[Show abstract][Hide abstract] ABSTRACT: The crystal and molecular structure of p-N,N 0-tetraacetylodiaminodurene (TADD) is reported based on the X-ray diffraction studies. The N-acetyl moieties are planar and all N-acetyl groups are perpendicular to the ring plane. Methyl groups both of acetyl moieties and of durene form a number of non-conventional hydrogen bonds with nitrogen and oxygen atoms. The vibrational spectra very well reflect the structure of molecules and their contacts. They are compared with calculated data by using various theoretical approaches. The neutron scattering spectra show two tunnel lines of low energy values (at ±0.9 and ±2.3 leV at 4 K), which can be ascribed to methyl groups of N-acetyl moieties, which behave more freely than those attached to the phenyl ring.
[Show abstract][Hide abstract] ABSTRACT: We revisit the nonuniversal aspect of polymer dynamics by considering both new and existing data on the zero-shear viscosity and linear viscoelastic response of various polymers, each with a wide range of molecular weights. Analysis of the zero-shear viscosity data in terms of the packing length p, whose role in entanglements has been discussed previously by Fetters and co-workers, reveals a behavior that is irreconcilable with our current understanding based on the tube model. Specifically, we find that the transition regime between Rouse and pure reptation dynamics, currently understood as the regime where contour length fluctuations are active, systematically shrinks as the packing length of the polymer increases. Further, we find that the slope of the loss moduli in the high-frequency wing of the terminal peak of well-entangled systems also decreases from the common −0.25 to −0.125 with increasing p. This is contrary to the single expected value of −0.25 from tube models which include contour length fluctuations or −0.5 from pure reptation. These findings hint on possible missing ingredients in our current understanding of polymer dynamics.
[Show abstract][Hide abstract] ABSTRACT: Large-scale domain motions in alcohol dehydrogenase (ADH) have been observed previously by neutron spin-echo spectroscopy (NSE). We have extended the investigation on the dynamics of ADH in solution by using high-resolution neutron time-of-flight (TOF) and neutron backscattering (BS) spectroscopy in the incoherent scattering range. The observed hydrogen dynamics were interpreted in terms of three mobility classes, which allowed a simultaneous description of the measured TOF and BS spectra. In addition to the slow global protein
diffusion and domain motions observed by NSE, a fast internal process could be identified. Around one third of the protons in ADH participate in the fast localized diffusive motion. The diffusion coefficient of the fast internal motions is around two third of the value of the surrounding D2O solvent. It is tempting to associate the fast internal process with solvent exposed amino acid residues with dangling side chains.
Full-text · Article · Aug 2015 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The short-time self-diffusion D of the globular model protein bovine serum albumin in aqueous (D2O) solutions has been measured comprehensively as a function of the protein and trivalent salt (YCl3) concentration, noted cp and cs, respectively. We observe that D follows a universal master curve D(cs,cp) = D(cs = 0,cp) g(cs/cp), where D(cs = 0,cp) is the diffusion coefficient in the absence of salt and g(cs/cp) is a scalar function solely depending on the ratio of the salt and protein concentration. This observation is consistent with a universal scaling of the bonding probability in a picture of cluster formation of patchy particles. The finding corroborates the predictive power of the description of proteins as colloids with distinct attractive ion-activated surface patches.
Full-text · Article · Jun 2015 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: The paired helical filaments (PHF) formed by the intrinsically disordered human protein tau are one of the pathological hallmarks of Alzheimer disease. PHF are fibers of amyloid nature that are composed of a rigid core and an unstructured fuzzy coat. The mechanisms of fiber formation, in particular the role that hydration water might play, remain poorly understood. We combined protein deuteration, neutron scattering, and all-atom molecular dynamics simulations to study the dynamics of hydration water at the surface of fibers formed by the full-length human protein htau40. In comparison with monomeric tau, hydration water on the surface of tau fibers is more mobile, as evidenced by an increased fraction of translationally diffusing water molecules, a higher diffusion coefficient, and increased mean-squared displacements in neutron scattering experiments. Fibers formed by the hexapeptide 306VQIVYK311 were taken as a model for the tau fiber core and studied by molecular dynamics simulations, revealing that hydration water dynamics around the core domain is significantly reduced after fiber formation. Thus, an increase in water dynamics around the fuzzy coat is proposed to be at the origin of the experimentally observed increase in hydration water dynamics around the entire tau fiber. The observed increase in hydration water dynamics is suggested to promote fiber formation through entropic effects. Detection of the enhanced hydration water mobility around tau fibers is conjectured to potentially contribute to the early diagnosis of Alzheimer patients by diffusion MRI.
