We present a comparison between theoretical predictions of the generalized Langevin equation for cooperative dynamics (CDGLE) and neutron spin echo data of dynamic structure factors for polyethylene melts. Experiments cover an extended range of length and time scales, providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights undergoing dynamics across the unentangled to entangled transition. Measured center-of-mass (com) mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The generalized Langevin equation for cooperative dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, of its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments.

Intracellular water dynamics in Haloarcula marismortui, an extremely halophilic organism originally isolated from the Dead Sea, was studied by neutron scattering. The water in centrifuged cell pellets was examined by means of two spectrometers, IN6 and IN16, sensitive to motions with time scales of 10 ps and 1 ns, respectively. From IN6 data, a translational diffusion constant of 1.3 ؋ 10 ؊5 cm 2 s ؊1 was determined at 285 K. This value is close to that found previously for other cells and close to that for bulk water, as well as that of the water in the 3.5 M NaCl solution bathing the cells. A very slow water component was discovered from the IN16 data. At 285 K the waterprotons of this component displays a residence time of 411 ps (compared with a few ps in bulk water). At 300 K, the residence time dropped to 243 ps and was associated with a translational diffusion of 9.3 ؋ 10 ؊8 cm 2 s ؊1 , or 250 times lower than that of bulk water. This slow water accounts for Ϸ76% of cell water in H. marismortui. No such water was found in Escherichia coli measured on BSS, a neutron spectrometer with properties similar to those of IN16. It is hypothesized that the slow mobility of a large part of H. marismortui cell water indicates a specific water structure responsible for the large amounts of K ؉ bound within these extremophile cells.Haloarcula marismortui ͉ water structure and dynamics ͉ extreme haplophile T he study of the specific properties of water in biological systems continues to yield fascinating surprises. Haloarcula marismortui, an archaeal extreme halophile isolated from the Dead Sea, attracted our attention some years ago because of its high selectivity for, despite a high membrane permeability (1). In distinction to other organisms, K ϩ was retained within the cell, even in the absence of metabolism, with a half-time of exchange with the outside medium of Ͼ24 h (2). At the time, the only systems known with a very high binding selectivity were antibiotics such as nonactin and valinomycin. The structure responsible for their specificity is a ''cage'' of six to eight carbonyl oxygen atoms arranged in space according to a definite pattern. If the same principle were to be responsible for K ϩ binding in H. marismortui, 24-32 moles oxygen per liter of cell water would be required, enormous amounts of oxygen atoms that cannot be supplied exclusively by the organic components of the cell. It is therefore suggested that the oxygen atoms required might be furnished by the ordering of water molecules forming a tertiary system of water, KCl, and cell proteins.Evidence in favor of more than a single phase of water in cell pellets of H. marismortui came initially from H-NMR measurements (3). The visible intensity of water in such pellets accounted for all of the total water present (101 Ϯ 7%). When the pellets were slowly cooled to below Ϫ19°C (the temperature at which the culture medium froze), 40% of the water signal remained visible. From analysis of the dynamic properties of the intracellular water, it was concluded ...

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 306 VQIVYK 311 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.hydration water | tau protein | amyloid fibers | intrinsically disordered proteins | neutron scattering A myloid fibers are the most stable forms of ordered protein aggregates. They have attracted much attention because of their implication in so-called conformational diseases, which include a variety of neurodegenerative disorders (1). Consequently, means of hindering or reversing fiber formation are actively researched (2). Pathological fibers are often formed by intrinsically disordered proteins (IDPs) that lack a well-defined 3D structure in their native state and are best described by an ensemble of different conformations (3). The human protein tau is an IDP that normally regulates microtubule stability in neurons. When tau aggregates, it forms paired helical filaments (PHF) that are one of the two histological hallmarks of Alzheimer disease (AD) (4, 5). As yet, and despite considerable effort over the past 30 y, the understanding of tau fibrillation in AD and other taupathies remains largely incomplete (6). The longest human tau isoform, htau40, is composed of 441 amino acid residues and is organized into several domains (see Fig. 1), including the repeat domains R1−R4 (residues 244-369) that constitute, together with the P1 and P2 domains, the microtubule binding regions (7). Essential for the nucleation of tau fibers is the presence of hexapeptides ( 275 VQIINK 280 and 306 VQIVYK 311 ) in R2 and R3 (8) that have a high propensity t...

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 Td. 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 > Td, 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 Td. (iii) The number of mobile side-chains increases sharply at Td 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.

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.

The dynamics of binary polymer blends of few labeled long chains in successively shorter matrix chains has been investigated by neutron spin echo (NSE) spectroscopy. For the first time the effect of constraint release on the chain relaxation has been directly observed on a microscopic scale. Decreasing the matrix chain length reduces the topological confinement until unconfined Rouse motion is observed, when the matrix chains are too short to confine the long chain in a tube. Whereas an analytical description of the effect is not yet available, a new simulation based on the slip-link model shows perfect agreement with the NSE data over the full range of matrix molecular weights. DOI: 10.1103/PhysRevLett.96.238302 PACS numbers: 83.80.Sg, 61.12.Ex The dynamics of linear polymer chains in the melt strongly depends on the chain length: For short, unentangled chains (and for any length at short times) the dynamics is determined by a balance of viscous and entropic forces which can be described by the Rouse model [1]. Here the chains interact solely by local friction with a heat bath, representing the neighboring chains. For long chains topological chain-chain interactions in terms of entanglements become important and are dominating the dynamical behavior. In the famous reptation model these constraints are described by a virtual tube which localizes a given chain and limits its motion to a one-dimensional Rouse motion inside the tube (local reptation) and a slow diffusive creep motion out of the tube (reptation) [2]. Neutron spin echo (NSE) spectroscopy is a powerful tool to explore the different dynamic regimes in polymer melts on a microscopic scale in space and time. While NSE experiments support the tube concept of topological confinement in long chain polymer systems [3,4], a close comparison of linear rheology data with predictions of the tube model indicates the existence of additional processes that release the topological confinement [5], such as fluctuating chain ends which escape the tube confinement (contour length fluctuations, CLF) and the relaxation of the tube itself (constraint release, CR). While CLF is an effect of the confined chain itself [6], CR stems from the movement of the chains building the tube, which of course undergo the same dynamical processes as the confined chain (see Fig. 1).In this Letter we present a neutron spin echo study on binary polymer blends in order to investigate the influence of constraint release on the tube confinement by a systematic variation of the matrix molecular weight. This study demonstrates on a molecular level the increasing loss of confinement with the reduction of the host molecular weight and proves the role of CLF in the constraint release process. With a new slip-links model [7] we simulate the dynamics of polymer melts under consideration of CLF and CR and obtain a good description of the NSE data.The different dynamic processes are reflected in the relaxation of the coherent single chain dynamic structure factor Sq; t. This dynamic structure facto...

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.

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