M. Paolantoni

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.

Hydration and Aggregation in Mono- and Disaccharide Aqueous Solutions by Gigahertz-to-Terahertz Light Scattering and Molecular Dynamics Simulations

The relaxation properties of hydration water around fructose, glucose, sucrose, and trehalose molecules have been studied by means of extended frequency range depolarized light scattering and molecular dynamics simulations. Evidence is given of hydration dynamics retarded by a factor ξ = 5-6 for all the analyzed solutes. A dynamical hydration shell is defined based on the solute-induced slowing down of water mobility at picosecond time scales. The number of dynamically perturbed water molecules N(h) and its concentration dependence have been determined in glucose and trehalose aqueous solutions up to high solute weight fractions (ca. 45%). For highly dilute solutions, about 3.3 water molecules per sugar hydroxyl group are found to be part of the hydration shell of mono- and disaccharide. For increasing concentrations, a noticeable solute-dependent reduction of hydration number occurs, which has been attributed, in addition to simple statistical shells overlapping, to aggregation of solute molecules. A scaling law based on the number of hydroxyl groups collapses the N(h) concentration dependence of glucose and trehalose into a single master plot, suggesting hydration and aggregation properties independent of the size of the sugar. As a whole, the present results point to the concentration of hydroxyl groups as the parameter guiding both sugar-water and sugar-sugar interactions, without appreciable difference between mono- and disaccharides.

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.

Solvent effect on the vibrational dephasing of the ν2 (CN) and ν4 (CC) stretching modes in liquid acetonitrile and acetonitrile-d3

Abstract The vibrational dynamics of CH 3 CN and CD 3 CN have been examined in pure substances and in mixtures with CCl 4 at constant, room temperature, analysing the band shapes and the frequency shifts of ν 2 (CN) and ν 4 (CC) stretching modes. The effective temperature of the scattering volume, instead of the thermostatic value, was used to calculate and eliminate some secondary structures present in all the examined profiles. By doing this, it has been possible to correctly extract the dynamic parameters from the Raman isotropic profiles and from the relevant time correlation functions. The nonideality of the CH 3 CN/CCl 4 mixture clearly emerges from the application of the Kubo theory in the frequency domain, following a calculation procedure previously developed in our laboratory. The concentration dependence of the bandwidths and the frequency shifts in ν 2 mode have been interpreted within the theory of the local concentration fluctuation.

Hydrophobic hydration of tert-butyl alcohol studied by Brillouin light and inelastic ultraviolet scattering

The longitudinal viscosity of diluted water-tert-butyl alcohol solutions in the 10 GHz frequency region has been measured by means of Brillouin light scattering and inelastic ultraviolet scattering. The main advantage of our hypersonic investigation compared to more traditional ultrasonic measurements is that in the gigahertz frequency range slow relaxation processes involving the alcohol dynamics are completely unrelaxed, so that the measured viscosity mainly originates from the hydrogen bond restructuring of water. In contrast with previous determinations, we estimate an activation energy which is independent from the alcohol mole fraction up to X = 0.1, and comparable to that of bulk water. A simple two-component model is used to describe the steep increase of viscosity with increasing alcohol mole fraction, and a retardation factor 1.7 ± 0.2 is found between the relaxation times of hydration and bulk water. These findings endorse a dynamic scenario where the slowing down of hydration water is mainly due to a reduction of configurational entropy and does not involve an arrested, icelike, dynamics.

Hydration properties of small hydrophobic molecules by Brillouin light scattering

We study the relaxation of water molecules next to hydrophobic solutes with different functional groups by Brillouin light scattering. Evidence is given for (i) water activation energy in trimethylamine-N-oxide, proline and t-butyl alcohol diluted solutions which is comparable to that of neat water, almost independent from solute mole fraction and (ii) moderate slowdown of relaxation time of proximal water compared to the bulk, which is consistent with excluded volume models. Assuming that the main contribution to viscosity comes from bulk and hydration water, a rationale is given of the phenomenological Arrhenius laws for the viscosity of diluted aqueous solutions.

Solvent Sharing Models for Non-Interacting Solute Molecules: The Case of Glucose and Trehalose Water Solutions

The concentration dependence of hydration numbers of molecules modelled as nearly spherical particles is studied by simple analytic and numerical approaches, in the ideal limit of absence of intermolecular interactions. It is shown that the random close-to-contact condition achieved by solute molecules, noticeably affects the average hydration numbers. Comparison with experimental results obtained by light scattering in glucose and trehalose water solutions shows a reduction of the hydration number that is twice faster than that calculated in absence of interactions, suggesting important aggregation phenomena to occur in both systems, even at relatively low solute concentration. The effect of concentration on shear viscosity is also reported, suggesting that the leading contribution to the increase of viscosity arises from hydration water.

Evidence of DMSO-Induced Protein Aggregation in Cells.

We report on a study of protein aggregation induced on different cell samples by dimethyl sulfoxide (DMSO) addition. DMSO is the most commonly used cryoprotectant because it is supposed to readily diffuse across lipid bilayers, thus reducing water activity within cells; despite its large use, the mechanism of penetration and even the main interaction features with cell components are far from being understood. In the present work, infrared absorption spectroscopy is successfully applied to real time detection of chemical and structural changes occurring in cells during dehydration from water and water/DMSO suspensions. As a most interesting result, DMSO is observed to favor protein aggregation both in cellular model systems, as cultured lymphocytes and fibroblasts, and in human samples for clinic use, as hematopoietic stem cells from cord blood. This effect is evidenced at low water content, analogously to what is observed for protein solutions. Such tendency is not specific of the type of protein and suggests one possible origin of DMSO toxicity.

Cryopreservation of cells: FT-IR monitoring of lipid membrane at freeze-thaw cycles.

In the present study, FTIR spectroscopy was used to monitor the freeze-thaw cycle of two cellular lines (HuDe and Jurkat) suspended in three different media: phosphate buffer solution (PBS); dimethylsulfoxide (DMSO)/PBS solution at 0.1 DMSO molar fraction; and CryoSure (0.1 DMSO molar fraction PBS solution+dextran 5% w/v) solution. The Trypan Blue test was also applied before freezing and after thawing each cell sample to estimate the recovery of membrane integrity after thermal treatment, and correlate this datum with spectroscopic results. By following the temperature evolution of two different spectral components (the libration and bending combination mode νc(H2O) at 2000-2500 cm(-1), and the methylene symmetric stretching vibration νsym(CH2) at about 2850 cm(-1)) in the -120÷28°C range, we evidenced the main transition of lipid membrane in connection with cell dehydration, as induced by ice formation in the extracellular medium. In particular, in DMSO/PBS and CryoSure samples we observed a transition to a more rigid state of the lipid membrane together with an increased amount of non-freezable water in the extracellular medium; these results are connected to the role of DMSO as a cryoprotective agent irrespective of the nature of cell type.

Hydrophobic Hydration in Water-tert-Butyl Alcohol Solutions by Extended Depolarized Light Scattering.

Molecular dynamics and structural properties of water-tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.