell morphology with different cross-linking density in PVCL microgels. Through the determination of the network mesh size, SANS was suitable to explain the increase of the values of the PVCL microgel swelling ratios by increasing the initial solids content of their synthesis.

22 février 2018

Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch and Néel skyrmions. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures. Direct imaging by Lorentz transmission electron microscopy shows field-stabilized antiskyrmion lattices and isolated antiskyrmions from 100 kelvin to well beyond room temperature, and zero-field metastable antiskyrmions at low temperatures. These results enlarge the family of magnetic skyrmions and pave the way to the engineering of complex bespoke designed skyrmionic structures.


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