By confining water in nanopores, so narrow that the liquid cannot freeze, it is possible to explore its properties well below its homogeneous nucleation temperature T H Ϸ 235 K. In particular, the dynamical parameters of water can be measured down to 180 K, approaching the suggested glass transition temperature T g Ϸ 165 K. Here we present experimental evidence, obtained from Nuclear Magnetic Resonance and Quasi-Elastic Neutron Scattering spectroscopies, of a well defined decoupling of transport properties (the self-diffusion coefficient and the average translational relaxation time), which implies the breakdown of the Stokes-Einstein relation. We further show that such a non-monotonic decoupling reflects the characteristics of the recently observed dynamic crossover, at Ϸ225 K, between the two dynamical behaviors known as fragile and strong, which is a consequence of a change in the hydrogen bond structure of liquid water.decoupling of transport properties ͉ dynamic crossover ͉ MCM-41 D espite its fundamental importance in science and technology, the physical properties of water are far from completely understood. The liquid state of water is unusual, especially at low temperatures (1-3). For example, contrary to other liquids, water behaves as if there exists a singular temperature toward which its thermodynamical properties, such as compressibility, thermal expansion coefficient, and specific heat, diverge (1). The efforts of scientists from many disciplines to seek a coherent explanation for this unusual behavior, in combination with its wide range of impacts, make water one of the most important open questions in science today. On the other hand, the nature of the glass transition (GT) of water represents another challenging subject for current research (4). Dynamical measurements in glassforming liquids have shown a dramatic slowdown of both macroscopic (viscosity and self-diffusion coefficient D) and microscopic (average translational correlation time ) observables, as temperature is lowered toward the GT temperature T g . Accordingly, a comprehension of the GT has been sought through the study of the dynamics at the molecular level, which, despite all efforts, has not yet been completely understood (5-8). Keeping in mind the ''complexities'' of both low-temperature water and its GT, we present here direct measurements of two dynamical parameters of water: the self-diffusion coefficient and the average translational relaxation time, in the temperature range from 280 to 190 K, obtained by NMR and quasi-elastic neutron scattering (QENS) experiments, respectively.Bulk water can be supercooled below its melting temperature (T M ) down to Ϸ235 K, below which it inevitably crystallizes; it is just in such supercooled metastable state that the anomalies in its thermodynamical properties are most pronounced, showing a power law divergence toward a singular temperature T S ϭ 228 K. At ambient pressure, water can exist in a glassy form below 135 K. Depending on T and P, glassy water has two amorphous phases with di...

By confining water in a nanoporous structure so narrow that the liquid could not freeze, it is possible to study properties of this previously undescribed system well below its homogeneous nucleation temperature T H ‫؍‬ 231 K. Using this trick, we were able to study, by means of a Fourier transform infrared spectroscopy, vibrational spectra (HOH bending and OH-stretching modes) of deeply supercooled water in the temperature range 183 < T < 273 K. We observed, upon decreasing temperature, the building up of a new population of hydrogen-bonded oscillators centered around 3,120 cm ؊1 , the contribution of which progressively dominates the spectra as one enters into the deeply supercooled regime. We determined that the fractional weight of this spectral component reaches 50% just at the temperature, T L Ϸ 225 K, where the confined water shows a fragile-to-strong dynamic cross-over phenomenon dynamic cross-over in water ͉ dynamic transitions in water ͉ Fourier transform infrared spectroscopy ͉ low-density liquid water ͉ Widom line in water W ater plays a fundamental and ubiquitous role on Earth and in all aspects of life phenomena. Understanding its properties is of paramount importance to mankind, and thus water is the most studied molecular system in science and technology. However, despite intense scrutiny over the years, scientists are still far from reaching a coherent understanding of all its unusual properties (1-3). Instead of behaving like other simple molecular liquids, many thermodynamic response functions of water, such as the isothermal compressibility, isobaric heat capacity, and thermal expansion coefficient, display counterintuitive trends as temperature is lowered. In particular, extrapolated from their values at moderately supercooled states, these functions all appear to diverge at a singular temperature around T S ϭ 228 K. Over the years, many plausible explanations for these strange behaviors have been proposed, starting from the two-state and the clathrate models (1-3). Three hypotheses are of active interest: (i) the stability limit (4), (ii) the percolation (5), and (iii) liquid-liquid (LL) critical point (6). The third approach has received support from various theoretical studies (7-9). However, in all three approaches, the main role is played by the local hydrogen-bond (HB) interaction pattern surrounding a typical water molecule in liquid state that governs the overall structure and dynamics (1) of water. Some of our recent experimental results (10, 11) on water confined in nanoporous structures as a function of temperature and pressure showed that the theoretical approach based on existence of the LL critical point is able to describe coherently many strange properties of water. By using the neutronscattering technique, we obtained evidence of the LL critical point (the second critical point predicted by the theory to be at T C ϳ 220 K and P C ϳ 1 kbar) located at T C ϭ 200 K and P C ϭ 1.5 kbar (10). This result was also subsequently confirmed qualitatively by an extensive molecular dynam...

