Structure of saccharides

The crystal structure of the Escherichia coli galactose-binding protein with a bound molecule of p-o-glucose was obtained. From the structure, it can be observed that recognition is achieved by sequestering the monosaccharide entirely beneath the protein surface and expelling bulk water from the active site. This not only fosters a favourable entropic stabilisation but also allows the surrounding amino acid residues unfettered access to the substrate. In the 

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The lamellar structure of saccharide-peptide block copolymers presents many analogies with the structure of copolymers with a hydrophobic polyvinyl block and a hydrophobic polypeptide block and exhibits a similar behaviour as a function of the solvent concentration. When the solvent concentration increases the total thickness of a sheet and the thickness of the carbohydrate layer both increase while the thickness of the polypeptide layer remains nearly constant284. ... 

Figure 3.13. Chemical composition and structure of saccharides, chlorides, amino sugars, amino acids, amides, and a heterocyclic amino acid.

Additional evidence on the selection of conformations by the exo-ano-meric effect is derived from the solid-state structures of saccharides. It was earlier observed that the actual orientation of the anomeric alkoxyl group in pyranosides in the solid state corresponds to the (+sc, +sc) or ap, —sc) conformer, and thus proved that these conformers respectively represent the most stable axial and equatorial forms. As already noted, a particularly clear illustration of the operation of the exo-anomeric effect comes from the nonreducing disaccharide 0 ,a-trehalose, in which the most stable orientation about both exocyclic, C-0 bonds corresponds to the (+sc, +sc) conformer. Analyses of carbohydrate structures - revealed regularities in the distribution of the torsional angle O that are consistent with a restriction of rotation about the exocyclic C-O bond. The torsional angle for equatorial isomers varies from — 50° to — 110 , with a mean value of—79.4°. For the axial isomers, the range is 30-130°, with a mean value of 84.5 ° (see Ref. 29). 

Figure 4.13 The Structures of Saccharides with Blood Group I and i Specificities. These are an example of the presence of antigenic sequences inside saccharide sequences, unlike A and B which are always terminal.

For all copolymers studied (copolymers containing between 18 % and 83 % of polypeptide] the liquid crystalline structures are always lamellar and are very similar to the structure of saccharide-peptide block copolymers. In the lamellar structure of copolymers with a polyvinyl block and a hydrophobic polypeptide block, each sheet of thickness d results from the superposition of two layers one of thickness d/ formed by the polyvinyl chains in a more or less random coil conformation, the other of thickness dg formed by the polypeptide chains, in an a helix conformation, arranged in an hexagonal array, and generally folded ( 7, 8]. ... 

In solution, the method of choice to study the three-dimensional structure of saccharides is NMR, through the parameters represented by chemical shifts, coupling constants, nuclear Overhauser effects (nOe), and also relaxation time measurements. Although the conformational dependence of the carbon chemical shifts is far from imderstood, coupling constants can be used to evaluate the magnitude of the torsion angles, and nOe measurements can provide estimations of distances between protons located in rather close proximity. In addition, relaxation time measurements give information on the mobility and the behavior of molecules in solution. 

Figure 3. Schematic chemical structure of saccharide-based PU.

The proximity and defined stereochemistry of hydroxyl groups in the ring structures of saccharides allows for a number of selective reactions. These include formation of... 

Ford, L., et al., 1974. Cry.stal. structure of a ly.sozyme-tetra.saccharide lactone complex. y(9wm z/ of Molecular Biology 88 349-371. 

A structural study on lipid A and the O-specific polysaccharide of the lipopoly-saccharide from a clinical isolate of Bacteroides vulgatus from a patient with Crohn s disease was conducted by Hashimoto and coworkers [39]. They separated two potent virulence factors, capsular polysaccharide (CPS) and lipopolysaccharide (LPS), from a clinical isolate of B. vulgatus and characterized the structure of CPS. Next, they elucidated the strucmres of O-antigen polysaccharide (OPS) and lipid A in the LPS. LPS was subjected to weak acid hydrolysis to produce the lipid A fraction and polysaccharide fraction. Lipid A was isolated by PLC, and its structure was determined by MS and NMR. 

Synthetic aspects for access to monovalent fullerene-carbohydrate hybrids were highlighted, but only a few biological applications were mentioned. In contrast, multivalent presentation of saccharides by multiple anchorages to the same structure, or their presentation as antennary glycodendrons, has generated promising results. 

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