Xylans types

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

Fig. 7 Stabilization effect of various xylan types isolated from beech wood (GXl and GX2), corn cobs (AGXl), rye bran (AXRl and AXR3), and corn hulls (AXCl and AXC2) on the protein (BSA) foam against thermal disruption foam volume before (V1) and after (V2) heating at 95 °C for 3 min [128]...

Kacurakova, M., Wellner, N., Ebringerova, A., Hromadkova, Z., Wilson, R. H., Belton, P. S. (1999). Characterisation of xylan-type polysaccharides and associated cell wall components by FT-IR and FT-Raman spectroscopies. FoodJJ tdrgggJl, 13, 35-41. 

Among the several cellulose derivatives are xylan-type hemicelluloses, which are the most abundant heteropolysaccharides derived as by-products from agricultural processes. Over the past few years their use in food packaging and other applications has increased considerably [121]. Hemicelluloses such arabinoxylan are hydrophilic polysaccharides and their films tend to show high water uptake, becoming a drawback in the production of materials for the food sector. In order to improve both mechanical and barrier properties, Sarossy et al. [122] adopted the... 

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). 

Both types of xyloglucans exhibit monolayer sorption onto cellulose (116) and tamarind xyloglucan exhibits maximum specific sorption onto cellulose less than that of coniferous xylan. By inference with other data, this is also less than that of glucomannan and hardwood xylan, but similar to many additives used in the paper industry. 

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Xylans as true homopolymers occur in seaweeds of the Palmariales and Nemaliales, however, their backbone consists of Xylp residues linked by -(1 3) (Type X3, Fig. la) or mixed -(1 3, 1 -> 4)-glycosidic linkages (Type Xmy Fig. lb). They are assumed mainly to have a structural function in the cell-wall architecture, but a reserve function cannot be ruled out [4]. From the microfibrils of green algae (Siphonales) such as Caulerpa and Bryop-sis sp., X3 was isolated and the structure confirmed by methylation analysis, C-NMR spectroscopy [7], as well as by mass spectrometry of enzymically released linear oligosaccharides up to a degree of polymerization (DP) of... 

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). 

Similarly, after a longer time of incubation, no significant changes in the cell proliferation rate was detected, as can be seen in the data for 72 h (Figure 13). In fact, this was expected due to the biocompatible nature of xylan. As a natural polyssacharide, this type of biomaterial is considered to be highly stable, non-toxic and hydrophilic (Liu et al., 2008). Accordingly, the alkaline extraction of xylan from corn has proved to be a safe approach for obtaining the polymer with no relevant toxicity (Unpublished data). 

This enzyme was shown to be specific for xylan oligomers and small acetylated synthetic substrates. Many characteristics have been published recently about this type of enzyme, purified from Trichoderma reesei, and A. oryzae [6], and a different A. n/ger preparation[7]... 

Prior to xylan removal it is possible to extract from the plant material those hemicelluloses soluble in dilute alkali. Soluble hemicelluloses of this type are the principal impurities extracted with xylan and consist for the most part of low molecular weight polysaccharides and polyuronides. Corn cob holocellulose may be freed of these compounds (called hemicellulose-B fraction, see Figure 2) by extraction with potas-... 

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. 

CBH I 497 core-BA aa sequence in part from protein and in full from gene (cbhl), number and location of SS bridges, region of O-glycosylation, types of carbohydrate, papain cleavage site, hydrophobic cluster analysis, computer model of active site, 2D-NMR on a synthetic tail fragment, SAXS on whole CBH I, head domain and xylan/CBH I complex... 

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