A broadband depolarized light scattering (DLS) study is performed on diluted lysozyme aqueous solutions as a function of temperature and concentration. The dynamical susceptibility, obtained in a wide spectral range (0.6-36000 GHz) through the coupled use of interferometric and dispersive devices, is interpreted and compared with neutron scattering and Raman-induced optical Kerr-effect literature data, thus giving a general picture of relaxation phenomena. We show that the proposed approach represents a suitable tool for investigating the hydration dynamics of protein-water solutions. A detailed analysis of the quasi-elastic scattering region evidences the existence of two distinct relaxational processes at picosecond time scales. The fast process (fractions of picosecond) is attributed to bulk water dynamics, while the slow one (few picoseconds) is attributed to dynamical rearrangements of water molecules strongly influenced by the protein (hydration water). The retardation effect here estimated of about 6-7 can be regarded as a direct measure of the increased protein-water and water-water hydrogen bond stability of the water molecules within the protein hydration shell. Interestingly, a similar effect was previously observed on small hydrophilic sugar molecules. Moreover, backbone and side chains torsional motions of the protein in the 600-5300 GHz frequency range are found to be insensitive to thermal variations and to eventual changes occurring in the premelting zone.

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.

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.

Brillouin and Raman scattering spectroscopy are established techniques for the nondestructive contactless and label-free readout of mechanical, chemical and structural properties of condensed matter. Brillouin-Raman investigations currently require separate measurements and a site-matching approach to obtain complementary information from a sample.Here we demonstrate a new concept of fully scanning multimodal micro-spectroscopy for simultaneous detection of Brillouin and Raman light scattering in an exceptionally wide spectral range, from fractions of GHz to hundreds of THz. It yields an unprecedented 150 dB contrast, which is especially important for the analysis of opaque or turbid media such as biomedical samples, and a spatial resolution on sub-cellular scale. We report the first applications of this new multimodal method to a range of systems, from a single cell to the fast reaction kinetics of a curing process, and the mechano-chemical mapping of highly scattering biological samples.

Biological interfaces characterized by a complex mixture of hydrophobic, hydrophilic, or charged moieties interfere with the cooperative rearrangement of the hydrogen-bond network of water. In the present study, this solute-induced dynamical perturbation is investigated by extended frequency range depolarized light scattering experiments on an aqueous solution of a variety of systems of different nature and complexity such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins. Our results suggest that a reductionist approach is not adequate to describe the rearrangement of hydration water because a significant increase of the dynamical retardation and extension of the perturbation occurs when increasing the chemical complexity of the solute.

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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.

We have studied the influence of the amphiphilic model peptide N-acetyl-leucine-methylamide (NALMA) on the dynamics of water using extended frequency range depolarized light scattering (EDLS), between 0.3 GHz and 36 THz. This technique allowed us to separate solute from solvent dynamics and bulk from hydration water, providing both characteristic times and relative fractions. In the temperature range 5-65 °C, a retardation factor from 9 to 7 is found for water hydrating NALMA. Moreover, in the same range, a hydration number from 62 to 50 is observed, corresponding to more than two hydration layers. This strong perturbation suggests the existence of a collective effect of amphiphilic molecules on surrounding water molecules.