Polysaccharides alternating -linked

Agarose is a polysaccharide (alternating 1,3-linked /3-D-galactose and 1,4-linked 3,6-anhydro-a-L-galactose units) obtained from seaweed (Figure 4.6). Above 50°C it dissolves in water, and if the solution is cooled below 30°C... 

Xanthan (Figure 11) is a commercially important polysaccharide produced by the bacterium Xanthomonas campestris.187 188 The xanthan backbone consists of a P(l-4)-linked D-glucopyranose chain with a trisaccharide side chain attached at C3 to alternate glucose residues. These side chains consist of an acetylated mannose residue, a glucuronic acid residue, and a pyruvate ketal linked to a terminal mannose residue. The acetate and pyruvate content depend on the fermentation and isolation conditions used by the supplier. 

Reeves and Goebel72 have shown that hydrolysis of the reduced methylated capsular polysaccharide of Type III pneumococcus yields 2,3,6-trimethyl-D-glucose and the anomeric forms of methyl 2,4-dimethyl-D-glucoside. The cellobiuronic acid units in the polysaccharide are thus linked through position 3 of the D-glucuronic acid residue, probably by /3-D-linkages. That is, the polysaccharide contains alternate 1,3-and l,4-/3-D-linkages. 

In potato pectic polysaccharides, the RG-I domain accounts for 75% of the total, a much higher proportion than in pectic polysaccharides of other species ( 20-35%) (Oomen et al., 2003 Mohnen et al., 2008). RG-1 consists of a backbone of alternating a-D-galacturonic acid and a-L-rhamnosyl residues, linked as indicated in Figure 3.3, which in potato accounts for 14% of the total RG-1 (Oomen et al., 2003). Although the galacturonic acid residues are probably not methyl-esterified, they may be acetylated. This is so in potato, where a preparation... 

In the more abundant a chitin the chains in alternate sheets have opposite orientations,101102 possibly a result of hairpin folds in the strands. Native chitin exists as microfibrils of 7.25 nm diameter. These contain a 2.8-nm core consisting of 15-30 chitin chains surrounded by a sheath of 27-kDa protein subunits. The microfibrils pack in a hexagonal array, but the structure is not completely regular. Several proteins are present some of the glucosamine units of the polysaccharide are not acetylated and the chitin core is often calcified.103 The commercial product chitosan is a product of alkaline deacetylation of chitin but it also occurs naturally in some fungi.102 Chitin is also present in cell walls of yeasts and other fungi. It is covalently bonded to a P-l,3-linked glycan which may, in turn, be linked to a mannoprotein (see Section D,2)97... 

A great variety of polysaccharides are present in the outer layers of other types of bacteria. For example, the mycobacteria have an alternating 5- and 6-linked P-D-Gal/polymer attached to their peptidoglycan. 

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