Faits marquants scientifiques 2019

The response of a position-sensitive Li-glass scintillator detector to alpha-particles from a collimated ²⁴¹Am source scanned across the face of the detector has been measured. Scintillation light was read out by an 8x8 pixel multi-anode photomultiplier and the signal amplitude for each pixel has been recorded for every position on a scan. The pixel signal is strongly dependent on position and in general several pixels will register a signal (a hit) above a given threshold. The effect of this threshold on hit multiplicity is studied, with a view to optimize the single-hit efficiency of the detector.

https://doi.org/10.1016/j.nima.2019.03.014

B. Annighöfer , A. Hélary, A. Brulet, A. Colas de la Noue, C. Loupiac and S. Combet
Review of Scientific Instruments 90 (2019) 025106

Abstract :

We report on a high pressure (HP) cell designed for the determination of the structure of molecular solutions by small-angle neutron scattering (SANS). The HP cell is fitted up with two thick metallic windows that make the device very resistant under hydrostatic pressures up to 600 MPa (or 6 kbar). The metallic windows are removable, offering the possibility to adapt the HP cell to a given study with the pressure desired on an appropriate spatial range to study the structure of various molecular solutions by SANS. In this context, we report the absorption, transmission, and scattering properties of different metallic windows. Finally, we describe, as a proof of principle, the solution structure changes of myoglobin, a small globular protein.

https://doi.org/10.1063/1.5051765

Daniela Russo, Maya Dimova Lambreva, Christiane Alba Simionesco, Pierre Sebban,and Giuseppina Rea

Studies on the dynamical properties of photosynthetic membranes of land plants and purple bacteria have been previously performed by neutron spectroscopy, revealing a tight coupling between specific photochemical reactions and macromolecular dynamics. Here, we probed the intrinsic dynamics of biotechnologically useful mutants of the green alga Chlamydomonas reinhardtii by incoherent neutron scattering coupled with prompt chlorophyll fluorescence experiments. We brought to light that single amino acid replacements in the plastoquinone (PQ)-binding niche of the photosystem II D1 protein impair electron transport (ET) efficiency between quinones and confer increased flexibility to the host membranes, expanding to the entire cells. Hence, a more flexible environment in the PQ-binding niche has been associated to a less efficient ET.Asimilar function/dynamics relationship was also demonstrated in Rhodobacter sphaeroides reaction centers having inhibited ET, indicating that flexibility at the quinones region plays a crucial role in evolutionarily distant organisms. Instead, a different functional/dynamical correlation was observed in algal mutants hosting a single amino acid replacement residing in a D1 domain far from the PQ-binding niche. Noteworthy, this mutant displayed the highest degree of flexibility, and besides having a nativelike ET efficiency in physiological conditions, it acquired novel, to our knowledge, phenotypic traits enabling it to preserve a high maximal quantum yield of photosystem II photochemistry in extreme habitats. Overall, in the nanosecond timescale, the degree of the observed flexibility is related to the mutation site; in the picosecond timescale, we highlighted the presence of a more pronounced dynamic heterogeneity in all mutants compared to the native cells, which could be related to a marked chemically heterogeneous environment.

DOI : https://doi.org/10.1016/j.bpj.2019.03.029

A. Theodoratou, L.-T. Lee, J. Oberdisse and A. Aubert-Pouëssel, Langmuir 35(20) (2019) 6620.

Abstract :

Nanofilms of about 2 nm thickness have been formed at the air–water interface using functionalized castor oil (ICO) with cross-linkable silylated groups. These hybrid films represent excellent candidates for replacing conventional polymeric materials in biomedical applications, but they need to be optimized in terms of biocompatibility, which is highly related to protein adsorption. Neutron reflectivity has been used to study the adsorption of two model proteins, bovine serum albumin and lysozyme, at the silylated oil (ICO)–water interface in the absence and presence of salt at physiologic ionic strength and pH and at different protein concentrations. These measurements are compared to adsorption at the air–water interface. While salt enhances adsorption by a similar degree at the air–water and oil–water interfaces, the impact of the oil film is significant with adsorption at the oil–water interface 3–4-fold higher compared to that at the air–water interface. Under these conditions, the concentration profiles of the adsorbed layers for both proteins indicate multilayer adsorption. The thickness of the outer layer (oil side) is close to the dimension of the minor axis of the protein molecule, ∼30 Å, suggesting a sideway orientation with the long axis parallel to the interface. The inner layer extends to 55–60 Å. Interestingly, in all cases, the composition of the oil film remains intact without significant protein penetration into the film. The optimal adsorption on these nanofilms, 1.7–2.0 mg·m^-2, is comparable to the results obtained recently on thick solid cross-linked films using a quartz crystal microbalance and atomic force microscopy, showing in particular that adsorption at these ICO film interfaces under standard physiological conditions is nonspecific. These results furnish useful information toward the elaboration of vegetable oil-based nanofilms in direct nanoscale applications or as precursor films in the fabrication of thicker macroscopic films for biomedical applications.

