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Pharmacognosy- 1 PHG 222 Prof. Dr. Amani S. Awaad Professor of Pharmacognosy Pharmacognosy Department, College of Pharmacy Salman Bin Abdulaziz University, Al-Kharj. KSA. Email: [email protected] Carbohydrates To know what is carbohydrates To Recognize Classification of carbohydrates To Identify types Monosaccharides To know Structural formulas for Monosaccharides To understand Reactions of Monosaccharides To know Polysaccharides To understand and identefiy Carbohydrate Antibiotics Carbohydrates • Carbohydrates [CX(H2O)Y] are usually defined as polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones. • Simple carbohydrates are known as sugars or saccharides (Latin saccharum, sugar) and the ending of the names of most sugars is –ose. For example: 1. 2. 3. 4. Glucose (for the principle sugar in blood) Fructose (for a sugar in fruits and honey) Sucrose (for ordinary table sugar) Maltose (for malt sugar) Carbohydrates Classification of carbohydrates • Monosaccharide (C6H12O6) it is a simple carbohydrate, one that one attempted hydrolysis is not cleaved to smaller carbohydrates. e.g. glucose • Disaccharide on hydrolysis is cleaved to two monosaccharide, which may be the same or different. e.g. sucrose • Oligosaccharide (oligos is a Greek word that means “few”) yields 3 10 monosaccharide unit on hydrolysis. e.g. raffinose • Polysaccharides are hydrolyzed to more than 10 monosaccharide units. Cellulose is a polysaccharide molecule that gives thousands of glucose molecules when completely hydrolyzed. Carbohydrates Classification of carbohydrates Monosaccharaides • Over 200 different monosaccharides are known. • Monosaccharides are classified according to: 1. The number of carbon atoms present in the molecule. 2. Whether they contain an aldehyde or ketone group. • A monosaccharide containing three carbon atoms is called a triose; one containing four carbon atoms is called a tetrose. • A monosaccharide containing an aldehyde group is called an aldose; one containing a keto group is called a ketose. Carbohydrates Classification of carbohydrates Monosaccharaides • The simplest monosaccharide is glyceraldehyde, which contains a stereocenter. Therefore, it exist in two enantiomeric forms. • In 1906, (+)-glyceraldehyde is designated D-(+)- glyceraldehyde • and ()- glyceraldehyde is designated L()- glyceraldehyde. • These two compounds serve as configurational standards for all monosaccharides. Carbohydrates Classification of carbohydrates Monosaccharaides • A monosaccharide whose highest numbered stereocenter (the penultimate carbon) has the same configuration as D-(+)glyceraldehyde is designated as a D sugar; one whose highest numbered stereocenter has the same configuration as L-()glyceraldehyde is designated as an L sugar. • D and L designations are not related to the optical rotations of the sugars to which they are applied. • One may encounter other sugars that are D-(+)- or D-()- and ones that are L-(+)- or L-()-. Carbohydrates Monosaccharaides Structural formulas for Monosaccharides • Fisher projection • Haworth formulas • anomer or anome O Pyran O Not all carbohydrates exist in Furan equilibrium with six-membered hemiacetal rings, in several instances the ring is five membered. If the monosaccharide ring is six membered, the compound is called a pyranose (e.g. -D-glucopyranose ); if the ring is five membered, the compound is designated as a furanose. (e.g. -D-ribofuranose). Carbohydrates Monosaccharaides Structural formulas for Monosaccharides Mutarotation • The spontaneous change that takes place in the optical rotation of and anomers of a sugar when they are dissolved in water. The optical rotations of the sugars change until they reach the same value. • the explanation for this mutarotation lies in the existence of an equilibrium between the open-chain form of D(+)-glucose and the and forms of the cyclic hemiacetals. • the concentration of open-chain D(+)-glucose in solution at equilibrium is very small. H2O H2O