Glycosides 1 and 2des PDF

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Glycosides- Definitions, medical importance, characteristics and classification...

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PHM 4453 8 GLYCOSIDES 1 (Introduction, ald glycs, alc/phenolic glys, Anthraq-intro.) by EDITH WAKORI 25th June 2018 1. GLYCOSIDES 1.1 Introduction Glycosides are (usually) non-reducing compounds, which on hydrolysis by reagents or enzymes yield one or more reducing sugars among the products of hydrolysis. A molecule in which a sugar is bound to another functional (non-sugar!) group via a glycosidic bond/linkage. The sugar group is known as the glycone and the non-sugar group as the aglycone or genin part of the glycoside. The glycone can consist of a single sugar group or several sugar groups. The sugars involved are glucose, fructose, galactose, mannose, rhamnose, digitoxose, cymarose, etc Examples of glycosides; salicin, arbutin, glucovanillin, barbaloin and oleandrin. Glycosides play numerous important roles in living organisms.

1.2 Role of glycosides in organisms Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme hydrolysis which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as medications. In animals and humans, poisons are often bound to sugar molecules as part of their elimination from the body. 1.3 Medicinal importance of glycosides  Source of analgesics e.g., gaultherin source of methylsalicylate ‘a hydrolytic product  Anti rheumatic: e.g., salicin.  Anti-inflammatory: e.g., the glycoside glycyrrhizin. Also has demulcent, expectorant and antispasmodic action.  Cardiac drugs: e.g., digitalis glycosides, strophanthus, squill.  Laxatives: e.g., anthraquinone glycosides of senna, aloes, rhubarb, cascara, frangula.  Counter irritants e.g., thioglycosides and their hydrolytic products ‘allylisothio cyanate’  Reduce the capillary fragility e.g., flavonoidal glycoside rutin.  More recently as an anticancer agent e.g., amygdalin

1. 4 Sugars in glycosides: Monosaccharide (glucose in salicin, digitoxose in digoxin, rhamnose in ouabain, ETC) Disaccharides (gentiobiose in amygdalin). Trisaccharides (strophanthotriose). Tetrasaccharides (purpurea glycosides) Rare sugars (deoxy sugars) Sugar generally linked in one position to the aglycone, rarely in 2 positions as cascarosides. 1.4 Physical & chemical properties Structures are diverse, however; Most glycosides are water soluble and soluble in alcohols. Either insoluble or less soluble in non polar organic solvents. More sugar units in a glycoside lead to more solubility in polar solvents. Glycosides do not reduce Fehling’s solution, but when are susceptible to hydrolysis give reducing sugars 1.5 Classification Glycosides can be classified by the glycone (type of sugar), by the type of glycosidic bond, and by the aglycone. a).By glycone If glucose, then the molecule is a glucoside; If fructose, then the molecule is a fructoside; If glucuronic acid, then the molecule is a glucuronide; etc. In the body, toxic substances are often bonded to glucuronic 1.5 Classification acid to increase their water solubility; the resulting glucuronides are then excreted. If galactose, galactosides. If the glycone is mannose, mannosides. If the glycone is arabinose, arabinosides. b). By type of glycosidic bondSugars exist in isomeric α and β forms. All natural glycosides are of the β Type. Some α linkage exists in sucrose, glycogen and starch. Also,in the glycoside Kstrophanthoside, strophanthidin-linked to cymarose + β-glucose + α- glucose. The most commonly found glycoside type is β-D-glucose

