Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic bonds. These diversified structural properties determine the functional properties of polysaccharides, such as solubility and rheological properties, which in turn benefit their extensive applications in both food and nonfood areas.
As most polysaccharides perform their function in aqueous solution, understanding the solubility of polysaccharides, therefore, becomes critically important [ 1 ]. This chapter specifically addressed the mechanisms of polysaccharide solubility from molecular level. The relationships between polysaccharide solubility and molecular structures were established. It also should be noted that the current chapter only focused on the polysaccharides in aqueous solution; the solubility of polysaccharides in other organic solvents was not covered.
Polysaccharides display a wide range of solubility; some are water insoluble, e. The dissolution of polysaccharides is different from that of the small crystalline molecules. The dissolution of most crystalline small molecules involved the disintegration of the crystalline structure and release of the separate atoms, ions, polysaccharides dissolution is a continuous hydration process with the conversion of inter-polysaccharide binding to polysaccharide-water binding, and most of the non-starch polysaccharides are in amorphous state.
The dissolution process is more or less assisted by entropy as the molecules assume lower-energy conformations. Polysaccharides have strong affinity to water molecules due to the presence of multi-OH groups. However, this also leads to a strong interaction among polysaccharide molecules via hydrogen bonding.
Therefore, the balance between molecule-molecule interaction and molecule-water interaction is the key to understand the polysaccharide solubility. For soluble polysaccharides, the interactions between polysaccharide molecules and water molecules are energetically favorable, and the solvent creates a solvating envelope around the polymer chain, which keeps the polysaccharide molecules away from each other.
For polysaccharides with poor water solubility, the intramolecular interactions between polymer segments dominate, leading to aggregation and eventually precipitation or gelation when ordered molecular structure, e. In between, under specific conditions, the polymer-polymer interaction can precisely compensate the polymer-water interaction, which referred as theta condition.
Under theta condition, the chain conformation is defined solely by bond angles and short-range interactions given by the hindrances to rotation about bonds and polymer coil dimensions [ 2 ]. Second virial coefficient A 2 , which describes the contribution of the pair-wise potential to the pressure of the gas, could reflect the polymer-water interaction. Good solvent, poor solvent, and theta condition can be indicated when the second virial coefficient is above, below, or equal to zero, respectively.
In a real experiment, the second virial coefficient of the polysaccharides in aqueous solution can be determined by static light scattering using Zimm plot Figure 1 based on the Eqs. Zimm plot of polysaccharide from seed of Artemisia sphaerocephala Krasch determined by SLS solvent, 0.
Adopted from Guo et al. For example, A 2 of xyloglucans from flaxseed kernel cell wall was reported as 3. This is because mild alkaline could break down the intermolecular hydrogen bonding, thus eliminating the aggregates in aqueous solution. The solubility of polysaccharides is determined by their molecular structures. Any structural feature that hinders the intermolecular association leads to higher solubility, such as branching structure, charged group carboxylate group, sulfate, or phosphate groups ; on the opposite, structural characters that promote the intermolecular association result in a poor solubility, such as linear chain, large molecular weight, and other regular structural characters.
Polysaccharides are polydisperse in molecular weight. Therefore the molecular weight of polysaccharides is mostly described in a statistic way, such as number average molecular weight Mn , weight average molecular weight Mw , and zeta average molecular weight Mz , as shown in the below equations Eqs.
Here Ci refers to the concentration of molecules that having molecular weight of Mi. The molecular distribution of polysaccharides can be described by the polydispersity index Eq. Most natural occurring polysaccharides demonstrated high PDI value above 2 :. The molecular weight and molecular weight distribution play a critical role for the solubility of 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. However, these molecules are not straight or totally linear.
At intervals along the starch molecule there are branches produced by another kind of glycosidic link between the C1 carbon on one sugar and the C6 carbon on the next sugar. When stored starch granules are removed from plants and placed in water they swell and release two types of material; amylose and amylopectin. Amylose is the simpler of the types of molecule and is largely linear chains of C1-to-C4 glysosides, several thousand units in length.
Amylopectin is more complex and these molecules are branched using a combination of C1-to-C4 bonds and C1-to-C6 bonds about every 25 glucose units along the chain.
Such large, complex molecules do not dissolve well in water. Glycogen is also made by linking together glucose molecules. Like starch, it is used by animals to store sugar and provide energy.
It is similar to amylopectin in structure, but branched with a C1-to-C6 glycosidic bond about every ten glucose units. Termites also contain cellulase-secreting microorganisms and thus can subsist on a wood diet. This example once again demonstrates the extreme stereospecificity of biochemical processes.
Certified diabetes educators come from a variety of health professions, such as nursing and dietetics, and specialize in the education and treatment of patients with diabetes. A diabetes educator will work with patients to manage their diabetes. This involves teaching the patient to monitor blood sugar levels, make good food choices, develop and maintain an exercise program, and take medication, if required.
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Starch is a storage form of energy in plants. It contains two polymers composed of glucose units: amylose linear and amylopectin branched. Glycogen is a storage form of energy in animals. It is a branched polymer composed of glucose units. It is more highly branched than amylopectin. Cellulose is a structural polymer of glucose units found in plants. Starch is the storage form of glucose energy in plants, while cellulose is a structural component of the plant cell wall.
Amylose and cellulose are both linear polymers of glucose units, but the glycosidic linkages between the glucose units differ.
Learning Objectives To compare and contrast the structures and uses of starch, glycogen, and cellulose. These branch points occur more often in glycogen.
Glycogen Glycogen is the energy reserve carbohydrate of animals. Cellulose Cellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls. Career Focus: Certified Diabetes Educator Certified diabetes educators come from a variety of health professions, such as nursing and dietetics, and specialize in the education and treatment of patients with diabetes.
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