Protein and solvent dynamics as studied by QENS and dielectric spectroscopy

We have investigated the dynamics of a protein (hemoglobin) in three different solvents by broadband dielectric spectroscopy and quasi-elastic neutron scattering (QENS). The solvents used were water, glycerol and methanol. From the dielectric measurements we were able to extract four relaxation processes, where the fastest process is due to solvent dynamics, and the three slower processes arise from protein motions of a different nature, and hence, visible on different time and temperature scales. The results from the mean square displacement (MSD) obtained by quasi-elastic neutron scattering indicate that the fast and local protein motions are strongly related (slaved) by the fastest and most local solvent motions in the case of water and glycerol. In contrast, for methanol we found the low temperature dynamics are dominated by the CH₃ group rotation, which does not promote any protein dynamics.     

Lattice dynamics and ion transport in structurally disordered chalcogenides of copper and silver

The results of studying the copper and silver selenides and their solid solutions are reported. The methods of inelastic neutron scattering and neutron diffraction analysis are used to study the dynamics and structure of the crystal lattice. The relation of the properties of dynamics and the crystal-lattice structure to the ion transport are analyzed.     

Analytical complex at the PIK reactor for studying the supra-atomic structure and dynamics of materials by neutron scattering

A project of the center for studying reactor materials and solving problems of materials science is presented which will be equipped with the following neutron instruments: a small-angle Membrana diffractometer, a spin-echo spectrometer, and a time-of-flight spectrometer. It is proposed to irradiate materials in the PIK reactor core and use neutron-scattering tools to analyze the structure and dynamics of these materials and investigate radiative defects in the complete experimental cycle (initial material-irradiation-strength tests, thermal loads, and other effects) using materials science techniques.     

On the structure of the molten mixture Ag(I0.3Br0.7)

We present neutron diffraction measurements of the total structure factor of the molten salt mixture Ag(I_0.3Br_0.7) at 650 °C, measured by using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the spallation neutron source of the Neutron Science Laboratory in the High Energy Accelerator Research Organization of Japan. We have compared the experimental data with molecular dynamics simulations using Vashishta–Rahman type potentials. Within the known constraints of pairwise rigid ion potentials, the results of the simulations are in fair agreement with experiment. We also present results using a linear combination of the experimental and molecular dynamics simulations of the structure factors of the AgI and AgBr melts. The latter are in somewhat better agreement with experiment.     

Structural and transport properties of the AgI–AgBr system in its superionic and molten phases by computer simulation

Molecular dynamics simulations were performed to investigate the structural properties of AgI–AgBr system in its superionic and molten phases. Parrinelo, Rahman and Vashishta (RVP) type potentials are used for the simulation. The obtained pair distribution functions are in good agreement with the neutron diffraction experimental data. The transport coefficients in these two phases are also calculated from the equilibrium and non-equilibrium molecular dynamics simulation method. In molten phase, the ratios of partial conductivities on constant concentration are found to be constant at different temperatures.     

Experimental investigation of the atomic dynamics of amorphous and icosahedral alloys

Most recent results from the experimental investigation of the single particle dynamics and the collective atomic motions in amorphous metals and in icosahedral alloys using neutron inelastic scattering techniques are briefly presented. The extension of the dispersion curve of short wavelength collective motions into the first pseudo Brillouin zone by neutron Brillouin scattering and the investigation of crystal field excitations in an amorphous metal and its dependence on structural relaxation are discussed. Comparative studies of the atomic dynamics of polydomain icosahedral alloys, approximant polycrystals, and polycrystals with similar chemical composition but different structure are discussed. It is shown that the generalized vibrational density of states and the total dynamic structure factor of the icosahedral alloys investigated so far are rather different from any crystalline sample except for the case of higher order approximants with very large units cells.     

