A neutron spin echo study of low-temperature water confined in the spherical silica pores of SBA-16.

The dynamic properties of heavy water (D2O) and light water (H2O) confined in porous silica SBA-16 were studied over a temperature range of 210-290 K by neutron spin echo measurements. SBA-16 has predominant spherical pores (7.1 nm in pore size), channels interconnecting the spherical pores, and micropores (corona). The coherent intermediate scattering function on D2O filled SBA-16 showed the rotational dynamics of confined water without significant translational motion over the temperature range measured. This finding is due probably to collective entities of water due to cooperativity of hydrogen-bonds among water molecules in SBA-16 pores. The relaxation time of the collective entities followed the Vogel-Fulcher-Tammann relation at temperatures down to the freezing temperature of 235 K, suggesting a behavior of fragile water in the spherical pore. A comparison with previous NSE measurements of D2O in MCM-41 showed that the collective entities of water in the SBA-16 spherical pores have higher rotational mobility than those in the MCM-41 cylindrical pores. On the other hand, the incoherent intermediate scattering function on H2O filled SBA-16 revealed the translational motion of individual water molecules in the collective entities. It has been found that water in micropores is not frozen and is mobile down to 210 K from data of both D2O and H2O in SBA-16. Phase changes of various water confined in SBA-16 with decreasing and increasing temperatures are discussed based on the obtained dynamic properties.

Neutron spin echo measurements of monolayer and capillary condensed water in MCM-41 at low temperatures.

Neutron spin echo measurements of monolayer and capillary condensed heavy water (D(2)O) confined in MCM-41 C10 (pore diameter 2.10 nm) were performed in a temperature range of 190-298 K. The intermediate scattering functions were analyzed by the Kohlrausch-Williams-Watts stretched exponential function. The relaxation times of confined D(2)O in the capillary condensed state follow remarkably well the Vogel-Fulcher-Tammann equation between 298 and 220 K, whereas below 220 K they show an Arrhenius type behavior. That is, the fragile-to-strong (FTS) dynamic crossover occurs, which has never been seen in experiments on bulk water. On the other hand, for monolayer D(2)O, the FTS dynamic crossover was not observed in the temperature range measured. The FTS dynamic crossover observed in capillary condensed water would take place in the central region of the pore, not near the pore surface. Because the tetrahedral-like water structure in the central region of the pore is more preserved than that near the pore surface, the FTS dynamic crossover would be concerned with the tetrahedral-like water structure.

Structural studies of water in hydrophilic and hydrophobic mesoporous silicas: an x-ray and neutron diffraction study at 297 K.

Water confined in a sol-gel network has been characterized by x-ray and neutron diffraction for two samples of mesoporous silica: one with a hydrophilic character (a nonmodified one) and another with a hydrophobic character (a modified one with a methylated internal pore surface). The pore size has been previously characterized [J. Jelassi et al., Phys. Chem. Chem. Phys. 134, 1039 (2010)] to have a mean pore diameter of approximately 55 Å. The diffraction measurements presented in this paper have been made at room temperature [293 K] for a filling factor of 0.45, giving a mean thickness of 8-9 Å for the water layer. The results show that the local order of the confined water molecules in the intermediate region of 3-6 Å is significantly different from that of the bulk water and also for the two different environments. For the hydrophilic sample, the siloxyl groups at the surface modify the water structure through the effects of interfacial hydrogen-bonding, which influences the orientational configuration of local water molecules and creates a modified spatial arrangement in the pore. In the case of the hydrophobic sample, there is no specific interaction with the pore wall, which is primarily van der Waals type, and the water molecules at the interface are differently oriented to create a hydrogen-bonded network linked more directly to the rest of the water volume. In the present circumstances, the thickness of the water layer has a relatively small dimension so that the interpretation of the measured diffraction pattern is not as straightforward as for the bulk liquids, and it is necessary to consider the effects of diffraction-broadening from a distributed sample volume and also the contribution from cross-terms that remain after conducting a "wet-minus-dry" analysis procedure. These analytic difficulties are discussed in the context of the present measurements and compared with the work of other groups engaged in the study of water confined in different environments. The present results, again, emphasize the complexity influencing the properties of water in a confined geometry and the strong influence of surface interactions on its behavior.

Temperature- and hydration-dependent internal dynamics of stripped human erythrocyte vesicles studied by incoherent neutron scattering.

