Anisotropic solvent structuring in aqueous sugar solutions

The details of solution structure have proven difficult to probe experimentally, but computer simulations allow solvent structuring to be examined directly. We report here molecular dynamics (MD) simulations of a series of pentose sugars in solution, describe in detail the three-dimensional structuring which these molecules impose on water, and relate this structuring to the solute molecular topologies. Well-defined first and second solvation shells are observed around the sugar molecules with specific locations determined by the arrangement of functional groups within each solute. Calculated molecular internal energies and solvent interaction energies generally correspond qualitatively with earlier models of sugar energetics based on stability factors. However, pairs of axial hydroxyl groups on the same side of-a sugar ring were found to be disfavored by solvation energy, not by internal energy, which is actually more favorable due to intramolecular hydrogen bonding, in accord with both quantum mechanical calculations and a crystal structure for a related molecule determined by diffraction. A single axial hydroxyl group could be favored or disfavored energetically by hydration, depending on the particular circumstance.