Some enzymes such as α-amylase can only hydrolyze α-linkages; others, such as emulsin, can only affect β-linkages. Types of linkages; C-linkage/glycosidic bond, where the sugar molecule is linked to carbon atomare non-hydrolysable by heating with dilute acids/alkalis. hydrolyzed by oxidative hydrolysis with FeCl3. include some anthraquinone glycosides such as cascarosides, aloin and some members of the flavones type of glycosides.  O-linkage/glycosidic bond. These are very common in higher plants, e.g senna, rhubarb.  N-linkage/glycosidic bond- glycosylamines, nucleosides.  S-linkage/glycosidic bond- thioglycosides, e.g, sinigrin and sinalbin. c). By aglyconeThe classification is according to the chemical nature of the aglycone. For purposes of biochemistry, pharmacognosy and pharmacology, this is the most useful classification. The following are various groups depending on aglycone type: i)Aldehyde glycosides- e.g, vanillin ii)Alcoholic glycosides- e.g, salicin/)Phenolic glycosides- the aglycone is a simple phenolic s tructure e.g, arbutin. iii)Anthraquinone glycosides- where aglycone group is a derivative of anthraquinone. iv)Cardiac glycosides-aglycone part is a C27 steroidal nucleus. v)Coumarin and furanocoumarin glycosides- the aglycone is coumarin or its derivative. vi)Cyanogenic glycosides- the aglycone contains a cyanide group, e.g , amygdalin. vii)Flavonoid and flavonol glycosides- the aglycone is a flavanoid. viii) Isothiocyanate glycosidesx)Saponins glycosides- These compounds give a permanent soapy froth when shaken with water. xi)Others e.g Steviol, quassia (under terpenoids) glycosides. Steviol glycosides have steviol as the aglycone part. d). By pharmacological action According to the physiological or pharmacological activity, e.g. Laxative, Cardiotonic, analgesic, anti-inflammatory glycosides, etc.

1.6 Stability of glycosides Acid hydrolysis:  Acetal linkage between the aglycone and glycone less stable than that between two individual sugars within the molecule.  All glycosides are hydrolysable by acids (except C-glycosides).  Glycosides containing 2-deoxy sugars are more unstable towards acid hydrolysis even at room temperature.  C-glycosides are very stable (need oxidative hydrolysis).  Enzymatic hydrolysis is specific. For each glycoside there is a specific enzyme that exerts a hydrolytic action on it.  The same enzyme is capable of hydrolyzing different glycosides, but α and β sterioisomers of the same glycoside are usually not hydrolysed by the same enzyme.  Emulsin is found to hydrolyze most β-glycoside linkages, those glycosides attacked by emulsin are regarded as β-glycosides.  Maltase and invertase are α-glycosidases, capable of hydrolyzing α-glycosides only. 1.6 Extraction & isolation Water mixed with different proportions of methanol or ethanol are suitable extracting solvents. Non-polar organic solvents are generally used for the de-fating process. Glycosides are not precipitated from aqueous solutions by lead acetate. For fresh material, it is necessary to destroy hydrolyzing enzymes. Sometimes ether saturated with water is used for dry material. General methods of isolation a). Destruction of hydrolyzing enzymes: Drying for 15-30 min. at 100 C˚. Place plant in boiling water or alcohol for 10-20 min. Boiling with acetone. Cold acid pH treatment Extraction at very low temperature b). De-fatting or purification of the plant material (in case of seeds). c). Extraction of the glycosidal constituents by alcohol, water or dilute alcohols. Sometimes ether saturated with water is used for dry material. d). Concentrate the alcoholic extract. e). Add water (or hot water) & filter any precipitate. f). Purify aqueous extract: Extract non glycosidal impurities by organic solvent. Water soluble impurities precipitated by lead acetate. g). Precipitate excess lead salts. h). Isolation of the glycosides from the purified aqueous solution, by crystallization.

1.7 Qualitative tests depending on the aglycone Steroidal or cardiac glycosides: Give +ve Liebermann’s test (steroidal structure). Anthraquinone aglycones: Give +ve Borntrager’s test, characteristic reddish coloration with alkalis. Flavonoidal glycosides and/or aglycones: Characteristic colors with, NH4OH, AlCl3, FeCl3. Cyanogenetic glycosides give upon hydrolysis, hydrocyanic acid which can be tested by changing Na picrate paper (yellow) to red color. Qualitative tests depending on the aglycone Sulphur containing glycosides give black precipitate of silver sulphate upon treatment with AgNO3 solution. Drugs of pharmaceutical significance: are covered as follows according to the chemistry of the aglycone.

