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Secondary Plant Metabolites

(Active Constituents)

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The secondary metabolites act as a protectiveagents.

They constitute as the waste product of

metabolism (detoxification).

They are biosynthesized to aid the producers'

survival.

Thus they are energy producer and

physiologically active. Secondary metabolites are divided into several

classes of various organic compounds.

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I- Carbohydrates and related

compoundsCompounds composed of carbon, hydrogen and oxygen as

polyhydroxy aldehydes or ketone alcohols: sucrose,lactose, starch, gums, mucilage and pectin.

1- Gums:

They are translucent, amorphous substances that arefrequently produced in higher plants as a protective afterinjury.

Gums are heterogeneous in composition. Upon

hydrolysis arabinose, galactose, glucose, mannose andvarious uronic acids are produced.

The uronic acids may form salts with calcium,magnesium and other cations. Examples of gums aregum Acacia and gum Tragacanth.

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2- Mucilages :

1- They are polysaccharide complexes formed

from sugar and uronic acid units.

2- They are insoluble in alcohol but forming a

viscous non-adhesive colloidal solution withwater.

3- They are stained red with Ruthenium Red

reagent (pectose type present in Senna, buchu),

and with solution ofCorallin soda (callose typefound in Squill). Others are stained by methylene

blue (neutral type of Fenugreek).

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3- Pectin:

It is formed of partially methoxylatedpolygalactouronic acids.

Pectin is a coarse or fine powder, yellowishwhite in colour, almost odourless and has a

mucilaginous taste. Soluble in 20 parts of water and the solution is

viscous, colloidal and acidic to litmus.

One part of pectin heated in nine parts of water

forms a stiff gel. Pectin is used as a suspending agent and is an

ingredient in many antidiarrheal formulations.

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II- Glycosides

Definition:

Glycosides are non-reducing organic compoundsthat yield on hydrolysis with acids, alkalis orenzymes:

1-A sugar (or glycone, formed of one or moresugar units).

2-A non-sugar moiety (also called aglycone orgenin).

Glycosides are widely distributed in nature. Theyusually accompanied with enzymes used ineither their synthesis or hydrolysis.

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Glycosides are considered as acetals or sugar ethers.

Glycosides are generally soluble in water and hydroalcoholicsolvents due to the hydrophilic nature of the sugar.

The higher the sugar content the less soluble in organicsolvent. The aglycones are soluble in organic solvents.

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Classification of glycosides

A-

Phenolic glycosides:

They produce phenolic

aglycone when

subjected to

hydrolytic processes.

The phenolic

aglycone may be:1- Simple phenolic

glycosides: e.g.

Arbutin present in Uva

ursi leaf

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2- Flavonoid glycosides:

They occur in plant in

both free aglycone and/or as glycosides.

This group is regardedas C6-C3-C6

compounds in whicheach C6 moiety is abenzene ring e.g.Diosmin glycoside ofBuchu and hespiridinglycoside ofCitrus

species.

Chemical test:flavonoides + KOH canary yellow colour

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3- Anthraquinoneglycosides:

They arepharmacologicallyactive constituentsof several laxatives

and purgatives ofplant origin e.g.Senna, Aloe,Cascara, Frangula,

Rhubarb.

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1- Borntrager's test for anthraquinone glycosides:

Boil the powder with dil. H

Cl, filter, cool, shake withorganic solvent, separate organic layer, shake

with NH4OH, the aqueous layer becomes rosepink or cherry red.

2- Modified Borntrager's test: in case of dianthronee.g. sennosoides:

a- Boil the powder with alc. KOH, filter.

b-Add dil. HCl to the filtrate and extract with ether.

c- Oxidize with H2O2.d-Add NH4OH to the ethereal extract and shake,

a rose red colour is produced in the aqueouslayer.

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4- Anthocyanin glycosides:

they are structurally related to flavonoidsand coloured.

They constitute the pigments of the colouredpetals of the flowers.

Their colours are affected by the pH of thecell sap and ranges from red in acidicmedium to blue in alkaline medium. e.g.Red Rose petals, karkadeh and corn silk.

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B- Cyanophore glycosides

(Cyanogenic) They yield HCN as one of the products of

hydrolysis.

Examples are; amygdalin in bitter almond,prunasin in wild cherry and linamarin in

linseed.

Chemical test: by Guignard test: turns the

yellow colour of sodium picrate paper into

orange.

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C- Thioglycosides

(thiocyanate or sulphated glycosides)

S-glycosides, S atom ispresent conjugated toglucose and a second Satom present in a

sulfonated oximegrouping.

Singrin from blackmustard seeds giving allylisothiocyanate (mustardirritant gas) uponhydrolysis.

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D- Steroidal cardioactive

glycosides These drugs are used inmedicine as mainly toincrease the tone andcontractility of the cardiacmuscle.

Also as diuretic due to theincreased renalcirculation.

Thet are present inleaves of Digitalis, seedsof Strophanthus and thebulb of Squill.

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Structure activity relationship (SAR)

Steroidal nucleus with alcoholic OH group atC

3 at which sugaris attached.

