Guar Galactomannan Research Articles

Borate complexes of guar galactomannan polymer and related model compounds

Borate complexes of guar galactomannan polymer and related model compounds

Non-linear viscoelasticity of polysaccharide solutions. 1: Guar galactomannan solutions

The viscoelastic properties of aqueous solutions of the polysaccharide guar galactomannan have been studied using dynamic oscillatory, steady and transient shear methods. At the concentrations studied, e.g. 1, 2 and 3% w/w, ([ η] = 8.8) a typical ‘entanglement’ network response has been observed, with liquid-like behaviour at low frequencies, linear strain dependence up to ∼0.4 and Cox-Merz superposition of η ∗(ω) and η( γ dot ). The evolution of shear stress during start shear experiments (stress overshoot) has been observed and the peak strain ( γ ∞) found to be independent of shear rate and within experimental error, independent of concentration, having a value of ∼2.1 strain units. For the highest concentration used, (3% w/w), the normal stress overshoot was also observed and the peak strain value found to be ∼20 strain units with significant dependence on shear rate. Stress relaxation measurements have shown measurable residual stress decaying within a few seconds to baseline noise. The effect of mechanical history on sample response was examined by monitoring the stress overshoot ratio after known ‘recovery’ times; this was found to stabilize after times of the order of 10 2 s, suggesting all of the temporary network structure could reform in this time. The results obtained are discussed in terms of theories for the molecular viscoelasticity of polymer solutions well known to workers in the area of synthetic polymers, but so far applied only to a very limited extent to aqueous polysaccharide solutions.

Non-linear viscoelasticity of polysaccharide solutions. 2: Xanthan polysaccharide solutions

The viscoelastic properties of aqueous solutions of the microbial polysaccharide xanthan have been studied using a wide range of rheological experiments including small deformation oscillatory, steady and transient shear techniques. Part I of this series described the classic ‘random-coil’ polymer response of guar galactomannan solutions under these experimental regimes. The high sensitivity of xanthan solutions to strain and to shear (strain) rate confirm previous observations that its rheological properties are more akin to those of a ‘weak gel’ than of a more conventional entanglement polymer solution, as exemplified by guar. In particular the strain at the peak shear stress γ ∞ (‘overshoot strain’) was found to be around 0.5, and with a minimal peak in the normal stress at γ N ⋍ 1 compared with values of γ ∞ around 2–3 and very high normal stress values of γ N for entanglement coupled semi-dilute solutions (including guar/water) and synthetic polymer melts/solutions. Stress relaxation measurements reveal extremely long (gel like) modes of residual stress in the relaxation spectrum. The observations are consistent with earlier data, and the postulated model of more specific inter-chain couplings between xanthan molecules occurring in solution.

Nuclear magnetic resonance evidence of the guarborate interaction

The formation of borate complexes, on addition of sodium tetraborate to solutions of guar galactomannan in D2O, was studied by 13C-n.m.r. spectroscopy. From the changes observed, some conclusions concerning the molecular motion of this polysaccharide and the preferential sites of a complexation have been drawn. These conclusions, which were substantiated by an 1H-n.m.r. study of borate-methyl α-d-galactopyranoside complexes in di2H3)methyl sulfoxide, may serve as a basis for understanding guar galactomannan-borate gelation.

Purification and characterization of a cell-associated, soluble mannanase from Bacteroides ovatus.

Bacteroides ovatus, a human colonic anaerobe, utilizes the galactomannan guar gum as a sole source of carbohydrate. Previously, we found that none of the galactomannan-degrading enzymes were extracellular, and we characterized an outer membrane mannanase which hydrolyzes the backbone of guar gum to produce large fragments. We report here the purification and characterization of a second mannanase from B. ovatus. This enzyme is cell-associated and soluble. Using ion-exchange chromatography, gel filtration, and chromatofocusing steps, we have purified the soluble mannanase to apparent homogeneity. The enzyme has a native molecular weight of 190,000 and a monomeric molecular weight of 61,000. It is distinct from the membrane mannanase not only with respect to cellular location but also with respect to stability and isoelectric point (pI of 6.9 for the membrane mannanase and pI of 4.8 for the soluble mannanase). The soluble mannanase, like the membrane mannanase, hydrolyzed guar gum to produce large fragments rather than monosaccharides. However, if galactosyl side chains were removed from the galactomannan fragments by alpha-galactosidase, both the soluble mannanase and the membrane mannanase could degrade guar gum to monosaccharides. Thus either or both of these two enzymes, working together with alpha-galactosidase, appear to be sufficient for the breakdown of guar gum to the level of monosaccharides.

