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).Here, we show by experiments, simulations, and model calculations that the dynamic properties of the hydrogen bond (HB) network at the protein-water interface exhibit not one, but two dynamic crossovers in the one-phase region at low pressure. We show how the two crossovers are related to the thermodynamics of water. We investigate the dielectric relaxation time of water protons, due to charged defects-such as H 3 O þ -moving with a diffusive or hopping mechanism along the HB network (6, 12). These measurements are a sensitive probe for HB breaking and formation (13). We perform dielectric relaxation experiments on lysozyme powder with hydration level h ¼ 0.30 g H 2 O∕g dry protein, over a broad frequency (10 −2 s −1 -10 8 s −1 ) and temperature range (150 K ≤ T ≤ 300 K). The experimental setup and the data analysis (14-18) are described in Methods and SI Text.In the diele...

The observation made by early naturalists that some organisms could tolerate extreme environmental condisions and "enjoy the advantage of real resurrection after death" [ Spallanzani , M. Opuscules de Physique Animale et Vegetale 1776 (translated from Italian by Senebier , J. Opuscules de Physique Animale et Vegetale 1787 , 2 , 203 - 285 )] stimulated research that still continues to this day. Cryptobiosis, the ability of an organism to tolerate adverse environments, such as dehydration and low temperatures, still represents an unsolved and fascinating problem. It has been shown that many sugars play an important role as bioprotectant agents, and among the best performers is the disaccharide trehalose. The current hypothesis links the efficiency of its protective role to strong modifications of the tetrahedral arrangement of water molecules in the sugar hydration shell, with trehalose forming many hydrogen bonds with the solvent. Here, we show, by means of state-of-the-art neutron diffraction experiments combined with EPSR simulations, that trehalose solvation induces very minor modifications of the water structure. Moreover, the number of water molecules hydrogen-bonded to the sugar is surprisingly small.