Derived Carbohydrate Structures

Many of the carbohydrates that we have considered do not occur naturally. Instead, they are found as derived or combined compounds and often play important roles in living systems. We will consider the structures of some of these compounds in this section. 

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Multiple-ion monitoring is, however, of considerable value in structural studies, but only if model compounds of known structure are available for comparison. Such an approach has been used in the study of the carbohydrate structures of glycoproteins from different tissues.50 Separation of glycopeptides obtained from various tissues was performed on columns of concanavalin A-Sepharose. Structural analysis by multiple-ion monitoring of partially methylated, alditol acetates derived from the various fractions indicated that the glycopeptides were separated according to the linkage pattern of mannose (see Fig. 1). 

In addition to these physical studies at the Bureau, Tipson was able to return to his synthetic interests, both alone and in collaboration with other staff members. He was especially pleased to prepare D-talose in crystalline form, an accomplishment that had eluded Emil Fischer. Pursuing his longstanding interest in the reaction of sulfonic esters with iodide and following an earlier observation that the tetratosyl ester of erythritol is converted into butadiene by the action of sodium iodide and zinc, he demonstrated (with A. Cohen) that nonterminal unsaturation may be conveniently introduced into alditol derivatives by reaction of contiguous secondary sulfonates with sodium iodide and zinc dust in boiling A.A-dimethylformamide. This Tipson-Cohen reaction subsequently proved of great utility in other hands for the conversion of more complex carbohydrate structures into vicinal dideoxy derivatives. 

The direct homologation technique was then extended to the synthesis of various uncommon carbohydrate structures. Thus, higher sugars of the L-series were obtained Starting from 2,3-0-isopropylidene-4-O-benzyl-L-threose (43) [39c] (Scheme 13), and the amino tetrose 47 and pentose 48 were prepared from the a-amino aldehyde 46 derived from L-serine [46a] (Scheme 14). These amino sugars were used as chiral building blocks for the... 

R. R. Schmidt and G. Effenberger, C-Glucosylarenes from O-a-D-glucosyl trichloroacetimi-dates. Structure of bergenin derivatives, Carbohydr. Res. 171 59 (1987). 

Perhaps the greatest advantage of using neoglycoproteins is that it is possible to obtain a series of derivatives that differ in specific structural details of the carbohydrate portion, thereby permitting an evaluation of the effect of carbohydrate structure on neoglycoprotein activ-... 

Oligolysine dendrimers with terminal carbohydrate structures were functionalised by solid-phase synthesis for well-defined construction of glycoclusters . Synthesis proceeded via coupling of lysine molecules. Eight amino groups are attached by peptide bonding to the periphery via a spacer-functionalised sialic acid derivative (Fig. 8.8). 

Monosaccharides are the simplest carbohydrate structure composed of one ring that can contain five C atoms (pentose, such as ribose, which is a constituent of RNA) or six C atoms (hexose, such as galactose derived from milk sugar lactose). 

The techniques developed for the LF 3600 N-terminal protein sequencer (Beckman Instr.) provide a fast and efficient way to positively identify Ser (0-linked Sac), -Thr (0-linked Sac) and Asn (N-linked Sac) carbohydrate structures during N-terminal sequence analysis (Gooley et al., 1995). Liquid phase anhydrous trifluoroacetic acid (TEA) is used to extract glycosylated, polar amino acid derivatives from the reaction cartridge. These amino acids are then converted into PTH derivatives which can be... 

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