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Glycosides

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Title: Glycosides


1
Glycosides Tannins
2
Glycosides
  • Glycosides consist of a sugar residue covalently
    bound to a different structure called the
    aglycone.
  • The sugar residue is in its cyclic form and the
    point of attachment is the hydroxyl group of the
    hemiacetal function.
  • The sugar moiety can be joined to the aglycone in
    various ways
  • Oxygen (O-glycoside)
  • Sulphur (S-glycoside)
  • Nitrogen (N-glycoside)
  • Carbon (Cglycoside)

3
  • ?-Glycosides and ?-glycosides are distinguished
    by the configuration of the hemiacetal hydroxyl
    group.
  • The majority of naturally-occurring glycosides
    are ?-glycosides.
  • O-Glycosides can easily be cleaved into sugar and
    aglycone by hydrolysis with acids or enzymes.
  • Almost all plants that contain glycosides also
    contain enzymes that bring about their hydrolysis
    (glycosidases).

4
  • Glycosides are usually soluble in water and in
    polar organic solvents, whereas aglycones are
    normally insoluble or only slightly soluble in
    water.
  • It is often very difficult to isolate intact
    glycosides because of their polar character.
  • Many important drugs are glycosides and their
    pharmacological effects are largely determined by
    the structure of the aglycone.

5
  • The term 'glycoside' is a very general one which
    embraces all the many and varied combinations of
    sugars and aglycones.
  • More precise terms are available to describe
    particular classes. Some of these terms refer to
  • the sugar part of the molecule (e.g. glucoside).
  • the aglycone (e.g. anthraquinone).
  • the physical or pharmacological property (e.g.
    saponin soap-like, cardiac having an action on
    the heart).

6
  • Modern system of naming glycosides using the
    termination '-oside' (e.g. sennoside).
  • Although glycosides form a natural group in that
    they all contain a sugar unit, the aglycones are
    of such varied nature and complexity that
    glycosides vary very much in their physical and
    chemical properties and in their pharmacological
    action.

7
1. Anthracene glycosides
  • A number of glycosides in which the aglycones are
    anthracene derivatives occur as the
    pharmacologically active constituents of several
    cathartics of plant origin e.g. cascara,
    rhubarb, aloe and senna.
  • These anthracene glycosides are sometimes
    referred to as the anthraquinone glycosides or
    the anthraglycosides.

8
  • These anthraquinone derivatives are glycosides,
    often glucosides or rhamnosides.
  • The presence of the sugar residue is a
    prerequisite for the pharmacological effects.
  • Anthraquinones are colored substances and many of
    them are used technically as dyes e.g. alizarin.
  • Reduced forms of anthraquinones, which exhibit
    keto-enol tautomerism, are often encountered.
  • The anthracene derivatives occur in vegetable
    drugs in different forms at different oxidation
    levels like anthraquinones, anthrones,
    anthranols, or oxanthrones.

9
Interrelationship of anthraquinone derivatives
10
  • These anthracene compounds occur in these drugs
    or plant materials in some cases as the aglycones
    of O-glycosides (e.g. frangulin), and in other
    cases as the aglycones of C-glycosides (e.g.
    aloin).
  • Biosynthesis natural anthraquinones are
    synthesized either via the acetate-malonate
    pathway (like the medicinally important purgative
    anthraquinones), or they are derived from
    shikimate and mevalonate (like alizarin).

11
A. Anthraquinones
  • Although anthraquinone is not used extensively in
    medical practice, it is the starting material for
    the preparation of several synthetic laxatives
    and represent the basic structure of a number of
    important laxatives and dyestuffs.
  • Borntragers test is often used for their
    detection.
  • The derivatives of anthraquinone present in
    purgative drugs may be dihydroxy phenols such as
    chrysophanol, trihydroxy phenols such as emodin
    or tetrahydroxy phenols such as carminic acid.

12
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13
B. Anthrones Anthranols
  • These reduced anthraquinone derivatives occur
    either free or combined as glycosides.
  • They are isomeric and one may be partially
    converted to the other in solution.
  • Anthranols are converted upon oxidation into
    anthraquinones. Oxidation takes place in the
    crude drug during storage especially if powdered.
  • Schontetens test is often used for anthranols
    (green fluorescence).
  • Anthranols and anthrones are the main
    constituents of chrysarobin, a mixture of
    substances.

