Search results for "Neutron scattering"
showing 10 items of 322 documents
Direct Evidence of the Amino Acid Side Chain and Backbone Contributions to Protein Anharmonicity
2010
Elastic incoherent neutron scattering has been used to study the temperature dependence of the mean-square displacements of nonexchangeable hydrogen atoms in powders of a series of homomeric polypeptides (polyglycine, polyalanine, polyphenylalanine and polyisoleucine) in comparison with myoglobin at the same hydration level (h = 0.2). The aim of the work was to measure the dynamic behavior of different amino acid residues separately and assess the contribution of each type of side chain to the anharmonic dynamics of proteins. The results provide direct experimental evidence that the first anharmonic activation, at approximately 150 K, is largely due to methyl group rotations entering the ti…
Rigid versus Flexible Protein Matrix: Light-Harvesting Complex II Exhibits a Temperature-Dependent Phonon Spectral Density
2018
Dynamics-function correlations are usually inferred when molecular mobility and protein function are simultaneously impaired at characteristic temperatures or hydration levels. In this sense, excitation energy transfer in the photosynthetic light-harvesting complex II (LHC II) is an untypical example because it remains fully functional even at cryogenic temperatures relying mainly on interactions of electronic states with protein vibrations. Here, we study the vibrational and conformational protein dynamics of monomeric and trimeric LHC II from spinach using inelastic neutron scattering (INS) in the temperature range of 20-305 K. INS spectra of trimeric LHC II reveal a distinct vibrational …
Small-Angle Neutron Scattering from Aqueous Solutions of C12E6 and C12E8: Critical Fluctuations and Micellar Growth
1986
Static SANS measurements have been performed on 0.028 M C12E8 in D2O over the temperature range 20–74.2 °C. Little micellar growth is observed; increases in the scattered intensities in the low angle region are due to critical concentration fluctuations of correlation length g. We have also reexamined our SANS data for C12E6 in D2O, in light of other worker’s claims that a sphere-to-cylinder transition in micellar shape is occurring in these solutions. We discuss in detail the inferences about C12E6 micellar size drawn from the results of several different experimental techniques.
Continuum elastic sphere vibrations as a model for low-lying optical modes in icosahedral quasicrystals
2004
The nearly dispersionless, so-called "optical" vibrational modes observed by inelastic neutron scattering from icosahedral Al-Pd-Mn and Zn-Mg-Y quasicrystals are found to correspond well to modes of a continuum elastic sphere that has the same diameter as the corresponding icosahedral basic units of the quasicrystal. When the sphere is considered as free, most of the experimentally found modes can be accounted for, in both systems. Taking into account the mechanical connection between the clusters and the remainder of the quasicrystal allows a complete assignment of all optical modes in the case of Al-Pd-Mn. This approach provides support to the relevance of clusters in the vibrational prop…
Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment
2008
The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light scattering techniques (Brillouin or low-frequency Raman scattering). We also report high resolution inelastic neutron scattering measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhan…
Lattice Dynamics in Wurtzite Semiconductors: The Bond Charge Model of CdS
1999
An extension of the adiabatic bond charge model of Rustagi and Weber is used to study the lattice dynamic properties of wurtzite-type compounds. The model has been applied to the description of the phonon dispersion of CdS, which has been recently measured by neutron scattering. The agreement with the neutron data is excellent with a small set of physically meaningful parameters. The eigenvector admixture of the E2 modes, calculated at the G-point, agrees with the experimental values obtained through the isotopic mass dependence of the optical modes and ab initio calculations.
Observation of a superfluid component within solid helium.
2011
We demonstrate by neutron scattering that a localized superfluid component exists at high pressures within solid helium in aerogel. Its existence is deduced from the observation of two sharp phonon-roton spectra which are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing the samples. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface.
Structural Study of Star Polyelectrolytes and Their Porous Multilayer Assembly in Solution
2015
Star polyelectrolytes with responsive properties to external stimuli, such as pH, temperature and ionic condition, were utilized to fabricate layer-by-layer (LbL) microcapsules . The microstructure of star polyelectrolytes was first studied in semi-dilute solution by in situ small-angle neutron scattering (SANS). These measurements show that with the addition of salts, arms of strong cationic star polyelectrolytes will contract and the spatial ordering of the stars would be interrupted. SANS measurements were also performed on the microcapsules in order to study their internal structure and responsive properties in solution. The results show that with the increase of shell thickness, microc…
Structure of diblock copolymers in supercritical carbon dioxide and critical micellization pressure
1999
This paper reports a small angle neutron scattering investigation of micelle formation by fluorocarbon-hydrocarbon block copolymers in supercritical ${\mathrm{CO}}_{2}{(\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2})$ at 65 \ifmmode^\circ\else\textdegree\fi{}C. A sharp unimer-micelle transition is obtained due to the tuning of the solvating ability of ${\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2}$ by profiling pressure, so that the block copolymer, in a semidilute solution, finds ${\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2}$ a good solvent at high pressure and a poor solvent at low pressure. At high pressure the copolymer is in a monomeric state…
Dynamic Anomalies and their Relation to the Glass Transition: A Neutron Scattering Study of the Glass Forming Van der Waals Liquid Ortho-terphenyl
1991
Neutron scattering experiments on the molecular glass former ortho-terphenyl reveal a dynamic anomaly at a temperature Tc ≈ 290 K well above the calorimetric glass temperature Tg = 243 K. Close above Tc the density autocorrelation function ΦQ(t) shows a two step decay over 4–5 decades in time. The slower component obeys the time-temperature superposition principle. Its line shape can be well parametrized by a Kohlrausch law and is strongly temperature dependent as its relaxation time scales with the shear viscosity. Thus this component is identified with the structural relaxation (α-process). The faster component (β-process) is much less temperature dependent. Its line shape factorizes in a…