Fingerprints of amorphous icelike behavior in the vibrational density of states of protein hydration water

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies

Molecular dynamics (MD) simulations are a powerful tool for detailed studies of altered properties of liquids in confinement, in particular, of changed structures and dynamics. They allow, on one hand, for perfect control and systematic variation of the geometries and interactions inherent in confinement situations and, on the other hand, for type-selective and position-resolved analyses of a huge variety of structural and dynamical parameters. Here, we review MD simulation studies on various types of liquids and confinements. The main focus is confined aqueous systems, but also ionic liquids and polymer and silica melts are discussed. Results for confinements featuring different interactions, sizes, shapes, and rigidity will be presented. Special attention will be given to situations in which the confined liquid and the confining matrix consist of the same type of particles and, hence, disparate liquid-matrix interactions are absent. Findings for the magnitude and the range of wall effects on molecular positions and orientations and on molecular dynamics, including vibrational motion and structural relaxation, are reviewed. Moreover, their dependence on the parameters of the confinement and their relevance to theoretical approaches to the glass transition are addressed.

Hydrogen-bond network distortion of water in the soft confinement of Nafion membrane

A Compton spectroscopy investigation is carried out in hydrated Nafion membranes, enabling identification of distortions in the hydrogen-bond distribution of the polymer hydrating water by means of the subtle changes reflected by the Compton profiles. Indeed, deformations of the Compton profiles are observed when varying hydration, and two different bonding kinds are associated with the water molecules: at low hydration, water surrounds the sulfonic groups, while on increasing hydration, water molecules occupy the interstitial cavities formed upon swelling of the membrane. The analysis is proposed in terms of averaged OH bond length variation. A sizable contraction of the OH distance is observed at low hydration (∼0.09 Å), while at higher hydration levels, the contraction is smaller (∼0.02 Å) and the OH bond length is closer to bulk water. An evaluation of the electron kinetic energy indicates that the spatial changes associated with the water distribution correspond to a consistent binding energy increase. Distinct temperature dependences of each water population are observed, which can be straightly related to water desorption into ice on cooling below the freezing point.

On the existence of soliton-like collective modes in liquid water at the viscoelastic crossover

The problem of large-density variations in supercooled and ambient water has been widely discussed in the past years. Recent studies have indicated the possibility of nanometer-sized density variations on the subpicosecond and picosecond time scales. The nature of fluctuating density heterogeneities remains a highly debated issue. In the present work, we address the problem of possible association of such density variations with the dynamics of terahertz longitudinal acoustic-like modes in liquid water. Our study is based on the fact that the subpicosecond dynamics of liquid water are essentially governed by the structural relaxation. Using a mode coupling theory approach, we found that for typical values of parameters of liquid water, the dynamic mechanism coming from the combination of the structural relaxation process and the finiteness of the amplitude of terahertz longitudinal acoustic-like mode gives rise to a soliton-like collective mode on a temperature-dependent nanometer length scale. The characteristics of this mode are consistent with the estimates of the amplitudes and temperature-dependent correlation lengths of density fluctuations in liquid water obtained in experiments and simulations. Thus, the fully dynamic mechanism could contribute to the formation and dynamics of fluctuating density heterogeneities. The soliton-like collective excitations suggested by our analysis may be relevant to different phenomena connected with supercooled water and can be expected to be associated with some ultrafast biological processes.

Diffusivelike Motions in a Solvent-Free Protein-Polymer Hybrid

The interaction between proteins and hydration water stabilizes protein structure and promotes functional dynamics, with water translational motions enabling protein flexibility. Engineered solvent-free protein-polymer hybrids have been shown to preserve protein structure, function, and dynamics. Here, we used neutron scattering, protein and polymer perdeuteration, and molecular dynamics simulations to explore how a polymer dynamically replaces water. Even though relaxation rates and vibrational properties are strongly modified in polymer coated compared to hydrated proteins, liquidlike polymer dynamics appear to plasticize the conjugated protein in a qualitatively similar way as do hydration-water translational motions.

