The hydration of sucrose

Abstract

The structural and dynamical features of the hydration of sucrose have been derived from a 500 ps molecular dynamics simulation with explicit water molecules. In order to obtain the degree and structure of the solvation of the sucrose solute, radial atomic pair distribution functions have been calculated between selected solute atoms and the water molecules. The analysis provides a molecular hydration number of 37.6 water molecules in the first hydration shell. If the ‘hydration criterion’ is restricted to include oxygen-oxygen distances less than 2.8 Å, an average hydration number of 7 is obtained, which is close to that derived experimentally from viscosity and apparent molar volume. This indicates that the firmly hydrogen bonded water molecules have more impact on the macroscopic properties than the shielding effect formed by the first hydration shell. Both the radial and orientational hydration of acetal and hydroxyl oxygens have been fully characterized. The analysis reveals significant differences between the glycosidic oxygen and the ring oxygen atoms. It also shows that the water structure around the three secondary hydroxyl groups of the pyranosyl moiety is more structured than around the rest of the molecule. This indicates a more perfect hydration of this relatively rigid part of sucrose. The residence times for polar water molecules around the sucrose solute were also characterized. Whereas typical values are in the order of 1 to 2 ps, some extremely long residence times of about 30 ps may occur in the vicinity of the O-3 hydroxyl group of the fructofuranosyl moiety. Extension of the analysis to significant bridging water molecules points towards the existence of two cases (O-2g … Ow … O-3f and O-2g … Ow … O-1f) being populated more than 41% and 66% of the time. In comparison, the direct interresidue hydrogen bond O-2g … O-1f is populated less than 4% of the time. The bridging water between O-2g and O-3f is found in the crystalline complex between sucrose and a lentil lectin. In an aqueous environment the ability of sucrose to establish these water-mediated hydrogen bonds between its two moieties may be relevant in explaining the oversaturation range which prevails before nucleation occurs. It may also have significant implications as far as the sweet-taste elicitation mechanism is concerned.