The temperature dependence of the density of water, (T ), is obtained by means of optical scattering data, Raman and Fourier transform infrared, in a very wide temperature range, 30 < T < 373 K. This interval covers three regions: the thermodynamically stable liquid phase, the metastable supercooled phase, and the low-density amorphous solid phase, at very low T. From analyses of the profile of the OH stretching spectra, we determine the fractional weight of the two main spectral components characterized by two different local hydrogen bond structures. They are, as predicted by the liquid-liquid phase transition hypothesis of liquid water, the low-and the highdensity liquid phases. We evaluate contributions to the density of these two phases and thus are able to calculate the absolute density of water as a function of T. We observe in (T ) a complex thermal behavior characterized not only by the well known maximum in the stable liquid phase at T ‫؍‬ 277 K, but also by a well defined minimum in the deeply supercooled region at 203 ؎ 5 K, in agreement with suggestions from molecular dynamics simulations.infrared and Raman scattering ͉ liquid-liquid phase transition ͉ supercooled and amorphous water ͉ Widom line in water U nderstanding the fundamental role that water plays on Earth and in all aspects of life phenomena represents one of the most challenging research problems in science and technology. In comparison with other simple molecular liquids, the thermodynamic properties of water (H 2 O) are characterized by a counterintuitive trend as temperature is lowered: examples are the isothermal compressibility, the isobaric heat capacity, the isobaric expansivity, and the density (1-3). The latter quantity, as is well known, exhibits a maximum at 277 K. Such a maximum is the only one occurring in liquids in their stable liquid phases just above the melting point. Over the years, many plausible explanations for these unusual behaviors have been proposed. In all of these explanations, the anomalies of water are invariably attributed to the role played by the hydrogen bond (HB) formation between water molecules (1). More precisely, the formation of HBs governs the overall structure and dynamics (1) of water, giving rise to, on decreasing T, a clustering process from which an open tetrahedrally coordinated HB network around each water molecule is gradually developed. It is such an increase in the HB structure that expands the liquid, compensating for the normal tendency of a liquid to contract as it is cooled. This finding is the basic reason for the occurrence of the density maximum phenomenon at 277 K (1-3).Among the many theoretical approaches (4-6) developed to explain water properties in a supercooled state, there is the liquid-liquid phase transition (LLPT) hypothesis (6), which has received the most substantial support from various theoretical (7-10) and experimental studies (11,12). For the LLPT model of water, the liquid state of water above the critical point is a mixture of two different local structures, ch...

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.

The link between the size of soluble amyloid β (Aβ) oligomers and their toxicity to rat cerebellar granule cells (CGC) was investigated. Variation in conditions during in vitro oligomerization of Aβ 1-42 resulted in peptide assemblies with different particle size as measured by atomic force microscopy and confirmed by the dynamic light scattering and fluorescence correlation spectroscopy. Small oligomers of Aβ 1-42 with a mean particle z-height of 1-2 nm exhibited propensity to bind to the phospholipid vesicles and they were the most toxic species that induced rapid neuronal necrosis at submicromolar concentrations whereas the bigger aggregates (z-height above 4-5 nm) did not bind vesicles and did not cause detectable neuronal death. Similar neurotoxic pattern was also observed in primary cultures of cortex neurons whereas Aβ 1-42 oligomers, monomers and fibrils were nontoxic to glial cells in CGC cultures or macrophage J774 cells. However, both oligomeric forms of Aβ 1-42 induced reduction of neuronal cell densities in the CGC cultures.

We study the dynamics of hydration water in the protein lysozyme in the temperature range 180 K

Using NMR, we measure the proton chemical shift ␦, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because ␦ is directly connected to the magnetic shielding tensor, we discuss the data in terms of the local hydrogen bond geometry and order. We argue that the derivative ؊(٢ ln ␦/٢T)P should behave roughly as the constant pressure specific heat CP(T), and we confirm this argument by detailed comparisons with literature values of CP(T) in the range 290 -370 K. We find that ؊(٢ ln ␦/٢T)P displays a pronounced maximum upon crossing the locus of maximum correlation length at Ϸ240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that CP(T) displays a maximum on crossing the Widom line.configurational specific heat ͉ nuclear magnetic resonance ͉ proteins ͉ proton chemical shift U nlike most fluids, water displays anomalies in thermodynamical properties such as compressibility, isobaric heat capacity, and thermal expansion coefficient, and their explanation on molecular basis remains a challenge (1-3). One hypothesis that has received support from various theoretical studies (4-7) is the liquid-liquid (LL) critical point hypothesis, but a proper test can be obtained only by studying the properties of liquid water well below its homogeneous nucleation temperature, T H ϭ 231 K. This is made possible by confining water inside nanoporous structures so small that the liquid cannot freeze.Among recent findings concerning water's dynamical properties at these low temperatures are (8-13): a fragile-to-strong crossover and the violation of the Stokes-Einstein relation, related to the crossing of the Widom line and to the existence of a low-density-liquid-like (LDL-like) local structure. The Widom line is the locus of maximum correlation length in the one-phase region beyond the liquid-liquid critical point, where thermodynamic response functions take their maximum values (12, 13). Scattering experiments (using neutrons and x-rays) have given precise values of the pair correlation function (PCF), providing important benchmarks for testing models of its structure. The PCF represents only an isotropically averaged measure of structure. Thus, in many cases, PCFs may not faithfully reproduce the subtle hydrogen bond geometry responsible for water's thermal anomalies. Our goal in this study is to provide additional information on the local hydrogen bond geometry and, in particular, the average number of the possible configurations of the local molecular hydrogen bonding geometry, by measuring the NMR proton chemical shift ␦. If a water molecule in a dilute gas is taken to be an isolated-state reference, the chemical shift ␦ accounts for the change of the value of the magnetic shielding with respect to that of such a reference. Hence the chemical shift is related to the ''non-dilute'' or ''virial'' interaction of a water molecule with its surroundings, providing a picture of the intermolecular geometry (14-19). Originally, it has been proposed, especially in the high ...

Proton nuclear magnetic resonance (1H NMR) experiments have been performed to measure the spin-lattice, T1, and spin-spin, T2, relaxation times of the three functional groups in water/methanol mixtures at different methanol molar fractions (XMeOH=0, 0.04, 0.1, 0.24, 0.5, 1) as a function of temperature in the range 205 K