W. Petry

Water-coupled low-frequency modes of myoglobin and lysozyme observed by inelastic neutron scattering

Conformational changes of proteins often involve the relative motion of rigid structural domains. Normal mode analysis and molecular dynamics simulations of small globular proteins predict delocalized vibrations with frequencies below 20 cm(-1), which may be overdamped in solution due to solvent friction. In search of these modes, we have studied deuterium-exchanged myoglobin and lysozyme using inelastic neutron scattering in the low-frequency range at full and low hydration to modify the degree of damping. At room temperature, the hydrated samples exhibit a more pronounced quasielastic spectrum due to diffusive motions than the dehydrated samples. The analysis of the corresponding lineshapes suggests that water modifies mainly the amplitude, but not the characteristic time of fast protein motions. At low temperatures, in contrast, the dehydrated samples exhibit larger motional amplitudes than the hydrated ones. The excess scattering, culminating at 16 cm(-1), is suggested to reflect water-coupled librations of polar side chains that are depressed in the hydrated system by strong intermolecular hydrogen bonding. Both myoglobin and lysozyme exhibit ultra-low-frequency modes below 10 cm(-1) in the dry state, possibly related to the breathing modes predicted by harmonic analysis.

Study of the glass transition order parameter in amorphous polybutadiene by incoherent neutron scattering

The temperature dependent elastic incoherent scattering from a glass forming polybutadiene was studied using high resolution neutron spectroscopy. This elastic scattering measures directly the non-ergodicity order parameter of the glass transition. We observed an anomalous decrease of this scattering setting in around 30 K below the thermodynamic glass transition,Tg, the temperature dependence of which is in agreement with the square root ofT prediction of the mode coupling approach. The critical temperature of 220 K lies about 30 K aboveTg. The missing elastic intensity reappears as inelastic scattering in the 1 meV range. Within the μeV resolution of the backscattering spectrometer no quasielastic scattering can be detected up to 20 K aboveTg. The observed inelastic scattering may be interpreted as resulting from a continous shift of the density of states towards low frequencies as a consequence of a general softening of the structure.

Vibrational frequency shifts as a probe of hydrogen bonds: thermal expansion and glass transition of myoglobin in mixed solvents

Abstract The contribution of hydrogen bonds to protein-solvent interactions and their impact on structural flexibility and dynamics of myoglobin are discussed. The shift of vibrational peak frequencies with the temperature of myoglobin in sucrose/water and glycerol/water solutions is used to probe the expansion of the hydrogen bond network. We observe a characteristic change in the temperature slope of the O–H stretching frequency at the glass transition which correlates with the discontinuity of the thermal expansion coefficient. The temperature-difference spectra of the amide bands show the same tendency, indicating that stronger hydrogen bonding in the bulk affects the main-chain solvent interactions in parallel. However, the hydrogen bond strength decreases relative to the bulk solvent with increasing cosolvent concentration near the protein surface, which suggests preferential hydration. Weaker and/or fewer hydrogen bonds are observed at low degrees of hydration. The central O–H stretching frequency of protein hydration water is red-shifted by 40 cm–1 relative to the bulk. The shift increases towards lower temperatures, consistent with contraction and increasing strength of the protein-water bonds. The temperature slope shows a discontinuity near 180 K. The contraction of the network has reached a critical limit which leads to frozen-in structures. This effect may represent the molecular mechanism underlying the dynamic transition observed for the mean square displacements of the protein atoms and the heme iron of myoglobin.

Relaxation and phonons in viscous and glassy orthoterphenyl by neutron scattering

We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from α-to β-relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureTc [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a multiple scattering simulation. Throughout the article, experimental difficulties are discussed in some detail.

Migration enthalpies in FCC and BCC metals

The authors present a model relating the migration enthalpy Hvm for nearest-neighbour vacancy jumps in cubic metals to the phonon dispersion. The migration enthalpy is split into two parts, one depending only on the lattice structure, the other on the vibrational properties of the particular metal. This latter term can be written in terms of the static lattice Green function, i.e. of the omega -2 moment of the spectrum. It can thus be calculated directly from measured phonon dispersion curves. For FCC metals, excellent agreement between calculated and measured values of Hvm is found. For BCC metals, where Hvm is known from experiments only in a few cases, predictions are made wherever the phonon dispersions are available. The model takes into account the unusually low-lying phonon branches in some of the BCC metals and yields, where phonon frequencies shift with temperature, temperature-dependent values of Hvm.

Localized Motion in Supercooled Glycerol as Measured by 2H-NMR Spin-Lattice Relaxation and Incoherent Neutron Scattering

Selectively deuterated glycerol has been subjected to 2H-NMR spin-lattice relaxation and quasi-elastic neutron scattering experiments. The measurements yield relaxation rates and a non-Gaussian Q-dependence of the Debye-Waller factor which are different for the two hydrogen sites. The data analysis shows that below the onset of the glass transition α-process the hydrogens perform a local motion (≈ 10-12 s) in addition to what is expected from harmonic phonons. The resulting mean-square displacements are highly temperature dependent but are significantly smaller than those found in van der Waals glasses. Amplitudes and activation energies of the carbon-bonded and oxygen-bonded hydrogens are different. A possible mechanism is discussed.

Reorientation of benzene in its crystalline state: A model case for the analogy between nuclear magnetic resonance spin alignment and quasielastic incoherent neutron scattering

The close analogy between 2H‐NMR spin alignment and 1H quasielastic incoherent neutron scattering [J. Chem. Phys. 84, 4579 (1986)] in determining the geometry and time scale of molecular reorientation is illustrated by an experimental example. Analysis of the final states of both methods show consistently that benzene in its (poly)crystalline state reorients by rotational jumps about the molecular sixfold symmetry axis. Emphasis is put on the quasielastic structure factor of incoherent neutron scattering, which excludes random jumps among the six orientations as the reorientation mechanism, allowing only single rotational jumps.

Dewetting of Thin Polymer-Blend Films Examined with GISAS

Abstract The morphology of dewetted thin polymer-blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) on top of silicon surfaces is investigated. The film thickness of the originally homogeneous films is varied between 19 and 104 A. Compared to the radius of gyration of the unperturbed molecule, R g =106 A , the as-prepared films are confined in the direction perpendicular to the sample surface. The dewetting results from the storage of the samples under toluene vapor atmosphere. Atomic force microscopy (AFM) and grazing incidence small-angle scattering (GISAS) are used. From the differences in the GISAS data measured with X-rays compared to data measured with neutrons a random distribution of the molecules inside the individual droplets is determined. Thus from dewetting under toluene atmosphere no periodicity in the internal structure exists. The, within all methods derived, most prominent in-plane length corresponds to the mean droplet distance. Its function of film thickness is explainable by the spinodal dewetting model.

From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H2O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions.

Phonon softening and martensitic transformation in α-Fe

Abstract The phonon dispersion of the low-temperature BCC-phase of pure iron (α-Fe) was investigated as a function of temperature. A strong softening of the entire T 1 [ ξξ 0] and T 1 [ ξξ 2 ξ ] branch is observed on approaching the martensitic α-γ-phase transition temperature. The eigenvectors of these phonons are in the direction of displacements needed for the transformation to the FCC- (γ−) phase. This indicates low potential barriers for displacemenes towards the closed-packed structure and can be interpreted as a dynamical precursor for the martensitic phase transition.