The Hemicelluloses

The seed sources of xyloglucan, from which the polysaccharide has been isolated, and characterized, include Tamarindus indica,183,186 nasturtium (Tropeoleum majus),182,186 and rape (Brassica campes- 

All of the aforementioned xyloglucans share common structural features, although the glycosyl composition varies somewhat with the source. They all possess a backbone of 4)-linked D-glucosyl resi- 

Arabinopyranose, a minor constituent of primary-wall xyloglucans, is attached to a few glucosyl residues of the polymer backbone.56,58,65 The glycosidic linkage is believed to be Arap-(1— 2)-Glc. 

The structure of the xyloglucans from the walls and culture medium of suspension-cultured sycamore-cells has been investigated by Alber-sheim and associates56 by utilizing purified endo-/ -(l— 4)-glucanase for 

Such products would be expected to arise from similar treatment of xyloglucans from dicots. However, if xyloglucan is present in primary cell-walls of monocots, it is clearly there in much smaller proportions than in dicot primary-walls, and it may not assume the structural significance that this polysaccharide has in the latter. 

Xyloglucan is perhaps the most thoroughly understood of the non-cellulosic polysaccharides of primary cell walls 31). Xyloglucans were first characterized as an amyloid component of seeds 21, 60, 61, 79, 

Glycosyl Residue Tamarindus Nasturtium Rape Seed r Rape Seed II BEPS REPS SEPS Sycamore cell wall 

Isolated from nasturtium (Tropeoleum majus) seeds (41, 67). 

Isolated from the cell walls of suspension-cultured sycamore Acer pseudoplatanus (calculated from the data presented in ref. 31). 

Considerably later, xyloglucans were isolated from the medium of suspension-cultured sycamore (24) cells, and, finally, from the primary cell walls of suspension-cultured sycamore cells (31). The basic structure of xyloglucans was elucidated by Kooiman (79) who studied the amyloid of Tamarindus indica seeds. The xyloglucans of primary cell walls were isolated and structurally characterized before it was recognized that the xyloglucans are very similar to the amyloids 31). The widespread occurrence of the amyloids 21, 67, 79, 115, 119) and xyloglucans 24, 29, 31) shows that polysaccharides isolated from tissues other than primary cell walls can, at times, serve as excellent models for the cell wall polysaccharides. 

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Of samples swollen with ethylene diamine, the graft yield at a 50 1 liquor ratio increases as the concentration of ethylene diamine increases. This is due to the increase of decrystallization of swollen samples, which helps the penetration velocity of the chemicals through the cellulosic chains. Graftability of the samples treated with 100% ethylene diamine is lower that of the sample treated with 75%. This is due to the dissolution of low DP chains and some of the hemicelluloses, which is detectable by the increase in DP of the sample teated with 100% ethylene diamine. 

Recently, the alkah-soluble hemicelluloses of hardwood dissolving pulps have been investigated [28]. Their composition and molecular properties depended on the pulp origin and steeping conditions. The MGX of the j8-fraction from press lye had a low uronic acid content (ratio of MeGlcA to Xyl is about 1 20). The molecular weight of the hemicellulose fractions varied between 5000 and lOOOOg/mol. 

The AIS of fresh and canned carrots were sequentially fractionated into water-soluble pectin (WSP), oxalate-soluble pectin (OXSP), acid-soluble pectin (HSP), alkah-soluble pectin (OHSP), hemicellulose and cellulose. The distribution of uronic acid among various fractions is presented in Table 1. WSP of fresh carrots accounted for 19.0% of the total pectin, while OXSP constituted 29.6%. HSP represented the lowest (12.0%) pectin fraction, whereas OHSP was the highest (35.6%) In the hemicellulose and cellulose fractions significant amounts of uronic acid were found. Heat treatments during canning altered the proportion... 

Rha, Ara and Gal are the neutral sugar components from all the fractions. Xyl is not present in Fla and is significantly present in the hemicellulose fractions, indicating that this monosaccharide is component of hemicellulosic polymers. Chemical composition of the water fractions were determined (Table V). High protein contents and the presence of O-acetyl-groups were observed in four aqueous fractions. Neutral sugar and uronic acid composition points to inclusion of these polymers in the class of pectic polysaccharides. 

The molecular weight distribution of cell wall polysaccharides was estimated by gel filtration with a TOSOH TSK gel G4000 PWXL (7.8 x 300 mm) column equilibrated and eluted with 0.05 M sodium acetate, 0.01 M EDTA, 0.05 M NaCl (pH 5.0) in polyuronide and 0.05 M sodium citrate, 0.1 M NaCl (pH 5.5) in the hemicellulose fraction. Samples (1 mg/ml) of 100 ml were injected. The eluate was monitored by a refractive index detector (Shimadzu R1D-6A, Kyoto, Japan) and collected at the fraction size of 0.4 ml. 

Influence of Metal Ions on Oxygen Chemisorption and Ignition of Chars. We have carried out extensive studies of the influence of metal ions in wood on pyrolysis mechanisms (5.6) and this approach has now been extended to oxygen chemisorption of the chars. The metal ions occur in wood predominantly as the counterions of the uronic acid components of the hemicelluloses (12). We have shown that they can be almost completely removed by very mild acid treatment without any other major change in the chemistry of the wood. Table II shows that the major metal ions in cottonwood are Ca, K and Mg. The acid-washing process removed 98X of the metal ions in... 

In chemical pulping, a significant part of the hemicelluloses is dissolved from the fibres into the pulping liquor. The rest remains in the fibre or is adsorbed into it, significantly affecting the properties of the cellulose fibres or paper produced... 

Polysaccharides from which 2,4,6-trimethyl-D-glucose can be prepared by methylation and hydrolysis include a polyglucose isolated from the cell wall of brewer s yeast,166 laminarin168 and the hemicellulose fraction of Iceland moss.188... 

The hemicelluloses are soluble in alkali, and can therefore be readily separated from the cellulose component by alkali extraction. However, this can only be done when the wood has first been delignified. This is because they are probably linked to lignin via covalent ester linkages (see Chapter 3) which need to be cleaved... 

The end group which is produced contains a carboxylic acid functionality which has an influence on the anionicity of pulp fibres (Chapter 6) but, in this form, it is resistant to further alkaline degradation. The hemicelluloses are also able to undergo the same type of peeling reaction but at different rates from each other and from cellulose. The /3-1,4-xylans, for example, are more stable to alkaline degradation than the glucomannans. 

Simultaneous saccharification and co-fermentation (SSCF) one-stage enzymatic hydrolysis of cellulose and fermentation of pentoses and hexoses all in one process step. The upstream hydrolysis of the hemicellulose takes place in a separate process step. 

The hemicellulose forms the glue around and between the cellulose bundles. It consists of shorter, branched polymer chains of various C6 and C5 sugars. The C6 sugars consist mainly of glucose together with mannose and galactose. The C5 sugars include xylose and arabinose. The steric hindrance offered by the side-... 

The space between the microfibrils is occupied by the hemicelluloses and by lignin. However, the incomplete filling of the intermicrofibrillar region results in the existence of what are usually referred to as micropores (or microvoids) in the cell wall. These have diameters of the order of nanometres and thus technically should be referred to as nanopores, but since the term micropores is the most commonly used in the literature, it will be used throughout this book. 

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