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Carbohydrates are the most abundant organic constituents of plants. They serve as the major source of chemical energy for living organisms (e.g. sugars and starch), as well as important constituents of supporting tissues (e.g. cellulose). Carbohydrates are usually defined as: “Polyhydroxy aldehydes or ketones, or substances that hydrolyze to yield polyhydroxy aldehydes or ketones”. Simple carbohydrates are also known as “sugars or saccharides” (latin: saccharum = sugar). Monosaccharides These consist of only one saccharide or sugar unit and they are non-hydrolysable. They are subclassified according to: 1-The number of carbon atoms present in their molecule and, 2-The type of carbonyl group they contain. Thus, a monosaccharide containing three carbon atoms is called a triose and that containing five is called a pentose and so on. A monosaccharide containing an aldehyde group is called an aldose and one containing a keto group is called a ketose. These two classifications are frequently combined: e.g. a five-carbon aldose, for example, is called an aldopentose, a six-carbon ketose is called a ketohexose Oligosaccharides These consist of 2 and up to 10 molecules of simple sugars and are hydrolysable. They are sub classified into di-, tri- and tetrasaccharides etc…, according to the number of molecules of simple sugars they yield on hydrolysis. Polysaccharides Polysaccharides are high molecular weight polymers of monosaccharides of very complex nature. They are hydrolysable and yield a large number of monosaccharides. Different groups of polysaccarides are distinguished according to the final products of hydrolysis: 1-The homosaccharides (or holosides)which yield on hydrolysis similar simple sugar units (units of the same monosaccharide). Those, which yield pentose sugar units on hydrolysis, are called pentosans and similarly those yielding hexose units are called hexosans. 2-The heterosaccharides (or heterosides), which yield upon hydrolysis dissimilar sugar units e.g. mixture of hexoses and pentoses and thus called pentohexosans.(put example) 3-The derived carbohydrates, which yield on hydrolysis monosaccharides in addition to other components such as uronic acids, sulfate esters or amino sugars. Physical Characters of CHO 1-Condition: monosaccharides and most disaccharides are white, crystalline in shape and with sharp melting points. 2-Taste: Most of the simple and low molecular weight sugars have a sweet taste. 3-Solubility Monosaccharides are soluble in cold water and hot alcohol. Starch, pectin, mucilages and glycogen are difficulty soluble in cold water, but more soluble in hot water and insoluble in alcohol. 4-Optical activity Monosaccharides and water-soluble oligosaccharides are optically active and determination of their specific rotation is useful for their identification. A compound is optically active when, in solution, it is capable to rotate the plane of polarized light either to right (dextrorotatory, + or d) or to the left (levorotatory, - or l) and the two stereoisomers are called enantiomers. This optical activity is due to presence of a chiral center in the molecule. Mutarotation When a sugar is dissolved in water, the specific rotation of the solution gradually changes until it reaches to a constant value, e.g. freshly prepared solution of glucose has a specific rotation +112 o. When this solution is allowed to stand (aged solution), the rotation falls to + 52.7 o, and remains constant at this value. This change in the specific rotation is known as the mutarotation phenomenon. All reducing sugars (except a few ketoses) undergo mutarotation.(why does it happen?) Reactions of carbohydrates: Reactions similar to carbonyl compounds Glycoside formation ( acetal formation ) Each carbonyl group (aldehydic or ketonic) reacts with two molecules of alcohol to give an acetal. O ROH OH OR Carbonyl compound Hemiacetal ROH OR OR Acetal + H2O Oxidation These reactions produce different products according to the reagent used. i-Bromine water; is a mild oxidizing reagent. It selectively oxidizes the -CHO group into -COOH, and converts aldoses to the corresponding aldonic acids ii-Nitric acid; is a strong oxidizing agent. It oxidizes both the -CHO and terminal -CH2OH of an aldose to -COOH groups, and these dicarboxylic acids are known as aldaric acids. CHO H HO CHO COOH OH H H Br2/H2O HO