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Molecular Modeling of Saccharides, Part VII.

The Conformation of Sucrose in Water: a Molecular Dynamics Approach

Stefan Immel and Frieder W. Lichtenthaler

Liebigs Ann. Chem. 1995, 1925-1937.

A molecular mechanics analysis of the conformational properties of sucrose in vacuo in terms of the intersaccharidic torsion angles F and Y revealed three energy minima, of which the global minimum-energy geometry closely resembles the solid-state structure. Most notably, the interresidue hydrogen bonding interaction 2g-O ... HO-1f present in the crystal, is retained under vacuum boundary conditions, indicating the molecular geometries adopted in the crystal lattice and in vacuo to be similar. For aqueous solutions, detailed molecular dynamics simulations of sucrose "soaked" with 571 water molecules in a periodic box (truncated octahedron) reveal this direct H-bond interaction to be replaced by an indirect, water-mediated one: an interresidue water-bridge of the 2g-O ... H2... HO-1f type prevailing with a high significance and a long life-time. This entails the linkage geometry of sucrose in water, despite of the absence of direct interresidue hydrogen bonds, to again resembling closely the solid-state and in-vacuo geometry in terms of the orientation of the glucose and the fructose unit relative to each other. The solution dynamics of, and the hydration around sucrose were analyzed in terms of pair distribution functions, that indicate strong hydrogen bonding between all sucrose hydroxyls (as donors and acceptors) and water within a first, well-defined hydration layer (hydroxyl-oxygen - water distances 1.8 - 3.5 Å), whereas the acetalic oxygens are engaged to a lesser extent as H-bond acceptors. The second hydration shell (> 4 Å) is rather diffuse and less pronounced, indicating those water molecules to be in a non-ordered state. The implications of the hydration shell and the water bridge on the crystallization process of sucrose and on binding towards transporter proteins and the sweet-taste receptor are discussed. To search for other sucrose conformations that may conceivably exist in aqueous solution, yet may have eluded the MD simulation, the umbrella sampling technique was applied for establishing the free energy profile as a function of the intersaccharidic torsion angles. The resulting concise picture of the dynamics of sucrose in aqueous solution encompassing the entire conformational space available, revealed only two energy minima of which the by far most populated global minimum structure corresponded to the most stable solution geometry as found by molecular dynamics.

Additional Graphics: Sucrose

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