M. E. Gallina

Light Scattering Spectra of Water in Trehalose Aqueous Solutions: Evidence for Two Different Solvent Relaxation Processes

Light scattering spectra on aqueous solutions of trehalose were recorded in a wide frequency range combining the use of a double monochromator and a multipass Fabry-Perot interferometer. Experimental results indicate the presence of a slow relaxation mode related to the solute dynamics, which is clearly separated from the solvent one. The spectral analysis reveals the existence of two separate solvent relaxation processes assigned to hydrating and bulk water molecules. The picosecond dynamics of water molecules directly interacting with the solute (proximal water) is consistently delayed with the corresponding relaxation time increase is about 5-6 times compared to the bulk. The slowing down induced by the sugar on the water dynamics mainly involves a restricted hydration layer constituted of 16-18 water molecules. These results improve our knowledge about the influence of carbohydrates on the fast rearrangement dynamics of water and may serve as a model to gain important insight on basic solvation properties of other biorelevant systems in aqueous media.

Rotational dynamics of trehalose in aqueous solutions studied by depolarized light scattering

High resolution depolarized light scattering spectra, extended from 0.5 to 2x10(4) GHz by the combined used of a dispersive and an interferometric setup, give evidence of separated solute and solvent dynamics in diluted trehalose aqueous solutions. The slow relaxation process, located in the gigahertz frequency region, is analyzed as a function of temperature and concentration and assigned to the rotational diffusion of the sugar molecule. The results are discussed in comparison with the data obtained on glucose solutions and they are used to clarify the molecular origin of some among the several relaxation processes reported in literature for oligosaccharides solutions. The concentration dependence of relaxation time and of shear viscosity are also discussed, suggesting that the main effect of carbohydrate molecules on the structural relaxation of diluted aqueous solutions is the perturbation induced on the dynamics of the first hydration shell of each solute molecule.

Structural properties of glucose-dimethylsulfoxide solutions probed by Raman spectroscopy

Raman spectroscopy was employed to achieve a molecular level description of solvation properties in glucose-dimethylsulfoxide (DMSO) solutions. The analysis of Raman spectra confirms the importance of the dipole-dipole interaction in determining structural properties of pure DMSO; the overall intermolecular structure is maintained in the whole 20-75 degrees C temperature range investigated. The blueshift of the CH stretching modes observed at higher temperatures points out that CH(3)...O contacts contribute to the cohesive energy of the DMSO liquid system. The addition of glucose perturbs the intermolecular ordering of DMSO owing to the formation of stable solute-solvent hydrogen bonds. The average number of OH...OS contacts (3.2+/-0.3) and their corresponding energy (approximately 20 kJ/mol) were estimated. Besides, the concentration dependence of the CH stretching bands and the behavior of the noncoincidence effect on the SO band, suggest that the dipole-dipole and CH(3)...O interactions among DMSO molecules are disfavored within the glucose solvation layer. These findings contribute to improve our understanding about the microscopic origin of solvent properties of DMSO toward more complex biomolecular systems.

The low frequency phonons dynamics in supercooled LiCl, 6H2O

We report the results of a series of ultrasound, Brillouin scattering, and optical heterodyne detected transient grating experiments performed on a LiCl, 6H(2)O solution from room temperature down to the vicinity of its liquid-glass transition, T(g) approximately 138 K. Down to T approximately 215 K, the supercooled liquid has a behavior similar to what is expected for supercooled water: its zero frequency sound velocity, C(0), continuously decreases while the corresponding infinite frequency velocity, C(infinity), sharply increases, reflecting the increasing importance of H bonding when temperature is lowered. Below 215 K, specific aspects of the solution, presumably related to the role of the Li(+) and Cl(-) ions, modify the thermal behavior of C(0), while a beta relaxation process also appears and couples to the sound propagation. The origin of those two effects is briefly discussed.

Density fluctuations of water–glucose mixtures studied by inelastic ultra-violet scattering

The dynamics of density fluctuations of aqueous glucose solutions were studied in the water-rich region by means of Brillouin ultra-violet scattering. The temperature dependence of the position and line-width of inelastic peaks gives evidence of a strong relaxation process located in the picosecond timescale. In the approximation of a single exponential process, the relaxation time, τ, was obtained for different glucose concentrations; its temperature dependence is well described by an Arrhenius law characterized by an activation energy comparable to that of pure water. This result supports the hypothesis that the microscopic mechanism responsible for the main relaxation process in sugar–water solutions is the continuum rearrangement of the hydrogen-bond network. A comparison is also reported with previous results on aqueous trehalose solutions, showing the acoustic absorption scales with the number of glucoside units for both glucose and trehalose aqueous solutions.

Experimental Determination of Structural Relaxation in Trehalose‐Water Solutions by Inelastic Ultraviolet Scattering

We report Brillouin ultraviolet scattering measurements on trehalose‐water solutions in a wide range of concentrations (φ = 0−0.74). A complete set of data as a function of temperature (−10 °C⩽T⩽100 °C) has been obtained for each concentration. The T‐φ evolution of the system has been analyzed in terms of energy position and linewidth of inelastic peaks. These results have been used to derive the structural relaxation time of the system, τ, which was found in the tens of ps timescale. Its T‐dependence can be described with an Arrhenius activation law, and, most importantly, a significant slowing down of the relaxation dynamics has been observed as trehalose concentration was increased. At low‐φ, the activation energy of the relaxation has been found to be consistent with literature data for pure water, and comparable with intermolecular hydrogen bond (HB) energy. This evidence strongly supports the hypothesis that the main microscopic mechanism responsible for the relaxation process in trehalose solutions...