1.0 ALDEHYDE GLYCOSIDESThe most important example of a plant containing aldehyde glycoside is vanilla pod-the glycoside is glucovanillin.  Vanilla and vanillin Bot source: Vanilla (vanilla pods) consists of the cured, mature, but unripe fruits of Vanilla planifolia (Madagascar, Mexico/Bourbon vanilla) or Vanilla tahitensis (Tahiti vanilla), family Orchidaceae. Geographical source Indigenous to Mexico, now cultivated in Mauritius, Tahiti, Java, Seychelles, Madagascar, Sri Lanka, Kelala (India). Constituents Glucovanillin is the glycosidal constituent of green vanilla pods. The fruits of the plant (pods) are carefully collected and cured to permit enzymatic action on the glycoside with Chemical constituents the liberation of vanillin (the aglycone) which is the principal flavouring constituent of the pods. Vanillin is widely used as a flavouring agent. It may be obtained from vanilla pod or prepared from the glycoside coniferin, lignin or from the phenolic volatile oil constituents eugenol.

Structures of: CHO

CHO

OCH3

OCH3 O-Gl

Glucovanillin

OH

Vanillin

Chemical constituents cont’d/uses Vanillin occurs as white or cream colored powder or crystals. It is soluble in fixed and volatile oils, alkali hydroxides, alcohol, chloroform and ether. It is sparingly soluble in water. Other sources of vanillin: The bulk of vanillin produced commercially is prepared from lignin. Lignin is obtained in extremely large amounts as a by product of timber industry Uses: Flavouring agent, aromatizer. 2. 0. ALCOHOLIC/PHENOLIC GLYCOSIDES Willow bark Bot source: Include various spp of Salix Official drug from S. daphoides, S.fragilis (Official in BHP,ESCOP, Complete German Commission E) Geog source of Salix spp (ca 300spp) in temperate zone Spp range from shrubs to tall trees. Commercial drug mainly from S.E Europe, Britain Constituents Salicin, a phenolic glycoside regarded as the forerunner to aspirin. Other phenolic glycosides in willow are salicortin, fragilin. Salicin is a primary alcohol, its aglycone phenolic. Flavonoids eg naringenin, chalcone, isoquercitin are present. BP requires the dried drug to contain a minimum of 1.5% total salicylic acid derivatives, calculated as salicin. Salicin structu re

Uses Salicin was used for many years as a remedy in the treatment of fever and rheumatism. Uses: It is used as an analgesic-antipyretic in case of periodic fever. It is better tolerated in the stomach than asprin and other antipyretics and anti-inflammatory agents.  Hops Bot source: Humulus lupulus (Cannabinaceae) Geog source: Produced from England, Germany, Belgium, France, Russia and USA-California Recently, there is considerable interest in the wide ranging biological activities of the constituents of hops. Hops principle use is an aromatic bitter in the preparation of beer. Constituents: Prenylated cpds, eg Xanthohumol and recently Acylphloglucinolglucopyranosides which have been variously reported to have; Hops-research interests Cytotoxic effects on human cancer cell lines Antiproliferative properties Antioxidant properties Oestrogenic properties Mild sedative properties of hops are due in part to 2-Methyl-3-buten-2-ol. 2.3 Arbutin & bearberry leaves Bot source Bear berry leaves, uva ursi are the dried leaves of Arectostaphyllos uva ursi, family Ericaceae. Bears relish the leaves as a meal! Geog Source Central and North Europe, North America, Canada & Scotland. Arbutin & bearberry leaves cont’d Arbutin is a phenolic glycoside that occurs in the bearberry leaves. When hydrolysed with acids or with emulsin it yields glucose and hydroquinone. Uva ursi leaf contains also methylarbutin (the methyl ether of arbutin), that also contributes to the diuretic and urinary antiseptic action of the leaves. Uses Urinary antiseptic Has diuretic and also bactericidal action. This activity is due to the hydroquinone given by hydrolysis. As an infusion, it is used in urethritis and cystitis.

