Extensive and high-quality quasi-elastic incoherent neutron scattering data were obtained for water in the temperature range extending from room temperature down to-20 C in the supercooled state. The analysis generally confirms findings of our previous experiment [S. H. Chen, J. Teixeira, and R. Nicklow, Phys. Rev. A 26, 3477 (1982)], but in particular three new results have been obtained: (a) two relaxation times are clearly identified, which are related to the short-time and intermediate-time diffusion of water molecules, respectively, and their temperature dependence has been determined; (b) one of these relaxation times is associated with jump diffusion of the protons, and the temperature dependence of the jump length has been qualitatively determined; (c) the Q dependence of the scattering intensity integrated over the quasi-elastic region gives a Debye-%'aller factor which is temperature independent.

Water is an essential participant in the stability, structure, dynamics, and function of proteins and other biomolecules. Thermodynamically, changes in the aqueous environment affect the stability of biomolecules. Structurally, water participates chemically in the catalytic function of proteins and nucleic acids and physically in the collapse of the protein chain during folding through hydrophobic collapse and mediates binding through the hydrogen bond in complex formation. Water is a partner that slaves the dynamics of proteins, and water interaction with proteins affect their dynamics. Here we provide a review of the experimental and computational advances over the past decade in understanding the role of water in the dynamics, structure, and function of proteins. We focus on the combination of X-ray and neutron crystallography, NMR, terahertz spectroscopy, mass spectroscopy, thermodynamics, and computer simulations to reveal how water assist proteins in their function. The recent advances in computer simulations and the enhanced sensitivity of experimental tools promise major advances in the understanding of protein dynamics, and water surely will be a protagonist.

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At ambient pressure, bulk liquid water shows an anomalous increase of thermodynamic quantities and apparent divergences of dynamic properties on approaching a temperature Ts of 228 K. At normal pressure, supercooled water spontaneously freezes below the homogeneous nucleation temperature, TH = 235 K. Upon heating, the two forms of Amorphous Solid Water (ASW), LDA (Low Density Amorphous Ice) and HDA (High Density Amorphous Ice), crystallise above TX = 150 K. As a consequence, up to now no experiment has been able to explore the properties of liquid water in this very interesting temperature range between 150 and 235 K. We present nanosecond-time-scale measurements of local rotational and translational dynamics of interfacial, non-crystalline, water from 77 to 280 K. These experimental dynamic results are combined with calorimetric and diffraction data to show that after exhibiting a glass transition at 165 K, interfacial water experiences a first-order liquid-liquid transition at 240 K from a low-density to a high-density liquid. This is the first direct evidence of the existence of a liquid-liquid transition involving water.

Quasielastic neutron scattering (QENS) spectra of water-filled MCM-41 samples (pore diameters: 21.4 and 28.4 Angstrom) were measured over the temperature range 238-298 K and the momentum transfer range 0.31-0.99 A(-1) to investigate the dynamics of confined water molecules. The spectra, which consist mainly of contributions from the translational diffusion of water molecules, were analyzed by using the Lorentzian and the stretched exponential functions. Comparison of the fits indicated that the latter analysis is more reliable than the former one. The fraction of immobile water molecules located in the vicinity of the pore walls, which give an elastic component, was found to be 0.044-0.061 in both pores. The stretch exponent beta was determined as 0.66-0.80. It was shown that the translational diffusion of water molecules in the pores is decelerated by confinement and that the deceleration becomes marked with a decrease in pore size. The ratios of the translational diffusion coefficient D(T) of confined water to that of bulk water at room temperature were within a range of 0.47-0.63.

A coherent-inelastic-neutron-scattering experiment was performed on liquid D20 at room temperature. We observe for the first time the collective high-frequency sound mode as predicted by computer molecular-dynamics simulations. We interpret the excitation to be a mode propagating within the hydrogen-bonded patches existing in liquid water, and distinct from the ordinary sound wave.PACS numbers: 61.12.Fy, 61.25.Em Short-wavelength collective excitations in dense atomic liquids are in general difficult to observe by coherent inelastic neutron scattering. We may define the evidence of the collective excitations by the existence of a peak or a shoulder in the dynamic structure factor S(Q, co) on each side of the central line, in the Q range of, say, 0& Q & 1.2 A '. The neutronscattering experiments performed so far indicate that, for neon at a liquid density 1.2 g cm 3 and T= 26.5 K, the excitation exists up to Qo-~1. 0, and for liquid rubidium and lead near their melting points up to Qo.~4. 0, 2 where a-is the effective hard-core diameter of the atoms. From the computer moleculardynamics simulations of hard spheres by Alley and coworkers, 4 it was established that the collective excitation exists up to Qo.~0.5. It was generally believed that the damping of the short-wavelength excitations depends critically on both the steepness of the repulsive part and the depth of the attractive part of the potential. For liquid metals, the repulsive part is generally softer and the attractive well deeper. This may be the reason why the excitation is observable up to higher Q. More recently, the existence of a collective mode in simple fluids has been discussed explicitly in terms of the kinetic theory of dense fluids. s These authors claimed that collective excitations can be defined in dense fluids up to Qo-~30 and that the behavior of S(Q, co) is dominated by these modes for all Q in which they exist. 5 In this paper we report for the first time an observation of new collective modes in a molecular fluid: water. From a point of view of the intermolecular interaction, water is a rather special case. Take, for instance, the ST2 model potential which has been shown to be successful in predicting various properties of water. 6 This potential consists of superposition of a Lennard-Jones potential between the molecular centers and directional electrostatic interactions which mimic the hydrogen bonding. A new feature of this potential, as compared with that of the simpler liquids, is the presence of a directional and strong attraction between molecules. Our experimental results lead us to conjecture the existence of a new collective mode which we shall call schematically the "high-frequency mode, " which is different from the ordinary sound wave in water. This kind of mode was already predicted in an earlier computer molecular-dynamics simulation (CMD) of Rahman and Stillinger and more recently also by Impey, Madden, and McDonald. We shall, in the following, compare our experimental results with these CMD predictions.The experiment...