Xylan determination

Partial etherification of the beech wood MGX with p-carboxybenzyl bromide in aqueous alkali yielded fully water-soluble xylan ethers with DS up to 0.25 without significant depolymerization the Mw determined by sedimentation velocity was 27 000 g/mol [400,401]. By combination of endo- 6-xylanase digestion and various ID- and 2D-NMR techniques, the distribution of the substituents was suggested to be blockwise rather than uniform. The derivatives exhibited remarkable emulsifying and protein foam-stabilizing activi-... 

In order to assess the effect of the corn cob xylan on the cell viability and proliferation rate, xylan solutions at concentrations of 0.1, 0.25, 0.50, 0.75, and 1 mg/ml were placed in contact with human cervical adenocarcinoma cells (HeLa cells) for 24 and 72 h. Finally, the cell viability was determined by the MTT assay. It was observed that regardless of the xylan concentration, the samples tested did not affect the viability of HeLa cells after incubation for 24 h (Figure 13) (Unpublished data). 

Determination of xylan is frequently made by estimation of furfural production. Data so obtained, even when appropriately corrected for furfural that arises from uronic acid, may be high if araban is present in the polysaccharide preparation. In the absence of interfering carbohydrates, furfural estimation may lead to accurate xylan values. 

Hydrolysis of xylan produces principally D-xylose.71 By hydrolysis of straw xylan in 3% nitric acid, Heuser and Jayme72 obtained crystalline D-xylose in 85% yield. Using a similar procedure, Hampton, Haworth and Hirst70 obtained a yield of 93% crystalline D-xylose from esparto xylan. A practically identical yield was indicated by reducing sugar determination made on the hydrolysis products of corn seedling xylan.68... 

The extent of oxidation, when determined by the reaction of the product with phenylhydrazine, is 85%. Hydrolysis of the oxidized xylan should produce approximately equimolar quantities of D-glyceraldehyde and glyoxal. Experimental determination of glyceraldehyde indicates 67 % of the theoretical when the oxidized xylan is distilled with sulfuric acid and the evolved methylglyoxal measured as the phenylosazone. Glyoxal is isolated in 63% yield, when separated as the phenylosazone or as the dioxime. Aldehyde groups in the oxidized xylan may be further... 

Recently a quantitative method for the determination of carbonyl groups in oxidized xylan has been developed.106 The method involves the reaction of the carbonyl group with O-methylhydroxylamine hydrochloride to form the corresponding oxime with the liberation of one mole of titratable acid. 

Acetyl values from fully acetylated xylan preparations indicate a D. P. of about 100 or greater. The theoretical acetyl value for a fully acetylated xylan molecule of infinite size is 39.8%. As the molecule decreases in length, its acetyl content increases because of the increasing proportion of triacetyl-bearing end units. Assuming an accuracy of 0.1% for the acetyl determination, a noticeable deviation from theory would be apparent in acetylated xylan of less than D. P. 100. 

Hardwood xylans and xylans of annual plants may contain up to 7% O-bound acetyl groups. Seven out of ten xylose residues in native hardwood xylan are acetylated on C-2 and/or C-3 (10). Because of the possible migration of O-acetyl groups between 2- and 3-positions during and after isolation of hemicellulose components, it is difficult to determine their original distribution in nature (11). The ratios reported for 2-, 3-, and 2,3-positions of acetyl groups in birch xylan have been 2 4 1 (3) and 2 2 1 (10) and in bracatinga xylan 3 3 1 (12). 

The specificity of a xylanase preparation may be determined from the isolation and characterization of oligosaccharides formed from xylan hydrolysis. While the oligosaccharides released were mainly dependent on the specificity of the xylanase, the complex structure of the heteroxylan also determines the extent of hydrolysis and the site of cleavage. Two types of xylooligosaccharides are usually produced acidic (from glucuronoxylans) and neutral (from arabino- and arabinoglucurono-xylans). The specificity of the action of several purified xylanases on various heteroxylans is shown below ... 

Figure 1. Induction of xylanases in Trichoderma harzianum. The ratios of xylanase to endocellulase activities were determined in the culture filtrates of T. harzianum grown on steam treated aspenwood. Aspen chips were steam treated from 20 to 240 s at 240° C to produce a series of samples with a variable content of xylan and cellulose. The specific content of these carbohydrates were expressed as the ratio of xylan to glucan. Enzyme activities were determined on culture filtrates of 300 mL batch cultures of T. harzianum grown on these wood samples at a loading of 1% (w/v) as described by Saddler and Mes-Hartree (66). Enzyme activities were also determined in culture filtrates of T. harzianum grown on Avicel, Solka Floe and oat spelts xylan.

The extent of hydrolysis of D-xylans by /S-D-xylosidase should denote the extent of substitution, but variation in the action patterns of enzymes may be reflected in the various results that have been obtained using enzymes from different sources. /S-D-Xylosidase hydrolyzed a soluble D-xylan from Shirakamba wood68 to the extent of 39%, and rice-straw D-xylan released D-xylose,69 but other D-xylans underwent no significant hydrolysis.70 /3-d-Xylosidase has been employed in partially determining the structures of... 

Although most polysaccharides may be acylated without degradation, acyl derivatives of xylans29 are often unsatisfactory for molecular-weight determinations owing to their poor solubility characteristics. Mixed esters of xylans143 are often superior in this respect, but there are obvious difficulties in the differential analysis of acyl groups. Hemicellulose nitrates have been used for measurements of molecular size,168 but xylan nitrates,29 like xylan acetates, are reported to have poor solubility characteristics. 

---