ABSTRACT
Oligosaccharides ..........................................................................294 14.6 IMS-MS Applications to Glycochemistry and Glycobiology ....................297
14.6.1 Application to Flavonoid Diglycoside Isomers ............................297 14.6.2 Application to Glycosaminoglycans ............................................297 14.6.3 Application to Glycosylation Analysis of Proteins ...................... 298
Carbohydrates are ubiquitous constituents of plants, animals, and microorganisms. Structure and conformation of carbohydrates strongly influence their physical properties, their chemical reactivity, and their biological functions. Monosaccharides are predominantly six-or five-membered cyclic compounds, formed via a semiacetal bond between a hydroxyl and a carbonyl group. The isobaric aldoses or ketoses differ within their subgroup by absolute configuration on chiral carbon atoms giving rise to 2n possible diastereomers. The stereochemistry on the anomeric carbon at the reducing end is fixed in an α-or β-position if its hydroxy group is substituted by another monosaccharide in a glycosidic linkage or by an aglycon. The carbohydrate chain can be further extended in a linear way and/or by branching, since the glycosidic linkages can be closed between monosaccharides and/or oligosaccharides in different positions. The majority of carbohydrates, as optically active compounds found on earth, belong to the D-series, of which D-glucose (Glc) is the most abundant. Biologically highly abundant carbohydrates also include D-mannose (Man), D-galactose (Gal), two pentoses, D-xylose (Xyl), and D-ribose (Rib), as well as a nonasaccharide N-acetyl-D-neuraminic acid (NeuAc). High-molecular-weight D-glucose polysaccharides like glycogen, cellulose and dextrans, and inulin containing D-fructose, are homopolymers with a defined linkage pattern of the main chain and branching. Carrageenans from seaweeds contain mainly galactose, which is also present in the modified forms, by lack of hydroxy groups at 3 and 6 positions and by sulfation at position 4. Hyaluronic acid, a major component of synovial fluid and connective tissue, composed of the disaccharide repeating unit D-glucuronic acidN-acetyl-D-glucosamine (GlcA-GlcNAc), can be prolonged to contain up to 10,000 monosaccharide units in the chain. Glycan chains attached to proteins via an N-or O-glycosidic linkage may contain 1-30 monosaccharide units and are structurally conserved. For all N-glycan types a pentasaccharide core structure is represented by two N-acetyl-D-glycosamines and three D-mannoses, where for O-glycans there are eight defined core structures, which all have an N-acetyl-D-galactosamine core attachment to a serin or a threonin in the protein chain in common (Figure 14.1). A high number of statistically possible variables of glycosylation is somewhat reduced according to defined biosynthetic rules, by which glycosyl transferases as primary gene products select the substrates for attachment of additional monosaccharides following their enzymatic specificity.
