Neutron Spin Echo Experiments Research Articles

Scintillator-based Timepix3 detector for neutron spin-echo techniques using intensity modulation.

A scintillator-based Timepix3 (TPX3) detector was developed to resolve the high-frequency modulation of a neutron beam in both spatial and temporal domains, as required for neutron spin-echo experiments. In this system, light from a scintillator is manipulated with an optical lens and is intensified using an image intensifier, making it detectable with the TPX3 chip. Two different scintillators, namely, 6LiF:ZnS(Ag) and 6LiI:Eu, were investigated to achieve the high resolution needed for spin-echo modulated small-angle neutron scattering (SEMSANS) and modulation of intensity with zero effort (MIEZE). The methodology for conducting event-mode analysis is described, including the optimization of clustering parameters for both scintillators. The detector with both scintillators was characterized with respect to detection efficiency, spatial resolution, count rate, uniformity, and γ-sensitivity. The 6LiF:ZnS(Ag) scintillator-based detector achieved a spatial resolution of 200μm and a count rate capability of 1.1 × 105 cps, while the 6LiI:Eu scintillator-based detector demonstrated a spatial resolution of 250μm and a count rate capability exceeding 2.9 × 105 cps. Furthermore, high-frequency intensity modulations in both spatial and temporal domains were successfully observed, confirming the suitability of this detector for SEMSANS and MIEZE techniques, respectively.

Dynamic structure factor of undulating vesicles: finite-size and spherical geometry effects with application to neutron spin echo experiments.

We consider the dynamic structure factor (DSF) of quasi-spherical vesicles and present a generalization of an expression that was originally formulated by Zilman and Granek (ZG) for scattering from isotropically oriented quasi-flat membrane plaquettes. The expression is obtained in the form of a multi-dimensional integral over the undulating membrane surface. The new expression reduces to the original stretched exponential form in the limit of sufficiently large vesicles, i.e., in the micron range or larger. For much smaller unilamellar vesicles, deviations from the asymptotic, stretched exponential equation are noticeable even if one assumes that the Seifert-Langer leaflet density mode is completely relaxed and membrane viscosity is neglected. To avoid the need for an exhaustive numerical integration while fitting to neutron spin echo (NSE) data, we provide a useful approximation for polydisperse systems that tests well against the numerical integration of the complete expression. To validate the new expression, we performed NSE experiments on variable-size vesicles made of a POPC/POPS lipid mixture and demonstrate an advantage over the original stretched exponential form or other manipulations of the original ZG expression that have been deployed over the years to fit the NSE data. In particular, values of the membrane bending rigidity extracted from the NSE data using the new approximations were insensitive to the vesicle radii and scattering wavenumber and compared very well with expected values of the effective bending modulus ([Formula: see text]) calculated from results in the literature. Moreover, the generalized scattering theory presented here for an undulating quasi-spherical shell can be easily extended to other models for the membrane undulation dynamics beyond the Helfrich Hamiltonian and thereby provides the foundation for the study of the nanoscale dynamics in more complex and biologically relevant model membrane systems.

Confined Dynamics in Spherical Polymer Brushes.

We investigate the dynamics of polymers grafted to spherical nanoparticles in solution using hybrid molecular dynamics simulations with a coarse-grained solvent modeled via the multiparticle collision dynamics algorithm. The mean-square displacements of monomers near the surface of the nanoparticle exhibit a plateau on intermediate time scales, indicating confined dynamics reminiscent of those reported in neutron spin-echo experiments. The confined dynamics vanish beyond a specific radial distance from the nanoparticle surface that depends on the polymer grafting density. We show that this dynamical confinement transition follows theoretical predictions for the critical distance associated with the structural transition from confined to semidilute brush regimes. These findings suggest the existence of a hitherto unreported dynamic length scale connected with theoretically predicted static fluctuations in spherical polymer brushes and provide new insights into recent experimental observations.

Intrinsic stiffening and dynamic softening by cholesterol.

The stiffness of a lipid bilayer is the key mechanical constant for understanding membrane reshaping in the cell. Cholesterol, present in the plasma membrane at high levels, has ambiguous effects on stiffness that appears to depend strongly on the lipid matrix. Inferred from long length and time scale experiments (e.g., X-ray diffraction and GUV fluctuations) cholesterol has little effect on a simple di-oleoyl-phosphatidylcholine (DOPC) bilayer. Inferred from dynamic experiments limited to nanosecond timescales (e.g., neutron spin echo) DOPC bilayers appear stiffer. In a saturated chain matrix like DMPC, both x-ray and neutron spin echo experiments indicate stiffening.

