  The dynamics of water in aqueous solutions of three homologous disaccharides,
namely trehalose, maltose and sucrose, has been analyzed by means of molecular
dynamics simulations in the 0-66 wt % concentration range. The low-frequency
vibrational densities of states (VDOS) of water were compared with the
susceptibilities chi" of 0-40 wt % solutions of trehalose in D2O obtained from
complementary Raman scattering experiments. Both reveal that sugars
significantly stiffen the local environments experienced by water. Accordingly,
its translational diffusion coefficient decreases when the sugar concentration
increases, as a result of an increase of water-water hydrogen bonds lifetimes
and of the corresponding activation energies. This induced slowing down of
water dynamics, ascribed to the numerous hydrogen bonds that sugars form with
water, is strongly amplified at concentrations above 40 wt % by the percolation
of the hydrogen bond network of sugars, and may partially explain their
well-known stabilizing effect on proteins in aqueous solutions.