Full-text · Article · May 2015 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: It is well-known that the packing model (Fetters and co-workers) shows the relationship between microscopic chain properties (chain stiffness, monomer bulkiness) with macroscopic rheological behavior via the entanglement molecular weight (Me). However, what is less recognized is that the critical molecular weight (Mc) and reptative molecular weight (Mr) which denote transitions between unentangled (Rouse) and entangled (tube) dynamics and reptation-contour length fluctuations (CLF) to pure reptation dynamics respectively, also show dependence on the packing length (p). In this contribution, we highlight two aspects of this dependence:
1. Based on zero-shear viscosity data for six different polymers, from our own measurements and from literature, we show that there's a packing length dependence of the two crossovers (Mc, Mr) and that the trend is for the two to merge at a certain critical value of the packing length (p*). Further, analysis of the loss moduli (G'') for several well-entangled polymers with different packing lengths reveals a p-dependence of the high-frequency slope of the terminal peak contrary to the predicted value of -1/4 by tube models with CLF.
2. Based on preliminary measurements of the dynamic structure factor by neutron spin echo spectroscopy, we show that the onset for the effect of entanglements on the dynamics (i.e. deviation from Rouse behavior) starts in fact from Me and manifests by suppression of long- wavelength modes. From measurements on a series of polymers with different molecular weights (around Mc) and with different packing lengths, we examine how this deviation from Rouse behavior might proceed differently depending on the packing length of the polymer.
Both aspects hint at possible missing ingredients in the current modeling framework (tube model) for entangled polymer melts, particularly on the understated importance of the packing length (p).
[Show abstract][Hide abstract] ABSTRACT: It is well-known that the transition between unentangled to entangled dynamics occurs at a critical molecular weight (Mc), typically assumed to be twice the entanglement molecular weight (Me). Recently, we have reiterated a previous finding that the ratio between Mc and Me doesn't assume a universal value of 2 for all polymer melts but in fact shows a dependence on the packing length (p). Not withstanding, the physical picture behind the independent existence of Me and Mc remains unclear.
Here, we reinvestigate the problem by probing the microscopic dynamics of polybutadiene melts near the transition by neutron spin echo spectroscopy. We analyze the dynamic structure factor within the framework of the Rouse model with modified mode spectrum. We find that suppression of long-wavelength modes in the spectra already occurs for melts with molecular weight above Me, even if they are still below Mc. We rationalize these results based on earlier ideas on entanglement formation. We also apply this framework in confronting our viscosity data for various polymers.
[Show abstract][Hide abstract] ABSTRACT: Proteins in solution move subject to a complex superposition of global translational and rotational diffusion as well as internal relaxations covering a wide range of time scales. With the advent of new high-flux neutron spectrometers in combination with enhanced analysis frameworks it has become possible to separate these different contributions. We discuss new approaches to the analysis by presenting example spectra and fits from data recorded on the backscattering spectrometers IN16, IN16B, and BASIS on the same protein solution sample. We illustrate the separation of the rotational and translational diffusion contribution, the accurate treatment of the solvent contribution, and the extraction of information on internal fluctuations. We also exemplify the progress made in passing from second- to third-generation backscattering spectrometers.