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15
C. Oxanthrones
  • These are intermediate products between
    anthraquinones and anthranols.
  • They give anthraquinones on oxidation with
    hydrogen peroxide.
  • An oxanthrone has been reported as a constituent
    of cascara bark.

16
D. Dianthrones
  • These are compounds derived from two anthrone
    molecules, which may be identical or different.
  • They are important aglycones in species of
    Cassia, Rheum and Rhamnus.
  • One of the best known is sennoside derived from
    two molecules of glucose and two molecules of
    rhein-anthrone.
  • On hydrolysis, sennoside yields the aglycone
    sennidin.

17
E. Aloin-type or C-glycosides
  • Aloin (Barbaloin) was obtained from species of
    Aloe.
  • It is strongly resistant to normal acid
    hydrolysis.
  • In aloin, the sugar is joined to aglycone with a
    direct C-C linkage (a C-glycoside).
  • Two aloins (A and B) are known and arise from the
    chiral centre at C-10.

18
2. Saponin glycosides
19
  • A group of plant glycosides known as saponins
    share in varying degrees, two common
    characteristics
  • (a) They foam in aqueous solution.
  • (b) They cause haemolysis of red blood cells.
  • The aglycones of the saponins are collectively
    referred to as Sapogenins. The more poisonous
    saponins are often called Sapotoxins.

20
  • Plant materials containing saponins have long
    been used in many parts of the world for their
    detergent properties for example, in Europe, the
    root of Saponaria officinalis (Fam.
    Caryophyllaceae) and in South America, the bark
    of Quillaia saponaria (Fam. Rosaceae). Such
    plants contain a high percentage of the
    glycosides known as saponins (Latin Sapo, means
    Soap) which are characterized by their property
    of producing a frothing aqueous solution.

21
  • Properties
  • Saponins form colloidal solution in water
    (hydrophilic colloids) which froths upon shaking.
    These substances modify and lower the surface
    tension and therefore foam when shaken. This has
    led to their use to increase the foaming of beer.
  • Practical industrial applications of saponins
    include their use in cleaning industrial
    equipment and fine fabrics and as powerful
    emulsifiers of certain resins, fats and fixed
    oils.

22
  • In general, they have a bitter, acrid taste and
    drugs containing them are usually sternutatory
    (causing or producing sneezing) and irritating to
    the mucous membranes of eyes and nose.
  • Characteristic for all saponins is their ability
    to cause haemolysis of red blood corpuscles and
    to destroy them. When injected into the blood
    stream, they are highly toxic.
  • When taken by mouth, Saponins are comparatively
    harmless, being not absorbed from the intestinal
    tract. Sarsaparilla, for example, is rich in
    saponins but is widely used in the preparation of
    nonalcoholic beverages.

23
  • Saponins are toxic especially to cold-blooded
    animals e.g. frogs. Many are used as
    fish-poisons.
  • The actual cause of the haemolysis
  • The red blood cells carry sterols in their
    membranes, and when brought into contact with
    saponins, the sterols of the RBCs are
    precipitated and the colloidal chemical
    properties of the membrane are so altered as to
    give hemoglobin passage to the surrounding
    medium.
  • Saponins have a high molecular weight and their
    isolation in a state of purity presents some
    difficulties.

24
  • Structure of Saponins
  • According to the structure of the aglycone or
    sapogenin, two kinds of saponin are recognized
  • The steroidal type (commonly tetracyclic
    triterpenoids, C-27).
  • The triterpenoid type (pentacyclic triterpenoids,
    C-30).
  • Both of these have a glycosidal linkage at C-3
    and have a common biosynthetic origin via
    mevalonic acid and isoprene units.

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26
A. Steroidal saponins
  • The steroidal saponins are less widely
    distributed in nature than the pentacyclic
    triterpenoid type.
  • Steroidal saponins are of great pharmaceutical
    importance because of their relationship to
    compounds such as the sex hormones, cortisone,
    diuretic steroids, vitamin D and the cardiac
    glycosides.
  • Examples Diosgenin (Dioscorea sylvatica),
    Sarsapogenin (Smilax sp.).

