Syntheses, Conformations and X‐Ray Structure Analyses of the Saccharide Chains from the Core Regions of Glycoproteins

The covalently bound carbohydrate moiety in glycoproteins can stabilize the protein molecule intramolecularly, or it may have an intermolecular function as receptor in biological recognition. The discovery of these biological phenomena has led to a renaissance of the chemistry and biochemistry of carbohydrates. Both N ‐glycoproteins as well as O ‐glycoproteins contain special, invariant oligosaccharide chains in the protein‐binding region, which occur again in all glycoproteins, and are described as the “core regions.” This review describes the various methods of oligosaccharide synthesis that may be used to arrive at the basic core structures by chemical means. Methods of oligosaccharide synthesis have improved so much that it is possible to synthesize complex lactosamine‐type structures, and “bisected”‐type structures up to nona‐ and undecasaccharides respectively. Oligosaccharide chains are considerably less flexible than peptide chains. Using modern methods of NMR spectroscopy, their preferred solution conformation can readily be determined. In the case of one branched octasaccharide, a comparison of the conformations in solution and in the crystal is possible. Oligosaccharides may be linked to the amide group of an asparagine, or to the hydroxyl groups of serine or threonine. By using suitable protecting groups, the glycosyl amino acids obtained can be extended with further amino acids at the N ‐ or C ‐terminus, thus arriving at the desired glycopeptide sequences. In the linkage region, glycopeptides prefer certain conformations. Future research into glycoprotein functions may involve the synthesis and biochemical study of modified glycoprotein segments.