In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Advances in the study of supercooled water

Topology and complexity of the hydrogen bond network in classical models of water

We study, with molecular dynamics simulations, a lysozyme protein immersed in a water-trehalose solution upon cooling. The aim is to understand the cryoprotectant role played by this disaccharide through the modifications that it induces on the slow dynamics of protein hydration water with its presence. The α-relaxation shows a fragile to strong crossover about 20° higher than that in the bulk water phase and 15° higher than that in lysozyme hydration water without trehalose. The protein hydration water without trehalose was found to show a second slower relaxation exhibiting a strong to strong crossover coupled with the protein dynamical transition. This slower relaxation time importantly appears enormously slowed down in our cryoprotectant solution. On the other hand, this long-relaxation in the presence of trehalose is also connected with a stronger damping of the protein structural fluctuations than that found when the protein is in contact with the pure hydration water. Therefore, this appears to be the mechanism through which trehalose manifests its cryoprotecting function.

Effect of trehalose on protein cryoprotection: Insights into the mechanism of slowing down of hydration water.

We study the dynamics of the first hydration shell of lysozyme to determine the role of hydration water that accompanies lysozyme thermal denaturation. We use nuclear magnetic resonance spectroscopy to investigate both the translational and rotational contributions. Data on proton self-diffusion and reorentational correlation time indicate that the kinetics of the lysozyme folding/unfolding process is controlled by the dynamics of the water molecules in the first hydration shell. When the hydration water dynamics change, because of the weakening of the hydrogen bond network, the three-dimensional structure of the lysozyme is lost and denaturation is triggered. Our data indicates that at temperatures above approximately 315 K, water behaves as a simple liquid and is no longer a good solvent.

Dynamical changes in hydration water accompanying lysozyme thermal denaturation

https://hal.archives-ouvertes.fr/hal-03358207/document

Molecular dynamics study on water and hydroxide transfer mechanisms in PSU-g-alkyl-TMA membranes at low hydration: Effect of side chain length

We report results on the translational dynamics of the hydration water of the lysozyme protein upon cooling obtained by means of molecular dynamics simulations. The self van Hove functions and the mean square displacements of hydration water show two different temperature activated relaxation mechanisms, determining two dynamic regimes where transient trapping of the molecules is followed by hopping phenomena to allow to the structural relaxations. The two caging and hopping regimes are different in their nature. The low-temperature hopping regime has a time scale of tenths of nanoseconds and a length scale on the order of 2-3 water shells. This is connected to the nearest-neighbours cage effect and restricted to the supercooling, it is absent at high temperature and it is the mechanism to escape from the cage also present in bulk water. The second hopping regime is active at high temperatures, on the nanoseconds time scale and over distances of nanometers. This regime is connected to water displacements driven by the protein motion and it is observed very clearly at high temperatures and for temperatures higher than the protein dynamical transition. Below this temperature, the suppression of protein fluctuations largely increases the time-scale of the protein-related hopping phenomena at least over 100 ns. These protein-related hopping phenomena permit the detection of translational motions of hydration water molecules longly persistent in the hydration shell of the protein.

https://www.mdpi.com/1420-3049/25/19/4570/pdf

Protein-Water and Water-Water Long-Time Relaxations in Protein Hydration Water upon Cooling-A Close Look through Density Correlation Functions.

Water hydrating biomolecules shows a more complex dynamical behavior when compared to the bulk. Its translational slow dynamics can be described by two mechanisms characterized by two well distinct time scales. One mechanism is the \(\alpha\)-relaxation typical of supercooled bulk water and other glass forming liquids. Upon cooling, this relaxation shows a fragile-to-strong crossover due to the activation of hopping phenomena which permits to the water molecules in the hydration layer to escape from nearest neighbors cage. The second mechanism is a much slower relaxation that is present only in hydration water and it is coupled with the biomolecule dynamics. This long-relaxation shows upon cooling a strong-to-strong crossover in coincidence with the well-known Protein Dynamical Transition. Structural rearrangements of biomolecules can trap hydration water molecules over length-scale larger than nearest neighbors distances. This causes a new hopping regime specific only of hydration water and already active at high temperature.

Lorenzo Tenuzzo

Papers

Protein-Water and Water-Water Long-Time Relaxations in Protein Hydration Water upon Cooling-A Close Look through Density Correlation Functions.

Slow Dynamics of Biological Water