Relaxation processes in supercooled confined water and implications for protein dynamics

Frontiers in biotransport: water transport and hydration

Biotransport, by its nature, is concerned with the motions of molecules in biological systems while water remains as the most important and the most commonly studied molecule across all disciplines. In this review, we focus on biopreservation and thermal therapies from the perspective of water, exploring how its molecular motions, properties, kinetic, and thermodynamic transitions govern biotransport phenomena and enable preservation or controlled destruction of biological systems.

Frontiers in Biotransport: Water Transport and Hydration

Biotransport, by its nature, is concerned with the motions of molecules in biological systems while water remains as the most important and the most commonly studied molecule across all disciplines. In this review, we focus on biopreservation and thermal therapies from the perspective of water, exploring how its molecular motions, properties, kinetic, and thermodynamic transitions govern biotransport phenomena and enable preservation or controlled destruction of biological systems.

Proteins remain soft at lower temperatures under pressure

The low-temperature behavior of proteins under high pressure is not as extensively investigated as that at ambient pressure. In this paper, we study the dynamics of a hydrated protein under moderately high pressures at low temperatures using the quasielastic neutron scattering method. We show that when applying pressure to the protein-water system, the dynamics of the protein hydration water does not slow down but becomes faster instead. The degree of "softness" of the protein, which is intimately related to the enzymatic activity of the protein, shows the same trend as its hydration water as a function of temperature at different pressures. These two results taken together suggest that at lower temperatures, the protein remains soft and active under pressure.

Quantum behavior of water protons in protein hydration shell

Quantum effects on the water proton dynamics over the surface of a hydrated protein are measured by means of broadband dielectric spectroscopy and deep inelastic neutron scattering. Dielectric spectroscopy indicates a reduced energy barrier for a hydrogenated protein sample compared to a deuterated one, along with a large and temperature-dependent isotopic ratio, in good agreement with theoretical studies. Recent deep inelastic neutron scattering data have been reanalyzed, and now show that the momentum distribution of water protons reflects a characteristic delocalization at ambient temperatures. These experimental findings might have far-reaching implications for enzymatic catalysis involving proton transfer processes, as in the case of the lysozyme protein studied in this report.

From shell to cell: neutron scattering studies of biological water dynamics and coupling to activity

An integrated picture of hydration shell dynamics and of its coupling to functional macromolecular motions is proposed from studies on a soluble protein, on a membrane protein in its natural lipid environment, and on the intracellular environment in bacteria and red blood cells. Water dynamics in multimolar salt solutions was also examined, in the context of the very slow water component previously discovered in the cytoplasm of extreme halophilic archaea. The data were obtained from neutron scattering by using deuterium labelling to focus on the dynamics of different parts of the complex systems examined.

A unified model of protein dynamics

Protein functions require conformational motions. We show here that the dominant conformational motions are slaved by the hydration shell and the bulk solvent. The protein contributes the structure necessary for function. We formulate a model that is based on experiments, insights from the physics of glass-forming liquids, and the concepts of a hierarchically organized energy landscape. To explore the effect of external fluctuations on protein dynamics, we measure the fluctuations in the bulk solvent and the hydration shell with broadband dielectric spectroscopy and compare them with internal fluctuations measured with the Mössbauer effect and neutron scattering. The result is clear. Large-scale protein motions are slaved to the fluctuations in the bulk solvent. They are controlled by the solvent viscosity, and are absent in a solid environment. Internal protein motions are slaved to the beta fluctuations of the hydration shell, are controlled by hydration, and are absent in a dehydrated protein. The model quantitatively predicts the rapid increase of the mean-square displacement above approximately 200 K, shows that the external beta fluctuations determine the temperature- and time-dependence of the passage of carbon monoxide through myoglobin, and explains the nonexponential time dependence of the protein relaxation after photodissociation.

Origins of apparent fragile-to-strong transitions of protein hydration waters

2H NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at Tx1 approximately 225 K (X1) and Tx2 approximately 200 K (X2). We show that neither X1 nor X2 are related to a fragile-to-strong transition. Our results rule out an existence of X1. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at T < Tx2, implying that X2 originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.

Protein dynamical transition does not require protein structure

Terahertz time domain spectroscopy shows that the protein dynamical transition, the rapid increase in protein dynamics occurring at approximately 200 K, needs neither tertiary nor secondary structure. Further, short chain alanine studies find a dynamical transition down to penta-alanine, with no transition observed for di-alanine or tri-alanine. These results reveal the temperature dependence arises strictly from the side-chain interaction with the solvent. The lack of a transition for shorter chain peptides may indicate a qualitative change in this interaction occurs at a specific peptide chain length.