Enzymatic modification of plant polysaccharides

Enzymes find widespread industrial use in the modification of the functional properties of plant polysaccharides both in vivo and in vitro. Reactions catalysed include depolymerization, debranching and de-esterification, depending on the specific enzyme or enzyme mixture employed and on the particular industrial requirement. Depolymerization of pentosans and/or barley β-glucans to destroy their viscosity-building properties is essential in starch and gluten manufacture, in the mashign of barley malt and in the production of maltosaccharide syrups. Depolymerization of pectin is required in juice clarification and to allow concentration. However, in other instances the aim may be to maintain or, at most, only slightly alter the molecular size of functional polysaccharides, i.e. in the conversion of guar galactomannan to a locust-bean type galactomannan and in the enzymic treatment of wheat-flour doughs. Enzymes may also be used to produce specific oligosaccharide fragments from polysaccharides and as diagnostic tools in the measurement of a particular polysaccharide in a mixture.

The fine structures of carob and guar galactomannans

The fine structures of carob and guar galactomannans

The fine structures of carob and guar galactomannans

The distribution of d-galactosyl groups along the d-mannan backbone (fine structure) of carob and guar galactomannans has been studied by a computer analysis of the amounts and structures of oligosaccharides released on hydrolysis of the polymers with two highly purified β- d-mannanases isolated from germinated guar seed and from Aspergillus niger cultures. Computer programmes were developed which accounted for the specific subsite-binding requirements of the β- d-mannanases and which simulated the synthesis of galactomannan by processes in which the d-galactosyl groups were transferred to the growing d-mannan chain in either a statistically random manner or as influenced by nearest-neighbour/second-nearest-neighbour substitution. Such a model was chosen as it is consistent with the known pattern of synthesis of similar polysaccharides, for example, xyloglucan; also, addition to a preformed mannan chain would be unlikely, due to the insoluble nature of such polymers. The d-galactose distribution in carob galactomannan and in the hot- and cold-water-soluble fractions of carob galactomannan has been shown to be non-regular, with a high proportion of substituted couplets, lesser amounts of triplets, and an absence of blocks of substitution. The probability of sequences in which alternate d-mannosyl residues are substituted is low. The probability distribution of block sizes for unsubstituted d-mannosyl residues indicates that there is a higher proportion of blocks of intermediate size than would be present in a galactomannan with a statistically random d-galactose distribution. Based on the almost identical patterns of amounts of oligosaccharides produced on hydrolysis with β- d-mannanase, it appears that galactomannans from seed of a wide range of carob varities have the same fine-structure. The d-galactose distribution in guar-seed galactomannan also appears to be non-regular, and galactomannans from different guar-seed varieties appear to have the same fine-structure.

Interaction properties of d-galactose-depleted guar galactomannan samples

The interaction of galactose-depleted guar galactomannan with agarose has been studied by optical rotation, and with xanthan using an Instron Materials Tester and a Rheometrics Mechanical Spectrometer. Samples were prepared by treatment of guar galactomannan with highly purified α- d-galactosidase. Modified guar galactomannans with a d-galactose content of 19–25%, in admixture with agarose, showed similar optical rotation changes on heating and cooling as did mixtures of agarose and carob galactomannan (23% d-galactose content). Modified samples with 13–16% d-galactose, in the presence of agarose, showed more pronounced optical rotation changes on heating and cooling, but samples with less than 13% d-galactose were only sparingly soluble even on autoclaving. The degree of interaction of galactose-depleted guar galactomannan samples with xanthan, as measured rheologically, increased as the d-galactose content decreased, paralleling the optical rotation changes with galactomannan/agarose mixtures. In the presence of xanthan, samples with a d-galactose content of 25% or less formed firm rubbery gels.

Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan

Molecular theories of the viscosity of dilute and more concentrated solutions of random-coil polymers have been applied to five molecular-weight fractions of guar galactomannan. The variation in intrinsic viscosity ([η]; dL.g −1) with molecular weight (M r) followed the relationship [η] = 3.8 × 10 −4) M r 0.723, consistent with random-coil behaviour. The intrinsic stiffness of the galactomannan backbone was estimated by evaluating the “characteristic ratio” C ∝, which is ∼ 12.6 and agrees well with results for carboxymethylcellulose, which has a closely similar, β-(1→4)-linked polymer backbone. At intermediate concentrations (c), up to specific viscosities of ∼ 10, η sp was proportional to c 1.3, while at higher concentrations, η sp was ∼c 5.1. This exponent is higher than is usually observed for polymers interacting purely by physical entanglement, and is evidence for more specific polymer-polymer interactions (“hyperentanglements”). The concentration dependence and the time-scale of entanglement coupling have been monitored by both steady-shear and oscillatory measurements, and a generality of response has been demonstrated.

Effect of galactose content on the solution and interaction properties of guar and carob galactomannans

Guar galactomannan has been modified by treatment with an α- d-galactosidase A preparation from lucerne seeds. This enzyme was purified by affinity chromatography on N-ϵ-aminocaproyl-α- d-galactopyranosylamine linked to Sepharose 4B, had a high activity towards galactomannans, and was completely devoid of β- d-mannanase. On incubation for 2 h, this enzyme removed ⪢ 75% of the galactose from guar galactomannan with no concurrent decrease in viscosity. Eventual decrease in viscosity was associated with the formation of insoluble, mannan-type precipitates. This phenomenon, although directly related to the galactose content of the galactomannan, was also time-dependent. The limiting viscosity numbers calculated for the “mannan backbones” of α- D-galactosidase-treated, guar galactomannan having galactose-mannose ratios of 38:62 to 15:85 were the same. Modified, guar galactomannan (at 0.4% w/v) having a galactose-mannose ratio of 20:80, or less, forms a gel on storage at 4° over several weeks. Also, gel particles form when solutions of these galactomannans are passed through a freeze-thaw cycle. Samples containing ⪡ 10% of galactose rapidly precipitate from solution even at 30°. The interaction of guar galactomannan with xanthan is greatly increased by removal of galactose residues. Samples having galactose-mannose ratios of ~19:81 interact with xanthan to essentially the same degree as carob galactomannan (Gal/Man = 23:77).

The relationship between the covalent structure of the Xanthomonas polysaccharide (Xanthan) and its function as a thickening, suspending and gelling agent

The ability of xanthan gum to form highly viscous and pseudoplastic solutions which are generally stable to variations in pH, temperature and salt concentrations is well documented. It has only recently been possible, however, to attempt an explanation for this behaviour on a molecular basis following publication of the revised chemical structure of the polysaccharide. The realisation that the polymer is, in essence, a derivative of cellulose has led to a number of propositions for the conformations adopted by the chains in solution. Temperature-induced changes of viscosity and optical rotation have been related to both inter- and intra-molecular conformational transitions, whilst viscosifying power has been found to depend on the degree of pyruvylation of the polysaccharide. From our work in this area we propose a rationalisation of these conflicting views and an explanation for the anomolous solution viscosity variations of the polyelectrolyte with ionic strength. The effect of molecular interactions on the suspending power of the solutions and the synergistic interactions with the galactomannans guar and locust bean gum is discussed.

Separation of polypeptide chains of ricin and the interaction of the A chain with cibacron blue F 3GA

From ricin bound to the galactomannan guar gum in a column, the nonbinding toxic A chain could be eluted by reduction with 2-mercaptoethanol and later the B chain by lactose. The presence of a nucleotide-binding domain on the toxic chain A could be demonstrated from its interaction with the blue dye Cibacron Blue F 3GA.

Additions and Corrections-The Structure of (I) Some Pectin Esters and (II) Guar Galactomannan

Additions and Corrections-The Structure of (I) Some Pectin Esters and (II) Guar Galactomannan

The Structure of (I) Some Pectin Esters and (II) Guar Galactomannan1a

The Structure of (I) Some Pectin Esters and (II) Guar Galactomannan^1a