Jean-Marc Zanotti

Evolution of the internal dynamics of two globular proteins from dry powder to solution.

Myoglobin and lysozyme picosecond internal dynamics in solution is compared to that in hydrated powders by quasielastic incoherent neutron scattering. This technique is sensitive to the motions of the nonexchangeable hydrogen atoms in a sample. Because these are homogeneously distributed throughout the protein structure, the average dynamics of the protein is described. We first propose an original data treatment to deal with the protein global motions in the case of solution samples. The validity of this treatment is checked by comparison with classical measurements of the diffusion constants. The evolution with the scattering vector of the width and relative contribution of the quasielastic component was then used to derive information on the amount of local diffusive motions and their characteristic average relaxation time. From dry powder to coverage by one water layer, the surface side chains progressively acquire the possibility to diffuse locally. On subsequent hydration, the main effect of water is to improve the rate of these diffusive motions. Motions with higher average amplitude occur in solution, about three times more than for a hydrated powder at complete coverage, with a shorter average relaxation time, approximately 4.5 ps compared to 9.4 ps for one water monolayer.

Hydration-Coupled Dynamics in Proteins Studied by Neutron Scattering and NMR: The Case of the Typical EF-Hand Calcium-Binding Parvalbumin

The influence of hydration on the internal dynamics of a typical EF-hand calciprotein, parvalbumin, was investigated by incoherent quasi-elastic neutron scattering (IQNS) and solid-state 13C-NMR spectroscopy using the powdered protein at different hydration levels. Both approaches establish an increase in protein dynamics upon progressive hydration above a threshold that only corresponds to partial coverage of the protein surface by the water molecules. Selective motions are apparent by NMR in the 10-ns time scale at the level of the polar lysyl side chains (externally located), as well as of more internally located side chains (from Ala and Ile), whereas IQNS monitors diffusive motions of hydrogen atoms in the protein at time scales up to 20 ps. Hydration-induced dynamics at the level of the abundant lysyl residues mainly involve the ammonium extremity of the side chain, as shown by NMR. The combined results suggest that peripheral water-protein interactions influence the protein dynamics in a global manner. There is a progressive induction of mobility at increasing hydration from the periphery toward the protein interior. This study gives a microscopic view of the structural and dynamic events following the hydration of a globular protein.

Slow dynamics of water molecules on the surface of a globular protein

Water is essential for the stability and function of biological macromolecules. High-resolution quasi-elastic neutron scattering studies of the translational dynamics of water molecules on the surface of a deuteriated protein are presented. The quasi-elastic spectra from the interfacial H2O are analysed by a confined diffusion model to obtain the elastic incoherent structure factor (EISF), the short-time self-diffusion constant (D) and the residence time, τ0, as functions of coverage and temperature. The combined effects of the hydration level and temperature on the retardation of the single-particle motions are discussed in the light of available NMR relaxation data and of a well known model of α-relaxation from the theory of kinetic glass transitions in dense supercooled fluids. The vibrational density of states of interfacial water is presented as a function of temperature and for two levels of hydration of the protein.

Dynamic Transition Associated with the Thermal Denaturation of a Small Beta Protein

We studied the temperature dependence of the picosecond internal dynamics of an all-beta protein, neocarzinostatin, by incoherent quasielastic neutron scattering. Measurements were made between 20 degrees C and 71 degrees C in heavy water solution. At 20 degrees C, only 33% of the nonexchanged hydrogen atoms show detectable dynamics, a number very close to the fraction of protons involved in the side chains of random coil structures, therefore suggesting a rigid structure in which the only detectable diffusive movements are those involving the side chains of random coil structures. At 61.8 degrees C, although the protein structure is still native, slight dynamic changes are detected that could reflect enhanced backbone and beta-sheet side-chain motions at this higher temperature. Conversely, all internal dynamics parameters (amplitude of diffusive motions, fraction of immobile scatterers, mean-squared vibration amplitude) rapidly change during heat-induced unfolding, indicating a major loss of rigidity of the beta-sandwich structure. The number of protons with diffusive motion increases markedly, whereas the volume occupied by the diffusive motion of protons is reduced. At the half-transition temperature (T = 71 degrees C) most of backbone and beta-sheet side-chain hydrogen atoms are involved in picosecond dynamics.

Water diffusion in a synthetic hectorite by neutron scattering—beyond the isotropic translational model

A quasi-elastic neutron scattering study of water dynamics confined in a model clay system (a synthetic hectorite with Na+ compensating counterions) is presented. The neutron spin echo (NSE) data obtained at ambient temperature for the monohydrated and bihydrated hectorite powder are analysed with the help of models taking into account the anisotropy of the system. A powder-averaged two-dimensional (2D) diffusion model applied to the monohydrated state yields a 2D diffusion coefficient in the plane of the clay layers () of 2.8 × 10−10 m2 s−1, where an isotropic analysis of the data would have underestimated the diffusion coefficient by approximately 25%. In the case of the bihydrated state, the analysis by a model including a possible diffusion perpendicular to the clay layers () leads to a range of values for between 3.5 and 8.7 × 10−10 m2 s−1, depending on the ratio . We conclude that simple isotropic analysis can only give a rough estimate (order of magnitude) of the water dynamics in the medium.