Role of hydration water in the onset of protein structural dynamics

Proteins are the molecular workhorses in a living organism. Their 3D structures are animated by a multitude of equilibrium fluctuations and specific out-of-equilibrium motions that are required for proteins to be biologically active. When studied as a function of temperature, functionally relevant dynamics are observed at and above the so-called protein dynamical transition (~240 K) in hydrated, but not in dry proteins. In this review we present and discuss the main experimental and computational results that provided evidence for the dynamical transition, with a focus on the role of hydration water dynamics in sustaining functional protein dynamics. The coupling and mutual influence of hydration water dynamics and protein dynamics are discussed and the hypotheses illustrated that have been put forward to explain the physical origin of their onsets.

Complex Dynamics of Water in Protein Confinement

This paper studies single-molecule and collective dynamics of water confined in protein powders by means of molecular dynamics simulations. The single-particle dynamics show a modest retardation compared to the bulk but become highly stretched in the powder, with the stretching exponent of ≃0.2. The collective dynamics of the total water dipole are affected by intermolecular correlations inside water and by cross-correlations between the water and the protein. The dielectric spectrum of water in the powder has two nearly equal-amplitude peaks: a Debye peak with ≃16 ps relaxation time and a highly stretched peak with the relaxation time of ≃13 ns and a stretching exponent of ≃0.12. The slower relaxation component is not seen in the single-molecule correlation functions and can be assigned to elastic protein motions displacing water in the powder. The loss spectrum of the intermediate scattering function reported by neutron-scattering experiments is also highly stretched, with the high-frequency wing scaling according to a power law. Translational dynamics can become much slower in the powder than in the bulk but are overshadowed by the rotational loss in the overall loss spectrum of neutron scattering.

Observation of dielectric universalities in albumin, cytochrome C and Shewanella oneidensis MR-1 extracellular matrix

The electrodynamics of metals is well understood within the Drude conductivity model; properties of insulators and semiconductors are governed by a gap in the electronic states. But there is a great variety of disordered materials that do not fall in these categories and still respond to external field in an amazingly uniform manner. At radiofrequencies delocalized charges yield a frequency-independent conductivity σ 1(ν) whose magnitude exponentially decreases while cooling. With increasing frequency, dispersionless conductivity starts to reveal a power-law dependence σ 1(ν)∝ν s with s < 1 caused by hopping charge carriers. At low temperatures, such Universal Dielectric Response can cross over to another universal regime with nearly constant loss ε″∝σ1/ν = const. The powerful research potential based on such universalities is widely used in condensed matter physics. Here we study the broad-band (1-1012 Hz) dielectric response of Shewanella oneidensis MR-1 extracellular matrix, cytochrome C and serum albumin. Applying concepts of condensed matter physics, we identify transport mechanisms and a number of energy, time, frequency, spatial and temperature scales in these biological objects, which can provide us with deeper insight into the protein dynamics.

Ergodicity breaking of iron displacement in heme proteins

We present a model of the dynamical transition of atomic displacements in proteins. Increased mean-square displacement at higher temperatures is caused by the softening of the force constant for atomic/molecular displacements by electrostatic and van der Waals forces from the protein-water thermal bath. Displacement softening passes through a nonergodic dynamical transition when the relaxation time of the force-force correlation function enters, with increasing temperature, the instrumental observation window. Two crossover temperatures are identified. The lower crossover, presently connected to the glass transition, is related to the dynamical unfreezing of rotations of water molecules within nanodomains polarized by charged surface residues of the protein. The higher crossover temperature, usually assigned to the dynamical transition, marks the onset of water translations. All crossovers are ergodicity breaking transitions depending on the corresponding observation windows. Allowing stretched exponential relaxation of the protein-water thermal bath significantly improves the theory-experiment agreement when applied to solid protein samples studied by Mössbauer spectroscopy.

Hydrogen Bond Network of Water around Protein Investigated with Terahertz and Infrared Spectroscopy

