Absence of anomalous dispersion features in the inelastic neutron scattering spectra of water at both sides of the melting transition

The glassy state of crambin and the THz time scale protein-solvent fluctuations possibly related to protein function

BACKGROUND

THz experiments have been used to characterize the picosecond time scale fluctuations taking place in the model, globular protein crambin.

RESULTS

Using both hydration and temperature as an experimental parameter, we have identified collective fluctuations (<= 200 cm(-1)) in the protein. Observation of the protein dynamics in the THz spectrum from both below and above the glass transition temperature (Tg) has provided unique insight into the microscopic interactions and modes that permit the solvent to effectively couple to the protein thermal fluctuations.

CONCLUSIONS

Our findings suggest that the solvent dynamics on the picosecond time scale not only contribute to protein flexibility but may also delineate the types of fluctuations that are able to form within the protein structure.

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line TW. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

Anisotropy on the collective dynamics of water confined in swelling clay minerals

Collective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si(7.3)Al(0.7)Mg(6)O(20)(OH)(4)Na(0.7) in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental results.

Intermolecular vibrational study in liquid water and ice by using far infrared spectroscopy with synchrotron radiation of MIRRORCLE 20

Far infrared absorption measurements for distilled water and ice Ih were performed in the frequency range from 20 cm(-1) to 1000 cm(-1) with Fourier Transform Infrared Spectrometer (FTIR) utilizing SR of a portable synchrotron. Four vibrational bands were separated from measured spectra in liquid water. We found that a peak frequency of 40±1 cm(-1) did not depend on the temperature in a range between 10.0°C and 70.0°C; however, any low energy excitation modes were not observed in the ice spectrum. It is concluded that this band is caused by collective vibrations specific to the dynamical structure in liquid water.

High-frequency dynamics of liquid and supercritical water

The dynamic structure factor S(Q,omega) of water has been determined by high-resolution inelastic x-ray scattering (IXS) in a momentum (Q) and energy (E) transfer range extending from 2 to 4 nm{-1} and from +/-40meV . IXS spectra have been recorded along an isobaric path (400bar) in a temperature (T) interval ranging from ambient up to supercritical (T>647K) conditions. The experimental data have been described in the frame of the generalized hydrodynamic theory, utilizing a model based on the memory function approach. This model allows identifying the active relaxation processes which affect the time decay of density fluctuations, as well as a direct determination of the Q , T , and density (rho) dependencies of the involved transport parameters. The experimental spectra are well described by considering three different relaxation processes: the thermal, the structural, and the instantaneous one. On approaching supercritical conditions, we observe that the microscopic mechanism responsible for the structural relaxation is no longer related to the making and breaking of intermolecular bonds, but to binary intermolecular collisions.

Collective acoustic modes as renormalized damped oscillators: unified description of neutron and x-ray scattering data from classical fluids

In the Q range where inelastic x-ray and neutron scattering are applied to the study of acoustic collective excitations in fluids, various models of the dynamic structure factor S(Q, omega) generalize in different ways the results obtained from linearized-hydrodynamics theory in the Q-->0 limit. Here we show that the models most commonly fitted to experimental S(Q, omega) spectra can be given a unified formulation. In this way, direct comparisons among the results obtained by fitting different models become now possible to a much larger extent than ever. We also show that a consistent determination of the dispersion curve and of the propagation Q range of the excitations is possible, whichever model is used. We derive an exact formula which describes in all cases the dispersion curve and allows for the first quantitative understanding of its shape, by assigning specific and distinct roles to the various structural, thermal, and damping effects that determine the Q dependence of the mode frequencies. The emerging picture describes the acoustic modes as Q-dependent harmonic oscillators whose characteristic frequency is explicitly renormalized in an exact way by the relaxation processes, which also determine, through the widths of both the inelastic and the elastic lines, the whole shape of collective-excitation spectra.

High-frequency longitudinal and transverse dynamics in water

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S (Q,omega) of liquid water have been performed for wave vectors Q between 4 and 30 nm(-1) in distinctly different thermodynamic conditions ( T=263-420 K ; at, or close to, ambient pressure and at P=2 kbar ). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with Q and the other almost nondispersive) is confirmed. The study of their temperature and Q dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the Q -dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes a complete transition from the adiabatic sound velocity ( c(0) ) (viscous limit) to the infinite-frequency sound velocity ( c(infinity) ) (elastic limit). On decreasing Q , as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in S (Q,omega) as soon as the purely viscous regime is left.

