Homopolysaccharides, Chemistry tutorial

Introduction:

Polysaccharides are the large high-molecular weight molecules prepared via combining monosaccharide units altogether via glycosidic bonds. They are at times termed as glycans. The most significant compounds in this class, cellulose, starch and glycogen are all polymers of glucose. This is simply illustrated by acid-catalyzed hydrolysis to the monosaccharide.

Homopolysaccharides:

These are the polysaccharides which comprise of a single kind of monosaccharides in their structures. Illustrations comprise starch, glycogen and cellulose.

Starch:

Starches are the carbohydrates in which 300 to 1000 glucose units join altogether. This is a polysaccharide that plants make use of to store energy for later use. Starch forms in grains by an insoluble outer layer that remains in the cell where it is made till the energy is required. Then it can be broken down to soluble glucose units. Starches are smaller as compare to cellulose units, and can be more readily employed for energy. In animals, the equivalent of starches is glycogen that can be stored in the muscles or in the liver for later use. Foods like rice, potatoes, corn and wheat include starch granules that are significant energy sources for humans. The human digestive procedure breaks down the starches to glucose units by the help of enzymes and those glucose molecules can circulate in the blood stream as the energy source.

Starch is the main form of stored carbohydrate in plants. Starch is comprised of a mixture of two substances: amylose, an in essence linear polysaccharide and amylopectin, a highly branched polysaccharide. Both forms of starch are polymers of α-D-Glucose. Natural starches have 10-20% amylose and 80-90% amylopectin. Amylose prepares a colloidal dispersion in hot water (that assists to thicken gravies) while amylopectin is fully insoluble. Most of the animals, including humans, depend on these plant starches for nourishment. The structure of starch is more complex than that of cellulose. The intact granules are insoluble in cold water, however grinding or swelling them in warm water causes them to burst. 

Amylose molecules comprise usually of 200 to 20,000 glucose units that form a helix as an outcome of the bond angles between the glucose units. Molecules of amylose are linear chains of some thousand glucose units linked by alpha C-1 to C-4 glycoside bonds. Amylose solutions are in reality dispersions of hydrated helical micelles. 

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Fig: Amylose 

Amylopectin varies from amylose in being highly branched. Short side chains of around 30 glucose units are joined by 1α→6 linkages around every 20-30 glucose units all along the chain. Amylopectin molecules might have up to two million glucose units.

The molecules of amylopectin are branched networks built from C-1 to C-4 and C-1 to C-6 glycoside links, and are necessarily water insoluble.

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Fig: Amylopectin

Starches are converted into numerous commercial products via hydrolysis by using acids or enzymes as catalysts. Hydrolysis is the chemical reaction in which water is employed to break long polysaccharide chains to smaller chains or to simple carbohydrates. The resultant products are assigned a Dextrose Equivalent (DE) value that is associated to the degree of hydrolysis. A DE value of 100 corresponds to fully hydrolyzed starch that is pure glucose (dextrose). 

Dextrins are a group of low-molecular-weight carbohydrates prepared by the hydrolysis of starch. Dextrins are the mixtures of polymers of D-glucose units joined by 1α→4 or 1α→6 glycosidic bonds. 

Maltodextrin is partly hydrolyzed starch which is not sweet and consists of a DE value less than 20. Syrups, like corn syrup made up from corn starch; encompass DE values from 20 to 91. Commercial dextrose consists of DE values from 92 to 99.

Corn syrup solids that might be labeled as soluble corn fiber or resistant maltodextrin are gently sweet semi-crystalline or powdery amorphous products by DEs from 20 to 36 made up by drying corn syrup in a vacuum or in spray driers. Resistant maltodextrin or soluble corn fiber are not broken down in the digestive system, however they are partly fermented via colonic bacteria therefore giving only two calories per gram rather than the 4 Calories per gram in corn syrup. 

High Fructose Corn Syrup (HFCS), generally employed to sweeten soft drinks, is prepared by treating corn syrup by enzymes to transform a part of the glucose into fructose. Commercial HFCS includes from 42% to 55% fructose, having the remaining percentage being mostly glucose. 

Modified starch is the starch which has been modified by mechanical methods or chemical treatments to stabilize the starch gels made up with hot water. Devoid of modification, gelled starch-water mixtures lose viscosity or become rubbery after some hours. 