The dynamical and structural properties of water at protein interfaces were characterized on the basis of the broadband complex dielectric constant (0.25 to 400 THz) of albumin aqueous solutions. Our analysis of the dielectric responses between 0.25 and 12 THz first revealed hydration water with retarded reorientational dynamics extending ∼8.5 Å (corresponding to three to four layers) out from the albumin surface. Second, the number of nonhydrogen-bonded water was decreased in the presence of the albumin solute, indicating protein inhibits the fragmentation of the water hydrogen-bond network. Finally, water molecules at the albumin interface were found to form a distorted hydrogen-bond structure due to topological and energetic disorder of the protein surface. In addition, the intramolecular O-H stretching vibration of water (∼100 THz), which is sensitive to hydrogen-bond environment, pointed to a trend that hydration water has a larger population of strongly hydrogen-bonded water molecules compared with that of bulk water. From these experimental results, we concluded that the "strengthened" water hydrogen bonds at the protein interface dynamically slow down the reorientational motion of water and form the less-defective hydrogen-bond network by inhibiting the fragmentation of water-water hydrogen bonds. Nevertheless, such a strengthened water hydrogen-bond network is composed of heterogeneous hydrogen-bond distances and angles, and thus characterized as structurally "distorted."

Effect of water polyamorphism on the molecular vibrations of glycerol in its glassy aqueous solutions

A glassy dilute glycerol-water solution undergoes a mutual polyamorphic transition relating to the transition between high- and low-density amorphous ices of solvent water. The polyamorphic transition behavior depends on the glycerol concentration, indicating that the glycerol affects the water polyamorphism. Here, we used the glassy dilute glycerol-water solution of the solute molar fraction of 0.07 and examined the effect of the polyamorphic change in solvent water on the molecular vibrations of glycerol via Raman spectroscopy. It is found that the molecular vibration of glycerol in high-density liquid like solvent water is different from that in the low-density liquid like solvent water and that the change in the molecular vibration of glycerol is synchronized with the polyamorphic transition of solvent water. The dynamical change of the solute molecule relates to the polyamorphic state of solvent water. This result suggests that the polyamorphic fluctuation of water structure emanated from the presumed liquid-liquid critical point plays an important role for the function of aqueous solution under an ambient condition such as the conformational stability of solute, the functional expression of solute, and so on.

Molecular properties of aqueous solutions: a focus on the collective dynamics of hydration water

When a solute is dissolved in water, their mutual interactions determine the molecular properties of the solute on one hand, and the structure and dynamics of the surrounding water particles (the so-called hydration water) on the other. The very existence of soft matter and its peculiar properties are largely due to the wide variety of possible water-solute interactions. In this context, water is not an inert medium but rather an active component, and hydration water plays a crucial role in determining the structure, stability, dynamics, and function of matter. This review focuses on the collective dynamics of hydration water in terms of retardation with respect to the bulk, and of the number of molecules whose dynamics is perturbed. Since water environments are in a dynamic equilibrium, with molecules continuously exchanging from around the solute towards the bulk and vice versa, we examine the ability of different techniques to measure the water dynamics on the basis of the explored time scales and exchange rates. Special emphasis is given to the collective dynamics probed by extended depolarized light scattering and we discuss whether and to what extent the results obtained in aqueous solutions of small molecules can be extrapolated to the case of large biomacromolecules. In fact, recent experiments performed on solutions of increasing complexity clearly indicate that a reductionist approach is not adequate to describe their collective dynamics. We conclude this review by presenting current ideas that are being developed to describe the dynamics of water interacting with macromolecules.

Water Determines the Structure and Dynamics of Proteins

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.

Protein dynamics: from rattling in a cage to structural relaxation

We present an overview of protein dynamics based mostly on results of neutron scattering, dielectric relaxation spectroscopy and molecular dynamics simulations. We identify several major classes of protein motions on the time scale from faster than picoseconds to several microseconds, and discuss the coupling of these processes to solvent dynamics. Our analysis suggests that the microsecond backbone relaxation process might be the main structural relaxation of the protein that defines its glass transition temperature, while faster processes present some localized secondary relaxations. Based on the overview, we formulate a general picture of protein dynamics and discuss the challenges in this field.

Rigidity, secondary structure, and the universality of the boson peak in proteins

Complementary neutron- and light-scattering results on nine proteins and amino acids reveal the role of rigidity and secondary structure in determining the time- and lengthscales of low-frequency collective vibrational dynamics in proteins. These dynamics manifest in a spectral feature, known as the boson peak (BP), which is common to all disordered materials. We demonstrate that BP position scales systematically with structural motifs, reflecting local rigidity: disordered proteins appear softer than α-helical proteins; which are softer than β-sheet proteins. Our analysis also reveals a universal spectral shape of the BP in proteins and amino acid mixtures; superimposable on the shape observed in typical glasses. Uniformity in the underlying physical mechanism, independent of the specific chemical composition, connects the BP vibrations to nanometer-scale heterogeneities, providing an experimental benchmark for coarse-grained simulations, structure/rigidity relationships, and engineering of proteins for novel applications.