OH H H HO COOH OH H H HNO3 OH HO H H OH H OH H OH H OH H OH H OH H OH H OH CH2OH Glucose CH2OH Gluconic acid CH2OH Glucose COOH Glucaric acid iii-Controlled oxidation: this is carried out by first protecting the -CHO group, followed by oxidation of the -CH2OH group, or in one step by the aid of a specific enzyme to give alduronic acid, e.g. oxidation of glucose into glucuronic acid. Reduction Aldoses and ketoses can be reduced with sodium borohydride into the corresponding alditols. Examples: glucose gives glucitol (-CHO is converted to – OH) or sorbitol, mannose gives mannitol, galactose gives dulcitol. Specific chemical reactions: Reaction with phenylhydrazine (Osazone formation): The carbonyl group of an aldose reacts with phenylhydrazine to give a crystalline phenyl osazone. Osazones are used in identification of mono- and disaccharides as they differ in physical characters (e.g. m.p. and shape of crystals). Monosaccharides give the same crystalline osazone on hot, while reducing disaccharides (lactose and maltose) give different crystalline shapes and on cold. Maltosazone Lactosazone Glucosazone Action of mineral acids Treatment with hot concentrated mineral acid (HCl or H2SO4) leads to dehydration of sugars. Pentoses and methylpentoses give furfural and methylfurfural, respectively by the action of hot hydrochloric acid, while ketoses and aldoses give hydroxymethylfurfural. The formation of furfural derivatives is the basis of the color reactions used for qualitative and quantitative determination of monosaccharides by coupling with phenolic compounds or amines. Examples: In Molisch’s test the phenol used is -naphthol. In Seliwanoff’s test (rapid furfural) the phenol is resorcinol. Reaction with oxidising cations All monosaccharides and reducing disaccharides are readily oxidized by metal ions such as Cu+2 (Fehling’s reagent). These reactions are used for identification and quantification of reducing sugars. Quantitative analysis of carbohydrates: Phenol-Sulfuric Acid Method Carbohydrates are destroyed by heat and acid. They are particularly sensitive to strong acids and high temperatures. Under these conditions, a series of complex reactions take place, beginning with a simple dehydration reaction. Phenol-Sulfuric Acid Method Continued heating in the presence of acid produces various furan derivatives. They will condense with various phenolic compounds, such as phenol, resorcinol, orcinol, to produce colored compounds that are useful for carbohydrate analysis. This method is simple, rapid, sensitive, accurate, specific for carbohydrates, and widely applied. The reagents are inexpensive, readily available, and stable. A stable color is produced, and concentration could be obtained from a standard calibration curve. The results are reproducible. Specific Analysis of Mono and Oligosaccarides High Performance liquid Chromatography is the method of choice for analysis of mono- and oligosaccharides and can be used for analysis of polysaccharides after hydrolysis. HPLC gives both qualitative analysis(identification of the carbohydrate) and also allows for quantitative analysis. HPLC analysis is rapid, can tolerate a wide range of sample concentrations, and provides a high degree of precision and accuracy. Monosaccharides The common monosaccharides belong to three groups: The pentoses, The hexsoses, and The deoxysugars (as deoxyribose in DNA). Pentoses These are simple sugars containing five carbon atoms. They give furfural when warmed with dilute acids. They are rarely present in the free state but generally occur as pentosans or form the sugar moiety of glycosides. Examples of pentoses: -D-Ribose found in all plant and animal cells as the carbohydrate part of nucleic acids e.g. ribonucleic acid (RNA). -D-Xylose (or wood sugar) prepared from corncobs (thick cylindrical part on which the grains grow), or any woody material. -L-Arabinose (or pectin sugar ): found in gums, and forms the sugar part of several glycosides. Hexoses Properties These contain six carbon atoms. They occur free, combined as oligo- and polysaccharides, or forming the sugar part of glycosides. Examples Two groups of commonly occurring hexoses are distinguished: The aldohexoses such as glucose, mannose and galactose. The ketohexoses such as fructose. CHO H HO OH CH 2OH O OH OH H H OH H OH O HO HO -D-glucose CH 2OH O CH 2OH HO H H H OH H OH CH 2OH O H H OH H OH HO HO H CH 