Full-text · Article · Jan 2015 · The European Physical Journal Conferences
[Show abstract][Hide abstract] ABSTRACT: The dynamics of proteins in solution is a complex and hierarchical process, affected by the aqueous environment as well as temperature. We present a comprehensive study on nanosecond time and nanometer length scales below, at, and above the denaturation temperature $T_d$. Our experimental data evidence dynamical processes in protein solutions on three distinct time scales. We suggest a consistent physical picture of hierarchical protein dynamics: (i) Self-diffusion of the entire protein molecule is confirmed to agree with colloid theory for all temperatures where the protein is in its native conformational state. At higher temperatures $T>T_d$, the self-diffusion is strongly obstructed by cross-linking or entanglement. (ii) The amplitude of backbone fluctuations grows with increasing $T$, and a transition in its dynamics is observed above $T_d$. (iii) The number of mobile side-chains increases sharply at $T_d$ while their average dynamics exhibits only little variations. The combination of quasi-elastic neutron scattering and the presented analytical framework provides a detailed microscopic picture of the protein molecular dynamics in solution, thereby reflecting the changes of macroscopic properties such as cluster formation and gelation.
Full-text · Article · Jan 2015 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The mechanism of proton conductivity in high temperature polymer electrolyte fuel cells (HT-PEFCs) has been investigated with macroscopic conductivity measurements and on a microscopic scale with quasielastic neutron scattering techniques. Polybenzimidazole membranes, which are used in HT-PEFCs, are doped with phosphoric acid (PA) to achieve the desired proton conductivity. Neutron spin echo experiments showed that the polymer matrix is very rigid incoherent scattering experiments, but incoherent scattering revealed rather fast diffusion processes, compatible with macroscopic conductivity measurements. The measured diffusion is faster than anticipated from the conductivity of a phosphoric acid doped PBI membrane, but slower than that expected for pure phosphoric acid. Over larger distances the fractal polymer membrane network slows down the locally fast diffusion to the macroscopic values. With elastic fixed window scans on a backscattering spectrometer an activation energy of 7.6 kJ/mol is obtained at typical conditions in an HT-PEFC.
No preview · Article · Dec 2014 · International Journal of Hydrogen Energy
[Show abstract][Hide abstract] ABSTRACT: We investigate the possibility of using the prompt γ rays emitted by aluminum windows in order to monitor the neutron flux of the beam. A Nal scintillation detector is used to detect the prompt γ rays. No additional material apart from the unavoidable Al windows along the flight path is placed in the beam. The performance of the monitor is compared to that of a standard BF3-monitor placed in the beam. Influences of a magnetic field on the photomultiplier of the Nal monitor is discussed, as well as the influence of activation gammas. At an instrument using a beam chopper the time behaviour is discussed.
No preview · Article · Jul 2014 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The functional efficacy of colocalized, linked protein domains is dependent on linker flexibility and system compaction. However, the detailed characterization of these properties in aqueous solution presents an enduring challenge. Here, we employ a novel, to our knowledge, combination of complementary techniques, including small-angle neutron scattering, neutron spin-echo spectroscopy, and all-atom molecular dynamics and coarse-grained simulation, to identify and characterize in detail the structure and dynamics of a compact form of mercuric ion reductase (MerA), an enzyme central to bacterial mercury resistance. MerA possesses metallochaperone-like N-terminal domains (NmerA) tethered to its catalytic core domain by linkers. The NmerA domains are found to interact principally through electrostatic interactions with the core, leashed by the linkers so as to subdiffuse on the surface over an area close to the core C-terminal Hg(II)-binding cysteines. How this compact, dynamical arrangement may facilitate delivery of Hg(II) from NmerA to the core domain is discussed.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the thermal behavior, local structure, and dynamics in a system where 25 wt % PEO [poly(ethylene oxide)] linear chains are mixed with 75 wt % PMMA [poly(methyl methacrylate)] soft nanoparticles. Calorimetric and wide-angle X-ray scattering experiments point to a weak penetration of the PEO chains in the nanoparticles, qualifying the mixture as a nanocomposite. Quasi-elastic neutron scattering (QENS) experiments on partially deuterated samples has selectively revealed the component dynamics in the system. The α-methyl group dynamics of PMMA, which fall within the QENS time scale in the temperature range investigated, are hardly affected by the presence of PEO except for hints of a more heterogeneous environment in the nanocomposite than in bulk PMMA. The effects on the dynamics of PEO are more interesting. The observation of dynamics in the microseconds range for the PEO component of the nanocomposite at temperatures at which the calorimetric experiments indicate the freezing of its segmental relaxation provides evidence for confined dynamics below the main glass transition of the mixture—attributable to the effective glass transition of the slow component. A parallel study on an equivalent blend of PEO and linear PMMA chains shows that these effects are independent of the topology of the PMMA. However, well above the effective glass transition of the slow component, the dynamics of PEO differ in both systems. In the linear blend, PEO segments move with the typical features of supercooled polymers in metastable equilibrium, while in the nanocomposite PEO dynamics exhibit an anomalously strong deviation from Gaussian behavior. This deviation grows with increased mobility of the nanoparticles. PEO segments are seemingly trapped in effective cages imposed by the nanoparticles for a very long time—more than 2 orders of magnitude longer than in bulk or when surrounded by linear PMMA chains—before the subdiffusive process leading to segmental relaxation sets in. We speculate that local loops in the surface of the nanoparticles may play an important role in this trapping mechanism.