27
B. Pentacyclic triterpenoid saponins
  • Triterpenoid saponins my be classified into three
    groups represented by ?-amyrin, ?-amyrin and
    lupeol.
  • Examples Primulagenin (Primula sp.), Quillaiac
    acid (Quillaia saponaria) and Glycyrrhetinic acid
    (Glycyrrhiza sp.).

28
3. Coumarin glycosides
  • The coumarins are shikimate-derived metabolites.
  • The majority of the coumarins are oxygenated at
    position C7.
  • Coumarins have a limited distribution in the
    plant kingdom and have been used to classify
    plants according to their presence
    (chemotaxonomy).

29
  • Coumarins are commonly found in the plant
    families Apiaceae, Rutaceae, Asteraceae and
    Fabaceae.
  • Some coumarins are phytoalexins and are
    synthesized de novo by the plant following
    infection by a bacterium or fungus.
  • Phytoalexins any of a group of compounds formed
    in plants in response to fungal infection,
    physical damage, chemical injury, or a pathogenic
    process. Phytoalexins inhibit or destroy the
    invading agent.

30
  • These phytoalexins are broadly antimicrobial for
    example, scopoletin is synthesized by the potato
    (Solanum tuberosum) following fungal infection.
  • Khellin is an isocoumarin (chromone) natural
    product from Ammi Visnaga (Apiaceae) and has
    activity as a spasmolytic and vasodilator.

31
  • It has long been known that animals fed sweet
    clover (Melilotus officinalis, Fabaceae) die from
    haemorrhaging. The poisonous compound responsible
    for this adverse effect was identified as
    dicoumarol.
  • A number of compounds have been synthesized based
    on the dicoumarol structure, e.g. warfarin, which
    is widely used as anticoagulant.

32
  • The psoralens are coumarins that possess a furan
    ring and are sometimes known as furanocoumarins.
    e.g. psoralen and bergapten.
  • These compounds may be produced by the plant as a
    protection mechanism against high doses of
    sunlight and some coumarins are formulated into
    sunscreens and cosmetics for this purpose.

33
4. Flavonoid glycosides
  • Biosynthesis
  • flavonoids are products from a cinnamoyl-CoA
    (C6C3, precursor from the shikimate pathway)
    starter unit, with chain extension using three
    molecules of malonyl-CoA.
  • Flavonoids are therefore of mixed biosynthesis,
    consisting of units derived from both shikimate
    and acetate pathways.

34
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35
  • The triketide starter unit undergoes cyclization
    by the enzyme chalcone synthase to generate the
    chalcone group of flavonoids. Cyclization can
    then occur to give a pyranone ring containing
    flavanone nucleus, which can either have the
    C2-C3 bond oxidized (unsaturated) to give the
    flavones or be hydroxylated at position C3 of the
    pyranone ring to give the flavanonol group of
    flavonoids. The flavanonols may then be further
    oxidized to yield the anthocyanins, which
    contribute to the brilliant blues of flowers and
    the dark colour of red wine.

36
  • The flavonoids contribute to many other colors
    found in nature, particularly the yellow and
    orange of petals even the colourless flavonoids
    absorb light in the UV spectrum (due to their
    extensive chromophores) and are visible to many
    insects. A chromophore is the part (or moiety)
    of a molecule responsible for its color.
  • It is likely that these compounds have high
    ecological importance in nature as colour
    attractants to insects and birds as an aid to
    plant pollination.

37
  • Certain flavonoids also markedly affect the taste
    of foods for example, some are very bitter and
    astringent such as the flavanone glycoside
    naringin, which occurs in the peel of grapefruit
    (Citrus paradisi). Interestingly. the closely
    related compound naringin dihydrochalcone, which
    lacks the pyranone ring of naringin, is
    exceptionally sweet, being some 1000 times
    sweeter than table sugar (sucrose).

