Two dynamic crossovers in protein hydration water and their thermodynamic interpretation (original) (raw)
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More Than One Dynamic Crossover in Protein Hydration Water
Studies of liquid water in its supercooled region have helped us better understand the structure and behavior of water. Bulk water freezes at its homogeneous nucleation temperature (approximately 235 K), but protein hydration water avoids this crystallization because each water molecule binds to a protein. Here, we study the dynamics of the hydrogen bond (HB) network of a percolating layer of water molecules and compare the measurements of a hydrated globular protein with the results of a coarse-grained model that successfully reproduces the properties of hydration water. Using dielectric spectroscopy, we measure the temperature dependence of the relaxation time of proton charge fluctuations. These fluctuations are associated with the dynamics of the HB network of water molecules adsorbed on the protein surface. Using Monte Carlo simulations and mean-field calculations, we study the dynamics and thermodynamics of the model. Both experimental and model analyses are consistent with the interesting possibility of two dynamic crossovers, (i) at approximately 252 K and (ii) at approximately 181 K. Because the experiments agree with the model, we can relate the two crossovers to the presence at ambient pressure of two specific heat maxima. The first is caused by fluctuations in the HB formation, and the second, at a lower temperature, is due to the cooperative reordering of the HB network. hydrated proteins | model calculations | dielectric relaxation | water dynamics | water specific heat R ecent experiments have studied water in the first hydration shell of globular proteins (1-5). Unlike bulk water, this water does not freeze until the temperature T is well below 235 K (6), a property that may be essential to biological functioning (7). Although quasi-elastic neutron scattering investigations (1) and molecular dynamics simulations (8, 9) support the presence of a dynamic crossover at approximately 220 K, other experiments and simulations do not (2-4, 10). It has been demonstrated that the suggested crossover could be related to the anomalous behavior of water, but that it is independent of any possible liquidliquid critical point at finite T (11).
Properties of hydration water and its role in protein dynamics
Journal of Physics: Condensed Matter, 2007
The low-temperature properties of confined water and the relation between protein and solvent dynamics have been studied by broadband dielectric spectroscopy with the aim to understand the role of hydration water for protein dynamics. At low temperatures (below approximately 200 K) confined water generally exhibits two relaxation processes: one process that is due to the local β-relaxation, and a faster and even more local process that is interpreted to arise from the motion of Bjerrum-type defects. These relaxation processes, showing Arrhenius temperature dependences, are also observed in glycerolwater solvents of myoglobin containing 50 wt% water. In the temperature regime below 200 K, only a local protein process is observed. The activation energies of this protein process and the β-relaxation in the solvent are similar, suggesting that this local protein process is determined by the β-relaxation in the solvent. At about 200 K the nature of the dynamics changes dramatically and an onset of cooperative and large-scale dynamics is observed for both the waterrich solvent and the protein. We believe that the reason for this crossover is that the β-relaxation in the solvent merges with the non-observable α-relaxation at this temperature, giving rise to a merged α-β-relaxation in the solvent at higher temperatures. Also above this temperature the fastest observed protein processes seem to be determined by the solvent dynamics, as suggested for 'solvent-slaved' protein motions.
Nanosecond Relaxation Dynamics of Hydrated Proteins: Water versus Protein Contributions
The Journal of Physical Chemistry B, 2011
We have studied picosecond to nanosecond dynamics of hydrated protein powders using dielectric spectroscopy and molecular dynamics (MD) simulations. Our analysis of hydrogen-atom single particle dynamics from MD simulations focused on "main" (τ main ≈ tens of picoseconds) and "slow" (τ slow ≈ nanosecond) relaxation processes that were observed in dielectric spectra of similar hydrated protein samples. Traditionally, the interpretation of these processes observed in dielectric spectra has been ascribed to the relaxation behavior of hydration water tightly bounded to a protein and not to protein atoms. Detailed analysis of the MD simulations and comparison to dielectric data indicate that the observed relaxation process in the nanosecond time range of hydrated protein spectra is mainly due to protein atoms. The relaxation processes involve the entire structure of protein including atoms in the protein backbone, side chains, and turns. Both surface and buried protein atoms contribute to the slow processes; however, surface atoms demonstrate slightly faster relaxation dynamics. Analysis of the water molecule residence and dipolar relaxation correlation behavior indicates that the hydration water relaxes at much shorter time scales.
