Mixed-linkage (1 — 3, 1 — 4) linear P-D-glucans (P-glucans) are major components of endosperm cell walls of commercially important cereals, such as oat, barley, rye and wheat. P-Glucans from cereals are linear homopoly-saccharides of D-glucopyranosyl residues (Glcp) linked via a mixture of P-(1 — 3) and P-(1 — 4) linkages. The structure features the presence of consecutive (1 — 4)-linked P-D-glucose in blocks (i.e. oligomeric cellulose segments) that are separated by single (1 — 3) linkages. Although most of the cellulose segments are trimers and tetramers, longer cellulosic oligosac-charides are also present in the polymeric chains.19-25 Cereal P-glucans exhibit diversity with respect to their molecular/structural features, such as molecular size, ratio of tri- to tetramers, amount of longer cellulosic oligomers and ratio of P-(1 — 4) : P-(1 — 3) linkages. These structural features seem to be important determinants of their physical properties, such as water solubility, viscosity, and gelation properties. The molecular size and fine structural features of P-glucans play an important role in the solubility and chain conformation, or shape and, therefore, in their rheological properties in solution. The chemical features of cereal P-glucans are reflected in their solubility in water and in their extended, flexible chain conformation.19 The cellulose-like portions of cereal P-glucans might contribute to the stiffness of the molecules in solution.22 Furthermore, P-glucans containing blocks of adjacent P-(1 — 4) linkages may exhibit a tendency for interchain aggregation (and hence lower solubility) via strong hydrogen bonds along the cellulose-like segments; the P-(1 — 3) linkages break up the regularity of the P-(1 — 4) linkage sequence, making it more soluble and flexible.26 On the other hand, it has been shown that a higher content of cellotriosyl units might impose some conformational regularity within the P-glucan chain, and consequently a higher degree of organization of these polymers in solution (i.e. lower solubility).27,28 Suggestions have been made that differences in the amount of cellotriosyl fragments, long cellulosic oligomers, and the ratio of (1 — 4) : (1 — 3) linkages, might explain solubility differences among cereal P-glucans in accord with the two previous aggregation mechanisms.21,29-31
Rheologically, solutions of cereal P-glucans fall into the category of vis-coelastic fluids behaving similarly to the well-characterized random-coil polysaccharides; i.e. a Newtonian region can be observed at low shear rates and a shear thinning flow at high shear rates. However, the preparation of P-glucan solutions, their storage time (i.e. waiting time before analysis) and their thermal history have been proven to affect their rheological behav-iour.32-37 Time-dependent rheological behaviour is revealed by thixotropic loop experiments for cereal P-glucans with certain structural features, implying the formation of intermolecular networks. Moreover, freshly prepared cereal P-glucan solutions exhibit typical random-coil flow behaviour and an increased storage time induces an unusual shear-thinning flow behaviour at low shear rates. This behaviour becomes more pronounced with increasing storage time before the rheological testing. A strong time-dependent behaviour has been observed for mixed-linked (1 — 3,1 — 4)-P-glucans with low molecular size and high amounts of cellotriose units and/or long cellulose-like oligomers in the polymeric chain. In addition, the departure from the usual flow behaviour was noticed in shorter storage periods for solutions with increasing concentration of P-glucans.34,35,37
As expected, an increased molecular weight induces an increase in the viscosity and the shear-thinning properties of P-glucan dispersions at equivalent polysaccharide concentrations.34,35,37,38 In addition to solution viscosity enhancement on storage, P-glucans have been shown to gel under certain conditions.34-45 Two different gelation models have been proposed in the literature for mixed-linkage (1 — 3,1 — 4)-P-glucans. One model involves the side-by-side interactions of cellulose-like segments of more than three contiguous P-(1 — 4)-linked glucosyl units, whereas the other model involves the association of chain segments with consecutive cellotriosyl units linked by P-(1 — 3) bonds.39 Among cereal P-glucans of equivalent molecular weight, the gelation time decreased and the gelation rate increased in the order of oat, barley and wheat P-glucans - reflecting the order of the molar ratio DP3/DP4 units.35,39,40,44 (NB, the enzyme lichenase, a (1 — 3, 1 — 4)-P-D-glucan-4-glucanohydrolase, specifically cleaves the (1 — 4)-glycosidic bond of the 3-substituted glucose residues in P-glucans, yielding oligomers with different degrees of polymerization (DP). The major products for the cereal P-glucans are 3-O-P-cellobiosyl-D-glucose (DP3) and 3-O-P-cellotriosyl-D-glucose (DP4).) Further to the fine struc ture, the molecular size of the polymer seems to have a strong impact on polysaccharide gelation ability. For samples with similar distribution of cellulose-like fragments the gelation time decreases and the gelation rate increases with decreasing molecular weight, possibly due to the higher mobility of the shorter chains that enhances diffusion and lateral interchain associations. 34,35,37-39,41,46,47 Although a slower gelation process is noted for the high molecular size P-glucans, the gel network structure consists of structural elements (microaggregates) with better organization and/or it involves interchain associations over longer chain segments.34'35'37
Variations in the mechanical properties of cereal P-glucan gels have also been revealed by large deformation compression tests. An increase in strength and a decrease in brittleness of the P-glucan gels cured at room temperature were found with increasing concentration, molecular size and DP3/DP4 ratio of the polysaccharide, whereas cereal P-glucans with high molecular size and low amounts of DP3 units promoted the formation of strong cryogels (formed under repeated freezing-thawing cycles) when tested under large deformation protocols, a fact that was attributed to differences in the nature of the network microstructure among samples.34,35,37,42
Overall, the structure-physical property relations for cereal P-glucan isolates have been largely explored. Structural features, such as distribution of cellulosic oligomers, ratio of DP3/DP4 units and molecular size of the polysaccharides were proven to be important determinants of their solubility, rheological behaviour in aqueous solution and gelation ability, as well as of the thermal and mechanical properties of hydrogels obtained by interchain associations either at room temperature or via repeated freeze-thaw cycles.
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