Oligosaccharides glycosidic links

Cylodextrins (CDs) are a class of chiral cyclic oligosaccharides that have molecule-sized cavities. They commonly comprise between six and eight D-glucopyranoside units that are linked via a-l,4-glycosidic links. Their bowl-shaped form is generally represented as a cylindrical funnel by analogy to the calixarenes family. There is a large number of cyclodextrin derivatives in the... 

As an example of the carbohydrate component of a glycoprotein, the structure of one of the oligosaccharide chains of immunoglobulin G (IgG see p.300) is shown here. The oligosaccharide has an N-glycosidic link to the amide group of an asparagine residue in the Fc part of the protein. Its function is not known. 

Establishing the complete structure of oligosaccharides and polysaccharides requires determination of branching positions, the sequence in each branch, the configuration of each monosaccharide unit, and the positions of the glycosidic links—a more complex problem than protein and nucleic acid analysis. 

In early attempts directed towards the solid-phase synthesis of oligosaccharides, glycosidic bonds were formed by treating pyranosyl bromides with partially protected carbohydrates (Figure 16.16). These syntheses could be performed with either the pyranosyl bromide (glycosyl donor) [184,185] or the alcohol (glycosyl acceptor) [186— 188] linked to the support. 

Five glycosidically linked oligosaccharides have been isolated from human-milk IgA. Their structures have been elucidated222 as /3-Gal-(1 -> 3)-GalNAc-ol, a-NeuAc-(2 -> 3)-/3-GaI-(l -> 3)-GalNAc-ol, /3-Gal-(1 -> 3)-[j8-GlcNAc-(l -> 6)]-GalNAc-ol, and /3-Gal-(l 3)-[/3-... 

Natural occurring 2,6-dideoxy oligosaccharides of the D-series are P-glycosidically linked, those of the L-series represent a-glycosides. The absolute configuration at the anomeric center, however, is similar in both cases. 

N-linked oligosaccharides are linked to the protein via N-glycosidic bonds, to the NH2 groups of asparagine side chains (Fig. lb) where the asparagine occurs in the sequence Asn-X-Ser (or Thr) where X is any amino acid except Pro and possibly Asp. 

Hounsell EF, Lawson AM, Feeney J, Gooi HC, Pickering NJ, Stoll MS, Lui SC, Feizi T. Structural analysis of the O-glycosidically linked core region oligosaccharides of human meconium glycoproteins which express oncofetal antigens. Eur. J. Biochem. 1985 148(2) 367-377. 

Carbohydrates (disaccharides) in aqueous solution are favorable examples for ROA studies furnishing an informative band structure over a wide range of the vibrational spectrum. Their complex and highly coupled normal modes generate strong ROA bands that produce patterns characteristic of the various types of structural units. ROA data can be used to confidently specify the central component of carbohydrate stereochemistry. For disaccharides [92] ROA can provide the nature and conformation of the glycosidic link, and can also probe extended secondary structures of oligosaccharides [93]. 

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