C. Branca

Anomalous cryoprotective effectiveness of trehalose: Raman scattering evidences

Results of Raman scattering measurements performed on aqueous solutions of the homologous disaccharides (trehalose, maltose, and sucrose) are reported. To get some insight into the effects of disaccharides on the hydrogen bond network of water, and to clarify the reasons that make trehalose the most effective in protecting organisms from dehydration and freezing, we investigate the intramolecular OH stretching mode. To carry out this study, two different approaches are employed: namely, a decomposition of the isotropic spectra into an “open” and a “closed” contribution, and a spectral stripping procedure to extract the “collective” contribution from the polarized spectra. Both procedures agree in suggesting that disaccharides promote, with a different strength, a destructuring effect on the tetrahedral H-bond network of pure water. This result makes plausible the hypothesis that disaccharides obstruct the crystallization process reducing the amount of freezable water, namely destroying the network of wate...

The anomalous behavior of the density of water in the range 30 K < T < 373 K

The temperature dependence of the density of water, ρ(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 high-density 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 ρ(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.

Transport properties of glass-forming liquids suggest that dynamic crossover temperature is as important as the glass transition temperature

It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity η on temperature T. In an Arrhenius plot, ln η vs 1/T, a strong liquid shows linear behavior whereas a fragile liquid exhibits an upward curvature [super-Arrhenius (SA) behavior], a situation customarily described by using the Vogel–Fulcher–Tammann law. Here we analyze existing data of the transport coefficients of 84 glass-forming liquids. We show the data are consistent, on decreasing temperature, with the onset of a well-defined dynamical crossover η×, where η× has the same value, η× ≈ 103 Poise, for all 84 liquids. The crossover temperature, T×, located well above the calorimetric glass transition temperature Tg, marks significant variations in the system thermodynamics, evidenced by the change of the SA-like T dependence above T× to Arrhenius behavior below T×. We also show that below T× the familiar Stokes–Einstein relation D/T ∼ η-1 breaks down and is replaced by a fractional form D/T ∼ η-ζ, with ζ ≈ 0.85.

NMR evidence of a sharp change in a measure of local order in deeply supercooled confined water

Using NMR, we measure the proton chemical shift δ, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because δ 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 −(∂ ln δ/∂T)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 −(∂ ln δ/∂T)P displays a pronounced maximum upon crossing the locus of maximum correlation length at ≈240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that CP(T) displays a maximum on crossing the Widom line.

Dynamical Crossover and Breakdown of the Stokes−Einstein Relation in Confined Water and in Methanol-Diluted Bulk Water

Using nuclear magnetic resonance and quasi-elastic neutron scattering spectroscopic techniques, we obtain experimental evidence of a well-defined dynamic crossover temperature T(L) in supercooled water. We consider three different geometrical environments: (i) water confined in a nanotube (quasi-one-dimensional water), (ii) water in the first hydration layer of the lysozyme protein (quasi-two-dimensional water), and (iii) water in a mixture with methanol at a methanol molar fraction of x = 0.22 (quasi-three-dimensional water). The temperature predicted using a power law approach to analyze the bulk water viscosity in the super-Arrhenius regime defines the fragile-to-strong transition and the Stokes-Einstein relation breakdown recently observed in confined water. Our experiments show that these observed processes are independent of the system dimension d and are instead caused by the onset of an extended hydrogen-bond network that governs the dynamical properties of water as it approaches dynamic arrest.

Clustering Dynamics in Water/Methanol Mixtures : A Nuclear Magnetic Resonance Study at 205 K < T < 295 K

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<T<295 K. The measured relaxation times in the mixtures, at all the methanol molar fractions, are faster than those of pure water and methanol because of strong interactions, resulting in a complex hydrogen bonding dynamics that determines their thermodynamic properties. In particular, we observe how the interplay between hydrophobicity and hydrophilicity changes with temperature and influences the peculiar thermal behavior of the NMR relaxation times of the solution. The obtained results are interpreted in terms of the existence of stable water-methanol clusters at high temperature whereas, upon cooling to low temperature, clusters of single species are present in the mixture.

Destructuring effect of trehalose on the tetrahedral network of water: a Raman and neutron diffraction comparison

Results on trehalose/water solutions by neutron diffraction are reported. The analysis of the partial structure factors obtained by neutron diffraction on trehalose/water solutions is interpreted in terms of a destructuring effect of trehalose on the tetrahedral H-bond network of water. This hypothesis, suggested by a previous analysis of the Raman intramolecular OH stretching band, can account for the greater cryoprotective action of trehalose on biological structures.

Conformational distribution of poly(ethylene oxide) in molten phase and in aqueous solution by quasi-elastic and inelastic light scattering

We report on measurements performed by Raman scattering, PCS and ultrasonic velocity measurements on poly(ethylene oxide) both in the molten phase and in aqueous solution. Increasing the polymerization degree, m, the Raman analysis of the D-LAM (disordered longitudinal acoustic mode) spectral contribution to the pure polymers reveals a behaviour of the centre frequency and linewidth which has been connected with an oligomer-polymer transition occurring at . In aqueous solutions the frequency increase towards values corresponding to the crystal ones and the sharpening of the D-LAM spectral contribution indicate that the addition of water destroys the intermolecular interactions and stiffens the coil structure. In addition evidence of a more ordered conformation with respect to the melt phase is presented. The temperature analysis of the D-LAM band and of the hydrodynamic radius, evaluated by PCS, reveals that the solvent power of water increases up to , decreasing at higher temperature. Interpreted in conjunction with ultrasonic data, these apparently differing findings provide a coherent interpretative key capable of encompassing the structural properties of our systems. Finally the role played by inter- and intra-molecular interactions is discussed within the framework of current theories.

Multi-component modeling of quasielastic neutron scattering from phospholipid membranes

We investigated molecular motions in the 0.3-350 ps time range of D2O-hydrated bilayers of 1-palmitoyl-oleoyl-sn-glycero-phosphocholine and 1,2-dimyristoyl-sn-glycero-phosphocholine in the liquid phase by quasielastic neutron scattering. Model analysis of sets of spectra covering scale lengths from 4.8 to 30 Å revealed the presence of three types of motion taking place on well-separated time scales: (i) slow diffusion of the whole phospholipid molecules in a confined cylindrical region; (ii) conformational motion of the phospholipid chains; and (iii) fast uniaxial rotation of the hydrogen atoms around their carbon atoms. Based on theoretical models for the hydrogen dynamics in phospholipids, the spatial extent of these motions was analysed in detail and the results were compared with existing literature data. The complex dynamics of protons was described in terms of elemental dynamical processes involving different parts of the phospholipid chain on whose motions the hydrogen atoms ride.

On the bioprotective effectiveness of trehalose: ultrasonic technique, Raman scattering and NMR investigations

Abstract The topic dealt in this article concerns with an experimental study, performed by means of ultrasonic techniques, Raman scattering and NMR, on aqueous solutions of ( α , α -trehalose, an effective bio-protector against dehydration and freezing. What emerges from this study is that the trehalose–water systems show a fragile character, namely a structural sensitivity to temperature and concentration changes. Futher, the addition of trehalose to water gives rise to a destructuring effect on the tetrahedric hydrogen bond network of water, that originates low density conformations similar to that of supercooled water.