Neutron Spin Echo Experiments Research Articles

Activation of Nanoscale Allosteric Protein Domain Motion Revealed by Neutron Spin Echo Spectroscopy

NHERF1 is a multidomain scaffolding protein that assembles signaling complexes, and regulates the cell surface expression and endocytic recycling of a variety of membrane proteins. The ability of the two PDZ domains in NHERF1 to assemble protein complexes is allosterically modulated by the membrane-cytoskeleton linker protein ezrin, whose binding site is located as far as 110 Ångstroms away from the PDZ domains. Here, using neutron spin echo (NSE) spectroscopy, selective deuterium labeling, and theoretical analyses, we reveal the activation of interdomain motion in NHERF1 on nanometer length-scales and on submicrosecond timescales upon forming a complex with ezrin. We show that a much-simplified coarse-grained model suffices to describe interdomain motion of a multidomain protein or protein complex. We expect that future NSE experiments will benefit by exploiting our approach of selective deuteration to resolve the specific domain motions of interest from a plethora of global translational and rotational motions. Our results demonstrate that the dynamic propagation of allosteric signals to distal sites involves changes in long-range coupled domain motions on submicrosecond timescales, and that these coupled motions can be distinguished and characterized by NSE.

KOHN ANOMALY ENERGY IN CONVENTIONAL SUPERCONDUCTORS EQUALS TWICE THE ENERGY OF THE SUPERCONDUCTING GAP: HOW AND WHY?

Kohn anomaly occurs in metals as a weak but discernible kink in the phonon spectrum around 2kF arising out of screened Coulombic interaction. Over the years this has been observed in a number of normal metallic systems. As a major surprise however, the recent neutron spin-echo experiments on elemental (conventional) superconductors Pb and Nb reveal a very important and striking relation that Kohn (anomaly) energy equals twice the energy of the superconducting gap. In this paper we explore the microscopic origin of this novel phenomenon and discuss its implication to the standard model BCS Theory.

Manifestation of the geometric phase in neutron spin-echo experiments

We show how the geometric (Berry's) phase becomes manifest on adiabatic rotation of the polarization vector in the magnetic field configuration in the arms in a neutron spin echo (NSE) experiment. When the neutron beam used is monochromatic, a geometric phase collected in one spin-echo arm can be exactly compensated in the other arm either by an opposite geometrical rotation or by adding/subtracting a dynamic (Larmor) phase. This is not possible in a white beam, because, contrary to the dynamic phase, the geometric phase is independent of wavelength. Therefore, the NSE pattern can be disturbed. We demonstrate that adiabatic resonant spin flippers inherently produce a geometric phase which can influence the performance of NSE setups based on such flippers. This effect can be avoided by a proper mutual symmetry of the gradient fields in these flippers.

Dynamic transition and glassy behaviour in hydrated proteins

The temperature dependence of functional kinetics of hydrated proteins has been related to changes in dynamic behaviour on the microscopic level, as reflected by the transition like variation of the mean square displacement ⟨u2⟩ of the hydrogen atoms. We have extended these explorations by a detailed study of the whole quasielastic and inelastic neutron scattering spectra over the broad energy transfer domain of 50 neV to 20 meV by the combination of high precision time-of-flight neutron spectroscopy (TOF) and neutron spin echo (NSE) experiments in H2O and D2O hydrated myoglobin. This allowed us to identify the basic character of the dynamic processes behind the observed un-harmonic variation of ⟨u2⟩. We have found that these processes are fully compatible with the extensively studied glassy type β fluctuations, which enter with increasing temperature the 0.2 ps – 10 ns combined TOF – NSE time window of the present study.

A Swollen Phase of Phospholipid Bilayers Induced by Pressure

Temperature, pressure, and ethanol concentration dependence of the structure and dynamics of dipalmitoyl phosphatidylcholine (DPPC) aqueous solution were investigated by small-angle neutron scattering and neutron spin echo experiments. A swollen phase, in which the mean repeat distance of lipid bilayers is larger than these of the other phases, is found between the liquid-crystalline phase and the interdigitated gel phase. The nature of the swollen phase is similar to the anomalous swelling state observed above the main transition temperature.