BACKGROUND: We focus on temperature- and hydration-dependence of internal molecular motions in stripped human red blood cell (RBC) vesicles, widely used as a model system for more complex biomembranes. METHODS: We singled out picosecond local motions of the non-exchangeable hydrogen atoms of RBC vesicles by performing elastic and quasielastic incoherent neutron scattering measurements in dry and heavy water (D2O)-hydrated RBC powders. RESULTS: In dry stripped RBCs, hydrogen motions remained harmonic all along the measured temperature range (100-310K) and mean-square displacements (MSDs) exhibited no temperature transition up to 310K. In contrast, MSDs of hydrated stripped RBCs (h ≈ 0.38g D2O/g dry powder) exhibited a pronounced transition near 260K, with the sharp rise of anharmonic diffusive motions of hydrogen atoms. This transition at ~260K was correlated with both the onset of nonvibrational (harmonic and nonharmonic) motions and the melting of crystallized hydration water. GENERAL SIGNIFICANCE: In conclusion, we have shown that MSDs in human RBC vesicles are temperature-and hydration-dependent. These results provide insight into biomembrane internal dynamics at picosecond timescale and nanometer length scale. Such motions have been shown to act as the "lubricant" of larger conformational changes on a slower, millisecond timescale that are necessary for important biological processes.

Temperature dependence on structure and dynamics of Bovine Pancreatic Trypsin Inhibitor (BPTI): a neutron scattering study.

We have studied the influence of temperature on the structure of BPTI in solution by small angle neutron scattering. We have investigated the variation of the radius of gyration and the modification of the shape of BPTI between ambient temperature and 368 K. Results have shown an increase of the radius of gyration from 10.9 A at ambient temperature up to 13.3 A at 368 K. Global and internal dynamics of BPTI in solution were studied by quasielastic neutron scattering. The analysis of neutron data in terms of intermediate scattering function reveals two relaxation times tau(1) and tau(2) related respectively to global translational diffusive motions and to internal motions of protein. Motions of protons belonging to lateral chains of residues located at the surface of the protein have been detected. The results are compared to the recently published results concerning the influence of pressure on structure and dynamics of BPTI in solution [Appavou MS et al. Biochimica et Biophysica Acta, 1764, 2006, pp 414-423].

Hydration water rotational motion as a source of configurational entropy driving protein dynamics. Crossovers at 150 and 220 K.

The existence of a protein dynamic transition around 220 K is widely known and the central role of the protein hydration shell is now largely recognized as the driving force for this transition. In this paper, we propose a mechanism, at the molecular level, for the contribution of hydration water. In particular, we identify the key importance of rotational motion of the hydration water as a source of configurational entropy triggering (i) the 220 K protein dynamic crossover (the so-called dynamic transition) but also (ii) a much less intense and scarcely reported protein dynamic crossover, associated to a calorimetric glass transition, at 150 K.

New sources and instrumentation for neutrons in biology.

Neutron radiation offers significant advantages for the study of biological molecular structure and dynamics. A broad and significant effort towards instrumental and methodological development to facilitate biology experiments at neutron sources worldwide is reviewed.

Picosecond dynamics of T and R forms of aspartate transcarbamylase: a neutron scattering study.

E. coli aspartate transcarbamylase (ATCase) is a 310 kDa allosteric enzyme which catalyses the first committed step in pyrimidine biosynthesis. The binding of its substrates, carbamylphosphate and aspartate, induces significant conformational changes. This enzyme shows homotropic cooperative interactions between the catalytic sites for the binding of aspartate. This property is explained by a quaternary structure transition from T state (aspartate low affinity) to R state (aspartate high affinity) accompanied by a 5% increase of radius of gyration of ATCase. The same quaternary structure change is observed upon binding of the bisubstrate analogue PALA (N-(phosphonacetyl)-L-aspartate. Owing to the large incoherent neutron scattering cross-section of the hydrogen atom and the abundance of this element in proteins, inelastic neutron scattering gives a global view of protein dynamics as sensed via the individual motions of its hydrogen atoms. We present neutron scattering results of the local dynamics (few angstroms), at short time (few tens of picoseconds), of ATCase in T and R forms. Compared to the T form, we observe an increased mobility of the protein in the R form that we associate to an increase of accessible surface area to the solvent. Beyond this specific result, this highlights the key role of the accessible surface area (ASA) in dynamic contribution to inelastic neutron data in the picosecond time scale. In particular, we want to stress out (i) that a difference at the picosecond time scale does not allow to conclude to a difference in the dynamics at a longer time scale and to address whether the T state is looser than the R state (ii) how challenging is, any comparison in terms of general dynamics (tense or relaxed) between dynamic values deduced from experimental neutron data on proteins with different sequences and therefore ASA. This caveat holds particularly when comparing dynamics of a mesophile with the corresponding extremophile.