Consequence of excess configurational entropy on fragility: the case of a polymer-oligomer blend.

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e., vibrational density of states, mean-square displacement, and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene-oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of the same Tg. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. Thus, the difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.

Dynamics of a globular protein as studied by neutron scattering and solid-state NMR

Abstract Effect of hydration on the dynamics of parvalbumin, a 11.5 kDa Ca2+/Mg2+-binding globular protein has been studied, at room temperature, by incoherent quasi-elastic neutron scattering and 13C solid-state NMR. Samples were protein powders hydrated at different hydration levels. The increase of the quasi-elastic signal observed in neutron scattering upon hydration is interpreted as an increase of the local mobility of charged side-chain protons (Asp, Glu, Lys) and is in agreement with a parallel study of parvalbumin by solid-state natural abundance 13C NMR under cross-polarization and magic angle spinning (CP MAS) conditions.

Competing coexisting phases in 2D water

The properties of bulk water come from a delicate balance of interactions on length scales encompassing several orders of magnitudes: i) the Hydrogen Bond (HBond) at the molecular scale and ii) the extension of this HBond network up to the macroscopic level. Here, we address the physics of water when the three dimensional extension of the HBond network is frustrated, so that the water molecules are forced to organize in only two dimensions. We account for the large scale fluctuating HBond network by an analytical mean-field percolation model. This approach provides a coherent interpretation of the different events experimentally (calorimetry, neutron, NMR, near and far infra-red spectroscopies) detected in interfacial water at 160, 220 and 250 K. Starting from an amorphous state of water at low temperature, these transitions are respectively interpreted as the onset of creation of transient low density patches of 4-HBonded molecules at 160 K, the percolation of these domains at 220 K and finally the total invasion of the surface by them at 250 K. The source of this surprising behaviour in 2D is the frustration of the natural bulk tetrahedral local geometry and the underlying very significant increase in entropy of the interfacial water molecules.

A unified approach to the dynamics of a polymer melt

Stretched exponentials are often used to describe quasi-elastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) relaxation data from polymer melts. In this paper, we attempt to derive a more physically meaningful model of the local (∼0.1 nm), short-time (∼10 ps) dynamics of linear polymers that takes into account (i) orientational diffusion along the polymer chain, (ii) local conformational transitions, and (iii) long-time, large-scale motions. The model takes into account the spatial component of the local dynamics, described in terms of the scattering vector Q. The model is applied to QENS results on highly entangled polyethylene oxide (PEO) melt at 373 K. We find the Q dependences of the three correlation times of the model to be consistent with Q°, Q -2 and Q -4 power laws, respectively. The high-Q limit of the model closely resembles the NMR-based DLM model (Dejean de la Batie et al 1988 Macromolecules 21 2045) but the physical interpretation is different. At 373 K, the polymer dynamics is described in terms of transverse motions of the chain segments over a distance of a few nm, with a local monomeric diffusion coefficient of 1.78 x 10 -9 m 2 s -1 . From this value, we derive a monomeric friction coefficient ξ 0 = 2.89 x 10 -12 N s m -1 that, used as numerical input to the Doi-Edwards theory, leads to a chain centre-of-mass diffusion coefficient D cm = 9.4 x 10 -15 m 2 s -1 . This value is in good agreement with pulsed field gradient NMR data (Appel and Fleischer 1993 Macromolecules 26 5520) and validates the proposed model.

Molecular dynamics of the self-organising strong hydrogen bonded 3,5-dimethylpyrazole

The family of pyrazoles containing only H and CH3 substituents displays a wide variation in physical properties which can be directly related to the manner in which the molecules self-organise in the solid state. Hydrogen-bonded multimeric motifs of the substituted pyrazoles are a recurring feature of this family. We have previously reported the use of quasielastic neutron scattering (QENS) to study 3,5-dimethylpyrazole which showed that the hydrogen-bonded amide protons within individual trimer units undergo a short range hopping motion between two equivalent sites straddling the direct N⋯H hydrogen-bond axis. This work was the first report of such a novel dynamic process in this family of materials. The current work extends the earlier study with additional QENS measurements of other isotopic variants, providing information on the methyl group dynamics, which lead us to the conclusion that the short-range amide motions are decoupled from the methyl torsions. Whilst the methyl groups were found to undergo 3-fold diffusive hopping motions on the QENS timescale (∼1011–1212 s−1), an appreciable non-mobile fraction of methyl groups was also detected at all temperatures studied.