Feasibility of study magnetic proximity effects in bilayer “superconductor/ferromagnet” using waveguide-enhanced polarized neutron reflectometry

A resonant enhancement of the neutron standing waves is proposed to use in order to increase the magnetic neutron scattering from a “superconductor/ferromagnet” (S/F) bilayer. The model calculations show that usage of this effect allows to increase the magnetic scattering intensity by factor of hundreds. Aspects related to the growth procedure (order of deposition, roughness of the layers etc.) as well as experimental conditions (resolution, polarization of the neutron beam, background etc) are also discussed. Collected experimental data for the S/F heterostructure Cu(32 nm)/V(40 nm)/Fe(1 nm)/MgO confirmed the presence of a resonant 60-fold amplification of the magnetic scattering.     

Scattering studies of photostructural changes in chalcogenide glasses

Structural changes which occur in chalcogenide glasses upon absorption of bandgap light have been investigated using neutron diffraction. The first part of this paper describes studies by small-angle neutron scattering of the irreversible photostructural changes which occur in obliquely evaporated amorphous chalcogenide films. The second part deals with the reversible photostructural effect observed in chalcogenide glasses, investigated by conventional neutron diffraction and EXAFS experiments. The various models which have been proposed for the photostructural effect are discussed in the light of these experimental results.     

Scintillation neutron detectors based on solid-state photomultipliers and lightguides

Neutron detectors based on scintillation screens ZnS(Ag)/LiF and solid-state photomultipliers have been developed. Lightguides are used to collect light. The application of a coincidence scheme provides a low dark count and a neutron detection efficiency as high as 70%. A scheme of x-y neutron detector based on wavelength shifting fibers is also proposed. Tests of the proposed versions of detectors in a neutron beam have shown their efficiency.     

Molecular dynamics simulation of anhydrous lithium acetate: crystalline and molten phases

The results of molecular dynamics simulations of the crystalline and molten phase of anhydrous lithium acetate are presented. The potential parameters were obtained from empirical fitting to the crystalline phases of the material. The simulations were carried out for 216 molecules in an NPT ensemble using the DLPOLY program. A structural model is proposed for both the crystalline and molten phases of lithium acetate. Calculated values of the melting point, diffusion coefficient and structural parameters of lithium acetate are in reasonable agreement with experimental results.     

Magnetic state, elastic constants, and longwave static displacements of atoms in γ-FeNi alloys

The experimental results on the concentration and temperature behavior of longwave (both transverse and longitudinal) static displacements of atoms, as well as the data on the elastic constants (derived from neutron and ultrasonic measurements) of γ-FeNi alloys, are reported. The relationship between the alloy magnetic state and the above-mentioned parameters is discussed and these parameters are classified with respect to the premartensitic phenomena and the Invar effect.     

Hydrogen dynamics in polyethersulfone: A quasielastic neutron scattering study in the high-momentum transfer region

In a previous quasielastic neutron scattering study [I. Quintana, A. Arbe, J. Colmenero, B. Frick, Macromolecules 38 (2005) 3999] we characterized the hydrogen dynamics in glassy polyethersulfone as due to ‘fast processes’ plus flips and oscillations of the phenylene rings. Here we have extended this investigation by using thermal neutrons, that allowed exploring also much higher values of the momentum transfer Q. In the dynamic window accessed, a characterization as best accurate as possible of the fast dynamics is imperative in order to describe the experimental results, especially at very local length scales. While for Q ≲ 2.2 Å⁻¹ (i.e., in the range previously studied) the proposed model perfectly describes the experimental data with the same parameters deduced from the earlier work, in the high Q-range (Q ≈ 3.5–4 Å⁻¹) deviations are found. These could be overcome by assuming some kind of ‘effective softening’ of the fast process and slight deviations from harmonic behavior in the Q-dependence of the associated Debye–Waller factor.     

Investigation of triple spin correlations and spin dynamics in ferromagnets

Data on the experimental detection and use of three-particle (chiral) spin dynamic correlations in ferromagnets are presented. The oblique-geometry method for investigating polarized neutron small-angle scattering is described, which gives the dependence that the scattering asymmetry has on the polarization P signs and the scattering angle θ. The following results of the dynamics investigation in the critical and ferromagnetic phases in the magnetic field are presented: the temperature dependence of the critical field H _c, the factorization of the momentum transfer dependence of three-particle vertices, the corroboration of the “hard” version of the dipole critical dynamics, and the dynamics of amorphous magnets and invars.     

