How does branching effect polysaccharides

High molecular weight molecules normally have a large excluded volume Eq. Almost all carbohydrate polymers with degrees of polymerization DP less than 20 are soluble in water [ 7 ].

Solubility decreases with the increase of molecular weight. For example, the amylose and amylopectin in starch are reluctant to dissolve in cold water due to high molecular weight, while maltodextrin starch after chain cleavage by acid or enzyme with the DP value less than 20 demonstrates very good solubility in cold water. The dissolution rate of polysaccharide samples is also highly affected by the molecular weight and molecular weight distribution.

Higher molecular weight usually leads to lower dissolution rate, as disentanglement from the particle surface and subsequent diffusion to the bulk solution of large molecules take a longer time compared to that of small molecules.

It has also been reported that samples with high polydispersity dissolved about twice as fast as monodisperse ones of the same Mn [ 1 ]:. Charged polysaccharides are referred to polysaccharides that carry charged groups in the molecules, which include both negatively acidic polysaccharides and positively charged polysaccharides. The charged groups help with the solubility of polysaccharides, which is achieved by 1 increasing the molecular affinity to water and 2 preventing the intermolecular association due to the electrostatic effects posed by the charged group.

Acidic polysaccharides are polysaccharides containing carboxyl groups e. The acidic group may be free or as a simple salt with sodium, potassium, calcium, or ammonium or naturally esterified with methanol. Therefore, most of the natural occurring pectin is readily soluble in water due to the charged group, although high in molecular weight.

It also should be noticed that adding salt or reducing pH value could shield the charged effect, which leads to gelation under some circumstances. For example, high methyl ester pectin gel at pH 3. Low methyl ester pectin can react with calcium ions to form gel, even under relative high pH environment.

Therefore, when dissolving the pectin into water, it is essential to avoid the gelling condition; similar to other hydrocolloids, the dissolution usually needs high shearing mixing [ 8 ]. Pectic polysaccharides from American ginseng. Adpated from Guo, et al. Adopted from Cui et al. As one typical positively charged polysaccharide Figure 4 , chitosan is derived from the deacetylation of chitin. The positively charged groups come from protonation of its free amino groups, which is the key to its water solubility.

Chitosan is insoluble in neutral and basic environments due to the lacking of a positive charge. However, in acidic environments, protonation of the amino groups increases the degree of water solubility. Following this property, chitosan has been widely used for drug delivery, e. Structural feature of chitosan. The linear polysaccharides with highly regular conformation that can form crystalline or partial crystalline structures are mostly insoluble in water, while branching structure could increase the solubility for two reasons: 1 the branching structure could weaken the intramolecular interaction due to the steric effects, which prevent the intermolecular association, and 2 the highly branched structure could also decrease the excluded volume when compared to polysaccharides with same molecular weight, which potentially increases the critical concentration and therefore improves the water solubility.

However, it can be modified by decreasing the Mw and introducing either charged sodium carboxymethyl cellulose CMC or branching groups methyl cellulose MC , hydroxylpropyl cellulose HPC , hydroxylpropyl methyl cellulose HPMC to increase the solubility Figure 5.

Schematic chart for cellulose derivatives. Starch contains both amylose and amylopectin. Amylopectin exhibits better solubility than amylose due to the highly branched structure, although the latter has relative low molecular weight amylose, 10 5 ; amylopectin, 10 7 —10 9. According to the structure and solubility difference, amylose and amylopectin can be separated from each other in starch granules according to the following procedure: firstly, starch granules are completely dispersed in hot water or aqueous dimethyl sulfoxide; amylose then can be precipitated by the addition of butanol as a crystalline complex due to the linear structure after cooling.

Afterward, amylopectin can be recovered from the supernatant by lyophilization [ 13 ]. Guar gum and locust bean gum both belong to the galactomannan family Figure 6 , while the degree of branching for guar gum galactose to mannose is higher than locust bean gum galactose to mannose about , which could easily prevent strong cohesion of the main backbones of different neighboring molecules, so that no extensive crystalline regions of guar gum can be formed, while locust bean gum is easy to form gel due to the naked region of the molecules, which favors the formation of junction zone [ 14 ].