Carbohydrates Monosaccharaides Structural formulas for Monosaccharides Glycoside Formation Hemiacetal: a functional group, consisting of a carbon atom bonded to an alkoxy group and to a hydroxyl group. Hemiacetals are synthesized by adding one molar equivalent of an alcohol to an aldehyde or a ketone. The reactions of aldehydes and ketones with alcohols parallel their reactions with water. Carbohydrates Monosaccharaides Structural formulas for Monosaccharides Glycoside Formation • Carbohydrate acetals, generally, are called glycosides, and an acetal of glucose is called a glucoside. • The methyl D-glucosides have been shown to have sixmembered ring, so they are properly named methyl -Dglucopyranoside and -Dglucopyranoside. • Glycosides are stable in basic solutions because they are acetals. • In acidic solutions, glycosides undergo hydrolysis to produce a sugar and an alcohol (aglycone). Carbohydrates Sugars that contain nitrogen 1. Glycosylamines Glycosylamines are a class of biochemical compounds consisting of an amine with a β-N-glycosidic bond to a carbohydrate, forming a cyclic hemiaminal ether bond (α-aminoether). Adenosine is an example of a glycosylamine that is also called a nucleoside. 2. Amino sugars A sugar in which an amino group replaces a nonanomeric OH group. e.g. D-glucosamine. •D-glucosamine can be obtained by hydrolysis of chitin, a polysaccharide found in the shells of lobsters and crabs and in the external skeletons of insects and spiders. Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Carbohydrates Reactions of Monosaccharides 1. Enolization, Tautomerization, and Isomerization OH Dissolving monosaccharides in aqueous base causes them to undergo enolizations and a series of keto-enol tautomerizations that lead to isomerizations. RC CR2 Enol O RC O CR2 RC Enolate ion Enols are in equilibrium with an isomeric aldehyde or ketone, but are normally much less stable than aldehydes and ketones. Enolate ion is the conjugate base of an enol. Enolate ions are stabilized by electron delocalization. Tautomerizm is a process by which two isomers are interconverted by a movement of an atom or a group. Enolization is a form of tautomerism Tautomerization creates structural isomers D-Glucose to D- Fructose Note: Formation of a glycoside prevents enolization CR2 Carbohydrates Reactions of Monosaccharides 2. Formation of Ethers The hydroxyl groups of monosaccharides are more acidic than those of ordinary alcohols because the monosaccharide contains so many electronegative oxygen atoms, all of which exert electron-withdrawing inductive effects on nearby hydroxyl groups. In aqueous NaOH, the hydroxyl groups are converted to alkoxide ions and each of these in turn, reacts with dimethyl sulfate to yield a methyl ether. Formation of Ethers – Exhaustive Methylation • SN2 Mechanism • Base-catalyzed Carbohydrates Reactions of Monosaccharides 3. Conversion to Esters Treating a monosaccharide with excess acetic anhydride and a weak base (such as pyridine or sodium acetate) converts all of the hydroxyl groups, including the anomeric hydroxyl, to ester groups O H OH O H O HO HO H OH H H OCCH3 (CH3CO)2O Pyridine, 0°C H O H3CCO H3CCO H OH O H OCCH3 O OCCH3 O Carbohydrates Reactions of Monosaccharides 4. Oxidation Reactions of Monosaccharides A. Fehling’s solution or Benedict’s reagent A characteristic property of an aldehyde function is its sensitivity to oxidation. Carbohydrates that give positive tests with Fehling’s or Benedict’s reagents are termed Reducing Sugars. Ketoses are also reducing sugars, since under the conditions of the test, ketoses equilibrate with aldoses by way of enediol intermediates, and the aldoses are oxidized by the reagents Fehling’s O O R CH Benedict’s 2Cu2+ 5HO Aldehyde From copper(II) Hydroxide sulfate ion R CO Carboxylate anion Cu2O 3H2O Copper(I) oxide Water Carbohydrates Reactions of Monosaccharides 4. Oxidation Reactions of Monosaccharides B. Bromine Water: the synthesis of aldonic acids Bromine water is a general reagent that selectively oxidizes