Attachment of unsaturated lactone ring at C17. the lactone ring ispresent either as a 5- membered ring (Cardinolides) e.g.lannatosides in Digitalis or as 6- membered ring (bufadienolides)e.g. Scillarin in Squill.

Deoxy sugar, e.g. digitoxose.

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Detection of cardiac glycosides

Several chemical tests could be used foridentification of cardiac glycosides

Colour reactions due to aglycone moiety:

A- Reactions due to (-CH2-) group of the lactonering:

These are characteristic for the 5-mamberedlactone ring of cardenolides:

1- Kedde's test: card. + Kedde's reagent (3,5-dinitrobenzoic acid + NaOH violet colour.

2- Baljet's test: card. +Baljet's reagent (picric acid+NaOH) orange or red.

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B- Reactions due to the steroidal nucleus:

Liebermann's test: card. in glacial acetic acid+ H2SO4 red, violet, blue to green.

Reactions due to the presence of2-deoxysugar unit in the sugar moiety:

Keller-Killiani's test: card. in glacial acetic

acid containing traces of FeC

l3, conc.H2SO4 on the wall of the test tube bluering is formed in between the two layers.

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E- Saponinse.g. saponins of Quillaia bark

This group of glycosides is characterized by the following:

1. it foams in aqueous solution forming froth.

2. It causes haemolysis of red blood cells.

3. Drugs containing saponins are usually sternutatory and irritatingthe mucous membrane of the eyes and the nose.

4. They are highly toxic when injected into blood stream.

5. They are used in cleaning industrial equipments and fabrics, alsoas a powerful emulsifier of certain resins, fats and fixed oils.

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III- Tannins

They are non-crystalline compounds that withwater form colloidal solutions possessing anacid reaction and a sharp astringent taste.

They precipitate gelatin and alkaloids.

They precipitate protein and can combine with

protein of animal hide (Gold beater's test),rendering them resistant to proteolytic enzymesand so prevent their putrefaction.

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Uses of drugs containing tannins:

Astringent in GIT and skin (haemostatic). In treatment of burns.

Tanning of leather.

Antidote of alkaloidal and heavy metals poisoning.

Preparation of paints and ink.

Types of true tannins:

True tannins: have molecular weight 1000-5000. thereare two main types of true tannins.

Pseudotannins:

compounds of lower molecular weight than true tannins,they don't respond to the gold beater's skin test.

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Hydrolysable

(Pyrogallol)

Condensed

(Catechol)Hydrolysis Hydrolysable Non-Hydrolysable

Dry distillation Gives pyrogallol Gives catechol

FeCl3 Blue-black colour Green colour

Br2 water No ppt. Ppt

Examples Gall, Clove,Rhubarb

Cinnamon, Tea,Henna.

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IV- Alkaloids

Alkaloids, which mean alkali-like, are basicnitrogenous compounds of biological origin, theyare physiologically active.

They occur in the form of salts with organic acids,or in combination with specific acids e.g. opiumalkaloids occur with meconic acid .

generally alkaloidal bases are insoluble in waterbut soluble in organic solvents. While alkaloidalsalts are soluble in water sparingly soluble inorganic solvents.

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Their physiological actions vary widely being

Analgesic and Narcotic e.g. morphine andcodeine

Central stimulant e.g. caffeine and strychnine

Mydriatic e.g. atropine

Myotic e.g. pilocarpine

Antiasthmatic e.g. ephedrine

Antihypertensive e.g. reserpine

smooth muscle relaxant e.g. atropine andpapaverine

skeletal muscle relaxants e.g. d-tubocurarine.

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Detection of alkaloids

Alkaloidal precipitants:

Most alkaloids are precipitated from neutral oracidic solution by a number of reagents which

contain certain heavy metals e.g. Mayer'sreagent (potassium mercuric iodide solution)gives creamy white ppt. with most alkaloidsexcept caffeine.

Alkaloidal colour reagents: The most common Dragendorff's reagent gives

orange colour.

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V- Volatile oils

Volatile or essential oils are volatile in steam.

They are secreted in oil cells, in secretion canals orcavities or in glandular hairs.

They are generally mixtures of hydrocarbons andoxygenated compounds derived from thesehydrocarbons.

These oxygenated constituents mainly determine theodour and taste of volatile oils.

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Volatile oils are used for flavoring e.g. oil of

Lemon, in perfumery e.g. oil of Rose

as spices e.g. Pepper, Clove , Cardamom

and for their therapeutic action as antiseptice.g. Thyme, antispasmodic e.g. Melissa,

Menthe, and carminative e.g. Chamomile.

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VI- Resins, Gum-resins and

similar substancesResins are more or less solid, amorphous substances of

complex chemical nature, on heating they soften andfinally melt. They are insoluble in water but dissolve inalcohol, chloroform and ether.

Resins are associated with volatile oil, so called oleoresin.

With gum and called gum resin.

With oil and gum and called oleo-gum resin.

Resins give brown colour with iodine reagent.

VII- Others:

Enzymes, vitamins, hormones, antibiotics, naturalinsecticides, etc.........