High-Pressure SANS and NSE Experiments in Microemulsion Systems

Small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy have been used to elucidate static and dynamic structures of a microemulsion system composed of a nonionic surfactant, water, and oil. Using the contrast variation neutron scattering technique, static structure parameters are evaluated, and the molecular volume change with pressure is calculated. The bending elastic modulus increases with increasing pressure. This tendency is similar to the microemulsion system composed of anionic surfactant, water, and oil. Therefore, a universal feature of the surfactant membrane as a response to pressure is clarified, that is, the surfactant membrane becomes rigid at high pressure due to the increase of density of the hydrophobic tail of surfactant molecules and their aggregates.

Cooperative Dynamics in Homopolymer Melts: A Comparison of Theoretical Predictions with Neutron Spin Echo Experiments

We present a comparison between theoretical predictions of the generalized Langevin equation for cooperative dynamics (CDGLE) and neutron spin echo data of dynamic structure factors for polyethylene melts. Experiments cover an extended range of length and time scales, providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights undergoing dynamics across the unentangled to entangled transition. Measured center-of-mass (com) mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The generalized Langevin equation for cooperative dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, of its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments.

Spin relaxation in nanophased manganites

We present recent results obtained by DC and AC susceptibility, muon spin relaxation and neutron spin echo experiments in magnetically ordered La0.7Pb0.3MnO3 and disordered La0.7Pb0.3Mn0.8Fe0.2O3. These results aim to cover an extraordinary equivalent frequency range (from quasi-static experiments to almost the THz region) and give an overall picture of the processes within. The different relaxations observed are related either to the macroscopic long-ranged interactions among magnetic clusters (susceptibility techniques) and to short-ranged intracluster spin diffusion processes (muon and neutron techniques).

Constraint Release in Entangled Binary Blends of Linear Polymers: A Molecular Dynamics Study

We present extensive molecular dynamics simulations of the dynamics of diluted long probe chains entangled with a matrix of shorter chains. The chain lengths of both components are above the entanglement strand length, and the ratio of their lengths is varied over a wide range to cover the crossover from the chain reptation regime to tube Rouse motion regime of the long probe chains. Reducing the matrix chain length results in a faster decay of the dynamic structure factor of the probe chains, in good agreement with recent neutron spin echo experiments. The diffusion of the long chains, measured by the mean square displacements of the monomers and the centers of mass of the chains, demonstrates a systematic speed-up relative to the pure reptation behavior expected for monodisperse melts of sufficiently long polymers. On the other hand, the diffusion of the matrix chains is only weakly perturbed by the diluted long probe chains. The simulation results are qualitatively consistent with the theoretical predictions based on constraint release Rouse model, but a detailed comparison reveals the existence of a broad distribution of the disentanglement rates, which is partly confirmed by an analysis of the packing and diffusion of the matrix chains in the tube region of the probe chains. A coarse-grained simulation model based on the tube Rouse motion model with incorporation of the probability distribution of the tube segment jump rates is developed and shows results qualitatively consistent with the fine scale molecular dynamics simulations. However, we observe a breakdown in the tube Rouse model when the short chain length is decreased to around N-S = 80, which is roughly 3.5 times the entanglement spacing N-e(P) = 23. The location of this transition may be sensitive to the chain bending potential used in our simulations.

Bending modulus of lipid bilayers in a liquid-crystalline phase including an anomalous swelling regime estimated by neutron spin echo experiments

Membrane fluctuations of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were investigated by neutron spin echo spectroscopy. The intermediate structure factor was analyzed in terms of the model proposed by Zilman and Granek (Phys. Rev. Lett. 77, 4788 (1996)), and the bending modulus of lipid bilayers was derived. The hardening of a lipid bilayer upon approaching the main transition point in the anomalous swelling regime was observed, which naturally connects the bending modulus in the gel phase below the main transition temperature.

