Structure cristalline du mélézitose monohydraté

Conformational analysis of furanoside-containing mono- and oligosaccharides.

A molecular mechanics analysis of the conformational properties of sucrose in vacuo in terms of the intersaccharidic torsion angles Φ and Ψ, revealed three energy minima. The geometry of the global minimum-energy 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), revealed this direct H-bond interaction to be replaced by an indirect, water-mentioned one: an interresidue water-bridge of the 2g-O…H2O…HO-1f type prevailed with a high significance and a long life-time. This means the linkage geometry of sucrose in water–despite the absence of direct interresidue hydrogen bonds–again closely resembles the solid-state and in vacuo geometry in terms of the orientation of the glucose and the fructose unit relative to one another. The solution dynamics of, and the hydration around sucrose were analyzed in terms of pair distribution functions. These 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 A), whereas the acetalic oxygens are engaged to a lesser extent as H-bond acceptors. The second hydration shell (>4 A) is rather diffuse and less pronounced, indicating those water molecules to be in a disordered 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. Other sucrose conformations that may conceivably exist in aqueous solution, may have eluded the MD simulation search. 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 these, the by far, most populated global minimum structure corresponded to the most stable solution geometry, as found by molecular dynamics.

Molecular modeling of saccharides, 7. The conformation of sucrose in water: A molecular dynamics approach†

The exo-anomeric effect: experimental evidence from crystal structures

Molecular modelling techniques have been applied to compute the conformation accessible to bacterial deep rough lipopolysaccharide of Escherichia coli (Re-LPS). Analyses of the results showed that the models typically exhibit a tilt of the diglucosamine backbone with respect to the membrane normal of 53 +/- 7 degrees while both the glucosamine ring planes are oriented approximately parallel to the membrane normal. Different models were found to show compact and elongated types of acyl chain arrangements, both producing anisotropic lateral dimensions of the models of 1.0-1.1 nm and 1.7-2.0 nm for the shorter and the longer side, respectively. The conformationally allowed range of the isolated dOclA(alpha-2-4)dOclA disaccharide (dOclA = 3-deoxy-D-mannooctulosonic acid) was found to be extremely limited. It appeared that the dOclA disaccharide (dOclA)2 is centred at the top of the Re-LPS molecule preferring two orientations stabilized by hydrogen bonds involving only one phosphate group of the lipid A moiety at a time. The effect of charges on the Re-LPS conformations has been studied in separate calculations. From these calculations it was obvious that charges have no significant effects on the conformations of the isolated lipid A and (dOclA)2 moieties. However, it was found that the orientation of (dOclA)2 with respect to the lipid A part is highly sensitive to charges, i.e. in the charged models the proximity of phosphate and carboxyl groups is prevented by strong electrostatic repulsion between these negatively charged groups. In order to rationalize the acyl chain packing of the models, a simple geometrical model which correlates the tilt of the diglucosamine backbone with the energically favoured close packing of the acyl chains is proposed. Furthermore, the possibility of a chelate-like complexation of divalent cations and its contribution to head group mobility is discussed.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1991.tb15962.x

Structure cristalline du mélézitose monohydraté

Citations

Conformational analysis of furanoside-containing mono- and oligosaccharides.

Molecular modeling of saccharides, 7. The conformation of sucrose in water: A molecular dynamics approach†

The exo-anomeric effect: experimental evidence from crystal structures

Molecular modelling of bacterial deep rough mutant lipopolysaccharide of Escherichia coli.

Chemical and Physical Properties of Fructans

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