CP/DD/MAS 13C NMR spectra have been obtained for different monosaccharides, oligosaccharides, and cellulose. It has been found that a simple linear relationship exists between the chemical shift of the CH2OH carbon and the torsion angle χ about the exo-cyclic C-C bond. The chemical shifts fall into three groups of 60–62.6 ppm, 62.5–64.5 ppm, and 65.5–66.5 ppm, which are related to gauche-gauche, gauche-trans, and trans-gauche conformations, respectively. On the basis of these results the conformation of the CH2OH carbon of cellulose is also discussed.

Solid-state 13C-NMR study of conformations of oligosaccharides and cellulose: Conformation of CH2OH group about the exo-cyclic C-C bond

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

Physical properties of the hyaluronate binding region of proteoglycan from pig laryngeal cartilage: Densitometric and small-angle neutron scattering studies of carbohydrates and carbohydrate-protein macromolecules

Isoenergy surfaces were calculated for the α- and β-anomers of isomaltose and gentiobiose, based on 46,656 conformers for each disaccharide. Low-energy regions exist for each of the three staggered positions about the C-5′ C-6′ bonds, and known crystal structures lie in two of these regions. As expected, the molecular partition function showed greater flexibility for these three-bond-linked disaccharides than for comparable two-bond-linked structures. A model miniature crystal of gentiobiose accounts for most of the remaining structural differences between the modeled isolated molecule and the crystal structure. Based on models of isolated molecules of isomaltose and gentiobiose, the predicted Boltzmann-weighted nmr coupling constants were satisfactory, as were predicted optical rotations for gentiobiose. © 1994 John Wiley & Sons, Inc.

T. Koops

Papers

The crystal structure of a disaccharide, α-melibiose monohydrate (O-α-d-galactopyranosyl-(1 → 6)-α-d-glucopyranoside)

Order–disorder phenomena in structures of carboxylic acids: the structures of fluoromalonic acid and hydroxymalonic acid at 20 and –150°C