MembraneDyn: simulating the dynamics of supported membrane stacks on the nanosecond timescale

The static structure factor and the undulation dynamics of a solid-supported membrane stack have previously been calculated by Romanov and Ul'yanov [Romanov & Ul'yanov (2002). Phys. Rev. E, 66, 061701]. Based on this prior work, the calculation has been extended to cover the membrane dynamics, i.e. the intermediate scattering function as a Fourier transform of the van Hove correlation function of the membrane stack. Fortran code which calculates the intermediate scattering function for a membrane stack on a solid support is presented. It allows the static and dynamic scattering functions to be calculated according to the derivation of Romanov and Ul'yanov. The physical properties of supported phospholipid bilayers can be examined in this way and the results can be directly compared with results obtained from grazing-incidence neutron spin-echo spectroscopy experiments.

Double-focusing geometry for phase correction in neutron resonance spin-echo spectroscopy

Neutron spin phase correction is an essential technique for achieving a high energy resolution in neutron spin-echo experiments. We propose a double-focusing geometry in which two identical ellipsoidal mirrors are arranged in series. This arrangement enables us to correct the path length differences for large divergent beams using a complete ellipsoidal focusing mirror. In this study, we evaluated the effectiveness of this geometry both analytically and numerically. A Fourier time of 1 μs is achievable using this geometry.

Neutron Spin Echo Spectroscopy as a Unique Probe for Lipid Membrane Dynamics and Membrane-Protein Interactions.

Lipid bilayers form the main matrix of cell membranes and are the primary platform for nutrient exchange, protein-membraneinteractions, and viral budding, among other vital cellular processes. For efficient biological activity, cell membranes should be rigid enough to maintain the integrity of the cell and its compartments yet fluid enough to allow membrane components, such as proteins and functional domains, to diffuse and interact. This delicate balance of elastic and fluid membrane properties, and their impact on biological function, necessitate a better understanding of collective membrane dynamics over mesoscopic length and time scales of key biological processes, e.g., membrane deformations and protein binding events. Among the techniques that can effectively probe this dynamic range is neutron spin echo (NSE) spectroscopy. Combined with deuterium labeling, NSE can be used to directly access bending and thickness fluctuations as well as mesoscopic dynamics of select membrane features. This paper provides a brief description of the NSE technique and outlines the procedures for performing NSE experiments on liposomal membranes, including details of sample preparation and deuteration schemes, along with instructions for data collection and reduction. The paper also introduces data analysis methods used to extract key membrane parameters, such as the bending rigidity modulus, area compressibility modulus, and in-plane viscosity. To illustrate the biological importance of NSE studies, select examples of membrane phenomena probed by NSE are discussed, namely, the effect of additives on membrane bending rigidity, the impact of domain formation on membrane fluctuations, and the dynamic signature of membrane-protein interactions.

Collective Motions and Mechanical Response of a Bulk of Single-Chain Nano-Particles Synthesized by Click-Chemistry.

We investigate the effect of intra-molecular cross-links on the properties of polymer bulks. To do this, we apply a combination of thermal, rheological, diffraction, and neutron spin echo experiments covering the inter-molecular as well as the intermediate length scales to melts of single-chain nano-particles (SCNPs) obtained through ‘click’ chemistry. The comparison with the results obtained in a bulk of the corresponding linear precursor chains (prior to intra-molecular reaction) and in a bulk of SCNPs obtained through azide photodecomposition process shows that internal cross-links do not influence the average inter-molecular distances in the melt, but have a profound impact at intermediate length scales. This manifests in the structure, through the emergence of heterogeneities at nanometric scale, and also in the dynamics, leading to a more complex relaxation behavior including processes that allow relaxation of the internal domains. The influence of the nature of the internal bonds is reflected in the structural relaxation that is slowed down if bulky cross-linking agents are used. We also found that any residual amount of cross-links is critical for the rheological behavior, which can vary from an almost entanglement-free polymer bulk to a gel. The presence of such inter-molecular cross-links additionally hinders the decay of density fluctuations at intermediate length scales.