the - CHO group to α-CO2H group Bromine Water: The Synthesis of Aldonic Acid Selectively oxidizes Aldehydes, Gives a positive Test for Aldose, and Gives a negative test for Ketose. Carbohydrates Reactions of Monosaccharides 4. Oxidation Reactions of Monosaccharides C. Nitric Acid Oxidation: Aldaric Acids CHO Dilute nitric acid – a stronger oxidizing agent than bromine water – oxidizes both the –CHO (CHOH)n group and the terminal –CH2OH group of an aldose to –CO2H group. These dicarboxylic CH2OH acids are known as aldaric acids. Aldose CO2H HNO3 (CHOH)n CO2H Aldaric acid Carbohydrates Reactions of Monosaccharides 4. Oxidation Reactions of Monosaccharides D. Periodate oxidations: oxidative cleavage of polyhydroxy compounds Compounds that have hydroxyl groups on adjacent atoms undergo oxidative cleavage when they are treated with aqueous periodic acid (HIO4). In these periodate oxidations that for every C-C bond broken, a C-O bond is formed at each carbon. When three or more –CHOH groups are contiguous, the internal ones are obtained as formic acid. For example, glycerol Stoichiometric cleavage Oxidative cleavage also takes place when an –OH group is adjacent to the carbonyl group of an aldehyde or ketone (but not that of an acid or an ester). For example, glyceraldehyde Periodic acid does not cleave compounds in which the hydroxyl groups are separated by an intervening –CH2 – group, nor those in which a hydroxyl group is adjacent to an ether or acetal function. O C OH C OH HIO4 2 C HIO4 H2O Carbohydrates Reactions of Monosaccharides 5. Reduction of Monosaccharides: Alditols Aldoses (and ketoses) can be reduced with sodium borohydride to compounds called alditols. For example, D-glucitol (or D-sorbitol) CHO (CHOH)n CH2OH NaBH4 (CHOH)n CH2OH CH2OH Aldose Alditol Disaccharides • Disaccharides are carbohydrates that yield two monosaccharide molecules on hydrolysis. e.g. 1)sucrose, 2)lactose, 3)maltose • Structurally, disaccharides are glycosides in which the alkoxy group attached to the anomeric carbon is derived from a second sugar molecule. Carbohydrates Sucrose Carbohydrates Disaccharides • Ordinary table sugar (C12H22O11) • Acid hydrolysis yields D-glucose and D-fructose. • Sucrose is a nonreducing sugar; it gives negative tests with Fehling’s solution because neither the glucose nor the fructose portion of sucrose has a hemiacetal group (both carbonyl groups are present as full acetals (i.e. as glycosides). Carbohydrates Polysaccharides • Polysaccharides, also known as glycans, consist of monosaccharides joined together by glycosidic linkages. • Polysaccharides that are polymers of a single monosaccharide are called homopolysaccharides; those made up of more than one type of monosaccharide are called heteropolysaccharides. • Homopolysaccharides are also classified on the basis of their monosaccharide units. • A homopolysaccharide consisting of glucose monomeric unit is called a glucan, one consisting of galactose units is a galactan, and so on. Carbohydrates Polysaccharides • Three important polysaccharides, all of which are glucans, are starch, glycogen, and cellulose. • Starch is the principle food reserve of plant. • Glycogen functions as a carbohydrate reserve for animals. • Cellulose serves as structural material in plants. Carbohydrates Carbohydrate Antibiotics Carbohydrate antibiotics refers to microbial metabolites (mainly from Actinomyces species) having antibacterial and antifungal properties and which are carbohydrates or contain a carbohydrate component in their chemical structure. Types include the aminoglycosides, macrolides, nucleoside analogs, and glycosylated aromatic structures NH NH HN • One of the important discoveries in carbohydrate chemistry was the isolation of the carbohydrate antibiotic called streptomycin. H2N NH OH O O L-Streptose H3C HO HO O O R • The glycosidic linkage is nearly always . Streptidine OH OH CHO • Streptomycin is made up of three unusual components. NH2 R' 2-Deoxy-2-methylamino-L-glucopyranose HO Streptomycin R = NHCH3 R' = CH2OH