Pyranose Sugars polysaccharide structures

Fig. 1 Schematic of possible polysaccharide structures from pyranose sugars. The C-1 carbon is in theBconfiguration and the "chair" pyranose ring is...

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

Acetal and ketal linkages are widely foimd in nat-inal sugars and polysaccharides. The structure of sucrose is a splendid example. Sucrose is a disaccharide, composed of two linked monosaccharide imits, glncose in pyranose ring form and frnctose in fmanose ring form. As we have seen above,... 

Polysaccharides prepared from 1,4-anhydro sugars contain both pyranose (6-membered) and fiiranose (5-membered) rings in the main chain l2). The presence of both structures was established by hydrolysis which showed that either of the chemically equivalent, C-l-O-bonds may be broken during polymerization ... 

Table 11 summarizes methylation analyses carried out on certain GXM listed in Table I. Analysis of the carboxy-reduced C. neoformans serotype A polysaccharide was recently performed at NRRC in cooperation with Dr. Errol Reiss of the Center for Disease Control, Atlanta. The results are in accord with a structure consisting of a main chain of (l->3)-linked g-mannosyl residues, two-thirds of which are also (1- 2)-linked by single residue branches of g-xylose and g-glucuronic acid. (All sugar residues referred to in text and figures are in the pyranose form.) The polysaccharide from the cell walls of T. fusiformis has a related structure. It appears, however, that there are also backbone... 

The rate of hydrolysis of a polysaccharide is a function of its structure. In general, a-n-glycosidic linkages are hydrolyzed more readily than 3-d-linkages. Polysaccharides, such as arabans and fructans, whose sugar units occur as furanosides may be rapidly and completely hydrolyzed under mild conditions, such as in 1 % mineral acid at 80°. Polysaccharides consisting of pentose units in the pyranose form are more resistant, but hot 3% nitric acid causes complete hydrolysis. More concentrated acids, especially hydrochloric acid, decompose the pentoses to furfural. Stronger hydrolytic conditions are necessary for polysaccharides of hexopyranoside units. Usually normal sulfuric or hydrochloric acid at 100° is sufficient, but in certain cases more drastic conditions are necessary. 

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