Observation of TOF–MIEZE Signals with Focusing Mirrors at BL06, MLF, J-PARC

A neutron spin echo experiment for tiny samples with high energy resolution can be realized by combining modulated intensity by zero effort (MIEZE) technique and ellipsoidal neutron focusing supermirrors. We observed TOF–MIEZE (the time-of-flight method with MIEZE) signals with an effective frequency of 108 kHz with focusing beam. The Fourier time range 0.48–5.3 ns was covered at once by using TOF technique with neutron wavelength 0.65–1.45 nm. The contrasts of TOF–MIEZE signal were 0.62 ± 0.02 and 0.50 ± 0.05 with each neutron wavelength of 0.68 nm and 1.35 nm, respectively.

Amyloid β-Peptide Interaction with Membranes: Can Chaperones Change the Fate?

The understanding of amyloid β-peptide (Aβ) interactions with cellular membranes is a crucial molecular challenge against Alzheimer's disease. Indeed, Aβ prefibrillar oligomeric intermediates are believed to be the most toxic species, able to induce cellular damagesdirectly by membrane damage. We present a neutron-scattering study on the interaction of large unilamellar vesicles (LUV), as cell membrane models, with both freshly dissolved Aβ and early toxic prefibrillar oligomers, intermediate states in the amyloid pathway. In addition, we explore the effect of coincubating the Aβ-peptide with the chaperonin Hsp60, which is known to strongly interact with it in its aggregation pattern. In fact, the interaction of the LUV with coincubated Aβ/Hsp60, right after mixing and after following the aggregation protocol leading to the toxic intermediates in the absence of Hsp60, is studied. Neutron spin echo experiments show that the interaction with both freshly dissolved and aggregate Aβ species brings about an increase in membrane stiffness, whereas the presence of even very low amounts of Hsp60 (ratio Aβ/Hsp60 = 25:1) maintains unaltered the elastic properties of the membrane bilayer. A coherent interpretation of these results, related to previous literature, can be based on the ability of the chaperonin to interfere with Aβ aggregation, by the specific recognition oftheAβ-reactive transient species. In this framework, our results strongly suggest that early in a freshly dissolved Aβ solution are presentsomespecies able to modify the bilayer dynamics, and the chaperonin plays the role of an assistant in such stochastic "misfolding events", avoiding the insult on the membrane as well as the onset of the aggregation cascade.

Collective Mesoscale Dynamics of Liquid 1-Dodecanol Studied by Neutron Spin-Echo Spectroscopy with Isotopic Substitution and Molecular Dynamics Simulation.

The collective dynamics of liquid 1-dodecanol was investigated at a length scale matching the mesoscale structure arising from the segregation of hydrophilic and hydrophobic domains. To this end, neutron spin-echo experiments were performed on a series of partially deuterated samples and the relevant collective dynamics of the hydroxyl groups with respect to the alkyl chains was extracted from the linear combination of the intermediate scattering functions of these samples. The resulting collective dynamics is slower than the single particle dynamics as determined by the measurement on the nondeuterated sample. The experimental results are in excellent agreement with molecular dynamics simulation, which allows further insight into the mechanism of the molecular motions. The results indicate that two factors are responsible for the slower collective dynamics. The first one is the slower dynamics of the hydroxyl group, with respect to the alkyl chains, owing to hydrogen bonding, and the second one is the presence of mesoscale structuring.

Influence of Concentration Fluctuations on Relaxation Processes in Spin Glasses

Using the unique combination of atomically resolved atom probe tomography (APT) and volume averaged neutron (resonance) spin echo (NRSE and NSE) experiments, the influence of nano-scaled clusters on the spin relaxation in spin glasses was studied. For this purpose, the phase transition from the paramagnetic phase to the spin glass phase in an Fe-Cr spin glass with a composition of Fe 17 . 8 Cr 82 . 2 was studied in detail by means of NRSE. The microstructure was characterised by APT measurements, which show local concentration fluctuations of Fe and Cr on a length scale of 2 to 5 nm, which lead (i) to the coexistence of ferro- and anti-ferromagnetic clusters and (ii) a change of the magnetic properties of the whole sample, even in the spin glass phase, where spins are supposed to be randomly frozen. We show that a generalized spin glass relaxation function, which was successfully used to describe the phase transition in diluted spin glasses, can also be used for fitting the spin dynamics in spin glasses with significant concentration fluctuations.

Polymer dynamics under cylindrical confinement featuring a locally repulsive surface: A quasielastic neutron scattering study.