38
  • the flavonoids have important dietary
    significance because, being phenolic compounds,
    they are strongly antioxidant.
  • Many disease states are known to be exacerbated
    by the presence of free radicals such as
    superoxide and hydroxyl, and flavonoids have the
    ability to scavenge and effectively mop up
    these damaging oxidizing species.

39
  • Foods rich in this group have therefore been
    proposed to be important in ameliorating diseases
    such as cancer and heart disease (which can be
    worsened by oxidation of low-density
    lipoprotein) quercetin, a flavonoid present in
    many foodstuffs, is a strong antioxidant.
    Components of milk thistle (Silybum marianum), in
    particular silybin, are antihepatotoxins
    extracts of milk thistle are generally known as
    silymarin.

40
Some action and therapeutic uses of flavonoids
  • Many flavonoid containing plants are
  • Diuretic.
  • Antispasmodic.
  • Diaphoretic.
  • Increase tensile strength of capillary walls.
  • Free radical scavengers.

41
5. Cyanogenetic glycosides(Cyanide glycosides)
  • Cyanogenesis is the ability of certain living
    organisms, plants in particular, to produce
    hydrocyanic acid (HCN, prussic acid).
  • Cyanogenesis in plants is a chemical defense
    mechanism against organism damaging or feeding on
    plant tissues and lead to release of HCN gas,
    which is toxic.
  • They are distributed in over 2000 plant species
    belonging to 110 families.

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43
  • These compounds, in presence of enzymes such as
    ?-glucosidase, lose their sugar portion to form a
    cyanohydrin which, in the presence of water and
    hydroxynitrile lyase, can undergo hydrolysis to
    give benzaldehyde and the highly toxic hydrogen
    cyanide (HCN).
  • The sugar portion of the molecule may be a
    monosaccharide or a disaccharide such as
    gentiobiose or vicianose. If a disaccharide,
    enzymes present in the plant may bring about
    hydrolysis in two stages, as in the case of
    amygdalin.

44
  • They are derivatives of ?-hydroxynitrile or
    2-hydroxynitrile (cyanohydrins).
  • In all cases the first sugar attached to the
    aglycone is ?-D-glucose.
  • R1 and R2 are often different residues resulting
    in pairs of C-2 epimers.
  • (Epimers are diastereomers that differ in
    configuration at only one of their stereogenic
    centers).

45
  • Most cyanogenetic glycosides are biosynthetically
    derived from the amino acids valine, leucine,
    isoleucine, tyrosine or phenylalanine.
  • Cyanogenetic glycosides are easy to detect with a
    strip of filter paper impregnated with reagents
    able to give a color reaction with the
    hydrocyanic acid released upon crushing the plant
    material (e.g., picric acid/sodium carbonate or
    benzidine/cupric acetate).
  • Although hydrocyanic acid is a violent poison, it
    is important to remember that oral intake of
    cyanogenetic drugs does not necessarily cause
    severe intoxication, this is because the range of
    dangerous concentrations (0.5-3.5 mg/kg) can only
    be achieved by rapid and massive ingestion of
    plant parts rich in cyanogenetic glycosides.

46
  • Examples
  • Amygdalin in bitter almonds (Prunus amygdalus).
    It is biosynthetically derived from
    phenylalanine.
  • Linamarin in linseed (Linum usitatissimum). It is
    biosynthetically derived from valine.

47
6. Steroidal cardioactive glycosides
  • Cardiac glycosides are a group of natural
    products characterized by their specific effect
    on myocardial contraction and atrioventricular
    conduction.
  • In large doses they are toxic and bring about
    cardiac arrest in systole, but in lower doses
    they are important drugs in the treatment of
    congestive heart failure.
  • They have a diuretic activity. Since, the
    improved circulation tends to improve renal
    secretion, which relieves the edema often
    associated with heart failure.

48
? Distribution in nature
  • Cardiac glycosides occur in small amounts in the
    seeds, leaves, stems, roots or barks of plants of
    wide geographical distribution, particularly of
    the Fam. Apocyanaceae (e.g. seeds of
    Strophanthus, roots of Apocynum and fruits of
    Acokanthera) others are found in the
    Scrophulariaceae (e.g. leaves of Digitalis sp.),
    Liliaceae (e.g. scales of the bulbs of Urginea
    and Convallaria), and Ranunculaceae (Adonis).
  • Cardiac glycosides are also found in animals only
    in exceptional cases Bufadienolides occur in
    toads (Bufo).

