Pharmacognosy- 1 PHG 222. Prof. Dr. Amani S. Awaad

Transcription

1 Pharmacognosy- 1 PHG 222 Prof. Dr. Amani S. Awaad Professor of Pharmacognosy Pharmacognosy Department, College of Pharmacy Salman Bin Abdulaziz University, Al-Kharj. KSA. amaniawaad@hotmail.com

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4 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

5 [C X (H 2 ) 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. Glucose (for the principle sugar in blood) 2. Fructose (for a sugar in fruits and honey) 3. Sucrose (for ordinary table sugar) 4. Maltose (for malt sugar)

6 Classification of carbohydrates Monosaccharide (C 6 H 12 6 ) 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 ligosaccharide (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.

7 Classification of carbohydrates Monosaccharaides ver 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.

8 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.

9 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. ne may encounter other sugars that are D-(+)- or D-( )- and ones that are L-(+)- or L-( )-.

10 Structural formulas for Monosaccharides Monosaccharaides Fisher projection Haworth formulas anomer or anome Pyran Not all carbohydrates exist in 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). Furan

11 Structural formulas for Monosaccharides Monosaccharaides 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. H 2 H 2

12 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.

13 Structural formulas for Monosaccharides Glycoside Formation Monosaccharaides 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).

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15 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 H 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 (C 6 H 13 N 5 ) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids.

16 Reactions of Monosaccharides 1. Enolization, Tautomerization, and Isomerization H Dissolving monosaccharides in aqueous base causes them to undergo enolizations and a series of keto-enol tautomerizations that lead to isomerizations. RC RC CR 2 Enol CR 2 RC Enolate ion CR 2 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

17 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 NaH, 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 S N 2 Mechanism Base-catalyzed

18 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 H H H H H H H H H (CH 3 C) 2 Pyridine,0 C H H 3 CC H 3 CC CCH 3 H H H CCH 3 CCH 3

19 Reactions of Monosaccharides 4. xidation Reactions of Monosaccharides A. Fehling s solution or Benedict s reagent A characteristic property of an aldehyde function is its sensitivity to oxidation. 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 Benedict s Fehling s R CH 2Cu 2+ 5H R C Cu 2 3H 2 Aldehyde From copper(ii) sulfate Hydroxide ion Carboxylate anion Copper(I) oxide Water

20 Reactions of Monosaccharides 4. xidation Reactions of Monosaccharides B. Bromine Water: the synthesis of aldonic acids Bromine water is a general reagent that selectively oxidizes the - CH group to α-c 2 H group Bromine Water: The Synthesis of Aldonic Acid Selectively oxidizes Aldehydes, Gives a positive Test for Aldose, and Gives a negative test for Ketose.

21 Reactions of Monosaccharides 4. xidation Reactions of Monosaccharides C. Nitric Acid xidation: Aldaric Acids Dilute nitric acid a stronger oxidizing agent than bromine water oxidizes both the CH group and the terminal CH2H group of an aldose to C2H group. These dicarboxylic acids are known as aldaric acids. CH (CHH)n CH 2 H Aldose HN 3 C 2 H (CHH)n C 2 H Aldaric acid

22 Reactions of Monosaccharides 4. xidation 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 (HI4). In these periodate oxidations that for every C-C bond broken, a C- bond is formed at each carbon. C C H H HI 4 2 C HI 4 H 2 When three or more CHH groups are contiguous, the internal ones are obtained as formic acid. For example, glycerol Stoichiometric cleavage xidative cleavage also takes place when an H 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.

23 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) CH CH 2 H (CHH)n NaBH 4 (CHH)n CH 2 H Aldose CH 2 H Alditol

24 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.

25 Sucrose Disaccharides rdinary table sugar (C 12 H ) 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).

26 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. Polysaccharides A homopolysaccharide consisting of glucose monomeric unit is called a glucan, one consisting of galactose units is a galactan, and so on.

27 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.

28 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 ne of the important discoveries in carbohydrate chemistry was the isolation of the carbohydrate antibiotic called streptomycin. Streptomycin is made up of three unusual components. The glycosidic linkage is nearly always. H H 2 N H 3 C R H CH H R' NH H Streptomycin HN H H NH 2 L-Streptose Streptidine 2-Deoxy-2-methylamino- L-glucopyranose R = NHCH 3 R' = CH 2 H

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