Mannose pentasaccharide

Fig. 1 Structures of oligosaccharides whose chemical shifts were predicted using CASPER program presented in CFG and 2D text notations (a) mannose pentasaccharide (M5), (b) (LacNAc-Le -LacNAc)-heptasaccharide, (c) (Le -Le )-heptasaccharide. Me - methyl, pTFA - para-trifluoroacetamidophenylethyl, AE - 2-azidoethyl.

D-Mannose is common, but L-mannose has only been found in a small group of extracellular polysaccharides of related structures, one of which is elaborated by Alcaligenes ATCC 31555. In these polysaccharides, it is a-linked and partially replaces an a-L-rhamnopyranosyl residue in the pentasaccharide repeating-unit. It seems possible that these sugar residues are scrambled, but the other possibility, that there are two populations of polysaccharides, has not yet been excluded. 

Mannose tri-, tetra- and pentasaccharide together with GM1 octasaccharide were a gift from Dr. M.A. Chester (Lund, Sweden). At the time of writing, these standards are no longer available. Pentasaccharides (from human urine) and sialylated tetra-saccharides (from human milk) are available from Sigma however, they are cost-prohibitive for routine use. 

In the first family, the pentasaccharide is substituted uniquely by mannose residues, and it was proposed14 that these glycans be called the oligomannosidic type. The glycans95-104 in Figs. 4 to 10 belong to this family. 

The octasaceharide which carries an additional A -acetylglucosamine at the reducing end in comparison to the heptasaccharide was prepared by an independent synthesis. This compound shows a different conformational behavior than the heptasaccharide [122]. The minimum energy conformation, as calculated by the GESA program, shows the trisaccharide at the 6-position of the P-mannose bent back towards the reducing end. This particular effect is in qualitative agreement with the observation of the conformational difference between the tri- and the pentasaccharide described above. The NMR analysis of this octasaceharide confirms the calculated structure. One important fact for the correct prediction of the conformational data was the simultaneous treatment of all independent parameters in the optimization procedure (Table 8). 

The NMR analysis and GESA calculation of the bisected pentasaccharide shows that the preferred conformation of the C5—C6 bond of the 0-D-mannose is gg (Table 12) [163]. Thus, the introduction of the (3-d-G1cAAc at the 4-position of the 3-d-Man results in a change of the preferred conformation of the C5—C6 bond from gt togg. 

Figure 3. The pentasaccharide repeating unit of xanthan showing the atom labeling. The unlettered residue and residue A are fi-D-glucose B is a-D-mannose C is fi-D-glucuronate and D is 4,6-0-(l-carboxyethyl)-[ -D-mannose.

Fig. 3. Examples of (a) high mannose type and (b) complex type oligosaccharides. In each case, the sugars that comprise the common pentasaccharide core are boxed. SA, sialic acid.

In the second family ofN-glycosylically linked carbohydrate chains, the core pentasaccharide aMan(l—>3)[aMan(l- 6)]/3Man(l—>4)-/3GlcNAc(l— 4)/3GlcNAc(l— N)Asn is substituted by mannose residues.1... 

Figure 11.19. -linked oligosaccharides. A pentasaccharide core (shaded yellow) is common to all A-linked oligosaccharides and serves as the foundation for a wide variety of A-linked oligosaccharides, two of which are illustrated (A) high-mannose type (B) complex type. Detailed chemical formulas and schematic structures are shown for each type. 

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