On the Coupling between the Collective Dynamics of Proteins and Their Hydration Water

Picosecond time scale dynamics of hydrated proteins has been connected with the onset of biological activity as it coincides with solvent-solute hydrogen bond rearrangements and amino acid rotational relaxation time scales. The presence and fluctuations of protein hydration water (PHW) largely influence protein motions that are believed to be slaved to those of the solvent, yet to date, how protein and hydration water dynamics are coupled remains unclear. Here, we provide a significant advance in characterizing this coupling; we present the first full study of both the longitudinal and transverse coherent collective motions in a protein-solvent system. The data show unexpectedly the presence in the water dynamics of collective modes belonging to the protein. The properties of these modes, in particular, their propagation velocities and amplitudes, indicate a strengthening of the interactions and a higher rigidity of the network of solvent molecules close to the protein surface. Accordingly, the present study presents the most compelling and clear evidence of a very strong dynamical coupling between a protein and its hydration water, previously suggested by studies using various experimental techniques.

Properties of water in the region between a tubulin dimer and a single motor head of kinesin

A kinesin is a molecular motor that can perform movement on a microtubule track in a stepping-like manner. This motion is connected with processes of association and dissociation of kinesin and tubulin. Water is an important participant in these kinds of molecular interactions. This is why we have decided to investigate the dynamical and structural properties of water in the region between the kinesin catalytic domain and the tubulin dimer. Using the molecular dynamics method, we found that these properties are different from the ones of bulk water. The changes in structure and dynamics are visible for water beyond the first solvation layers, even for the longest analyzed distance between proteins equal to 2.0 nm. However, these changes are not always enhanced compared to the situation when only one protein surface is present. One factor that distinguishes the investigated situation from the one with a single protein is the presence of an additional electric field originating from the second protein. The tendency of vectors of dipole moments of water molecules between the proteins to follow the vectors of electric field generated by the proteins causes a distortion of the water-water hydrogen bond network. It has been shown that this distortion affects the properties of water in this region: it induces structural changes in solvation water, and leads to increased water density and increased stiffness of the water structure.

More Is Different: Experimental Results on the Effect of Biomolecules on the Dynamics of Hydration Water

Biological interfaces characterized by a complex mixture of hydrophobic, hydrophilic, or charged moieties interfere with the cooperative rearrangement of the hydrogen-bond network of water. In the present study, this solute-induced dynamical perturbation is investigated by extended frequency range depolarized light scattering experiments on an aqueous solution of a variety of systems of different nature and complexity such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins. Our results suggest that a reductionist approach is not adequate to describe the rearrangement of hydration water because a significant increase of the dynamical retardation and extension of the perturbation occurs when increasing the chemical complexity of the solute.

Alteration of water structure by peptide clusters revealed by neutron scattering in the small-angle region (below 1 Å(-1))

Solution scattering of neutrons and x-rays can provide direct information on local interactions of importance for biomolecular folding and structure. Here, neutron scattering experiments are combined with molecular-dynamics simulation to interpret the scattering signal of a series of dipeptides with varying degrees of hydrophobicity (GlyAla, GlyPro, and AlaPro) in concentrated aqueous solution (1:20 solute/water ratio) in which the peptides form large segregates (up to 50-60 amino acids). Two main results are found: 1), the shift to lower Q of the so-called water-ring peak (Q ≈ 2 Å(-1)) arises mainly from an overlap of water-peptide and peptide-peptide correlations in the region of 1.3 <Q< 2 Å(-1), rather than from a shift of the water signal induced by the presence of the clusters; and 2), in the low-Q region (Q ≈ 0.6 Å(-1)) a positive peak is observed originating from both the solute-solute correlations and changes in the water structure induced by the formation of the clusters. In particular, the water molecules are found to be more connected than in the bulk with hydrogen-bonding directions tangential to the exposed hydrophobic surfaces, and this effect increases with increasing peptide hydrophobicity. This work demonstrates that important information on the (hydrophobic) hydration of biomolecules can be obtained in the very-small-angle region.

