Unique similarity of the asymmetric trehalose solid-state hydration and the diluted aqueous-solution hydration

The structural and dynamical features of the hydration of the disaccharide alpha,alpha-trehalose have been derived from a 2.5 ns molecular dynamics with an explicit representation of the water molecules. The study aims at establishing a comprehensive understanding of the hydration pattern of trehalose and comparing such features with those displayed by sucrose. The homonuclear and heteronuclear coupling constants, the overall molecular tumbling time, and self-diffusion coefficient of the trehalose in aqueous solutions were established from the molecular dynamics simulations and compare well with experimental data. While the calculated translational diffusion of trehalose is very similar to that of sucrose, the calculated rotational diffusion is much slower; The presence of water in the simulation induces significant changes in the mean potential acting on trehalose. It generates an asymmetric mean structure between the two glucose rings, in the otherwise symmetrical trehalose. The analysis of the hydration characteristics provides an average molecular hydration number of 7.8 water molecules in the first hydration shell which is close to that derived experimentally from viscosity and apparent molar volume. Average and maximum residence times for water molecules around the trehalose solute were also characterized. The analysis revealed that the water molecules around the O-2 hydroxyl groups were the most resident and that the water molecules around the acetalic oxygens in the "central cavity" of trehalose were particular mobile. 2D radial pair distributions were calculated to analyze the solute surroundings for localized water densities, e.g., bridging water molecules between the two pyranose rings. This analysis revealed no strong first hydration shell interactions, as found in the case of sucrose, but revealed that the water molecules of the dihydrate solid-state structure are largely capable of satisfying the "hydration requirements" of the solute.