Internal molecular motions of bacteriorhodopsin: hydration-induced flexibility studied by quasielastic incoherent neutron scattering using oriented purple membranes.

Quasielastic incoherent neutron scattering from hydrogen atoms, which are distributed nearly homogeneously in biological molecules, allows the investigation of diffusive motions occurring on the pico- to nanosecond time scale. A quasielastic incoherent neutron scattering study was performed on the integral membrane protein bacteriorhodopsin (BR), which is a light-driven proton pump in Halobacterium salinarium. BR is embedded in lipids, forming patches in the cell membrane of the organism, which are the so called purple membranes (PMs). Measurements were carried out at room temperature on oriented PM-stacks hydrated at two different levels (low hydration, h = 0.03 g of D2O per g of PM; high hydration, h = 0.28 g of D2O per g of PM) using time-of-flight spectrometers. From the measured spectra, different diffusive components were identified and analyzed with respect to the influence of hydration. This study supports the idea that a decrease in hydration results in an appreciable decrease in internal molecular flexibility of the protein structure. Because it is known from studies on the function of BR that the pump activity is reduced if the hydration level of the protein is insufficient, we conclude that the observed diffusive motions are essential for the function of this protein. A detailed analysis and classification of the different kinds of diffusive motions, predominantly occurring in PMs under physiological conditions, is presented.

Spin dependence of slow neutron scattering by deuterons /

Work performed at the Argonne National Laboratory.

"Scientific opportunities with advanced facilities for neutron scattering" : report of a workshop held at Shelter Island, New York, October 23-26, 1984 /

On cover: Shelter Island Workshop, October 23-26, 1984.

Measuring and predicting the dynamics of linear monodisperse entangled polymers in rapid flow through an abrupt contraction. A small angle neutron scattering study

Small-angle neutron scattering measurements on a series of monodisperse linear entangled polystyrene melts in nonlinear flow through an abrupt 4:1 contraction have been made. Clear signatures of melt deformation and subsequent relaxation can be observed in the scattering patterns, which were taken along the centerline. These data are compared with the predictions of a recently derived molecular theory. Two levels of molecular theory are used: a detailed equation describing the evolution of molecular structure over all length scales relevant to the scattering data and a simplified version of the model, which is suitable for finite element computations. The velocity field for the complex melt flow is computed using the simplified model and scattering predictions are made by feeding these flow histories into the detailed model. The modeling quantitatively captures the full scattering intensity patterns over a broad range of data with independent variation of position within the contraction geometry, bulk flow rate and melt molecular weight. The study provides a strong, quantitative validation of current theoretical ideas concerning the microscopic dynamics of entangled polymers which builds upon existing comparisons with nonlinear mechanical stress data. Furthermore, we are able to confirm the appreciable length scale dependence of relaxation in polymer melts and highlight some wider implications of this phenomenon.

Gamma rays associated with neutron scattering in fusion reactor materials

The gamma-rays produced by the inelastic scattering of 14 MeV neutrons. in fusion reactor materials have been studied using a gamma-ray spectrometer employing a sodium iodide scintillation detector. The source neutrons are produced by the T(d,n)4He reaction using the SAMES accelerator at the University of Aston in Birmingham. In order to eliminate the large gamma-ray background and neutron signal due to the sensitivity of the sodium iodide detector to neutrons, the gamma-ray detector is heavily shielded and is used together with a particle time of flight discrimination system based on the associated particle time of flight method. The instant of production of a source neutron is determined by detecting the associated alpha-particle enabling discrimination between the neutrons and gamma-rays by their different time of flight times. The electronic system used for measuring the time of flight of the neutrons and gamrna-rays over the fixed flight path is described. The materials studied in this work were Lithium and Lead because of their importance as fuel breeding and shielding materials in conceptual fusion reactor designs. Several sample thicknesses were studied to determine the multiple scattering effects. The observed gamma-ray spectra from each sample at several scattering angles in the angular range Oº - 90° enabled absolute differential gamma-ray production cross-sections and angular distributions of the resolved gamma-rays from Lithium to be measured and compared with published data. For the Lead sample, the absolute differential gamma-ray production cross-sections for discrete 1 MeV ranges and the angular distributions were measured. The measured angular distributions of the present work and those on Iron from previous work are compared to the predictions of the Monte Carlo programme M.O.R.S.E. Good agreement was obtained between the experimental results and the theoretical predictions. In addition an empirical relation has been constructed which describes the multiple scattering effects by a single parameter and is capable of predicting the gamma-ray production cross-sections for the materials to an accuracy of ± 25%.

Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst

Peer reviewed

Neutron Scattering Studies on Complex Magnetic Structures in Magnetoelectric Materials

In this thesis, the magnetic properties of four transition-metal oxides are presented. Their multiferroic and magnetoelectric phases have been investigated by means of different neutron scattering techniques. The materials TbMnO3 and MnWO4 belong to the group of spin-induced multiferroics. Their ferroelectric polarization can be explained by the inverse DzyaloshinskiiMoriya interaction. Another common feature of both materials is the presence of subsequent magnetic transitions from a spin-density wave to a spin spiral. The features of the phase transitions have been studied in both materials and it could be shown that diffuse magnetic scattering from the spin spiral is present even in the ordered spin-density wave phase. The excitation spectrum in the multiferroic phase of TbMnO3 was investigated in detail and a comprehensive dataset was obtained using time-of-flight spectroscopy. A spin-wave model could be obtained which can quantitatively describe the full dispersion. Furthermore, the polarization of the zone-center excitations could be derived which fit well to data from inelastic neutron spectroscopy and infrared spectroscopy. With the combination of spherical neutron polarimetry and a poling of the sample by an electric field, it was possible to observe the chiral magnetic component of the magnetic excitations in TbMnO3 and MnWO4. The spin-wave model for TbMnO3 obtained in this thesis is able to correctly describe the dispersion of this component. The double tungstate NaFe(WO4)2 is isostructural to the multiferroic MnWO4 and develops a complex magnetic phase diagram. By the use of neutron diffraction techniques, the zero-field structure and high-field structures in magnetic field applied along the b-axis could be determined. The data reveal a direct transition into an incommensurate spin-spiral structure. The value of the incommensurability is driven by anharmonic modulations and shows strong hysteresis effects. The static and dynamic properties in the magnetoelectric spin-glass phase of Ni0.42Mn0.58TiO3 were studied in detail. The spin-glass phase is composed of short-ranged MnTiO3 and NiTiO3-type order. The antiferromagnetic domains could be controlled by crossed magnetic and electric fields, which was visualized using spherical neutron polarimetry. A comprehensive dataset of the magnetic excitations in the spin-glass phase was collected. The dataset revealed correlations in the hexagonal plane which are only weakly coupled along the c-axis. The excitation spectra could be simulated by taking into account the MnTiO3-type order.

Neutron-mapping polymer flow: Scattering, flow visualization, and molecular theory

Flows of complex fluids need to be understood at both macroscopic and molecular scales, because it is the macroscopic response that controls the fluid behavior, but the molecular scale that ultimately gives rise to rheological and solid-state properties. Here the flow field of an entangled polymer melt through an extended contraction, typical of many polymer processes, is imaged optically and by small-angle neutron scattering. The dual-probe technique samples both the macroscopic stress field in the flow and the microscopic configuration of the polymer molecules at selected points. The results are compared with a recent tube model molecular theory of entangled melt flow that is able to calculate both the stress and the single-chain structure factor from first principles. The combined action of the three fundamental entangled processes of reptation, contour length fluctuation, and convective constraint release is essential to account quantitatively for the rich rheological behavior. The multiscale approach unearths a new feature: Orientation at the length scale of the entire chain decays considerably more slowly than at the smaller entanglement length.