2OH OH OH -D-galactose OH O HO HO O CH 2OH CHO OH OH OH -D-mannose HO OH CH 2OH H OH OH H HO OH CH 2OH O CHO HO H O -D-glucose H OH HO OH HO HO CH 2OH O CHO OH O HO HO OH OH -L-galactose OH -D-Glucose (dextrose, grape sugar, blood sugar) Source D-glucose is found free in sugar juices or forms the sugar part of many glycosides. Glucose can also be obtained from honey, which contains a mixture of glucose, fructose and sucrose, or by inversion of sucrose. Properties D-Glucose is white crystalline, sweet, readily soluble in water. It reduces Fehling's and Barfoed's solutions. It gives an osazone that crystallizes on hot. Glucosazone occurs in the form of golden yellow, needle -shaped crystals. Uses It has a great pharmaceutical importance as ingredient in i.v. injections, as nutrient, diuretic and sweetening agent. It is used in ice-cream and candy industries. Ketohexoses β-D-Fructose (levulose or fruit sugar) Source It is found free in honey and in fruits juices, or as constituent of polysaccharide e.g. inulin. Properties Fructose occurs as crystals with intense sweet taste. It reduces Fehling's, and Barfoed's solutions It forms an osazone similar to that of glucose It gives a positive Seliwanoff’s test for ketoses (rapid furfural). Uses Fructose is used as food for diabetics and in infant feeding formulae (more easily digested than glucose). Diabetes is a disorder affecting the way the body produces and uses insulin and how it handles blood glucose. Insulin is essential for aiding glucose transport into cells. People with type I diabetes do not produce insulin, whereas those with type II diabetes either do not produce enough insulin or cannot efficiently use the insulin their bodies make. Factors such as overweight and obesity, lack of physical activity, and genetic predisposition all increase the risk for type II diabetes. People with diabetes must pay attention to the amount of all carbohydrates—sugars and starches—they consume. Because fructose does not require insulin, individuals with diabetes can often tolerate it better than other sugars. Disaccharides According to the position of the linkage between the sugar units, disaccharides are classified into non-reducing such as sucrose and reducing such as maltose and lactose. Non-reducing disaccharides Sucrose (saccharose, cane sugar or beet sugar) Source Sucrose is the most widely occurring disaccharide, it is found in many fruit juices, seeds, leaves, roots and honey. Properties It has a sweetening power more than glucose and less than fructose. On heating from 200 to 250oC, sucrose changes into an amorphous brown substance known as caramel (a decomposition product widely used as flavoring and coloring matter). It gives positive results with cobalt nitrate test (violet). It does not reduce Fehling’s solution. It does not form an osazone. The enzymatic or dilute acid hydrolysis of sucrose is called “inversion” due to the fact that: sucrose ([α]25D= +66.5o) is hydrolyzed to an equimolecular mixture of D-(+)-glucose ([α]25D= +52.7o) and D-(-)fructose ([α]25D = -92o), because of the high negative rotatory power of fructose, the final solution has [α]25D= - 20.4o . The sign of rotation being changed from (+) in the original solution of sucrose into (-) in the hydrolyzed solution, the process is called inversion. Uses of sucrose In Pharmaceutical industries, sucrose is used in syrup preparation and tablet manufacture. It is used as nutrient and demulcent. Reducing disaccharide Maltose, or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) linkage. It is the second member of an important biochemical series of glucose chains. The addition of another glucose unit yields maltotriose; further additions will produce dextrins (also called maltodextrins) and eventually starch (glucose polymer). Lactose (or milk sugar) Source Lactose is the principal sugar of mammalian milk and is not present in higher plants. Structure It consists of galactose and glucose, linked by a β(1— 4) linkage . Preparation Lactose is obtained from whey (a by-product from cheese manufacture) after concentration, upon which deposits of lactose crystallize out. Uses Lactose is used as nutrient in infant foods, since it is less sweet than sucrose and more easily digested. It is also used as inert diluent for other drugs. Homosaccharides (Homopolysaccharides or holosides) Examples The most common holosaccharides are: The glucans (glucosans) such as starch, dextrin , glycogen and cellulose. The fructosans such as inulin Starch Source Starch is the most abundant and widely distributed plant substance next to cellulose and hemicellulose. It occurs as microscopic granules in the seeds, fruits, tubers and roots of plants. The most common commercial sources are the graminaceous fruits (e.g. rice and wheat ), maize and potato tubers. Structure It is an α-glucan ( or glucosan ) polysaccharide. The final product of hydrolysis is glucose. On heating starch with water, the starch granules swell and produce a colloidal suspension from which two major components can be isolated: amylose and amylopectin. Uses Starch has a wide variety of applications in food and drug industries it is used as: Antidote for iodine poisoning. Diluent in powders and tablets manufacture. Nutrient, demulcent, protective and adsorbent. Starting material in the manufacture of glucose, liquid glucose, maltose, and dextrins. The Mucopolysaccharides Mucopolysaccharides are polysaccharides which on hydrolysis yields amino sugar units. Amino sugars are derived from monosaccharides by replacement of a hydroxyl group by an amino group. ex. Heparin Source Heparin is the powerful blood anticoagulant. Uses Heparin and heparin analogues are used as anticoagulants. They have multiplicity of biological activities such as, anti-inflammatory, anti- cancer, antiviral, as well as in treatment of Alzheimer’s disease. Biologically Active Carbohydrates According to recent researches carbohydrates promise to be a major source of drug discovery. The diversity and complexity of carbohydrates explain their wide range of biological functions. There are several established carbohydrates-based products with “biopharmaceutical” application, as well as, other new products with potential application in medicine, e.g., development of specific cancer vaccines, new nonsteroidal anti-inflammatory drugs, and many other examples. I-Antibiotics This class of useful and potential therapeutics has gained attention with the appearance of complex nucleoside antibiotics, which exhibit a variety of biological activities. Of these are natural the thiosugars, aminoglycosides and macrolide antibiotics produced by various species of bacteria and fungi. II-Antitumour agents Only some non-toxic antitumour polysaccharides derived from bacteria, fungi, and algae have demonstrated good antitumour activity. Example (1-3)- β-D-glucans, such as lentinan isolated from the fruiting body of Lentinus edodes. Schizophyllan (Sonifilan®) is an extracellular of product of the fungus Schizophyllum commune. It is an immunostimmulant related to lentinan. It is useful in combination with other antineoplastic treatments in the management of carcinomas of the lungs, stomach, uterus and breasts. III-Laxatives: Lactulose is used in treatment of chronic constipation. IV-Sucralfate is an aluminum hydroxide complex of sucrose sulfate that is used in the therapy of duodenal ulcers. V-Sweeteners Carbohydrates play an important factor in increasing the incidence of diabetes, obesity and dental caries. There is increasing need for other alternatives to sucrose as sweetening agents for medical purposes, especially in case of diabetes, and for diet improvement. These agents should have high solubility in water, good stability, and a relative sweetness close or equivalent to that of sucrose. They should be safe, low caloric, and non-carcinogenic. Sweeteners are either of “natural” or “artificial” origin: Natural sweeteners Natural sweeteners are classified into two broad classes: The bulk sweeteners, having sweetening effect nearly similar to sucrose. The intense sweeteners having sweetening effect many times as that of sucrose. Bulk sweeteners These are the traditional sweeteners such as sucrose, glucose, fructose and the polyols or sugar alcohols e.g. sorbitol, mannitol, xylitol and lactitol. Intense sweeteners These are either synthetic e.g. saccharin, aspartame, cyclamate or natural e.g. steviol glycosides, glycyrrhizin. Steviol glycosides These are a group of diterpene glycosides obtained from Stevia rebaudiana, Family Asteraceae. The most important of which is stevioside. They are 100- 300 times sweeter than sucrose. Glycyrrhizin This is a triterpene glycoside isolated from Glycyrrhiza glabra, Family Leguminosae. It is an example of intense sweetener, sweetness 50 times as sucrose.