49
? Structure of glycosides
  • The structure comprise a steroidal aglycone of
    the (C23) cardenolide type or of the (C24)
    bufadienolide type, and a sugar moiety, most
    often an oligosaccharide.

50
A. Structure of the aglycones
  • All of the aglycones have in common the classic,
    tetracyclic, steroidal nucleus.
  • The A, B, C and D rings normally have a
    cis-trans-cis configuration or less often, a
    trans-trans-cis configuration.
  • Also common to all the aglycones is the presence
    of two hydroxyl groups one is a 3? secondary
    alcohol, the other is a 14? tertiary alcohol.
  • All of the aglycones have a ? constituent at
    C-17 an ?,?-unsaturated lactone.

51
  • The size of the lactone ring distinguishes two
    groups of aglycones the C23 cardenolides with an
    ?,?-unsaturated ?-lactone ( butenolide) and the
    C24 bufadienolides with a di-unsaturated
    ?-lactone ( pentadienolide).

52
B. Structure of the sugar moiety
  • The sugar moiety is generally linked to the
    aglycone through the hydroxyl group at C-3.
  • The majority of the saccharides found in cardiac
    glycosides are highly specific
  • 2,6-dideoxyhexoses, e.g. D-digitoxose
  • 2,6-dideoxy-3-methylhexoses, e.g. D-diginose
  • 6-deoxyhexoses, e.g. L-rhamnose
  • 6-deoxy-3-methylhexoses, e.g. D-digitalose
  • Hexose, e.g. glucose (when these is a glucose
    unit, it is always terminal).
  • The sugars can modify the activity (potency,
    toxicity), the solubility, the diffusion through
    membranes, the rate of absorption and
    transportation of the glycosides.

53
C. Structure-Activity Relationships (SAR)
  • The cardiac activity is linked to the aglycone.
  • The sugar moiety does not participate directly in
    the activity, but its presence enhances the
    activity and modulates it by modifying the
    polarity of the compound.
  • The presence of a certain number of structural
    elements is required for, or at least favorable,
    to the activity
  • The lactone at C-17, and it must be in the ?
    configuration.
  • The configuration of the rings. The activity is
    maximized when the A, B, C and D rings are in the
    cis, trans, cis configuration. The C and D rings
    must be cis fused.
  • The substituents. The inversion of the
    configuration at C-3 diminishes the activity, but
    3-deoxy compounds are not completely inactive.

54
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? Biosynthetic origin
  • Aglycone of the cardiac glycosides are derived
    from mevalonic acid but the final molecules arise
    from a condensation of a C21 steroid with a C2
    unit (the source of C-22 and C-23).
    Bufadienolides are condensation products of a C21
    steroid and a C3 unit.

56
? Color reactions
  • They can be due to the sugars or to the aglycone
  • Color reactions of the sugars. The only color
    reactions of the sugars that are of interest are
    those specific to 2-deoxyhexoses. e.g.
    Keller-Kiliani test.
  • Color reactions of the aglycones (steroidal
    nucleus). These are positive with any compound
    containing a steroidal nucleus including
    cardenolides or bufadienolide
  • Antimony trichloride (SbCl3)
  • Liebermann's test (for bufadienolides)

57
  • Color reactions of the aglycones (lactone ring).
  • These are characteristic for cardenolides having
    a five-membered lactone ring
  • Legal's test
  • Raymond's test
  • Kedde's test
  • Baljet's test

58
Pharmacological properties
  • Cardiac glycosides increase the force and speed
    of contraction of the heart. In patients with
    cardiac insufficiency, this positive inotropic
    effect translates into 1an increase in cardiac
    output, 2an increase in cardiac work capacity
    without any increase in oxygen consumption, 3a
    decrease in heart rate, and, indirectly, 4a
    decrease in arterial resistance. (MOA) The
    glycosides are thought to act at the membrane
    level, by inhibition of the Na-K ATPase, which
    would result in an increase of the intracellular
    calcium ion concentration.