Structure of arbutin & methyl arbutin OH

OCH3

O-Gl

O-Gl

Arbuti n

Methyl arbutin

3. 0 ANTHRAQUINONES Anthraquinones are anthra-9,10- quinones. Their aglycones are anthracene derivatives. 3.1 Anthraquinone & glycosides; Introduction Only a few anthraquinones occur at all regularly in plants.  Emodin is the most frequent, occurring in at least six higher plant families.

Structure of emodin 1,3,8-Trihydroxy-6-methyl-9,10-anthracenedione/1,3,8-Trihydroxy-6-methylanthraquinone (EMODIN)

 Aloes contain O- and C- glycosides (eg barbaloin, the C- glycoside of aloe-emodin)  Ethylacetate-methanol-water; 100:16.5:13.5—on silica plate gives good separation.  Anthraquinones detected on TLC plates by their visible UV colors. By spraying plates with 10% KOH, the original yellow and yellow-brown colors change to red, violet, green or purple.

 They are mainly found in dicot families e g Euphorbiaceae, Ericaceae, Lythraceae, Polygonaceae, Rhamnaceae, Rubiaceae, Leguminosae, Verbenaceae, Scrophularaceae, Saxifragaceae.  In the monocots, anthraquinone glycosides are found only in the family Liliaceae-genus Aloe, in the form of unusual C-glycoside barbaloin.  They occur in certain fungi and lichens.  The fungal anthraquinone pigments are nearly all chrysophanol or emodin derivatives.  Anthraquinone derivatives are often orange-red compounds which may sometimes be observed in situ-e.g in medullary rays of rhubarb and cascara.  May be; o Dihydroxy phenols such as chrysophanol, o Trihydroxy phenols such as emodin, o Tetrahydroxy phenols such as carminic acid.  Other groups are often present e,g methyl in chrysophanol. Hydroxyl-methyl in aloeemodin and carboxyl in rhein and carminic acid.  The sugars in anthraquinones glycosides are; glucose, rhamnose, glucose and rhamnose, primeverose, arabinose-attached in various positions.  They are usually soluble in hot water or dilute alcohol.  Some plants contain oxanthrone which is the intermediate substance from anthraquinone to anthranol.  In some plants, the anthrone molecule orients in dimeric form called dianthrone which is therapeutically more important.  The anthrone molecules may be identical or different.  They are important aglycones in spp of Cassia, Rheum and Rhamnus. Interrelationship between anthraquinone, anthrone, anthranol & oxanthrone Anthraquinone

Anthrone

O

O 1

8 9

7 6

Anthranol

2

6

3

10 5

9

7

4H

2 3

10 5

4

OH

1

8

H

O

H

4 H

O

2H

1

8 7

9

2

6

10

3

5

H

OH

Oxanthrone

4

O

Gl O 8

OH 1

9

Gl O

O

8

9

2

10 5

CH2 OH

4

OH 1 2

10

O

8

9

4

5

O

Aloe-emodin-8-glycoside

OH 1 2

10

COOH

5

O

Gl O

4

CH3

O

Rhein-8-glycoside

Chrysophanol-8-glycoside

Types of glycosides in anthraquinones; i- O-glycosides (anthraq) ii- O-glycoside where aglycone is oxanthrone (Oxanthrone-9-glycoside)

Gl OH H

O

1

9

8

7 6

OH 2 3

10 5

4 O

iii- C-glycoside where the aglycone structure is anthrone derivatives (Barbaloin -the Cglycoside of aloe-emodin)