The Journal of …, 2008
A super-Arrhenius-to-Arrhenius dynamic crossover phenomenon has been observed in the translational R-relaxation time and in the inverse of the self-diffusion constant both experimentally and by simulations for lysozyme hydration water in the temperature range of T L ) 223 ( 2 K. MD simulations are based on a realistic hydrated powder model, which uses the TIP4P-Ew rigid molecular model for the hydration water. The convergence of neutron scattering, nuclear magnetic resonance and molecular dynamics simulations supports the interpretation that this crossover is a result of the gradual evolution of the structure of hydration water from a high-density liquid to a low-density liquid form upon crossing of the Widom line above the possible liquid-liquid critical point of water.
The Journal of Chemical Physics, 2013
In this work, we compare experimental data on myoglobin hydrated powders from elastic neutron scattering, broadband dielectric spectroscopy, and differential scanning calorimetry. Our aim is to obtain new insights on the connection between the protein dynamical transition, a fundamental phenomenon observed in proteins whose physical origin is highly debated, and the liquidliquid phase transition (LLPT) possibly occurring in protein hydration water and related to the existence of a low temperature critical point in supercooled water. Our results provide a consistent thermodynamic/dynamic description which gives experimental support to the LLPT hypothesis and further reveals how fundamental properties of water and proteins are tightly related.
Nature of the water specific relaxation in hydrated proteins and aqueous mixtures
Chemical Physics, 2013
The dynamic transition found by Mössbauer spectroscopy and neutron scattering in hydrated and solvated proteins has been an active research area for the past three decades. By now a consensus among some researchers has been reached that it originates exclusively from relaxation of the hydration water (HW) coupled to the protein. The dynamic transition temperature T d depends on energy resolution of the spectrometer and is higher than the glass transition temperature T g. Recently demonstrated is the presence of yet another transition at T g , which is independent of the resolution of the spectrometer and coexists with the dynamic transition at a higher temperature T d. The transition at T g is similar to that found in various kinds of glass-formers by neutron and dynamic light scattering at short times when molecules are mutually caged via the intermolecular potential. Like in the case of conventional glass-formers, the transition at T g of hydrated proteins has been explained by the sensitivity of the extent of the caged dynamics to change of specific volume and entropy on crossing T g. The caged dynamics are terminated by the onset of relaxation of HW, which in turn gives rise to the dynamic transition at T d > T g. Despite these important roles played by the caged dynamics and the HW relaxation in the observed dual transitions of the hydrated proteins, their exact nature is still unclear. In this paper we clarify their nature in hydrated proteins by use of various experimental data, with the assist of the results from studies of mixtures of water with hydrophilic solutes, taking advantage of the fact that the properties are similar in both systems.
Dynamical Transition of Protein-Hydration Water
Physical Review Letters, 2010
Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasielastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly.
Resolving the ambiguity of the dynamics of water and clarifying its role in hydrated proteins
Philosophical Magazine, 2011
The dynamics of water in aqueous mixtures with various hydrophilic solutes can be probed over practically unrestricted temperature and frequency ranges, in contrast to bulk water where crystallization preempts such study. The characteristics of the dynamics of water and their trends observed in aqueous mixtures on varying the solutes and concentration of water, in conjunction with that of water confined in spaces of nanometer size, lead us to infer the fundamental traits of the dynamics of water. These include the universal secondary relaxation, here called the-relaxation, the low degree of intermolecular coupling/cooperativity, and the 'strong' character of the structural primary relaxation. The dynamics of hydration water in hydrated proteins at sufficiently high hydration levels are similar in every respect to that in aqueous mixtures. In particular, the-relaxation of hydration water has a relaxation time nearly the same as that of the-relaxation of aqueous mixtures above and below the glass transition temperature. This can explain the dynamics transition observed by Mo¨ssbauer spectroscopy and neutron scattering. The fact that it is coupled to atomic motions of the hydrated protein, like similar situation in aqueous mixtures, explains why the dynamic transition is observed by neutron scattering at the same temperature whether the hydration water is H 2 O or D 2 O. The possibility that the-relaxation of the solvent is instrumental for biological function of hydrated biomolecules is suggested by the comparable temperature dependences of the ligand escape rate and the reciprocal of the-relaxation time.
Hydration-dependent dynamic crossover phenomenon in protein hydration water
Physical Review E, 2014
The characteristic relaxation time τ of protein hydration water exhibits a strong hydration level h dependence. The dynamic crossover is observed when h is higher than the monolayer hydration level h c = 0.2-0.25 and becomes more visible as h increases. When h is lower than h c , τ only exhibits Arrhenius behavior in the measured temperature range. The activation energy of the Arrhenius behavior is insensitive to h, indicating a local-like motion. Moreover, the h dependence of the crossover temperature shows that the protein dynamic transition is not directly or solely induced by the dynamic crossover in the hydration water.