Hydrogenated glucose syrup (HGS) is prepared by hydrolyzing starch and then hydrogenating the resultant syrup to form sugar alcohols such as maltitol and sorbitol, all along by the hydrogenated oligo- and polysaccharides. 

Polydextrose (that is, poly-D-glucose) is a synthetic, highly-branched polymer with numerous kinds of glycosidic linkages made by heating dextrose by an acid catalyst and purifying the resultant water-soluble polymer. Polydextrose is employed as a bulking agent as it is tasteless and is identical to fiber in terms of its resistance to digestion. The name resistant starch is applied to dietary starch which is not degraded in the stomach and small intestine, however is fermented via microflora in the large intestine.  

Glycogen:

Glucose is stored as glycogen in animal tissues via the method of glycogenesis. Whenever glucose can't be stored as glycogen or employed instantly for energy, it is transformed to fat. Glycogen is a polymer of α-D-Glucose similar to amylopectin; however the branches in glycogen tend to be shorter (around 13 glucose units) and more common. The glucose chains are organized globularly similar to branches of a tree originating from the pair of molecules of glycogenin, a protein having a molecular weight of 38,000 which acts as a primer at the core of the structure. Glycogen is simply transformed back to glucose to give energy.

Glycogen is the glucose storage polymer employed by animals. It consists of a structure similar to amylopectin, however is even more highly branched (around each and every tenth glucose unit). The degree of branching in such polysaccharides might be measured via enzymatic or chemical analysis.

Dextran:

Dextran is the polysaccharide identical to amylopectin, however the main chains are made by 1α→6 glycosidic linkages and the side branches are joined by 1α→3 or 1α→4 linkages. Dextran is the oral bacterial product which adheres to the teeth, making a film termed as plaque. It is as well employed commercially in confections, in lacquers, as food additives and as plasma volume expanders.

Cellulose:

Cellulose is the polymer of β-D-Glucose that in contrast to starch is oriented with -CH2OH groups alternating above and beneath the plane of the cellulose molecule therefore producing long, unbranched chains. The absence of side chains lets cellulose molecules to lay close altogether and form rigid structures. Cellulose is the main structural material of plants. Wood is mostly cellulose, and cotton is nearly pure cellulose. Cellulose can be hydrolyzed to its constituent glucose units via microorganisms which inhabit the digestive tract of termites and ruminants.

Cellulose might be modified in the laboratory via treating it by nitric acid (HNO3) to substitute all the hydroxyl groups having nitrate groups (-ONO2) to make cellulose nitrate (that is, nitrocellulose or guncotton) that is an explosive component of the smokeless powder. Partly nitrated cellulose, termed as pyroxylin, is employed in the preparation of collodion, plastics, lacquers and nail polish.

Over half of the net organic carbon in the earth's biosphere is in cellulose. Cotton fibers are necessarily pure cellulose and the wood of bushes and trees is around 50% cellulose. As a polymer of glucose, cellulose consists of the formula (C6H10O5)n where 'n' ranges from 500 to 5,000, based on the source of the polymer. Cellulose molecules tend to be straight chains and the fibers that yield from collections of cellulose molecules encompass the strength to form the supporting structures of plants. Even although human digestion can't break down cellulose for make use of as a food; animals like cattle and termites rely on the energy content of cellulose. They encompass protozoa and bacteria having the essential enzymes in their digestive systems. Cellulose in the human diet is required for fiber.

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Fig: Cellulose structure

Cellulose Gum or Carboxymethyl Cellulose (CMC) is the chemical derivative of cellulose where some of the hydroxyl groups (-OH) are replaced by carboxymethyl groups (-CH2COOH). The properties of cellulose gum based on the degree of substitution and the length of the cellulose chains. The degree of substitution (DS) is the number of carboxymethyl groups per glucose unit and might differ in commercial products from 0.4 to 1.5. Cellulose gum is non-toxic and becomes extremely viscous whenever joined by water. It is employed as a thickener for foods and as an emulsion stabilizer in products such as ice-cream. Cellulose gum is as well employed in personal lubricants, water-based paints, diet pills, detergents and paper coatings. 

