4-0-methyl glucuronoxylan

Figure 4. MALDI-MS spectrum (negative-ion mode) of a 4-0-methyl-glucuronoxylan from a sulfite pulp (spruce). The mass peaks correspond to xylooligosaccharides of increasing lengths containing an odd (o) or even (x) number of4-0-methylglucuronic acid residues. Reproduced from Reference 3, Copyright 2001 American Chemical Society,...

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). 

Softwood xylans (arabino-4-O-methyl-glucuronoxylans) are composed of shorter chains with a degree of polymarization between 70 and 130 (Fig. 11.4-5). Unlike hardwood xylan, the softwood xylan has a higher content of4-0-methyl-glucuronic acid. The acetyl groups are replaced by a-L-arabinofuranose units, which are linked by a-l,3-glycosidic bonds to the C-3 position of xylose 143. ... 

Figure 11.4-4. O-Acetyl-4-O-metHyl-glucuronoxylan from hardwood.

Figure IL The SEC molar mass distribution curves for arabino-4-O-methyl-glucuronoxylan (from spruce holocellulose), arabinohexenuronoxylan (kraft pulp) and two different 4 0-methylglucuronoxylans (sulfite and dissolving pulp) obtained by the SEC/MALDI-MS procedure. Each xylan was analyzed separately employing the alkaline (pH 13) buffer SEC system and subsequently graphed on the same MALDI-MS calibrated absolute molar mass scale.

Figure 7. Thickness - temperature curves for 0-acetyF4-0-methyl glucuronoxylans. Plasticity develops at flowpoint. . (Adaptedfrom ref 7)...

Contribution of the Moiecular Architecture of 4-<9-Methyl Glucuronoxylan to Its Aggregation Behavior in Soiution... 

Most of the glucuronoxylans have single 4-0-methyl-o -D-glucopyranosyl uronic acid residues (MeGlcA) attached always at position 2 of the main chain Xylp units (Fig. 2). This structural type is usually named as 4-0-methyl-D-glucurono-D-xylan (MGX). However, the glucuronic acid side chain may be present in both the 4-0-methylated and non-methylated forms (GlcA). 

Glucuronoxylans can be partly acetylated and have units substituted with a-(l—>2)-4-0-methyl-D-glucopyranosyl uronic acid (MeGlcUA). They are found in hardwood, depending on the treatment. 

Figure 2. Hydrolysis products of beechwood O-acetylglucuronoxylan (Fig. 2a), beechwood glucuronoxylan (Fig. 2b), and wheat straw arabinoxylan (Fig. 2c), as analyzed by gel permeation chromatography. The hydrolysis was carried out at pH 5 at 45°C for 24 h using 10.000 nkat of the 20 kDa xylanase of T. reesei. X = xylose X2 = xylobiose Xmga — 4-O-methyl-glucuronosyl substituted xylo-oligosaccharides X = xylo-oligosaccharides DP > 20.

Fig. 3-15. Abbreviated formula of glucuronoxylan. Sugar units /3-D-xylopyranose (Xylp), and 4-O-methyl-a-D-glucopyranosyluronic acid (GlcpA). R is an acetyl group (CH3CO).

Nagy, T., Emami, K., Fontes, C. M., Ferreira, L. M., Humphry, D. R., and Gilbert, H. J., The membrane-bound alpha-glucuronidase from Pseudomonas cellulosa hydrolyzes 4-O-methyl-D-glucuronoxylooligosaccharides but not 4-O-methyl-D-glucuronoxylan. JBacteriol 2002, 184 (17), 4925-9. 

Glucosyl phosphate, D-, 361 Glucuronic acid, n-, 280, 315, 316 6,3-Iactone, 329 —, 2-amino-2-deoxy-D-, 293 —, 4-0-methyl-D-, 73 Glucuronoxylan, 75... 

Figure 18 O-Acetyl-4-O-methyl-D-glucuronoxylan, a type of hemicellulose, structure from angiosperms (Dekker, 1985) (reproduced by permission of Academic Press from Biosynthesis and Biodegradation of...

In this Section, the term xylanase will refer to those enzymes capable of hydrolyzing the (1 — 4)-/3-D-xylopyranosyl linkages of the (1 — 4)-/3-D-xylans, namely, arabinoxylan, arabinoglucuronoxylan, arabino-4-O-methyl-D-glucuronoxylan, and glucuronoxylan. D-Xylanases of this type have been assigned the Enzyme Commission numbers 3.2.1.8 [(1 — 4)-/3-D-xylan xylanohydrolase, endo-xylanase] and 3.2.1.37 [(1 —> 4)-/3-D-xylan xylohydrolase, exo-xylanase]. 

In hardwood, the xylan, or glucuronoxylan, consists mainly of D-xylose residues, most of which are acetylated, and 4-0-methyl-D-glucuronic acid residues (Fig. 2). Minor amounts of other constituents, such as L-rhamnose and D-galacturonic acid, can also be found. The molar ratio of 4-0-methyl-a-D-glu-curonic acid, acetyl groups, and xylose has been estimated to be 1 7 10 [59]. Hardwood also contains small amounts (2-5% dry weight (DW)) of gluco-mannan, composed of D-glucose and o-mannose. 

Table 6.7. Infrared Spectrum of 4-O-Methyl-D-glucuronoxylan (Marchessault and Liang, 1962)...

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