Schematic chart for galactomannan structure. Xylans of all higher plants possess 1—4 linked D-xyl P residues as the backbone, substituted by various degrees with sugar units including arabinose, xylose, and glucuronic acid 4-O-methyl. However, with the increase of degree of substitution such as arabinose arabinoxylan as shown in Figure 7 , its solubility dramatically increased [ 15 ].

Schematic chart of arabinoxylan. A: T-Ara P. Gum arabic has a highly branched structure Figure 8. Gum arabic has been commercially used as emulsifiers due to the covalent bond with protein, in which protein functioned as hydrophobic group attached to the oil droplet and keeps the whole emulsion system stable. According to methylation analysis [ 16 ], the most branched sugar residues in gum ghatti were 3,4,6-Gal p The good solubility of gum ghatti could also be attributed to the 1—6 linked glycosidic bonds, which will be discussed in the later session.

Proposed structure of gum arabic Acacia senegal , adopted from Nie et al. Proposed structure of gum ghatti, adopted from Kang et al. Similar to branching effects, the presence of some hydrophobic groups, e. O -acetyl substituents are present on many cell wall polymers including various hemicelluloses Figure 10 , the pectic polysaccharides and the polyphenol lignin, which have been previously summarized [ 19 ]. O -acetylation of cell wall polysaccharides. A Generic representation of O -acetyl group as found at different -OH positions in many cell wall polysaccharides.

Note the structural similarity between O -acetyl- and methyl ester groups that decorate carboxylic acid residues in polygalacturonic acid.

B Occurrence of O -acetyl groups in cell wall matrix polysaccharides. Adopted from Pawar et al. It has been reported that the totally acetylated xylan DA 2. Similar conclusion has also been drawn from acetylated glucomannan. One example is polysaccharides from Dendrobium officinale traditional Chinese herbs , which belong to glucomannan family but highly acetylated, as shown in Figure This polysaccharide is readily dissolved in water. The solubility, however, is significantly decreased if the acetyl group was either removed through alkaline treatment or increased through acetylation reaction.

It has also been reported that the immunomodulating effect of this polysaccharide was also affected by the acetyl group [ 21 ]. Proposed structure of polysaccharides from Dendrobium officinale. Adopted from Xing et al. The conformation can be simply regarded as the way that polymer chains align themselves in solution to adopt an orientation with lower energies.

Two general types of conformation for polysaccharides can be simply divided—ordered conformation and disordered conformation—which is decided by the regularity of the molecular structure. In aqueous solution, most of non-starch polysaccharides with heterogeneous structure demonstrated disordered conformation, including random coil, rigid, and spherical conformation.

High-performance size exclusion chromatography HPSEC could be used to study the conformational properties of polysaccharides in aqueous solution. The double logarithmic plot of the molecular weight vs. The logarithmic plot of the molecular weight versus intrinsic viscosity of heteropolysaccharide isolated from seeds of Artemisia sphaerocephala Krasch is shown in Figure 12 , from which a random coil conformation of this polysaccharide was determined.

However, the curve in Figure 12 is not exact linear, a slightly decreased slope was observed with the increase of Mw, which was attributed to the increased percentage of side chains in the high Mw fraction by the authors [ 3 ]. Logarithmic plot of the molecular weight vs. If the values of the torsion angles are fixed by cooperative interactions between residues, such as in solid or gel states, an ordered conformation can be adopted.

Two general ordered conformations are ribbon-like and helix conformations Figure Polysaccharide with ribbon-like conformation is most easily aligned and closely packed through numerous hydrogen bonds and van der waals forces. The resultant compact structures essentially prevent solvent penetration and retain insolubility in water. Polysaccharides such as cellulose, xylan, and mannan, which contained the zig-zag type linkages, all belong to this type.

Compared to ribbon-like conformation, the hollow helix conformation showed a relative better solubility, but still not comparable with disordered conformation random coil , e. Linkage patterns and conformation types of polysaccharides: a ribbon-like and b hollow helix.

Adopted from Cui and Wang [ 23 ]. Type your answer in the space below and click on the Check button. Please enter your answer in the space at left. Now compare your answer with the one below.

The enzymes that build up and break down glycogen and starch act on the free ends of the polysaccharides. Having a great deal of branching ensures that plants and animals can quickly add to their energy supply when energy is plentiful, or break it down the storage molecules when energy is in short supply. Polysaccharides Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Cellulose is the most abundant organic molecule on earth, since it is the main component of plant cell walls.