https://doi.org/10.1021/acs.langmuir.9b00186

"Dynamics properties of photosynthetic microorganisms probed by incoherent neutron scattering"
Daniela Russo, Maya Dimova Lambreva, Christiane Alba Simionesco, Pierre Sebban, and Giuseppina Rea
Biophysical Journal, 116 (9) (2019) 1759-1768

Studies on the dynamical properties of photosynthetic membranes of land plants and purple bacteria have been previously performed by neutron spectroscopy, revealing a tight coupling between specific photochemical reactions and macromolecular dynamics. Here, we probed the intrinsic dynamics of biotechnologically useful mutants of the green alga Chlamydomonas reinhardtii by incoherent neutron scattering coupled with prompt chlorophyll fluorescence experiments. We brought to light that single amino acid replacements in the plastoquinone (PQ)-binding niche of the photosystem II D1 protein impair electron transport (ET) efficiency between quinones and confer increased flexibility to the host membranes, expanding to the entire cells. Hence, a more flexible environment in the PQ-binding niche has been associated to a less efficient ET.Asimilar function/dynamics relationship was also demonstrated in Rhodobacter sphaeroides reaction centers having inhibited ET, indicating that flexibility at the quinones region plays a crucial role in evolutionarily distant organisms. Instead, a different functional/dynamical correlation was observed in algal mutants hosting a single amino acid replacement residing in a D1 domain far from the PQ-binding niche. Noteworthy, this mutant displayed the highest degree of flexibility, and besides having a nativelike ET efficiency in physiological conditions, it acquired novel, to our knowledge, phenotypic traits enabling it to preserve a high maximal quantum yield of photosystem II photochemistry in extreme habitats. Overall, in the nanosecond timescale, the degree of the observed flexibility is related to the mutation site; in the picosecond timescale, we highlighted the presence of a more pronounced dynamic heterogeneity in all mutants compared to the native cells, which could be related to a marked chemically heterogeneous environment.

B. Annighöfer , A. Hélary, A. Brulet, A. Colas de la Noue, C. Loupiac and S. Combet
Review of Scientific Instruments 90 (2019) 025106

Abstract :

We report on a high pressure (HP) cell designed for the determination of the structure of molecular solutions by small-angle neutron scattering (SANS). The HP cell is fitted up with two thick metallic windows that make the device very resistant under hydrostatic pressures up to 600 MPa (or 6 kbar). The metallic windows are removable, offering the possibility to adapt the HP cell to a given study with the pressure desired on an appropriate spatial range to study the structure of various molecular solutions by SANS. In this context, we report the absorption, transmission, and scattering properties of different metallic windows. Finally, we describe, as a proof of principle, the solution structure changes of myoglobin, a small globular protein.

https://doi.org/10.1063/1.5051765

Andrés Marcoleta, Frank Wien, Véronique Arluison, Rosalba Lagos, Rafael Giraldo
Bacterial Amyloids (2019)

Amyloids are supramolecular protein assemblies based on fibrillar arrangements of β‐sheets that were first found as linked to neurodegenerative and systemic human diseases. However, there is now overwhelming evidence on alternative roles of amyloids as functional assemblies and as epigenetic determinants of beneficial traits, both in Fungi and Metazoa. Bacteria also use amyloids as functional devices, mainly as extracellular scaffolds in biofilms, but there is increasing evidence for functional roles of amyloids in the bacterial cytosol, and these have enabled to engineer minimal models of a ‘generic’ amyloid disease. Amyloids are thus key players in the physiology of bacteria and versatile building blocks in synthetic biology.