[Show abstract][Hide abstract] ABSTRACT: We have selectively studied the component dynamics in a nanocomposite where 25 wt % of PMMA [poly(methyl methacrylate)] soft nanoparticles (SNPs) are dispersed in PEO [poly(ethylene oxide)] by means of quasi-elastic neutron scattering (QENS) experiments on partially deuterated samples. We have covered a time range from subpico to nanosecond regime and a momentum transfer range 0.5 ≤ Q ≤ 1.8 Å–1 by combining three different spectrometers. Complementary diffraction measurements with polarization analysis have facilitated the data analysis, by providing the coherent and incoherent contributions to the scattered intensities. Regarding the SNPs, the α-methyl group dynamics of PMMA—to which the QENS experiments are most sensitive in the temperature range investigated—turn to be faster than in bulk PMMA. This could be due to the plasticization effect induced by the fast PEO chains. In fact, calorimetric measurements show the coexistence of two glass-transition temperatures in the system, associated with each of the components, but modified with respect to those in the neat materials. The QENS results on the PEO component for large length scales reveal Rouse-like dynamics slowed down by the presence of the SNPs with respect to that in the bulk. With decreasing temperature indications for distributed chain mobilities are found, probably due to the enhancement of the concentration fluctuations. At local scales, deviations from Rouse-like dynamics occur, that could be attributed to an extra-friction related to the local potentials, and also to non-Gaussian effects arising from the discrete character of the elementary processes underlying the subdiffusive dynamics in the polymer. The deviations take place in a very similar way as in bulk PEO.
[Show abstract][Hide abstract] ABSTRACT: The resolution of neutron backscattering spectrometers deteriorates at small scattering angles where analyzers deviate from exact backscattering. By reducing the azimuth angle range of the analyzers, the resolution can be improved with little loss of peak intensity. Measurements at the spectrometer SPHERES are in excellent agreement with simulations, which proves the dominance of geometric effects.
No preview · Article · Nov 2013 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: By means of quasielastic neutron scattering we have investigated the hydrogen dynamics in poly(alkylene oxide)s (PAOs) with different side-chain lengths at temperatures below as well as above the glass-transition. The combination of results from three different spectrometers (a time-of-flight and two backscattering instruments) has allowed covering almost 4 orders of magnitude in time—from the ps to ns range—with spatial resolution. The results evidence the simultaneous occurrence of vibrations and localized side-group motions at low temperatures and additional diffusive-like (segmental) dynamics at high temperatures. The localized processes of the side groups show (i) stretching of the scattering function, (ii) associated activation energies similar to those found for single and cooperative bond rotations of polyethylene, and (iii) spatial extents that increase with increasing temperature. Compared with poly(ethylene oxide) (PEO), the diffusive segmental process in PAOs presents (i) the same spectral shape, (ii) slower characteristic times—antiplasticization—(iii) similar deviations from Gaussian behavior. For comparison, we also report on backscattering results on the side-group dynamics of poly(n-hexyl methacrylate) in the same temperature range, that show evidence for confinement effects. We suggest that the dynamic asymmetry in systems with intrinsic dynamic heterogeneities between constituent parts is the key ingredient leading to both plasticization and confinement effects.
