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

Abstract

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.

Broadband dielectric spectroscopy of glucose aqueous solution: Analysis of the hydration state and the hydrogen bond network

“…Nevertheless, our statement that the glucose relaxation component can be separated from water relaxation is consistent with recent depolarized scattering measurements, where solute relaxations are distinctly present because their spectrum does not represent a dipole moment but a polarizability anisotropy. [14][15][16][17][18] As can be seen in Figure 4, the apparent dipole moment of glucose µ eff exhibits little concentration dependence, which is in line with an earlier dielectric spectroscopic result. 23 Once we introduce the Kirkwood correlation g factor in order to describe the short-range dipole-dipole self-correlation with the relationship µ eff 2 = g µ 0 2 (µ 0 : dipole moment of isolated solutes), the dependence of the g factor on the glucose concentration is shown in the inset of Figure 4.…”

“…Indeed, they measured a "static" aspect of the hydrated water, focusing on the compressibility or melting point of water molecules, while the criterion for distinction between the bulk and hydrated water lies in its dynamical aspects in our analysis. Also, the n 0 hyd = 21.0 ± 0.7 is slightly larger than the recent depolarized light scattering measurements (14)(15)(16), 17,18 probably owing to the distinctive molecular dynamics probed by that technique, as described above. Recently, Havenith et al 32,33 used THz vibrational spectroscopy around 2.5 THz to measure the saccharide hydration state, based on the dipole fluctuation at a sub-picosecond time scale (corresponding to 2.5 THz).…”

“…and MD simulations (2.5). 6 On the other hand, Lupi et al 17,18 have reported a larger retardation factor, ξ = 5-6, for both mono-and disaccharides using depolarized light scattering. This discrepancy may arise from the different molecular dynamics probed.…”

“…65 As an earlier MD simulation has pointed out, 6 the translational and rotational dynamics of water in the hydration shell are decoupled, reasonably explaining the difference in ξ between our and Lupi et al results. 17,18 In this context, interestingly, the fact of a smaller retardation in the orientational than the translational direction evokes an image of more restricted collective relaxation dynamics in the translational direction in the saccharide hydration layer. Furthermore, a retardation factor of up to 2.8 in this study does not have to call on the existence of dynamically immobilized or icelike hydrated water molecules in the hydration shell.…”

“…Although the results from a number of experimental methods, including the Raman spectroscopy, [8][9][10] depolarized light scattering, [14][15][16][17][18] nuclear magnetic resonance (NMR), 19 neutron diffraction, 20 infrared (IR) spectroscopy, 9 polarizationresolved femtosecond spectroscopy, 21 and dielectric spectroscopy, [22][23][24] have been compared to those from MD simulations to characterize the hydration state and the destructuring effect of saccharides in aqueous solution, each has its limitations.…”