Xylan isolation

Potential resources of xylans are by-products produced in forestry and the pulp and paper industries (forest chips, wood meal and shavings), where GX and AGX comprise 25-35% of the biomass as well as annual crops (straw, stalks, husk, hulls, bran, etc.), which consist of 25-50% AX, AGX, GAX, and CHX [4]. New results were reported for xylans isolated from flax fiber [16,68], abaca fiber [69], wheat straw [70,71], sugar beet pulp [21,72], sugarcane bagasse [73], rice straw [74], wheat bran [35,75], and jute bast fiber [18]. Recently, about 39% hemicelluloses were extracted from vetiver grasses [76]. 

Xylans from beech wood, corncobs, and the alkaline steeping liquor of the viscose process have been shown to be applicable as pharmaceutical auxiliaries [3]. Micro- and nanoparticles were prepared by a coacervation method from xylan isolated from corncobs [150]. The process is based on neutralization of an alkaline solution in the presence of surfactant, which was shown to influence both the particle size and morphology. They are aimed at applications in drug delivery systems. 

Relatively pure xylan isolated from the holocellulose of aspen (Populus) wood is said to contain 85% of xylose residues.78 One of the characteristic properties of xylan is its ease of hydrolysis. Because it hydrolyzes much more readily than cellulose, mild acid treatment may be employed to bring about preferential hydrolysis of xylan from plant material. Xylose is ordinarily prepared in the laboratory by direct sulfuric acid hydrolysis of the native xylan in ground corn cobs.74 Hydrolysis in hydrochloric acid proceeds rapidly, but decomposition to furfural also occurs to some extent.76 A commercial method for the production of D-xylose from cottonseed hulls76 and straw77 and from corn cobs17 78 has been described. 

Rassow and Dorr have also nitrated pure xylan, isolated from straw cellulose. By applying a concentrated nitrating acid, mixed at the proportions mentioned above they prepared nitroxylan in the form of a white powder containing 8.76% N, in 115.7% yield. 

Some of the main structural features of the xylans isolated from the Gramineae are outlined in Table I. 

From methylatioii data and molecular-weight determinations, a structure for esparto-grass xylan ([a] D —92°, in aqueous XaOH) has been suggested by Hirst and associates. The molecule consists of about 75 (3-D-xylopyranose units linked 4- 1, with a single branch point (3 —1) linkage. A similar structure has been proposed for a xylan isolated from... 

For size exclusion chromatography (SEC) analysis, 3 mg xylan was dissolved in 0.15 ml DMSO (75 C, 2h) and 2.85 ml 0.5% LiCl/DMAc was then added. The molecular weight of the xylan isolated was analyzed using... 

Acetyl signals at 2.2 ppm, as recently reported for 0-acetyl-(4-0-methylglucurono)xylan isolated from aspenwere not detected. Such O-acetyl groups are most probably removed under the alkaline conditions used here during the extraction of the xylan. 

Obt. by the action of endoxylanases from Cryptococcus albidus and Streptomyces on Rhodymenan a water sol. xylan isol. from the marine alga Rhadymenia palmata also by action of P-xylosidase from Penicillium wortmarmi IFO 7237 on P(1 4) xylo-biose. Cryst. 

In general, HP or AP xylan derivatives (33), produced sharp transitions in surface tension with respect to bulk solution concentration and distinct CMC values (Figure 3D), whereas most (Figure 3D), but not all (Figure 3B), xylan isolated from natural... 

Figure 6. The CP/MAS C-NMR spectrum recorded on xylan isolated from bleached birch krqft pulp. The xylan sample is a colloidal sol (Reproduced with permission from reference 16, Copyright 1999).

More recently, the quartemization of xylan isolated fh)m beech wood, com cobs, and rye bran was investigated (15), The reactions were carried out with a sodium hydroxide activated xylan and CHTMAC as alkylating reagent. The results show clearly that the DS of TMAHP groups depends on the molar ratios CHTMAC/xylan and NaOH/CHTMAC as well as on the xylan type used (Table 3). 

The sodium periodate oxidation of xylan isolated from Palmaria decipiens yields 2,3-dialdehyde xylan (26). The dialdehyde moieties can be converted into a Schiff base type compound by the reaction with p-chloroaniline (Figure. 4). 

Isolated from the medium of suspension-cultured sycamore (Acer pseudoplatanus) cells (24). Isolated from the medium of suspension-cultured sycamore (Acer pseudoplatanus) cells (75). Also contains 14% xylosyl residues which are thought to arise from a contaminating xylan. Isolated from soybean (Glycine max) meal (7, 98, 99). 

The sidechain reported most frequently is a terminal 4-0-methyl-glucuronosyl residue (22, 28, 40, 53, 120, 125). Xylans characterized by the presence of these monomethylated mono-uronosyl sidechains have been isolated from rapeseed cotyledon meal (Brassica campestris), from midrib of tobacco (Nicotiana tabacum), from aspen wood (Populus tremuloides) and from the bark of white willow (Salix alba L.). The uronosyl residues are always linked to the C-2 of xylosyl residues. One uronosyl residue is linked to one in every 7 to 10 xylosyl residues (22, 120, 125). The xylan isolated from soybean (Glycine max) hull cell wall contains only one unmethylated glucuronosyl residue attached to about one in every 30 xylosyl residues (17). 

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