Most of the animals can't digest cellulose as a food, and in the diets of humans this portion of our vegetable intake functions as roughage and is removed largely unchanged. Some of the animals (like cow and termites) harbor intestinal microorganisms which breakdown cellulose to monosaccharide nutrients via the use of beta-glycosidase enzymes.

Cellulose is generally accompanied via a lower molecular weight, branched, amorphous polymer termed as hemicellulose. In contrary to cellulose, hemicellulose is structurally weak and is simply hydrolyzed via dilute acid or base. As well, most of the enzymes catalyze its hydrolysis. Hemicelluloses are comprised of numerous D-pentose sugars, having xylose being the main component. Mannose and mannuronic acid are frequently present, and also galactose and galacturonic acid.

Synthetic Modification of Cellulose:

Cotton, possibly the most helpful natural fiber, is almost pure cellulose. The manufacture of textiles from cotton comprises physical manipulation of the raw material by carding, combing and spinning chosen fibers. For fabrics the best cotton has long fibers, and short fibers or cotton dust are eliminated. Crude cellulose is as well available from wood pulp via dissolving the lignan matrix surrounding it. Such less desirable cellulose sources are broadly employed for making paper.

In order to expand the ways in which the cellulose can be put to practical purpose, chemists have devised methods for making solutions of cellulose derivatives which can be spun to fibers, spread to a film or cast in various solid forms. A main factor in these transformations are the three free hydroxyl groups on each glucose unit in the cellulose chain, --[C6H7O(OH)3]n--. Esterification of such functions leads to polymeric products having very different properties compared by cellulose itself.

Cellulose Nitrate, first made over 150 years ago via treating cellulose by nitric acid, is the earliest synthetic polymer to observe general use. The completely nitrated compound, --[C6H7O(ONO2)3]n--, termed as guncotton, is explosively flammable and is a component of smokeless powder. Partly nitrated cellulose is termed pyroxylin. Pyroxylin is soluble in ether and at one time was employed for photographic film and lacquers. The high flammability of pyroxylin caused many tragic cinema fires during its period of use. Moreover, slow hydrolysis of pyroxylin yields nitric acid, a process which contributes to the deterioration of early motion picture films in storage. 

Cellulose Acetate, --[C6H7O(OAc)3]n--, is less flammable than pyroxylin, and has substituted  it in most applications. This is prepared by reaction of cellulose having acetic anhydride and an acid catalyst. The properties of the product differ with the degree of acetylation. A few chains shortening take place unavoidably in the preparations. An acetone solution of cellulose acetate might be forced via a spinneret to produce filaments, termed as acetate rayon which can be woven into fabrics. 

Viscose Rayon is made by the preparation of an alkali soluble xanthate derivative which can be spun into a fiber which reforms the cellulose polymer via acid quenching. The given general equation describes these transformations. The product fiber is known as viscose rayon.

ROH ↔ (NaOH) ↔ RO(-) Na(+) + S = C = S → RO-CS2(-) Na(+) → (H3O+) → ROH

Cellulose                                                       Viscose solution                    rayon

Hemicellulose:

The word 'hemicellulose' is applied to the polysaccharide components of plant cell walls other than cellulose, or to polysaccharides in plant cell walls that are extractable via dilute alkaline solutions. Hemicelluloses include approximately one-third of the carbohydrates in woody plant tissue.

The chemical structure of hemicelluloses comprises of long chains of a variety of pentoses, hexoses, and their respective uronic acids. Hemicelluloses might be found in plant stems, fruit and grain hulls. However hemicelluloses are not digestible, they can be fermented via yeasts and bacteria. The polysaccharides resulting pentoses on hydrolysis are known as pentosans. Xylan is an illustration of a pentosan comprising of D-xylose units by 1β→4 linkages. 

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Fig: Xylan

Beta-Glucan:

Beta-glucans comprise of linear unbranched polysaccharides of β-D-Glucose such as cellulose, however with one 1β→3 linkage for each and every three or four 1β→4 linkages. Beta-glucans form long cylindrical molecules having around 250,000 glucose units. Beta-glucans take place in the bran of grains like barley and oats, and they are acknowledged as being advantageous for reducing heart disease via lowering cholesterol and reducing the glycemic response. They are employed commercially to transform food texture and as fat substitutes. 

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Fig: Beta-Glucan

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