59
Therapeutic indications
  • Cardiac glycosides are currently indicated for
  • Cardiac insufficiency with low output (generally
    in combination with diuretics), particularly when
    there is atrial fibrillation.
  • Supraventricular rhythm abnormalities to slow
    down or decrease atrial fibrillation or flutter.

60
Examples
  • Strophanthus glycosides
  • The name Strophanthus is derived from the Greek
    strophos (a twisted cord or rope) and anthos (a
    flower).
  • e.g. Strophanthus kombe
  • The principle glycosides are
  • K-strophanthoside
  • K-strophanthin-?
  • Cymarin

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62
  • Squill glycosides
  • Urginea maritima (L.)
  • 0.1 ? 2.4 total bufadienolides, ?15
    glycosides
  • White variety average 0.2-0.4
  • proscillaridin A, scillaren A, glucoscillaren A
    (aglycone scillarenin)
  • scilliphaeoside, scilliglaucoside
  • Red variety lt 0.1
  • scilliroside and glucoscilliroside (aglycone
    scillirosidin) proscillaridin A and scillaren A
    as in the white variety

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Pharmacological properties of squill
  • White squill
  • it is an expectorant, but it also possesses
    emetic, cardiotonic (proscillaridin A), and
    diuretic properties.
  • Red squill
  • it is used as a rat poison (scilliroside),
    because rodents lack the vomiting reflex, which
    makes red squill particularly lethal to these
    animals.

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  • Digitalis glycosides
  • Several species of Digitalis yield
    pharmacologically active principles. The most
    important of these species are Digitalis purpurea
    and Digitalis lanata.
  • Digitalis purpurea folium (Red foxglove leaves)
  • 0.15 ? 0.4 total cardenolides, ? 30 glycosides
    Purpurea glycosides A and B (?60), digitoxin
    (?12), gitoxin (?10) and gitaloxin (?10).
  • Digitalis lanata folium (White foxglove leaves)
  • 0.5 ? 1.5 total cardenolides, ? 60 glycosides
    Lanatosides A and C (?50), lanatosides B, D, E
    as well as digoxin and digitoxin.

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  • Digitoxin is a cardiotonic glycoside obtained
    from D. purpurea, D. lanata.
  • It is the most lipid-soluble of the cardiac
    glycosides used in therapeutics.
  • The major pharmacokinetic parameters for
    digitoxin include complete oral absorption, which
    distinguishes it from other cardiac glycosides.
  • Digitoxin may be indicated in patients with
    impaired renal function.
  • Digoxin is the most widely used of the
    cardiotonic glycosides, and it is obtained from
    the leaves of D. lanata.
  • It is a highly potent drug and should be handled
    with exceptional care.
  • Digoxin tablets are 60 to 80 absorbed.
  • Digoxin is indicated when the risk of digitalis
    intoxication is great, since it is relatively
    short-acting and rapidly eliminated when compared
    with digitoxin.

69
Digitalis purpurea
70
7. Tannins
  • Historically, the importance of tannin-containing
    drugs is linked to their tanning properties, in
    other words their ability to transform fresh
    hides into an imputrescible material leather.
  • Tannins are "phenolic natural products that
    precipitate proteins from their aqueous
    solutions".

71
  • The consequence of tanning is the formation of
    bonds between the collagen fibers in the hide,
    which imparts resistance to water, heat, and
    abrasion. This capability of tannins to combine
    with macromolecules explains why they precipitate
    cellulose, pectins, and proteins it also
    explains their characteristic astringency and
    tartness by precipitating the glycoproteins
    contained in saliva, tannins make the latter lose
    its lubricating power.
  • Most true tannins have molecular weights from
    about 1000 ? 5000.

72
Pseudotannins
  • They are compounds of lower molecular weight than
    true tannins and they do not respond to the
    goldbeater's skin test.
  • Examples of drugs containing Pseudotannins are
  • Gallic acid Rhubarb
  • Catechins Guarana, Cocoa
  • Chlorogenic acid Mate, Coffee
  • Ipecacuanhic acid ipecacuanha

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Function of tannins in plants
  1. Tannins are considered the source of energy
    through their oxygen content.
  2. They serve as a protective to the plant (plant
    antiseptics).
  3. They may have function in respiratory activity,
    i.e. in the mechanisms of hydrogen transfer in
    plant cells.
  4. Tannins play an important part in the acceptance
    of many foods and beverages by consumers e.g.
    tea, cocoa.

