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268 Advances in textile biotechnology

11.2 Xyloglucans: a family of functional
plant polysaccharides

11.2.1 Xyloglucan in the vegetative cell wall

The XGs comprise a plant-specific family of polysaccharides based on a
highly xylose-substituted β(1 → 4) glucan (cellulose) backbone (Carpita
and McCann, 2000; Hoffman et al., 2005). Fuco-galacto-xyloglucans (Fig.
11.1) are widely distributed among land plants (excepting many grass
species), in which they act as the primary crosslinking glycans of cellulose

microfibrils in the primary cell wall (Carpita and Gibeaut, 1993; Carpita and
McCann, 2000; Popper, 2008). As such, XGs constitute up to one-quarter of
the dry weight of dicot cell walls (Busato et al., 2001 and references therein),
and are intimately associated with cellulose by adsorption onto and entrap-
ment within the paracrystalline structure (Pauly et al., 1999a). Indeed, XGs

have a demonstrably tight and specific binding to cellulosic substrates,
which is unique among polysaccharides (Zykwinska et al., 2005, 2008).
Moreover, this binding is effectively irreversible over a broad pH range (de
Lima and Buckeridge, 2001; Lima et al., 2004); strongly basic solutions (e.g.
2 M NaOH) are required to release cellulose-bound XGs, presumably
through partial ionization of the polysaccharide chains (Edwards et al.,
1985, 1986).

This remarkable, inherent affinity of XGs for cellulose forms the basis
for their use in biofi bre modification. Although an exact structural explana-

tion is still lacking, elucidation of the molecular details of the strong cel-
lulose–XG interaction has been an area of continued interest since the
mid-1970s (Hanus and Mazeau, 2006, and references therein; Valent and
Albersheim, 1974). The seminal study of Vincken et al. (1995) is particularly
illuminating in the context of the practical utilization of XG as a cellulose

modification reagent, and the subsequent development of the XET/XG-
based technology. This study was the first to demonstrate that quantitative

binding of XG to microcrystalline cellulose occurs when the polysaccharide
is comprised of four or more xyloglucan oligosaccharide (XGO) repeats,
i.e. when the XG chain has a backbone of 16 or more Glc residues (n > 3,

Fig. 11.1). Subsequent studies have confirmed and extended these results to
indicate that the binding of XG to microcrystalline cellulose is largely inde-
pendent of pH over the range 2–8 and temperature of 5–60 °C (de Lima
and Buckeridge, 2001; Lima et al., 2004).

11.2.2 Xyloglucan in seeds

In addition to their structural role in the vegetative cell wall, XGs have been
recruited as the primary storage carbohydrates in the seeds of certain

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