Pressure effects on bending elasticities of surfactant monolayers in a ternary microemulsion composed of aerosol-OT∕D2O/decane

Pressure effects on the bending elasticities of surfactant monolayers have been investigated in a microemulsion system composed of aerosol-OT (AOT), D2O, and deuterated decane by means of small angle neutron scattering, neutron spin echo (NSE), and dynamic light scattering (DLS). In this system, a water-in-oil droplet structure, at ambient temperature and pressure, decomposes into two phases, under both increasing temperature and pressure. The authors' previous study showed that the bending modulus kappa of monolayers slightly decreased with increasing temperature, while it increased with increasing pressure. Temperature and pressure dependencies of kappa were explained in terms of a microscopic model, which takes into account the interactions between surfactant molecules. In this paper, the authors present the temperature and pressure dependencies of kappa obtained by the analysis combined with DLS and NSE experiments. The values of the bending modulus and mean displacement of the second-order droplet deformation are reasonable. It was further confirmed that an increase in the attractive interaction between hydrocarbon tails of AOT molecules with increasing pressure could be the origin of the pressure-induced phase transition.

Friction Dependence on α-Relaxations in a Tethered Polymer Monolayer

Functionalized low-molecular-weight polyisoputylene is commonly used as a lubricant additive to control low-velocity friction in clutches. The friction of tethered monolayers as a function of temperature and velocity was examined using lateral force microscopy. The friction force was found to obey time-temperature superposition, which indicates that the friction originates from a single relaxation mode. The scaling factors are identical to those obtained from neutron spin echo experiments of alpha-relaxations, linking the friction directly to segmental motion.

Starlike dendrimers in solutions: Structural properties and internal dynamics

We measured the shape and the internal dynamics of starlike dendrimers under good solvent conditions with small-angle neutron scattering and neutron spin-echo (NSE) spectroscopy, respectively. Architectural parameters such as the spacer length and generation were varied in a systematic manner. Structural changes occurring in the dendrimers as a function of these parameters are discussed, i.e., in terms of the fractal dimension and deviations of the radius of gyration from the Gaussian value. A first cumulant evaluation of the NSE spectra for each scattering vector q separately yields the length scale dependent relaxation rates. We observe a local minimum in the normalized relaxation rates Omega(q)q(3) on length scales corresponding to the overall dendrimer dimension. The dynamics is discussed within a Rouse-Zimm approach generalized to the case of starlike dendrimers of arbitrary geometry. The model allows an identification of the modes contributing to the relaxation of the dendrimer in the q and time range of the NSE experiment. The local minimum is due to collective breathing motions of (parts of) the dendrons relative to each other. Shape fluctuations are not observed.

Dynamic heterogeneity in hydrogen-bonded polymers

We report on neutron spin echo experiments on hydrogen-bonded polymers and compare the experimentally found dynamical structure factor with theoretical predictions. Surprisingly, we find that in the melt phase the expected scaling of the Rouse dynamics is not satisfied. We propose an explanation based upon the large spatial volume occupied by the connecting groups. When the effects of these bulky groups on the local friction are taken into account, the usual scaling behavior is restored.

Self- and Collective Dynamics of Syndiotactic Poly(methyl methacrylate). A Combined Study by Quasielastic Neutron Scattering and Atomistic Molecular Dynamics Simulations

We have investigated the molecular dynamics of syndiotactic poly(methyl methacrylate) well above the glass transition by combining quasielastic neutron scattering and fully atomistic computer simulations. The incoherent scattering measured by backscattering on a sample with deuterated ester methyl groups has revealed the single-particle motions of hydrogens in the main chain and in the α-methyl groups. Moreover, with neutron spin-echo experiments on the fully deuterated sample we have accessed the collective motions at the two first maxima of the structure factor. The simulated cell, which has been previously validated regarding the structural properties [Genix, A.-C.; et al. Macromolecules 2006, 39, 3947], shows a dynamical behavior that, allowing a shift in temperature, reproduces very accurately all the experimental results. The combined analysis of both sets of data has shown that: (i) The segmental relaxation involving backbone atoms deviates from Gaussian behavior. (ii) The dynamics is extremely heterogeneous: in addition to the subdiffusion associated with the α-process and the methyl group rotations, we have found indications of a rotational motion of the ester side group around the main chain. (iii) At a given momentum transfer and depending on the molecular groups considered, the time scales for collective motion are spread over about 1 order of magnitude, the correlations involving the main chain decaying much more slowly than those relating side groups. (iv) At the length scale characteristic for the overall periodicity of the system (that corresponding to the first structure factor peak), the experimentally observed collective dynamics relates to the backbone motions and is of interchain character; there, coherency effects are observed for all correlations, though side groups display weaker collectivity. (v) At the second structure factor peak, coherency remains only for correlations involving the main chains.

