Anomalous Dynamics of Hydration Water in Carbohydrate Solutions

Molecular dynamics simulations have been carried out to investigate the dynamics of fructose aqueous solutions up to 70 wt % concentration. We find that the hydrogen (H)-bonded network of fructose molecules extends with increasing sugar content and forms a structurally heterogeneous system around and above 45 wt % concentration, characterized as a percolated-like solute domain permeated by patchy regions of solvent. The presence of such aggregates in concentrated solutions promotes the slowing down of water translational, reorientational, and H-bonding dynamics, typical of many biomolecular environments. Analysis of the effects of the topological and energetic disorder of the sugar aggregates on vicinal water dynamics, similar to that recently carried out for the hydration layer of proteins by Pizzitutti et al. (J. Phys. Chem. B 2007, 111, 7584), reveals many similarities between the dynamical anomaly of the hydration layers of both systems. Like a protein surface, topological and energetic disorders of the sugar aggregates both contribute to the translational diffusion anomaly. However, unlike in the vicinity of a protein surface, the rotational relaxation is also hindered by the topological disorder created by the intertwined, percolating sugar clusters in concentrated solutions.