Investigating hydration dependence of dynamics of confined water: monolayer, hydration water and Maxwell-Wagner processes

The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 A, respectively, is examined by broadband dielectric spectroscopy (10(-2)-10(9) Hz) and differential scanning calorimetry for a wide temperature interval (110-340 K). The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.

Nonlinear viscoelastic dynamics of nanoconfined wetting liquids

The viscoelastic dynamics of nanoconfined wetting liquids is studied by means of atomic force microscopy. We observe a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids. We show that the origin of the measured nonlinear viscoelasticity in nanoconfined water and silicon oil is a strain rate dependent relaxation time and slow dynamics. By measuring the viscoelastic modulus at different frequencies and strains, we find that the intrinsic relaxation time of nanoconfined water is in the range 0.1-0.0001 s, orders of magnitude longer than that of bulk water, and comparable to the dielectric relaxation time measured in supercooled water at 170-210 K.

Conductivity in hydrated proteins: no signs of the fragile-to-strong crossover

Dielectric spectroscopy studies of hydrated protein demonstrate smooth temperature variations of conductivity. This observation suggests no cusplike fragile-to-strong crossover (FSC) in the protein's hydration water. The FSC at T approximately 220 K was postulated recently on the basis of neutron scattering studies [Chen, Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] and was proposed to be the main cause for the dynamic transition in hydrated proteins. Following Swenson et al. , we ascribe the neutron results to a secondary relaxation. We emphasize that no cusplike solvent behavior is required for the protein's dynamic transition.

Coincidence of dynamical transitions in a soluble protein and its hydration water: direct measurements by neutron scattering and MD simulations

The coupling between protein dynamics and hydration-water dynamics was assessed by perdeuteration, temperature-dependent neutron scattering, and molecular dynamics simulations. Mean square displacements of water and protein motions both show a broad transition at 220 K and are thus coupled. In particular, the protein dynamical transition appears to be driven by the onset of hydration-water translational motion.

Universal features of water dynamics in solutions of hydrophilic polymers, biopolymers, and small glass-forming materials

A systematic investigation by dielectric spectroscopy of 18 different water-rich mixtures with very different hydrophilic substances shows universal features for the water dynamics. The temperature dependence of the relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior at the T(g) range of the mixtures. Furthermore, the temperature dependence of the relaxation times presents a universal behavior both above and below the crossover temperature. We also show that these features suggest that the observed crossover is associated with the emergence of confinement effects.

Coupling of protein and hydration-water dynamics in biological membranes

The dynamical coupling between proteins and their hydration water is important for the understanding of macromolecular function in a cellular context. In the case of membrane proteins, the environment is heterogeneous, composed of lipids and hydration water, and the dynamical coupling might be more complex than in the case of the extensively studied soluble proteins. Here, we examine the dynamical coupling between a biological membrane, the purple membrane (PM), and its hydration water by a combination of elastic incoherent neutron scattering, specific deuteration, and molecular dynamics simulations. Examining completely deuterated PM, hydrated in H(2)O, allowed the direct experimental exploration of water dynamics. The study of natural abundance PM in D(2)O focused on membrane dynamics. The temperature-dependence of atomic mean-square displacements shows inflections at 120 K and 260 K for the membrane and at 200 K and 260 K for the hydration water. Because transition temperatures are different for PM and hydration water, we conclude that ps-ns hydration water dynamics are not directly coupled to membrane motions on the same time scale at temperatures <260 K. Molecular-dynamics simulations of hydrated PM in the temperature range from 100 to 296 K revealed an onset of hydration-water translational diffusion at approximately 200 K, but no transition in the PM at the same temperature. Our results suggest that, in contrast to soluble proteins, the dynamics of the membrane protein is not controlled by that of hydration water at temperatures <260 K. Lipid dynamics may have a stronger impact on membrane protein dynamics than hydration water.

Separable cooperative and localized translational motions of water confined by a chemically heterogeneous environment

We report quasi-elastic neutron scattering experiments at two resolutions that probe timescales of picoseconds to nanoseconds for the hydration dynamics of water, confined in a concentrated solution of N-acetyl-leucine-methylamide (NALMA) peptides in water over a temperature range of 248 K to 288 K. The two QENS resolutions used allow for a clean separation of two observable translational components, and ultimately two very different relaxation processes, that become evident when analyzed under a combination of the jump diffusion model and the relaxation cage model. The first translational motion is a localized beta-relaxation process of the bound surface water, and exhibits an Arrhenius temperature dependence and a large activation energy of approximately 8 kcal mol(-1). The second non-Arrhenius translational component is a dynamical signature of the alpha-relaxation of more fluid water, exhibiting a glass transition temperature of approximately 116 K when fit to the Volger Fulcher Tamman functional form. These peptide solutions provide a novel experimental system for examining confinement in order to understand the dynamical transition in bulk supercooled water by removing the unwanted interface of the confining material on water dynamics.