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Chapter 12
Contribution of the Molecular Architecture of 4-O-Methyl Glucuronoxylan to Its Aggregation Behavior in Solution 1
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Johannes P. Roubroeks , Bodo Saake , Wolfgang G. Glasser , and Paul Gatenholm 1
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Biopolymer Technology, Department of Materials and Surface Chemistry, Chalmers University of Technology, SE-412 58, Göteborg, Sweden Institute of Wood Chemistry and Chemical Technology of Wood, Federal Research Centre of Forestry and Forest Products, 21031 Hamburg, Germany Department of Wood Science and Forest Products, Virginia Polytechnic and State University, Blacksburg, V A 24061-0323
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Alkali extracted glucuronoxylan from aspen wood, showed a polydisperse fractionation pattern after size exclusion chromatography. The glucuronic acid distribution along the chain showed that sparsely substituted glucuronoxylan chains contributed to aggregate formation while removal of lignin moieties decreased aggregation. It was observed that glucuronoxylan contained both bound-and unbound lignin. The unbound lignin was mainly syringyl in the erythro form.
© 2004 American Chemical Society
In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction
There has been a considerable interest in the interactions between different polymeric constituents of the cell wall. In the approximation of comparing wood to reinforced composites, the fibrillar skeleton of cellulose is embedded in an amorphous matrix, consisting of lignin and hemicellulose (Frey-Wyssling,
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: October 7, 2003 | doi: 10.1021/bk-2004-0864.ch012
1968) (/). Both the cellulose skeleton and the matrix have an enormous impact on chemical, physical and mechanical properties. In recent literature, it has been emphasized that the cell wall design is basically a helicoidal pattern analog to cholesteric order (Roland et al., 1987; Satiat-Jeunemaitre, 1987; Neville, \993)(2,3,4). The helicoidal wall pattern and its consolidation have been extensively described by Reis et al. (1994)(5). The tight interaction between glucuronoxylan and cellulose with colloidal gold labelling (Reis et al., 1992)(6) confirms the occurrence of a charged coat, along the microfibrils that is likely to play an important role in assembly. The existence of chemical linkages between lignin and hemicelluloses have been suggested (Watanabe et al., 1993; Kosikova and Ebringerova, 1994) (7,8), and the interaction between hydrophobic and hydrophilic moieties creates possibilities for the design of functional nanostructures according to the concept of supramolecular assembly (Akiyoshi et al., 2000) (9). The major component of the secondary walls of dicotyledons is glucuronoxylan. They constitute 20-35% of the dry weight of woody tissues (Zinbo and Timell, 1965)(/0). Glucuronoxylans are composed of a linear backbone of (l-4)-p-linked xylopyranosyl residues, substituted with short mobile side chains. This could be a-glucuronic acid (or its methyl ester) mainly at 0-2. The distribution of these side groups is in average one per ten xylose residues (Jacobs et al., 2002)(VV). Native glucuronoxylan is acetylated in 0-2 and/or 0-3 and the acetyl content is around 70% (Aspinall, 1980; Carpita and Gibeaut, 1993)(72,/3). The degree of substitution determines the solubility of the polymer and also affects the capability of interact with other polymers (Reis et al., 1994)(J). The molecular properties of xylans from various wood species have been elucidated by static light scattering experiments (Goring and Timell, 1969) (14). These authors already reported about the colloidal character of the xylan solution. The tendency for xylans to form associated material in aqueous solutions was also reported by Blake and Richards (1971)(/5). Solubility of xylans in water containing salts, in comparison to dissolution in organic solvents like D M S O or T H F have shown that light scattering signals were removed or shifted and the distribution of xylan became more Gaussian than in the water system (Glasser et al., 2000; Saake et al., 2001)(7tf,77). The amphiphilic behaviour of glucuronoxylans can also be derived from the hypothesis that glucuronoxylan could play a key role in the twisted morphogenesis of the cell
In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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walls (Nieduszinski and Marchessault, 1972; Reis et al., 1994)(5,7