Brillouin neutron scattering in heavy water: evidence for two-mode collective dynamics

A high resolution (1.5 meV) inelastic neutron scattering experiment was carried out, aiming at an accurate investigation of the high frequency and low momentum dynamic response in heavy water. The experimental data confirm the existence of a dispersionless mode, besides the ordinary longitudinal collective dynamics. A simplified model, based on the interaction of two vibrational branches, is proposed to interpret the observed features of the dynamic spectra. The validity extent of this scheme is proved by applying it to room temperature neutron and x-ray data, to temperature and pressure dependent x-ray data, and to room temperature neutron data of vibrational density of states. The overall successfull results provided by this model, in conjunction with the combined analysis of the x-ray and neutron data on collective dynamics, enable a deeper insight into the complex mechanisms of the water dynamics and provide a simple phenomenological explanation for the transition from ordinary to fast sound.

High-resolution neutron scattering measurement of the dynamic structure factor of heavy water

The low momentum collective dynamics of heavy water has been investigated by means of neutron three-axis spectroscopy. Working at the resolution limit of this instrument, an energy resolution of 2.6 meV was achieved for a constant analyzer energy of 120 meV. This good resolution allowed us to establish definitely that the collective dynamics of water is dominated by the presence and interaction of opticlike and acousticlike branches, coupling at wave-vector transfers of 0.3 to 0.35 A(-1). The transition from slow to fast sound has been attributed to the interaction between opticlike modes and a solidlike fast acoustic mode.

Nature of self-diffusion and viscosity in supercooled liquid water

The nature of the simplest transport processes in water, namely, self-diffusion and shear viscosity, is analyzed on the basis of a version of the microinhomogeneous structure model. The study predicts the existence of locally ordered groups of molecules, taking into account considerations of acoustic properties, light scattering, and computer simulation findings. In particular, it is shown that the anomalous properties of water in supercooled states are mainly connected with the existence of quasiordered regions, which we call clusters. Furthermore, the spatial sizes and evolution times of the crystal-like clusters, as well as the temperature dependence of their fraction volume, are established. Special invariants of the characteristic parameters of molecular motion are pointed out. Finally, it is shown that the self-diffusion in supercooled water is caused by the processes of formation and destruction of crystal-like clusters, while the processes of internal partial reconstruction give the main contribution to the shear viscosity coefficient.

Analysis of inelastic x-ray scattering spectra of low-temperature water

We analyze a set of high-resolution inelastic x-ray scattering (IXS) spectra from H2O measured at T=259, 273, and 294 K using two different phenomenological models. Model I, called the "dynamic cage model," combines the short time in-cage dynamics described by a generalized Enskog kinetic theory with a long-time cage relaxation dynamics described by an alpha relaxation. This model is appropriate for supercooled water where the cage effect is dominant and the existence of an alpha relaxation is evident from molecular-dynamics (MD) simulation data of extended simple point charge (SPC/E) model water. Model II is essentially a generalized hydrodynamic theory called the "three effective eigenmode theory" by de Schepper et al. 11. This model is appropriate for normal liquid water where the cage effect is less prominent and there is no evidence of the alpha relaxation from the MD data. We use the model I to analyze IXS data at T=259 K (supercooled water). We successfully extract the Debye-Waller factor, the cage relaxation time from the long-time dynamics, and the dispersion relation of high-frequency sound from the short time dynamics. We then use the model II to analyze IXS data at all three temperatures, from which we are able to extract the relaxation rate of the central mode and the damping of the sound mode as well as the dispersion relation for the high-frequency sound. It turns out that the dispersion relations extracted from the two models at their respective temperatures agree with each other giving the high-frequency sound speed of 2900+/-300 m/s. This is to be compared with a slightly higher value reported previously, 3200+/-320 m/s, by analyzing similar IXS data with a phenomenological-damped harmonic oscillator model 22. This latter model has traditionally been used exclusively for the analysis of inelastic scattering spectra of water. The k-dependent sound damping and central mode relaxation rate extracted from our model analyses are compared with the known values in the hydrodynamic limit.