Rheological properties of xyloglucans from different plant species

Abstract

The rheological properties of three xyloglucans (XGs) from the extracellular medium of suspension cultured Nicotiana plumbaginifolia cells, apple pomace and tamarind seeds, with different structural features and molecular weights have been studied. The molecular weight (weight average) of the Nicotiana, apple pomace and tamarind seed XGs determined by multi-angle laser light scattering were 129, 219 and 833 kDa, respectively. Tamarind seed XG had the highest viscosity and Nicotiana XG had the lowest viscosity, with that of apple pomace XG intermediate. The viscosity of apple pomace XG at 5% w/v was almost equivalent to that of tamarind seed XG at 2% w/v, but their behaviour at high shear rates differed; both XGs were non-Newtonian in their rheological properties, but that from tamarind seeds showed more pronounced shear-thinning. The viscosity of Nicotiana XG at 5% w/v was almost equivalent to tamarind seed XG at 0.5% w/v, displaying Newtonian behaviour. Modification of the molecular weight of the XGs and their degree of branching revealed that differences in viscosity between the molecules, and their shear-field behaviour, was due primarily to differences in molecular weight. Removal of fucose residues from apple pomace XG decreased the viscosity of solutions from 8 to 4 mPa·s, whereas removal of both fucose and galactose from apple pomace XG, resulted in precipitation from solution. Deacetylation of Nicotiana XG also resulted in precipitation from solution.

Introduction

Xyloglucans (XGs) are a family of plant polysaccharides consisting of a backbone of 1,4-β-d-Glcp residues substituted at C(O)6 with side-chains of α-d-Xylp (Darvill et al., 1980; McNeil et al., 1984; Bacic et al., 1988; Fry, 1989; Hayashi, 1989). Most XGs are composed of the heptasaccharide unit structure XXXG (using nomenclature of Fry et al., 1993). Tamarind seed (Tamarindus indica) XG is composed of Glc:Xyl:Gal:Ara in the ratio of 4.0:3.4:1.5:0.3 (Gidley et al., 1991). XGs from many species are composed of the same structural features as tamarind seed XG but also contain α-l-Fucp-(1→2)-β-d-Galp disaccharides attached to O-2 of some of the Xylp residues; one such XG has been isolated from parenchymatous tissues of apples (Malus domestica; Ruperez et al., 1985; Renard et al., 1992) and from apple pomace which has been treated enzymically to remove pectin (Renard et al., 1995). Several features distinguish the XGs produced by solanaceous plants. An unusual XG which is branched at less than half of the backbone β-d-Glcp residues has been purified from cell walls of Nicotiana tabacum (Eda and Kato, 1978; Mori et al., 1979), the cell walls of potato (Solanum tuberosum; Vincken et al., 1996), the cell walls and extracellular polysaccharides (ECPs) of suspension cultures of N. tabacum (Eda et al., 1983; Akiyama and Kato, 1982; York et al., 1996) and the ECPs of N. plumbaginifolia (Sims and Bacic, 1995; Sims et al., 1996) and tomato (Lycopersicon esculentum; York et al., 1996). These XGs are highly arabinosylated and contain side-chains of α-d-Xylp and α-l-Araf-(1→2)-α-d-Xylp; the tomato XG contains, in addition, β-Araf-(1→3)-α-l-Araf-(1→2)-α-d-Xylp (York et al., 1996). In addition, N. plumbaginifolia XG bears O-acetyl groups on slightly less than half of the non-reducing 4-Glcp residues that are not xylosylated (Sims et al., 1996); a feature of other solanaceous arabinosylated XGs (York et al., 1996).

Studies on the location of substituents on the backbone of XGs show that their distribution is predominantly regular (York et al., 1990; Renard et al., 1992; Sims et al., 1996; York et al., 1996). From these data, generalized repeat structures of the XGs from tamarind seeds, apple pomace and suspension cultures of Nicotiana have been deduced, showing the differences in their side-chain substituents (Fig. 1). Incubation of tamarind seed XG with endo-(1→4)-β-d-glucanase yields four oligosaccharides with the structures XXXG (13% of total digest), XLXG (9%), XXLG (28%) and XLLG (50%)(Fig. 1A; York et al., 1990). Renard et al. (1992)have shown that XG from apple fruits was composed predominantly of approximately equal proportions of XXXG, XXFG and XLFG (Fig. 1B). Comparison of the monosaccharide composition of apple fruit XG (Ruperez et al., 1985; Renard et al., 1992) with that of apple pomace XG (Renard et al., 1995) suggested that these XGs were composed of similar repeats. The major oligosaccharide subunits obtained from endo-(1→4)-β-d-glucanase digestion of Nicotiana plumbaginifolia XG are based on the subunits XXGG (34% of total digest) and XXGGG (27%). Almost 60% of these subunits are further substituted with one terminal Araf residue, and almost 15% are substituted with two terminal Araf residues attached to O-2 of the Xylp residues (Fig. 1C; Sims et al., 1996). O-Acetyl groups are attached to C-6 of 44% of the non-reducing 4-Glcp residues that are not xylosylated, and to C-5 of 15% of the terminal Araf residues (Sims et al., 1996). Thus, it appears that in these XGs, blocks of unsubstituted 4-Glcp are short (up to four Glcp residues in Nicotiana XG), and are probably distributed regularly along the backbone.

The rheological properties of polysaccharides in solution are influenced by a number of parameters, including their molecular weight, and the presence, number and location of substituents (glycosyl and non-glycosyl) along the backbone. Hwang and Kokini (1991)have reviewed the effects of side branches on functional properties, including the effects of branching on viscosity, solubility, gelling, retrogradation, freeze–thaw stability and film formation. Izydorczyk and Biliaderis, 1992a, Izydorczyk and Biliaderis, 1992bhave demonstrated that the viscosity of wheat arabinoxylan increases with increasing molecular weight. In addition, they showed that shear-thinning behaviour, a characteristic of non-Newtonian fluids, is most pronounced in fractions with the higher molecular weights, with lower molecular weight fractions exhibiting near-Newtonian behaviour (i.e, no shear-thinning) at low shear rates.

Tamarind seed gum, a crude extract of tamarind seeds, has been used as a replacement for starch in cotton sizing, and as a wet-end additive in the paper industry, where it replaces starches and galactomannans (Glicksman, 1986). In Japan, where it is a permitted food additive, refined tamarind seed XG is used as a thickening, stabilizing and gelling agent in the food industry (Glicksman, 1986; Gidley et al., 1991). Tamarind seed XG is a high-molecular-weight polysaccharide (880 kDa), which forms viscous solutions when dissolved in water; the solution properties of tamarind seed XG have been studied (Gidley et al., 1991). The removal of terminal β-d-galactopyranosyl residues from tamarind seed XG by β-d-galactosidase results in a gradual decrease in viscosity, followed by an increase and then gelation (Reid et al., 1988). The formation of the gel is a function of the molecular weight of the tamarind seed XG, and if the molecular weight is decreased by digestion with endo-(1→4)-β-d-glucanase during removal of terminal β-d-galactopyranosyl residues, then precipitation occurs rather then gel formation (Reid et al., 1988).

In this study we report on the solution properties of an arabinosylated XG isolated from ECPs of suspension-cultured N. plumbaginifolia cells (Nicotiana XG) and a fucosylated XG isolated from apple pomace (apple pomace XG) and compared them with those of commercially available tamarind seed XG. The changes in viscosity properties of apple pomace and N. plumbaginifolia XGs were also investigated following enzymic and chemical modification, and the role of the side-chain substituents discussed.