M. Ruminy, S. Guitteny, J. Robert, L.-P. Regnault, M. Boehm, P. Steffens, H. Mutka, J. Ollivier, U. Stuhr, J. S. White, B. Roessli, L. Bovo, C. Decorse, M. K. Haas, R. J. Cava, I. Mirebeau, M. Kenzelmann, S. Petit, and T. Fennell

Tb2Ti2O7 presents an ongoing conundrum in the study of rare-earth pyrochlores. Despite the expectation that it should be the prototypical unfrustrated noncollinear Ising antiferromagnet on the pyrochlore lattice, it presents a puzzling correlated state that persists to the lowest temperatures. Effects which can reintroduce frustration or fluctuations are therefore sought, and quadrupolar operators have been implicated. One consequence of strong quadrupolar effects is the possible coupling of magnetic and lattice degrees of freedom, and it has previously been shown that a hybrid magnetoelastic mode with both magnetic and phononic character is formed in Tb2Ti2O7 by the interaction of a crystal field excitation with a transverse-acoustic phonon. Here, using polarized and unpolarized inelastic neutron scattering, we present a detailed characterization of the magnetic and phononic branches of this magnetoelastic mode, particularly with respect to their composition, the anisotropy of any magnetic fluctuations, and also the temperature dependence of the different types of fluctuation that are involved. We also examine the dispersion relations of the exciton branches that develop from the crystal field excitation in the same temperature regime that the coupled mode appears, and find three quasidispersionless branches where four are expected, each with a distinctive structure factor indicating that they are nonetheless cooperative excitations. We interpret the overall structure of the spectrum as containing four branches, one hybridized with the phonons and gaining a strong dispersion, and three remaining dispersionless.

https ://doi.org/10.1103/PhysRevB.99.224431

A. S. Sukhanov, Praveen Vir, A. S. Cameron, H. C. Wu, N. Martin, S. Mühlbauer, A. Heinemann, H. D. Yang, C. Felser, and D. S. Inosov

The cubic chiral helimagnets with the P213 space group represent a group of compounds in which the stable skyrmion-lattice state is experimentally observed. The key parameter that controls the energy landscape of such systems and determines the emergence of a topologically nontrivial magnetic structure is the Dzyaloshinskii-Moriya interaction (DMI). Chemical substitution is recognized as a convenient instrument to tune the DMI in real materials and has been successfully utilized in studies of a number of chiral magnets, such as MnSi, FeGe, MnGe, and others. In our study, we applied small-angle neutron scattering to investigate how chemical substitution influences the skyrmionic properties of an insulating helimagnet Cu2OSeO3 when Cu ions are replaced by either Zn or Ni. Our results demonstrate that the DMI is enhanced in the Ni-substituted compounds (Cu,Ni)2OSeO3, but weakened in (Cu,Zn)2OSeO3. The observed changes in the DMI strength are reflected in the magnitude of the spin-spiral propagation vector and the temperature stability of the skyrmion phase.

https://doi.org/10.1103/PhysRevB.100.184408

N. Martin, I. Mirebeau, C. Franz, G. Chaboussant, L. N. Fomicheva, and A. V. Tsvyashchenko

We study the helimagnetic ground state of the MnGe cubic alloy using small-angle neutron scattering and a high-resolution method, the so-called MIEZE spectroscopy. Upon cooling below the Néel temperature TN = 170(5) K, we observe the proliferation of long-wavelength gapless spin fluctuations, concomitant with a continuous evolution of the helical correlation length. These fluctuations disappear at Tcom = 32(5) K when the helical period becomes commensurate with the lattice. We propose to describe this intermediate phase as a soliton lattice, promoting nonlinear collective modes, or phasons, over a large temperature interval. We discuss the possible relevance of our results to the previously observed magnetotransport anomalies.

https://doi.org/10.1103/PhysRevB.99.100402

M. Deutsch, W. Peng, P. Foury-Leylekian, V. Balédent, S. Chattopadhyay, M. T. Fernandez-Diaz, T. C. Hansen, A. Forget, D. Colson, M. Greenblatt, M.-B. Lepetit, S. Petit, and I. Mirebeau

The magnetic structure of TbMn2O5 and DyMn2O5 multiferroics has been studied by high-pressure neutron diffraction in a large pressure range up to 6.6 GPa. In both cases, we observe a pressure-induced commensurate magnetic phase with propagation vector ( 1/2 0 1/2 ), growing with pressure at the expense of the ambient pressure phases. Being previously observed in YMn2O5 and PrMn2O5, this phase is most likely a generic feature of the RMn2O5 multiferroic family. A simple model is proposed to explain qualitatively the emergence of this pressure-induced phase. Differences between TbMn2O5 and DyMn2O5 behaviors at ambient and low pressures provide clues on the interaction scheme.

DOI : http://dx.doi.org/10.1103/PhysRevB.98.024408