Is the rigid-body assumption reasonable?: Insights into the effects of dynamics on the electrostatic analysis of barnase-barstar

Electrostatically-driven association of proteins is important to many biological functions, and understanding which amino acid residues contribute to these interactions is crucial to protein design. Theoretical calculations that are used to elucidate the role of electrostatics in association are typically based on a single experimentally determined protein structure, while an underlying rigid-body assumption is adopted. The goal of this study was to investigate the role of conformational fluctuations on electrostatic interaction energies, as applied to the electrostatic analysis of barnase-barstar. For our calculations, we apply theoretical alanine-scan mutagenesis to introduce charge perturbations by replacing every ionizable residue with alanine, one at a time. Electrostatic clustering and free energy calculations based on the Poisson-Boltzmann method are used to evaluate the effects of each perturbation. Molecular dynamics simulations are performed for the barnase-barstar parent complex and seven experimental alanine mutations from the literature, in order to introduce relaxation before and after mutation. We discuss the effects of dynamics, in the form of pre- and post-mutation relaxation, on electrostatic clustering and free energies of association in light of experimental data. We also examine the utility of nine electrostatic similarity methods for clustering of barnase alanine-scan mutants. Our calculations suggest that the rigid-body assumption is reasonable for electrostatic calculations of barnase-barstar.     

Polarized neutron reflectometry at the IBR-2 pulsed reactor

Polarized neutron reflectometry as a method for investigating layered nanostructures and its implementation at the IBR-2 pulsed reactor (Dubna, Russia) are described. The experimental data illustrating the studies of magnetic layered nanostructures and the development of the method of polarized neutron reflectometry on the polarized neutron spectrometer are presented. The directions of further development of the method of polarized neutron reflectometry are analyzed.     

Larmor procession reflectometry for studying multilayers

A new experimental method (angular encoding) based on the combination of reflectometry and neutron spin-echo methods, which makes it possible to use widely collimated neutron beams and provides an angular resolution of about 0.01°, has been proposed. It was used to study magnetic thin films. In particular, the effect of Larmor pseudoprecession of neutron polarization upon reflection from a ⁵⁷Fe film with a thickness of about 100 nm was investigated, and it was shown that this method allows one to determine the absolute magnetization direction; this cannot be done using conventional reflectometry.     

Relaxation dynamics and acoustic properties in simple liquids

An overview of the relaxation dynamics in simple liquids is undertaken, in view of the recent experimental results achieved by means of inelastic X-ray and neutron scattering. We address some basic issues related to the extension of ordinary hydrodynamic in the Thz domain, such as the coupling of density and entropy fluctuations and the origin of anomalous acoustic dispersion within the general context of disordered systems dynamics. Specifically, we show that the Landau Placzeck ratio do not describe the quasielastic component of the dynamic structure factor and that the positive dispersion is a universal feature which can be understood in terms of a disorder induced relaxation process.     

Study on the influence of isotope exchange of hydrogen with deuterium on the vibrational spectrum of lysozyme by inelastic neutron scattering

The influence of isotope exchange of hydrogen with deuterium on the lysozyme dynamics was studied by incoherent inelastic neutron scattering. The generalized vibrational densities of states G(ω) were constructed from experimental results for protonated and deuterated protein samples at 200, 280, and 311 K. The major isotope effect was observed in G(ω) in the frequency region higher than 100 cm^?1. At all temperatures, both the Debye mode and the region of G(ω), whose spectral dimension corresponds to the fracton mode, are observed in the low-frequency region of the densities of states of both protonated and deuterated lysozyme. The influence of the hydrogen isotope exchange on the low-frequency region of G(ω) is insignificant.