Water structuring properties of carbohydrates, molecular dynamics studies on 1,5-anhydro-D-fructose

Molecular dynamics simulations of 1,5-anhydro-D-fructose in aqueous solution have been carried out at ambient temperature using the DL POLY program to determine the carbohydrate's water-structuring properties. Three different force-fields were applied to the molecule, and the SPC water model was chosen to represent the solvent. Five properties describing the water structure were calculated to examine the immediate aqueous environment of the carbohydrate: the available volume radial distribution functions; the angular distribution functions; the average coordination and hydrogen bond numbers; and the water oxygen probability densities. In order to evaluate the results, calculations were also carried out using alpha -D-glucose and alpha -D-mannose as reference molecules. Two of the force-fields contain repulsive terms between adjacent hydroxy groups to prevent intramolecular hydrogen bonding and differ from each other only in the values of these terms. These force-fields have been shown to be less appropriate for the simulation of carbohydrates in aqueous solution, whereas the third force-field, which allows limited intramolecular attractive interactions, has proven to be more suitable. With this forcefield, 1,5-anhydro-D-fructose generates a water structure that is significantly different from the solvent structure observed around the reference monosaccharides and this can be ascribed to the different arrangement of functional groups, in that 1,5-anhydro-D-fructose contains a more hydrophilic ring oxygen than glucose and mannose and a clearly hydrophobic ring methylene group. Additionally, the hydration spheres of the two geminal hydroxy groups interfere with each other leading to a weaker solvent structure close to these groups.