Influence of pressure on structure and dynamics of bovine pancreatic trypsin inhibitor (BPTI): small angle and quasi-elastic neutron scattering studies.

We have studied the influence of pressure on structure and dynamics of a small protein belonging to the enzymatic catalysis: the bovine pancreatic trypsin inhibitor (BPTI). Using a copper-beryllium high-pressure cell, we have performed small angle neutron scattering (SANS) experiment on NEAT spectrometer at HMI (Berlin, Germany). In the SANS configuration, the evolution of the radius of gyration and of the shape of the protein under pressures up to 6,000 bar has been studied. When increasing pressure from atmospheric pressure up to 6,000 bar, the pressure effects on the global structure of BPTI result on a reduction of the radius of gyration from 13.4 A down to 12.0 A. Between 5,000 and 6,000 bar, some transition already detected by FTIR [N. Takeda, K. Nakano, M. Kato, Y. Taniguchi, Biospectroscopy, 4, 1998, pp. 209-216] is observed. The pressure effect is not reversible because the initial value of the radius of gyration is not recovered after pressure release. By extending the range of wave-vectors to high q, we have observed a change of the form factor (shape) of the BPTI under pressure. At atmospheric pressure BPTI exhibits an ellipsoidal form factor that is characteristic of the native state. When the pressure is increased from atmospheric pressure up to 6,000 bar, the protein keeps its ellipsoidal shape. The parameters of the ellipsoid vary and the transition detected between 5,000 and 6,000 bar in the form factor of BPTI is in agreement with the FTIR results. After pressure release, the form factor of BPTI is characteristic of an ellipsoid of revolution with a semi-axis a, slightly elongated with respect to that of the native one, indicating that the pressure-induced structural changes on the protein are not reversible. The global motions and the internal dynamics of BPTI protein have been investigated in the same pressure range by quasi-elastic neutron scattering experiments on IN5 time-of-flight spectrometer at ILL (Grenoble, France). The diffusion coefficients D and the internal relaxation times of BPTI deduced from the analysis of the intermediate scattering functions show a slowing down of protein dynamics when increasing pressure.

Dynamics from picoseconds to nanoseconds of trehalose in aqueous solutions as seen by quasielastic neutron scattering.

We present a study of the dynamical behavior of trehalose, a cryoprotecting agent, in concentrated aqueous solutions. Dynamics in a wide time range from picoseconds to nanoseconds has been observed using both neutron time of flight and neutron spin-echo techniques. Fast dynamics has been described using a simple diffusion model, while dynamical processes at longer times show a more complex behavior, described by a stretched exponential decay. Obtained relaxation times show a good agreement with data from viscosity measurements on aqueous trehalose solutions by Magazu et al. [Branca, Magazu, Maisano et al., J. Phys.: Condens. Matter 11, 3823 (1999)]. Experimental data provide us with some insight into the cryoprotecting properties and processes of trehalose. We conclude that an increase of the solvent viscosity in embedded biological material due to the production or the presence of trehalose might prevent biomolecules from damage.

Status of experiments probing the dynamics of water in confinement.

In many relevant situations, water is not in its bulk form but is instead attached to some substrates or filling some cavities. We shall call water in the latter environment "confined or interfacial water" as opposed to bulk water. This confined water is essential for the stability and function of biological macromolecules. In this review paper, we present the more recent up to date account of the dynamics of confined water as compared with that of bulk water. Various techniques are used to study the dynamics of confined water. Among them, quasi-elastic and inelastic neutron scattering is a powerful tool to study translational and rotational diffusion as well as vibrational density of states of confined water. Various examples involving water confined in porous media, adsorbed on surface of ionic crystals, in the presence of organic solutes and at the surface of biological molecules are presented. The combined effects of the hydration level and the temperature on the retardation of the water molecules motions are discussed on the basis of phenomenological models as well as of power law fits based on the Mode Coupling Theory.

Age-dependent dynamics of water in hydrated cement paste.

Self-dynamics of water molecules has been studied in hydrated tricalcium silicate as functions of temperature, aging, and in the presence of an additive. A dynamical model taking into account the existence of "immobile water" and "glassy water" has been used to analyze quasielastic neutron spectrometer spectra. We deduced the fraction of the immobile water (p), the stretch exponent (beta), and the average relaxation time (tau) of the glassy water. A quantitative picture for an aspect of the kinetics of the curing process and the structural relaxation parameters of the glassy water have been established.

Radially softening diffusive motions in a globular protein.