OH 7

8

6 5

H

O

OH

9

1

2

10

4

3

C6 H11 O5

CH2 OH

iv- O-glycosides where the aglycone moiety is di-anthrone derivative (i.e. Sennoside A & B of rhein)

3.2. Test for anthraquinones: Borntrager’s test 1. The drug is powdered and further extracted with ether or any water-immiscible organic solvent. 2. It is filtered. 3. The filtered ethereal extract is made alkaline either with caustic soda or aqueous ammonia. 4. After shaking, a pink, red or violet color in the aqueous layer indicates the presence of free anthraquinone derivatives. 5. The aqueous layer assumes the color because of the formation of anthraquinone salts. 6. If the drug being tested contains either very stable anthraquinone glycosides or reduced derivatives of the anthranol type, Borntrager test will be negative. Hence the test should be modified by first hydrolyzing with alcoholic KOH or 2M acid. NB: anthraquinones are usually soluble in hot water or alcohol. Borntrager’s test Borntrager’s reaction can distinguish anthraquinones from anthrones and anthranols which do not give the test unless they are converted to anthraquinone by oxidation with mild oxidants such as hydrogen peroxide or ferric chloride. 3.3. Formation and distribution of anthraquinone glycosidesThereThere is ontogenic sequence in the formation of these compounds. Chrysophanol is the first anthraquinone formed in young seedlings. Then aloe-emodin appears followed by rhein. This is in keeping with the expected biogenetic order, which involves the successive oxidation of the 3-methyl grp of chrysophanol. In the presence of light, glycosylation follows. During fruit development, the amounts of aloe-emodin and rhein glycoside fall markedly, and sennosides accumulate in the pericarp.

3.4. EvaluationThe BP/EP determines the total senna leaf glycosides in terms of sennoside B- not less than 2.5%. The glycosides and free anthraquinones are extracted from the leaves. The free aglycones are removed. The remaining sennosides and other glycosides are hydrolysed, oxidized to give rhein and some aloe-emodin, which are then determined spectrophotometrically. The BP limits the fragments of rachis, petiole and stalk to 3.0 %. These are determined by hand-picking and weighing in the whole drug, while in the powdered drug, quantitative microscopy is exploited. The leaves are officially required to give an acid-insoluble ash of not more than 2.5%.

PHM 4453 9 (FOP) GLYCOSIDES 2

3.5.0. Official natural anthraquinone drugs Examples of Anthraquinone Drugs  Senna leaf & senna fruit (pod).  Cascara tablets, elixir, dry exract, liquid extract  Frangula bark  Rhubarb powdered, tincture.  Aloes. 3.5.1-Senna Leaf & Pod 3,5.1.1 Senna Leaf Consists of the dried leaflets of Alexandrian or Khartoum senna, Cassia senna (C. acutifolia), Tinnevelly senna, C. angustifolia, family Leguminosae. C angustifolia is indigenous to Somaliland, Arabia, Punjab and is cultivated in South India (Tinnevelly) and is known as Tinnevelly (India) senna, in commerce. C senna is collected from wild and cultivated plants while C angustifolia is grown, carefully collected and compressed into bales in South India, N W Pakistan and Jammu. Being a legume, it adds nitrogen to the soil. Vein islet numbers, palisade ratio and stomatal indices can be used to distinguish the two species. Two naphthalene glycosides- 6-hydroxymusizin glucoside and tinnevellin glucoside- were isolated from the leaves and pods of Alexandrian and Indian senna , respectively. This difference has been used as a distinguishing feature of the commercial varieties. Constituents: Pharmacologically active include; Sennosides A and B Sennosides C and D- the glycosides of heteroanthrones involving rhein and aloe-emodin. Aloe-emodin dianthrone –diglycoside. Rhein-anthrone-8-glucoside.

Aloe-emodin-8-glucoside. Aloe-emodin-anth...