Hemicellulose B

Prior to xylan removal it is possible to extract from the plant material those hemicelluloses soluble in dilute alkali. Soluble hemicelluloses of this type are the principal impurities extracted with xylan and consist for the most part of low molecular weight polysaccharides and polyuronides. Corn cob holocellulose may be freed of these compounds (called hemicellulose-B fraction, see Figure 2) by extraction with potas-... 

Fig. 6.2 Structural formulas of cellulose (a), example of hemicelluloses (b), and an example of lignin (c), showing a repeating unit as possible model compound.

Hemicelluloses are quite difficult to extract from cell walls of softwoods (9,10) and are usually destroyed or depolymerized during the chemical pulping of these raw materials. However, other hemicelluloses, primarily xylans, can be extracted by cold, dilute sodium hydroxide from grasses and many hardwoods in very high yields (9,77). These xylans are deacetylated in an alkaline medium and are for the most part insoluble (hemicellulose A). A partially water soluble fraction (hemicellulose B) has also been... 

Tejero. Olive polysaccharides. IX. Characterization of hemicellulose B OE092 from the pulp of Hojiblanca variety of olive. An Quim Ser C 1987 83(1) ... 

Some hemicelluloses are partially extractable with water, but they are usually extracted with alkaline solutions following removal of lipids and lignin. Delignified plant material is termed, holocellulose. Neutralization of the alkaline extract effects precipitation of the more linear and less acidic hemicelluloses, termed the hemicellulose A [63100-39-0] fraction. The more acidic and more branched material, termed hemicellulose B [63100-40-3], is precipitated with ethanol (70%). Hemicellulose B types type are usually water-soluble after extraction. 

Hemicellulose. The hemicelluloses consisting of hexoses, pentoses, and uronic acids may be separated into two fractions, A and B. Hemicellulose B usually contains a higher proportion of uronic acid, mainly 4-methyl-D-glucuronic acid, than the A fraction. This methyl derivative of D-glucuronic acid is most frequently isolated in combined form as the... 

As previously mentioned, Kauss (40) has shown that the methyl donor for the formation of 4-methyl-D-glucuronic acid of hemicellulose B proved to be S-adenosyl-L-methionine, the same as in pectin. A particulate preparation from immature com cobs containing hemicellulose B was found capable of transferring the 14C-labeled methyl group from S-adeno-syl-L-methionine to a macromolecular acceptor present in the particles. The radioactive product was shown to be hemicellulose B labeled in the 4-methyl-D-glucuronic acid residues. It was isolated chiefly as 4-methyl-glucuronosyl-( 1 - 2)-D-xylose. 

In another experiment with the particulate enzyme from the same plant he obtained 4-methyl D-glucuronosyl-D-galactose together with 4-methyl D-glucuronosyl-D-xylose. Kauss (43) also found that the particulate enzyme preparation from immature corn cobs contains, in addition to the methyl transferase, an enzyme which introduces the D-glucu-ronic acid group from UDP-D-glucuronic acid into hemicellulose B. 

Rao, M. V. S. S. T. S. and Muralikrishna, G. (2006b). Hemicelluloses of ragi (finger millet, Eleusine coracana, Indaf-15) Isolation and purification of an alkali-extractable arabinox-ylan from native and malted hemicellulose B. J. Agric. Food Chem. 54, 2342-2349. 

Fehling solution. It contained anhydro-D-xylose units and 11% of it was uronic acid anhydride. D-Glucuronic acid was identified in hydrolyzates, but a test for D-galactose was negative. Hemicellulose-B (Hd —120° in 0.5% sodium hydroxide) yielded 4.8% of galactose (galactaric acid test), an undetermined amount of L-arabinose, and 64 % of uronic acid identified as galacturonic acid. ... 

Hemicellulose-B(Table V) from sapwood gave a blue color with iodine, which disappeared after treatment with Takadiastase under carefully controlled conditions. The solution was found to contain n-glucose, and its reducing value was equivalent to 24.6% of hexosan. On the basis of analytical data, heartwood hemicellulose-B appeared to be similar in constitution to the polysaccharide (6 anhydro-n-xylose units and 1 0-methyl-hexuronic acid per structural unit) isolated- after prolonged enzymic hydrolysis of heartwood hemicellulose-A. 

The arabinosidase activity of D-xylanase HC I was suppressed when the enzyme was pre-incubated with X3 prior to attack on hemicellulose B. The presence of X3 did not, however, prevent further scission of the D-xylan chain, which suggests that participation of two active sites is involved in the hydrolysis of hemicellulose B by the action of xylanase HC I. 

The D-xylanases of rumen microbial origin are all of the endo type, and degrade D-xylan, arabinoxylan, hemicellulose B, and xylo-oligosaccharides of d.p. 2=3, mainly to X2 and D-xylose X2 remains unattacked. The D-xylanases were found38,42 to have a higher affinity for linear D-xylans than for the branched hemicelluloses, namely, arabinoxylan and a branched B fraction from Trifolium pratense (red clover). The presence of L-arabinose, D-galactose, and uronic acid substituents on the D-xylan backbone thus appears to inhibit the action of these D-xylanases. The D-xylan was not effectively hydrolyzed until the side-group substituents had mostly been removed, for example, by arabinofuranosidase action on arabinoxylan.44... 

The alkaline extracts are acidified to pH 5 with glacial HOAc, dialyzed (or ultrafiltered) to remove salts, and any precipitated polymers (hemicel-lulose A) are removed by centrifugation. The supernatant fluid is concentrated by evaporation under reduced pressure, redialyzed and concentrated to give a solution of the hemicellulose B fraction an aliquot is freeze-dried for analytical purposes. Alcohol precipitation is avoided to... 

As previously mentioned, the methyl donor " for the formation of 4-O-methyl-D-glucuronic acid of hemicellulose B proved to be S-adenosyl-L-methionine, as in the formation of the methyl ester of pectin. ... 

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