We investigated the effect of intermediate cylindrical confinement with locally repulsive walls on the segmental and entanglement dynamics of a polymer melt by quasielastic neutron scattering. As a reference, the corresponding polymer melt was measured under identical conditions. The locally repulsive confinement was realized by hydrophilic anodic alumina nanopores with a diameter of 20 nm. The end-to-end distance of the hydrophobic infiltrated polyethylene-alt-propylene was close to this diameter. In the case of hard wall repulsion with negligible local attraction, several simulations predicted an acceleration of segmental dynamics close to the wall. Other than in attractive or neutral systems, where the segmental dynamics is slowed down, we found that the segmental dynamics in the nanopores is identical to the local mobility in the bulk. Even under very careful scrutiny, we could not find any acceleration of the surface-near segmental motion. On the larger time scale, the neutron spin-echo experiment showed that the Rouse relaxation was not altered by confinement effects. Also the entanglement dynamics was not affected. Thus at moderate confinement conditions, facilitated by locally repulsive walls, the dynamics remains as in the bulk melt, a result that is not so clear from simulations.

A Computational Approach for Modeling Neutron Scattering Data from Lipid Bilayers.

Biological cell membranes are responsible for a range of structural and dynamical phenomena crucial, which are crucial to a cell's well-being and its associated functions. Due to the complexity of cell membranes, lipid bilayer systems are often used as biomimetic models. These systems have led to significant insights into vital membrane phenomena such as domain formation, passive permeation, and protein insertion. Experimental observations of membrane structure and dynamics are, however, limited in resolution, both spatial and temporal. Importantly, computer simulations are starting to play a more prominent role in interpreting experimental results, enabling a molecular understanding of lipid membranes. In particular, the synergy between scattering experiments and simulations offers opportunities for new discoveries in membrane physics, as the length and time scales probed by molecular dynamics (MD) simulations parallel those of experiments. Here, we describe a coarse-grained MD simulation approach that mimics neutron scattering data from large unilamellar lipid vesicles over a range of bilayer rigidities. Specifically, we simulate vesicle form factors and membrane thickness fluctuations determined from small angle neutron scattering (SANS) and neutron spin echo (NSE) experiments, respectively. Our simulations accurately reproduce trends from experiments and lay the groundwork for studies of more complex membrane systems.

Nanostructuration of ionic liquids: impact on the cation mobility. A multi-scale study.

When probed at the macroscopic scale, Ionic Liquids (ILs) behave as highly dissociated (i.e. strong) electrolytes while, at the molecular scale, they show clear characteristics of weak ionic solutions. The multi-scale analysis we report in this paper reconciles these apparently at odds behaviors. We investigate by quasi-elastic neutron scattering (QENS) and neutron spin-echo (NSE), the nanometer/nanosecond dynamics of OMIM-BF4, an imidazolium-based IL showing strong nanostructuration. We also probe the same IL on the microscopic (μm and ms) scale by pulsed field gradient NMR. To interpret the neutron data, we introduce a new physical model to account for the dynamics of the side-chains and for the diffusion of the whole molecule. This model describes the observables over the whole and unprecedented investigated spatial ([0.15-1.65] Å-1) and time ([0.5-2000] ps) ranges. We arrive at a coherent and unified structural/dynamical description of the local cation dynamics: a localized motion within the IL nanometric domains is combined with a genuine long-range translational motion. The QENS, NSE and NMR experiments describe the same long-range translational process, but probed at different scales. The associated diffusion coefficients are more than one order of magnitude different. We show how this apparent discrepancy is a manifestation of the IL nanostructuration.

Role of Dynamic Asymmetry on the Collective Dynamics of Comblike Polymers: Insights from Neutron Spin-Echo Experiments and Coarse-Grained Molecular Dynamics Simulations

We investigate the collective dynamics of comblike polymers displaying nanosegregation in side groups and main-chain rich domains, where side groups and backbones are dynamically equivalent. Neutron spin-echo experiments on poly(alkylene oxide)s with increasing side-group length have been focused on the decay of inter- and intradomain correlations and are complemented with molecular dynamics simulations on a coarse-grained model. In general, a good qualitative agreement between simulation and experiment is found. The collective times at inter- and intradomain levels converge with decreasing temperature, implying a common glass transition, and separate at high temperatures, revealing an increasing role of connectivity. The features of collective relaxations—regarding stretching, characteristic time, and temperature dependence—are independent of branching. The standard stretching for side-group motions and the closeness of collective and self-motions show that the anomalies exhibited by dynamically asymmetr...

