Birch, acidic xylan from

Xyloisosaccharinic acid [2,4-dihydroxy-2-(hydroxymethyl)butanoic acid] is one of the major, alkaline-degradation products of wood xylan, in particular, that of birch. The disaccharide, 2-O-D-xylopyranosyl-L-arabinose, which was isolated as a hydrolysis product of corn-cob hemicellulose, is readily degraded at 100° in 15 mM Ca(OH)2 to acidic products, primarily saccharinic acids. Xylan oligosaccharides from corn-cob hemicellulose produced 2,4-dihydroxy-2-(hydroxymethyl)buta-noic acid when exposed to 0.02 M Ca(OH)2 at 25°. However, it was noted that the xylan, itself, was stable at 100° in Af NaOH. The major acidic component of the hemicellulose fraction of slash pine Pinus el-liotti) after acid hydrolysis was identified as 4-O-methyl-D-glucuronic... 

In an attempt to isolate a water-soluble 0-acetyl-(4-0-methylglucurono)-xylan from a birch chlorite holocellulose, Marchessault and coworkers kept the time of contact between the ammonia and the holocellulose at a minimum. Successive extractions of the pretreated holocellulose with water and methyl sulfoxide removed the xylan in yields of 7.1 and 14.6%, respectively, based on the weight of the original wood. The products still contained most of the iironic acid, but only one third of the 0-acetyl groups of the native xylan. 

One of the first polymers to be obtained in the form of single crystals was a xylan from wood of white birch reported by Yundt in 1949. The crude xylan was purified by heating in boiling 0.2% oxalic acid solution, to give a highly swollen, water-insoluble fraction in a yield of 7.4% of the wood. Further purification was effected by autoclaving in water at 120° and... 

Acidic polysaccharides are also precipitated but at higher concentrations of Ba(OH)2. Meier (1958) found that birch glucuronoxylan was precipitated when the Ba(OH)2 concentration was 0.15 M, probably due to the formation of the insoluble barium salt of the acidic xylan. Pectic acid can be separated almost quantitatively from the neutral (pectic) polysaccharides as the insoluble barium salt. Therefore the use of Ba(OH)2 as a precipitant will provide a complement to the other methods. 

Previous papers have investigated the suitable use of xylan in papermaking [10] and textile printing [21]. In the drug delivery field, xylan extracted from birch wood has been used for the production of nanoparticles after structural modification by the addition of different ester moieties, namely those with furoate and pyroglutamate functions [21]. On the other hand, the esterification of xylan from beech wood via the activation of the carboxylic acid with iV,iV -carbonyldiimidazole has been carried out in order to produce prodrugs for ibuprofen release [10, 21, 22, 37]. 

Figure 7. Positive- (upper) and negative- (lower) ion MALDI mass spectra of the oligosaccharides obtained from birch wood xylan by mild acid hydrolysis. In positive ion mode, both neutral and acidic oligosaccharides are detected, whereas acidic oligosaccharides only are detected by negative-ion MALDI-MS. Reproduced from Reference 6. Copyright 2001 American Chemical Society.

The xylans used were non-substituted linear xylan from beech (Lenzing AG, Austria), 4-0-methylglucuronoxylan from birch (MeGlc-xylan) (Roth, Germany). Hexenuronic acid xylan (HexA-xylan) was alkali treated 4-0-... 

The distribution of the 0-acetyl groups along the xylan chains has not been studied. The figures quoted above are very close to what would be expected if the acetate groups were distributed according to the laws of chance. An 0-acetylated xylan has also been isolated from the same birch wood by way of a peroxyacetic acid holocellulose. ... 

The positive-ion MALDI mass spectrum obtained for the oligo- and polysaccharides obtained from a hardwood (birch) xylan after mild acid hydrolysis is shown in Figure 7 (upper spectrum). This spectrum consists of several partially over-lapping series of peaks, which were identified on the basis of their exact molar masses as corresponding to neutral xylosaccharides with DP up to 30 and acidic xylosaccharides with one or two 4-0-MeGlcA substituents. 

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