Dynamics of protein and its hydration water: neutron scattering studies on fully deuterated GFP

We present a detailed analysis of the picosecond-to-nanosecond motions of green fluorescent protein (GFP) and its hydration water using neutron scattering spectroscopy and hydrogen/deuterium contrast. The analysis reveals that hydration water suppresses protein motions at lower temperatures (<~ 200 K), and facilitates protein dynamics at high temperatures. Experimental data demonstrate that the hydration water is harmonic at temperatures <~ 180-190 K and is not affected by the proteins' methyl group rotations. The dynamics of the hydration water exhibits changes at ~ 180-190 K that we ascribe to the glass transition in the hydrated protein. Our results confirm significant differences in the dynamics of protein and its hydration water at high temperatures: on the picosecond-to-nanosecond timescale, the hydration water exhibits diffusive dynamics, while the protein motions are localized to <~3 Å. The diffusion of the GFP hydration water is similar to the behavior of hydration water previously observed for other proteins. Comparison with other globular proteins (e.g., lysozyme) reveals that on the timescale of 1 ns and at equivalent hydration level, GFP dynamics (mean-square displacements and quasielastic intensity) are of much smaller amplitude. Moreover, the suppression of the protein dynamics by the hydration water at low temperatures appears to be stronger in GFP than in other globular proteins. We ascribe this observation to the barrellike structure of GFP.

Water dynamics as affected by interaction with biomolecules and change of thermodynamic state: a neutron scattering study

The dynamics of water as subtly perturbed by both the interaction with biomolecules and the variation of temperature and pressure has been investigated via neutron scattering spectroscopy. A measurement of inelastic neutron scattering devoted to the study of the coherent THz dynamics of water in a water-rich mixture with DNA (hydration level of 1 g DNA/15 g D(2)O) at room temperature is reported. The DNA hydration water coherent dynamics is characterised by the presence of collective modes, whose dispersion relations are similar to those observed in bulk water. These dispersion relations are well described by the interaction model developed in the case of bulk water, and the existence of a fast sound is experimentally demonstrated. The behaviour of the collective water dynamics was complemented by studying the single-particle dynamics of bulk water along the isotherm T = 298 K in the pressure range 0.1-350 MPa by means of incoherent scattering. This experiment is an attempt to simulate the change of the water molecular arrangement due to the interaction with DNA, by increasing the pressure as the presence of the biomolecule produces an increase in the density. An anomaly is found in the behaviour of the relaxation time derived from the quasi-elastic scattering signal, which can be related to the hypothetical second critical point in water. This anomaly and the transition from slow to fast sound take place in the same Q range, thus suggesting that the two phenomena could be related at some microscopic level.

Increase of both order and disorder in the first hydration shell with increasing solute polarity

Monte Carlo simulations of a small model solute in an aqueous solution are used to examine the effects of solute polarity on hydration structure. A judicious definition of the orientational order parameter leads to reinterpretation of the conventional picture of hydration. As the solute varies from hydrophobic to hydrophilic the ordered first shell water simultaneously fractionates into a more highly ordered and a more disordered component. The hydrogen-bond network rearranges such that the more ordered component relaxes to configurations of optimal intermolecular angles, the other fraction being released from the network.

Collective density fluctuations of DNA hydration water in the time-window below 1 ps

The coherent density fluctuations propagating through DNA hydration water were studied by neutron scattering spectroscopy. Two collective modes were found to be sustained by the aqueous solvent: a propagating excitation, characterised by a speed of about 3500 m/s, and another one placed at about 6 meV. These results globally agree with those previously found for the coherent excitations in bulk water, although in DNA hydration water the speed of propagating modes is definitely higher than that of the pure solvent. The short-wavelength collective excitations of DNA hydration water are reminiscent of those observed in protein hydration water and in the amorphous forms of ice.