[Show abstract][Hide abstract] ABSTRACT: The diffusive motions of covalently tethered 1, 3 diphenylpropane (DPP) via a silyl-aryl-ether linkage in the mesopores of MCM-41 were studied by quasielastic neutron scattering. The geometric effect of pore radius was investigated with samples having pores that ranged from 1.6 to 3.0 nm in diameter and highest achievable DPP grafting density. The effect of molecular crowding was investigated in 3.0 nm diameter pores for surface coverage ranging from 0.60 to 1.61 DPP/nm2. Temperature dependence was determined for large pore diameter samples from 240 K to 370 K. As the DPP molecules remain attached over this entire temperature range, data were analyzed in terms of a model of localized diffusion inside a sphere. Only the motions of the DPP hydrogen atoms were considered because of the high sensitivity of neutron scattering to the presence of hydrogen. As atoms far from the attachment point have a greater range of motion than those nearer the tether, the radius of the sphere limiting the motion of individual hydrogen atoms was allowed to increase based on the atom s distance from the tether point. Both smaller pore diameters and higher DPP surface coverage resulted in larger amplitude motion while the diffusion coefficient was greatest in the largest pores at highest surface coverage. These observations support a model where the DPP molecules prefer an orientation allowing close proximity to the MCM-41 pore surface and are forced into the pore interior by either the steric effect of small pore diameter or by increased competition for surface area at high molecule surface coverage.
No preview · Article · Jan 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Transitions within the tunneling multiplet of CH(4) in phase II have been measured in an experiment at the backscattering instrument BASIS of the Neutron Source SNS. They all involve transitions from or to T-states. A statistical model is put forward which accounts for local departures from tetrahedral symmetry at the sites of ordered molecules. Different from previous work, in which discrete sets of overlap matrix elements have been studied, now large numbers of elements as well as the ensemble of T-states are considered. The observed neutron spectra can be explained rather well, all based on the pocket state formalism of A. Hüller [Phys. Rev. B 16, 1844 (1977)]. A completely new result is the observation and simulation of transitions between T-states, which give rise to a double peaked feature close to the elastic position and which reflect the disorder in the system. CH(2)D(2) molecules in the CH(4) matrix are largely responsible for the disorder and an interesting topic for their own sake. The simple model presented may lend itself to a broader application.
No preview · Article · Dec 2011 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The transition metal based oxide YBaCo3FeO7 is structurally related to the
mineral Swedenborgite SbNaBe4O7, a polar non-centrosymmetric crystal system.
The magnetic Co3Fe sublattice consists of a tetrahedral network containing
kagome-like layers with trigonal interlayer sites. This geometry causes
frustration effects for magnetic ordering, which were investigated by
magnetization measurements, M\"ossbauer spectroscopy, polarized neutron
diffraction, and neutron spectroscopy. Magnetization measurement and neutron
diffraction do not show long range ordering even at low temperature (1 K)
although a strong antiferromagnetic coupling (~2000 K) is deduced from the
magnetic susceptibility. Below 590 K, we observe two features, a spontaneous
weak anisotropic magnetization hysteresis along the polar crystallographic axis
and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the
interlayer sites remain idle. Below ~50 K, the onset of a hyperfine field shows
the development of moments static on the M\"ossbauer time scale also for the Fe
interlayer sites. Simultaneously, an increase of spin correlations is found by
polarized neutron diffraction. The relaxation part of the dynamic response has
been further investigated by high-resolution neutron spectroscopy, which
reveals that the spin correlations start to freeze in below ~50 K. Monte Carlo
simulations show that the neutron scattering results at lower temperatures are
compatible with a recent proposal that the particular geometric frustration in
the Swedenborgite structure promotes quasi one dimensional partial order.