Polysaccharide heterogeneous hydrolysis

With the heterogeneous hydrolysis of polysaccharides like cellulose, these general considerations are valid, too, of course, but the rate of cleavage is slowed down by one or two orders of magnitude by the limited accessibility of the acetalic O atoms. The rate of reaction depends largely on the physical structure of the original samples and on the state... 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

The gel filtration chromatography is employed as an important technique to isolate heterogeneous LPS extracted from bacterial cells, especially for isolation of truncated LPS without lipid A moiety. The truncated LPS can be obtained by hydrolysis with 1% acetic acid at 100°C for 1 h, or 0.1 M NaOH in 99% ethanol at 37°C for 45 min (Chester and Meadow, 1975 Muller-Seitz et al., 1968 Prehm et al., 1975). The former releases the polysaccharide from the lipid A moiety with exclusive spitting of the extremely acid-labile Kdo linkage (Luderitz et al., 1966 Muller-Seitz et al., 1968) the latter removes ester-linked fatty acids from the lipid A, reducing non-polar interactions between LPS components and facilitating their separation (Chester and Meadow, 1975). 

Since food systems usually contain a heterogeneous mixture of proteins and polysaccharides differing in chemical nature, conformation, chain rigidity, size, and shape of molecules, degrees of hydrolysis, denaturation, dissociation and aggregation, information on model systems containing two biopolymers is not sufficient to interpret phase behaviour and functionality of real dispersed food systems. 

Jones and coworkers, in studies on mannose-containing polysaccharides, have isolated, from seeds of Iris ochroleuca and Iris sibirica, heterogeneous polysaccharides yielding on hydrolysis D-glucose and D-mannose in approximately equal amounts, as well as D-galactose (m. 3 %). These workers consider the possibility that the polysaccharides are mixtures of glucomannan with galactomannan, but, because of the similarity (in composition) between the two iris polysaccharides, tend to the view that they are... 

Bacterial polysaccharides are a very heterogeneous group and are clearly of several biosynthetic types. Many are closely equivalent to the polysaccharide (O-antigenic-type) chains of lipopolysaccharide and have presumably been released by hydrolysis, rather than transferred to lipid A or core structures. Some, such as bacterial hyaluronate, somewhat resemble wall polymers (some teichuronic acids, in this case), but have no exact counterparts. Others, such as bacterial cellulose and colominic acid (polysialic acid) are very different from... 

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