Xylan uronic acid content

The presence of uronic acid groups significantly reduces the hydrolysis rate of glycosidic linkages (Table 2). Thus, high yields of aldobiuronic acid dimers were obtained on partial hydrolysis of xylans [18, 19, 242]. The hydrolysis rate of different hardwood xylans was closely related to their uronic acid content [257]. A higher stability of softwood xylan as compared to... 

Evidently, the indiAddual xylan molecules do not contain the same relative number of acid side-chains, and it is to be expected that these groups will have a more decisive influence on the solubility of the polysaccharide than will its molecular weight. Similar variations in uronic acid content of birch-xylan fractions have been reported by other investi-gators. ... 

Tables XIII and XIV present more detailed analyses of woods Table XIII contains data on 30 North American species, and Table XIV contains data on 32 species from the southeastern United States. The lignin values in Table XIV are the sum of Klason and acid-soluble lignins. Pectin (Table XIV) is mainly galacturonic acid. It is the measured total uronic acid value minus the estimated glucuronic acid value. Glucuronic acid content can be estimated from the xylan content by assuming a ratio of xylose to 4-O-methylglucuronic acid of 10 1 see Table I and Figure 5). The reported values of the carbo-...

Attempts to estimate the xylan content of wood from pentosan analyses are fraught with difficulties of many kinds. In this method, which has previously been discussed in detail in this Series, arabinose and xylose residues are converted by strong hydrochloric acid into 2-furaldehyde, which is determined by one of several available methods. The values for pentosan and (4-0-methylglucurono) xylan in Table III show, with one exception, good agreement but this is probably fortuitous, since it is known that uronic acids, under the conditions used, give rise to one third of the theoretical yield of 2-furaldehyde. A previous claim that 4-0-methylgluc-... 

The new MALDI-MS based analytical procedures described in this chapter have been employed successfully for (i) determining the accurate molar mass and molar mass distribution for different types of xylans and glucomannans, (ii) characterizing the distribution of uronic acid residues in hard- and softwood xylans and (iii) determining the content and distribution of 0-acetyl substituents in xylans and glucomannans. 

Since the days of Pasteur and Freudenberg, the value of optical rotation [a]o, as a means of characterizing polysaccharides has been appreciated. Timell isolated and purified glucuronoxylans and arabinoxylans to provide a series of samples whose optical rotation could be recorded in 10% aqueous sodium hydroxide. Figure 1 is a schematic of the molecular structure of the 4-0-methyl-glucuronoxylans which he isolated with a variable content of uronic acid. Figure 1 also shows the Equation which we used to account for the observed molar rotation of xylans (3). 

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