Single Chain Dynamic Structure Factor of Linear Polymers in an All-Polymer Nano-Composite

We present neutron spin echo (NSE) experiments on the single chain dynamic structure factor of long poly(ethylene oxide) (PEO) linear chains in the presence of poly(methyl methacrylate)-based single-chain nanoparticles (SCNPs). A complementary structural characterization of the system discards a significant interpenetration of the components and reveals a close to globular conformation of the SCNPs when surrounded by PEO chains. Analogous NSE measurements on blends of PEO with the linear precursor chains of the SCNPs are taken as a reference for the dynamics study. For short times (below approximately 5 ns) PEO in both mixtures exhibits slowed down Rouse dynamics with respect to bulk PEO behavior. The similar deceleration observed in both environments suggests this effect to be due to the large dynamic asymmetry in the mixtures as evidenced by DSC experiments. More interestingly, the NSE results at longer times reveal a spectacular increase of the explored volume of PEO chains in the all-polymer nanocompo...

Antimicrobial Peptide Impacts the Lateral Diffusion and Bending Rigidity of Phospholipid Membrane

Antimicrobial peptides (AMPs) are the first line of defense system against foreign microorganisms. They are well known for their strong interaction with the cell membrane of pathogens. Quasielastic neutron scattering (QENS) and Neutron spin echo (NSE) measurements have been carried out on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membrane to understand the effect of melittin, an important AMP on the dynamical and mechanical behavior of membrane. Even at low concentration (0.2 mol%), the addition of melittin is found to influence the membrane dynamics significantly in a different way, depending on the phase of the lipid bilayers. In the gel phase, it acts as a plasticizer, enhancing the lateral motion of DMPC. However, in the fluid phase, it acts as a stiffening agent, hindering the lateral motion of the lipid molecules. Various interesting results on the impact of melittin on the phase behavior and lateral diffusion of the membrane will be discussed here. Their importance originate from the fact that many cellular functions, including cell signaling, membrane trafficking, and cell recognition are controlled to a great extent by the lateral diffusion of lipids in the membrane. NSE measurements indicate that incorporation of melittin modify the bending rigidity of the membrane, which is found to decrease with the concentration of melittin. Cholesterol is a vital component of erythrocyte membranes, which is a natural target for melittin. To understand the role of cholesterol, both QENS and NSE experiments have also been carried out on cholesterol-supplemented DMPC membrane in the absence and presence of melittin. Our measurements show that the destabilizing effect of melittin on the membranes can be mitigated by the presence of cholesterol. This study might provide new insights into the mechanism of action of antimicrobial peptides and their selective toxicity towards foreign microorganisms.

Diffusion of Isobutane in Silicalite: A Neutron Spin–Echo and Molecular Dynamics Simulation Study

The diffusion of isobutane in silicalite was studied using neutron spin–echo (NSE) experiments and molecular dynamics (MD) simulations between 444 and 550 K. The experimental and simulated diffusion coefficients showed agreement well within an order of magnitude, as did the activation energies of diffusion which agreed to within 3.4 kJ mol–l (∼15%). Jump diffusion was observed by NSE with a jump distance of 10 A, also observed by the MD simulations showing that the residence time was spent in a small section of the sinusoidal channel, rather than the intersections between channels as reported in previous simulations. The level of sinusoidal trapping diminishes in the simulations with increasing temperature, where the isobutane develops isotropic diffusion through both channel systems, as observed by NSE experiments.

Unstable spin-ice order in the stuffed metallic pyrochlorePr2+xIr2−xO7−δ

Specific heat, elastic neutron scattering, and muon spin rotation ($\mu$SR) experiments have been carried out on a well-characterized sample of "stuffed" (Pr-rich) Pr$_{2+x}$Ir$_{2-x}$O$_{7-\delta}$. Elastic neutron scattering shows the onset of long-range spin-ice "2-in/2-out" magnetic order at $T_M = 0.93$ K, with an ordered moment of 1.7(1)$\mu_\mathrm{B}$/Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 \AA\ and 0.7 ns, respectively. $\mu$SR experiments yield an upper bound 2.6(7) mT on the local field $B_\mathrm{loc}^{4f}$ at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7$\mu_B$ moments (120--270 mT, depending on muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr$^{3+}$ ions by the $\mu^+$-induced lattice distortion. For this to be the only effect, however, $\sim$160 Pr moments out to a distance of $\sim$14 \AA\ must be suppressed. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat, invokes slow correlated Pr-moment fluctuations in the ordered state that average $B_\mathrm{loc}^{4f}$ on the $\mu$SR time scale (${\sim}10^{-7}$ s), but are static on the time scale of the elastic neutron scattering experiments (${\sim}10^{-9}$ s). In this picture the dynamic muon relaxation suggests a Pr$^{3+}$ $4f$ correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.