XANTHAN PROPERTIES FOR SPECIFIC APPLICATIONS
3.2 CHEMICAL STRUCTURE AND CONFORMATION OF XANTHAN CHAINS
Xanthan is an anionic heteropolysaccharide with molecular weight ranging from 2 million to 50 million Da (Rosalam and England, 2006) and branched structure composed of repeated pentasaccharide units as shown in Figure 3.1.
FIGURE 3.1 Primary structure of xanthan. Reprinted with permission from Brunchi, C.
E.; Morariu, S.; Bercea, M. Intrinsic Viscosity and Conformational Param eters of Xanthan in Aqueous Solutions: Salt Addition Effect. Colloids Surf. B 2014, 122, 512–519. © 2014 Elsevier.
The repeated units consist of two glucose residues (as backbone), two mannose residues (namely, α-D-mannose and β-D-mannose), and one β-D-glucuronic acid residue (as a side chain linked at every second glucose residue of the backbone). The α-D-mannose (inner mannose) is linked to the backbone and it may contain an O-acetyl group at the C-6 position, while the β-D-mannose (outer mannose) may contain a pyruvate ketal at the C-4 and C-6 positions (Hassler and Doherty, 1990). The glucuronic acid residue is located between α-D- and β-D-mannose. Some studies suggest that the content and location of functional groups in the side chain struc- ture may undergo various changes (Kulicke and van Eikern, 1992; Kool et al., 2013, 2014; Bilanovic et al., 2016). The various types of xanthan were reported as a function of the acetate and pyruvate content from the repeated units: (1) low-pyruvate xanthan in which one pyruvyl group is found at 20 repeated units, (2) high-pyruvate xanthan with one pyruvyl group to each repeated unit, (3) high-acetate xanthan that has one acetyl group to
each repeated unit, and (4) low-acetate xanthan which contains one acetyl group to 20 repeated units (Bilanovic et al., 2016). In many cases, xanthan is presented with a structure in which the inner mannose is nearly stoichio- metric acetylated (about 70–85%) and the outer one is nonstoichiometric pyruvate (about 30–50%) (Rocherfort and Middleman, 1987; Cadmus et al., 1976; Orentas et al., 1963; Sutherland, 1981; Bilanovic et al., 2015).
Also, it was hypothesized (Hassler and Doherty, 1990; Kool et al., 2014) the existence of six-sided chain containing only acetyl or pyruvyl groups, both types of groups or none of them.
If pyruvyl group is located exclusively at the outer mannose, the acetyl group may be found at the inner mannose unit or at the outer one (in this position the acetylation is about 24%) or at both mannose units (Hassler and Doherty, 1990). Generally, it is assumed that the acetyl group from inner mannose is involved (by its methyl groups) in hydrogen bonding with adjacent hemi-acetal oxygen atom of the backbone promoting thus the intra-molecular association and stabilizing the ordered rigid rod-like conformation (structure) of xanthan (Tako and Nakamura,1989; Fitzpat- rick et al., 2013). As it turns, the pyruvate group has a destabilizing effect of ordered structure due to the increase of the electrostatic repulsion inter- actions (between COO− groups) (Viebke, 2005; Fitzpatrick et al., 2013), but by methyl group may contribute to the intermolecular association of xanthan macromolecules into quaternary structure (Tako and Nakamura, 1989).
Thus, the acetate and pyruvate content can be seen as an indicator of the xanthan rheological quality, high pyruvate xanthan is more viscous than that with low pyruvate content (Borges et al., 2009). By elimination of terminal mannose residue, the thickening ability is reduced compared with standard xanthan, whereas an opposite effect was observed by elimi- nation of both terminal mannose and glucuronic acid residues (Hassler and Doherty, 1990). The ordered xanthan structure with stiffness (Tinland and Rinaudo,1989; Brunchi et al., 2013) intermediate that of double- stranded DNA (Smidsrød and Haug, 1971) and triple-stranded collagen or schizophyllan (Zirnsak et al., 1999) is generally accepted, but the question if the ordered xanthan is formed by a single (Norton et al., 1984; Milas and Rinaudo, 1986) or double (Lui and Norisuye, 1988; Camesano and Wilkinson, 2001; Holzwarth and Prestridge, 1977) helix or as dimmers obtained by association of single chains in solution (Foss et al., 1987;
Gamini et al., 1991) is still debated. However, it was established that,
depending on solute nature, xanthan exists both in single- and double- stranded conformation in solution. Dintzis et al. (1970) reported that single-stranded xanthan is dissolved in urea, while Southwick et al. (1980) suggested the self-association of xanthan chains in aqueous solutions is induced by the presence of urea. This uncertainness related to the xanthan conformation may be due, on the one hand, to the limitations of analytical techniques used in solution as a result of the formed aggregates that mask the individual behavior of macromolecules and, on the other hand, to the lack of evidences of smaller fractions implied in deviation from the average behavior (the properties being determined in bulk solution) (Camesano and Wilkinson, 2001). Atomic force microscopy (AFM) studies have shown that, at equilibrium, the double-helix configuration of xanthan molecules is based on intramolecular (antiparallel) or inter-molecular (antiparallel or parallel) associations (Moffat et al., 2016) (Fig. 3.2).
FIGURE 3.2 AFM images of xanthan chains (a) with unraveled ends and (b) electronic zoom of the marked region. Reprinted with permission from Moffat, J., et al. Visualization of Xanthan Conformation by Atomic Force Microscopy. Carbohydr. Polym. 2016, 148, 380–389. https://creativecommons.org/licenses/by/4.0/
However, it is unanimously accepted that xanthan exists in two ordered structures (i.e., native and re-natured) and one disordered (de-natured) (Milas et al., 1996; Capron et al., 1998; Matsuda et al., 2009).
3.3 STRATEGIES FOR CHANGING XANTHAN PROPERTIES