Dynamical properties of the hydration shell of fully deuterated myoglobin

Freeze-dried perdeuterated sperm whale myoglobin was kept in a water-saturated atmosphere in order to obtain a hydration degree of 335 H(2)O molecules per one myoglobin molecule. Incoherent neutron scattering was performed at the neutron spectrometer TOFTOF at the FRM II in an angular range of q from 0.6 to 1.8 Å(-1) and a temperature range from 4 to 297 K. We used neutrons with a wavelength of λ αE 6 Å and an energy resolution of about 65 μeV corresponding to motions faster than 10 ps. At temperatures above 225 K, broad lines appear in the spectra caused by quasielastic scattering. For an explanation of these lines, we assumed that there are only two types of protons, those that are part of the hydration water (72%) and those that belong to the protein (28%). The protons of the hydration water were analyzed with the diffusion model of Singwi and Sjölander [Phys. Rev. 119, 863 (1960)]. In this model, a water molecule stays for a time τ(0) in a bound state performing oscillatory motions. Thereafter, the molecule performs free diffusion for the time τ(1) in a nonbound state followed again by the oscillatory motions for τ(0) and so forth. We used the general formulation with no simplifications as τ(0)≫τ(1) or τ(1)≫τ(0). At room temperature, we obtained τ(0) αE 104 ps and τ(1) αE 37 ps. For the protein bound hydrogen, the dynamics is described by a Brownian oscillator where the protons perform overdamped motions in limited space.

Dynamics of biological macromolecules: not a simple slaving by hydration water

We studied the dynamics of hydrated tRNA using neutron and dielectric spectroscopy techniques. A comparison of our results with earlier data reveals that the dynamics of hydrated tRNA is slower and varies more strongly with temperature than the dynamics of hydrated proteins. At the same time, tRNA appears to have faster dynamics than DNA. We demonstrate that a similar difference appears in the dynamics of hydration water for these biomolecules. The results and analysis contradict the traditional view of slaved dynamics, which assumes that the dynamics of biological macromolecules just follows the dynamics of hydration water. Our results demonstrate that the dynamics of biological macromolecules and their hydration water depends strongly on the chemical and three-dimensional structures of the biomolecules. We conclude that the whole concept of slaving dynamics should be reconsidered, and that the mutual influence of biomolecules and their hydration water must be taken into account.

Study of solvent-protein coupling effects by neutron scattering

The present work aims to characterize the dynamical behavior of proteins immersed in bio-preserving liquids and glasses. For this purpose, the protein dUTPase was chosen, while the selected solvents were glycerol, a triol, and some homologous disaccharides, i.e., trehalose, maltose, and sucrose, which are known to be very effective bio-preserving agents. The results highlight that the disaccharides show a slowing down effect on the water dynamics, which is stronger for trehalose than in the case of the other disaccharides. Furthermore, a characterization of the medium which hosts the protein is performed by using an operative definition of fragility based on the mean square displacement extracted by elastic incoherent neutron scattering, which is directly connected to Angell's kinetic fragility based on the viscosity. Finally, a study of the dynamics of the protein sequestered within the solvents is performed. The result shows that the protein dynamics is coupled with that of the surrounding matrix.

Coupled relaxations at the protein-water interface in the picosecond time scale

The spectral behaviour of a protein and its hydration water has been investigated through neutron scattering. The availability of both hydrogenated and perdeuterated samples of maltose-binding protein (MBP) allowed us to directly measure with great accuracy the signal from the protein and the hydration water alone. Both the spectra of the MBP and its hydration water show two distinct relaxations, a behaviour that is reminiscent of glassy systems. The two components have been described using a phenomenological model that includes two Cole-Davidson functions. In MBP and its hydration water, the two relaxations take place with similar average characteristic times of approximately 10 and 0.2 ps. The common time scales of these relaxations suggest that they may be a preferential route to couple the dynamics of the water hydrogen-bond network around the protein surface with that of protein fluctuations.

Collective dynamics of protein hydration water by brillouin neutron spectroscopy

By a detailed experimental study of THz dynamics in the ribonuclease protein, we could detect the propagation of coherent collective density fluctuations within the protein hydration shell. The emerging picture indicates the presence of both a dispersing mode, traveling with a speed greater than 3000 m/s, and a nondispersing one, characterized by an almost constant energy of 6-7 meV. In agreement with molecular dynamics simulations [Phys. Rev. Lett. 2002, 89, 275501], the features of the dispersion curves closely resemble those observed in pure liquid water [Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2004, 69, 061203]. On the contrary, the observed damping factors are much larger than in bulk water, with the dispersing mode becoming overdamped at Q = 0.6 A(-1) already. Such novel experimental findings are discussed as a dynamic signature of the disordering effect induced by the protein surface on the local structure of water.