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.

Careful studies of liquid water in its supercooled region have led to many insights into the structure and behavior of water in all temperature regions. While bulk water freezes at its homogeneous nucleation temperature of approximately 235 K, for protein hydration water, the binding of water molecules to the protein avoids crystallization. Here we study the slow dynamics of water at low temperatures by dielectric relaxation experiments on lysozyme hydration water over an extremely broad range of frequencies and temperatures. We probe the temperature dependence of the relaxation time due to water protons in order to gain insight into the structure and connectivity of the hydrogen bond network of water molecules adsorbed on the protein surface. We observe two dynamic crossovers: one at about 252 K, assigned to a change of the diffusive regime of water protons along the hydrogen bond network; the other, at about 181 K assigned to a structural re--arrangement of the water network. We support this interpretation by comparing the experimental results with Monte Carlo simulations and mean--field calculations on a cell model of water. The model allows an interpretation in terms of thermodynamic features of water, predicting the presence of two specific heat maxima at ambient pressure. The first is due to fluctuations in the hydrogen bond formation, and the second, at lower temperature, due to cooperative reordering of the hydrogen bond network.

We report the results of a broadband (10−2–107Hz) dielectric spectroscopy study on a solvent system (glycerol–water solution) confined in a porous silica matrix. The dielectric relaxation of the system is studied as a function of both temperature (120–280K) and solvent composition (0–36 glycerol molar percentage), at constant matrix composition. Our data show that glycerol–water systems confined inside silica gel are characterized by a very complex dynamics quite different from that observed in solution, thus indicating that confinement may deeply modify solvent dynamics. Indeed in addition to the relaxation processes similar to those occurring in bulk samples, new dielectric relaxations are detected: two non-collective relaxations, attributed to water molecules strongly interacting with pore surfaces and to the glycerol trapped within the matrix structure, respectively; a relaxation in the glycerol free sample (and in samples at very low glycerol content) almost coincident with that observed in other different confinement conditions and governed by geometrical confinement per se. Moreover, at high glycerol content we observe two non-Arrhenius processes at least 4 order of magnitude slower than solution-like main relaxation; at low glycerol content the two above relaxations merge and show a fragile to strong transition at about 200K.