Molecular dynamics simulations of glycerol glass-forming liquid

Structural and dynamical properties of liquid glycerol have been investigated by Molecular Dynamics simulations. An improved model based on a slight reparametrisation of the all-atoms AMBER force field used in [R. Chelli, P. Procacci, G. Cardini, R.G.D. Valle, S. Califano, Phys. Chem. Chem. Phys. 1 (1999) 871] is presented. The structure remains satisfactory, qualitatively similar to that obtained from the original model. This new model is also found to reproduce significantly better the diffusion coefficient and the correlations times as they can be deduced from neutron spin echo (NSE) experiments. Structural heterogeneities revealed as a pre-peak of the static structure factor S( Q) close to Q ∼ 0.6 Å −1 are observed. Our results are also found compatible with predictions of the Mode Coupling Theory.

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Complementary neutron spin-echo and x-ray experiments and molecular-dynamics simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2) glassy crystal. Static, single-molecule reorientational dynamics and collective dynamics properties are investigated. Our results confirm the strong analogy between molecular liquids and plastic crystals. The orientational disorder is characterized at different temperatures and a change in the nature of rotational dynamics is observed. A careful check of the rotational diffusion model is performed using self-angular correlation functions Cl with high l values and compared to results obtained on molecular liquids composed of A-B dumbbells. Below the crossover temperature at which slow dynamics emerge, we show that some scaling predictions of the mode coupling theory hold and that alpha-relaxation times and nonergodicity parameters are controlled by the nontrivial static correlations.

Light, Small Angle Neutron and X‐Ray Scattering from Gels

AbstractThis paper gives two examples of experiments that demonstrate the power of small angle scattering techniques in the study of swollen polymer networks. First, it is shown how the partly ergodic character of these systems is directly detected by neutron spin echo experiments. The observed total field correlation function of the intensity scattered from a neutral gel allows the ergodic contribution to be directly distinguished from the non ergodic part, at values of transfer wave vector q that lie well beyond the range of dynamic light scattering. The results can be compared with those obtained at much lower q from visible light scattering.Second, a recent application of small angle X‐ray (SAXS) and neutron (SANS) scattering is described for a polyelectrolyte molecule, DNA, in semi‐dilute solutions under near‐physiological conditions. For these observations, the divalent ion normally present, calcium, is replaced by an equivalent ion, strontium. The comparison between SANS and SAXS yields a quantitative picture of the cloud of divalent counter‐ions around the central DNA core. At physiological conditions, the cloud is thinner than that predicted on the basis of the Debye screening length but thicker than if the counter‐ions were condensed on the DNA chain.

Structural relaxation in supercooled (Na 2O–Li 2O)–2P 2O 5 : a neutron spin-echo study

The structural relaxation of viscous (Na 2O–Li 2O)–2P 2O 5 is investigated by neutron spin-echo experiments. The spectra show stretching and time–temperature scaling as known from α-relaxation in supercooled liquids and derived from mode-coupling theory (MCT). The momentum transfer dependence of the correlation times is found to exhibit a maximum value at the first maximum of the structure factor S( Q) in agreement with predictions of MCT. In contrast, the Q-dependence of the stretching exponent β does not seem to oscillate in phase with S( Q) in this network glass.

Structural relaxation in supercooled (Na2O?Li2O)?2P2O5 : a neutron spin-echo study

The structural relaxation of viscous (Na2O–Li2O)–2P2O5 is investigated by neutron spin-echo experiments. The spectra show stretching and time–temperature scaling as known from α-relaxation in supercooled liquids and derived from mode-coupling theory (MCT). The momentum transfer dependence of the correlation times is found to exhibit a maximum value at the first maximum of the structure factor S(Q) in agreement with predictions of MCT. In contrast, the Q-dependence of the stretching exponent β does not seem to oscillate in phase with S(Q) in this network glass.