Molecular dynamics simulation, quasielastic neutron scattering and analytical theory are combined to characterize diffusive motions in a hydrated protein, C-phycocyanin. The simulation-derived scattering function is in approximate agreement with experiment and is decomposed to determine the essential contributions. It is found that the geometry of the atomic motions can be modeled as diffusion in spheres with a distribution of radii. The time dependence of the dynamics follows stretched exponential behavior, reflecting a distribution of relaxation times. The average side chain and backbone dynamics are quantified and compared. The dynamical parameters are shown to present a smooth variation with distance from the core of the protein. Moving outward from the center of the protein there is a progressive increase of the mean sphere size, accompanied by a narrowing and shifting to shorter times of the relaxation time distribution. This smooth, "radially softening" dynamics may have important consequences for protein function. It also raises the possibility that the dynamical or "glass" transition with temperature observed experimentally in proteins might be depth dependent, involving, as the temperature decreases, progressive freezing out of the anharmonic dynamics with increasing distance from the center of the protein.

Probing the binding sites of exchanged chlorophyll a in LH2 by Raman and site-selection fluorescence spectroscopies.

In this work we have selectively released the 800 nm absorbing bacteriochlorophyll a molecules of the LH2 protein from the photosynthetic bacterium Rhodopseudomonas acidophila, strain 10050, and replaced them with chlorophyll a (Chla). A combination of low-temperature electronic absorption, resonance Raman and site-selection fluorescence spectroscopies revealed that the Chla pigments are indeed bound in the B800 binding site; this is the first work that formally proves that such non-native chlorins can be inserted correctly into LH2.

Effect of high pressure on the photochemical reaction center from Rhodobacter sphaeroides R26.1.

High-pressure studies on the photochemical reaction center from the photosynthetic bacterium Rhodobacter sphaeroides, strain R26.1, shows that, up to 0.6 GPa, this carotenoid-less membrane protein does not loose its three-dimensional structure at room temperature. However, as evidenced by Fourier-transform preresonance Raman and electronic absorption spectra, between the atmospheric pressure and 0.2 GPa, the structure of the bacterial reaction center experiences a number of local reorganizations in the binding site of the primary electron donor. Above that value, the apparent compressibility of this membrane protein is inhomogeneous, being most noticeable in proximity to the bacteriopheophytin molecules. In this elevated pressure range, no more structural reorganization of the primary electron donor binding site can be observed. However, its electronic structure becomes dramatically perturbed, and the oscillator strength of its Q(y) electronic transition drops by nearly one order of magnitude. This effect is likely due to very small, pressure-induced changes in its dimeric structure.

Bacteriochlorin-protein interactions in native B800-B850, B800 deficient and B800-Bchla(p)-reconstituted complexes from Rhodopseudomonas acidophila, strain 10050.

Recently, a method which allows the selective release and removal of the 800 nm absorbing bacteriochlorophyll a (B800) molecules from the LH2 complex of Rhodopseudomonas acidophila strain 10050 has been described [Fraser, N.J. (1999) Ph.D. Thesis, University of Glasgow, UK]. This procedure also allows the reconstitution of empty binding sites with the native pigment Bchla(p), esterified with phytol. We have investigated the bacteriochlorophylla-protein interactions in native, B800 deficient (or B850) and in B8110-bacteriochlorophylla(p)-reconstituted LH2 complexes by resonance Raman spectroscopy. We present the first direct structural evidence which shows that the reconstituted pigments are correctly bound within their binding pockets.

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.

Structure of supercooled and glassy water under pressure.

We use molecular-dynamics simulations to study the effect of temperature and pressure on the local structure of liquid water in parallel with neutron-scattering experiments. We find, in agreement with experimental results, that the simulated liquid structure at high pressure is nearly independent of temperature, and remarkably similar to the known structure of the high-density amorphous ice. Further, at low pressure, the liquid structure appears to approach the experimentally measured structure of low-density amorphous ice as temperature decreases. These results are consistent with the postulated continuity between the liquid and glassy phases of H2O.

Measurement of coherent Debye-Waller factor in in vivo deuterated C-phycocyanin by inelastic neutron scattering.

Quasielastic neutron scattering measurements of dry and 35% D2O hydrated amorphous protein powder of C-phycocyanin were made as a function of temperature ranging from 313K down to 100K. The protein is grown from blue-green algae cultured in D2O and is deuterated up to 99%. The scattering is thus dominated by coherent scattering. Within the best energy resolution of the time-of-flight instrument, which is 28 mueV FWHM, the scattering appears entirely elastic. For this reason we are able to extract a coherent Debye-Waller factor by making an independent measurement of the static structure factor. We observe a considerable difference in the q dependence of the Debye-Waller factor between the dry and hydrated proteins; furthermore, there is an interesting temperature dependence of the Debye-Waller factor that is quite different from that predicted for dense hard-sphere liquids.