75
Classification of tannins
  • In higher plants, two groups of tannins are
    generally distinguished, which differ by their
    structure, as well as their biosynthetic origin
    hydrolysable tannins and condensed tannins.
  • Hydrolysable tannins
  • Hydrolysable tannins are esters of a sugar (or
    related polyol) and of a variable number of
    phenolic acid molecules.

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  • The sugar is most generally glucose.
  • The phenolic acid is either gallic acid, in the
    case of gallitannins, or Ellagic acid, in the
    case of the tannins conventionally referred to as
    ellagitannins.
  • Ellagic acid can arise by lactonization of
    hexahydroxydiphenic acid ( HHDP) during chemical
    hydrolysis of the tannin.
  • Hydrolysable tannins were formerly known as
    pyrogallol tannins, because on dry distillation
    gallic acid and similar components are converted
    into pyrogallol.

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  • Biosynthetically, gallic acid (
    3,4,5-trihydroxybenzoic acid) arises from the
    metabolism of shikimic acid.
  • Examples of drugs containing Hydrolysable
    tannins
  • Gallitannins rhubarb, cloves, Chinese galls,
    Turkish galls, hamamelis, chestnut and maple.
    Ellagitannins pomegranate rind, pomegranate
    bark, eucalyptus leaves, and oak bark.

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  • Condensed tannins (proanthocyanidins)
  • Condensed tannins or proanthocyanidins are
    polymeric flavans. They consist of flavan-3-ol
    units linked together by carbon-carbon bonds,
    most often 4?8 or 4?6, which result from coupling
    between the electrophilic C?4 of a flavanyl unit
    from a flavan-4-ol or flavan-3,4-diol and a
    nucleophilic position (C-8, less commonly C-6) of
    another unit, generally a flavan-3-ol.
  • Unlike hydrolysable tannins, these are not
    readily hydrolyzed to simpler molecules and they
    do not contain a sugar moiety.

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  • Biosynthetically, flavonoids are derived from
    acetate and shikimate pathways.
  • Condensed tannins occur due to polymerization
    (condensation) reactions between flavonoids.
  • The polymers may include up to 50 monomer units.
  • On treatment with acids or enzymes condensed
    tannins are converted into red insoluble
    compounds known as phlobaphenes. Phlobaphenes
    give the characteristic red colour to many drugs
    such as red cinnamon bark.

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  • Examples of drugs containing Condensed tannins
  • Some drugs (e.g. tea, hamamelis leaves and
    hamamelis bark) contain both hydrolysable and
    condensed tannins. The following are rich in
    condensed tannins.
  • (1) Barks cinnamon, wild cherry, cinchona,
    willow, acacia, oak and hamamelis
  • (2) Roots and rhizomes krameria (rhatany) and
    male fern
  • (3) Flowers lime and hawthorn
  • (4) Seeds cocoa, guarana, and kola
  • (5) Leaves hamamelis, hawthorn and tea,
    especially green tea
  • (6) Extracts and dried juices catechu, acacia
    and mangrove cutches

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Properties and tests of tannins
  • Tannins are soluble in water, dilute alkalis,
    alcohol, glycerol and acetone, but generally only
    sparingly soluble in other organic solvents.
  • Solutions precipitate heavy metals, alkaloids,
    glycosides and gelatin.
  • With ferric salts, gallitannins and ellagitannins
    give blue-black precipitates and condensed
    tannins brownish-green ones. If a very dilute
    ferric chloride solution is gradually added to an
    aqueous extract of hamamelis leaves (which
    contains both types of tannin), a blue colour is
    produced which changes to olive-green as more
    ferric chloride is added. Other useful tests are
    the following

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  • Goldbeater's skin test
  • Soak a small piece of goldbeater's skin in 2
    hydrochloric acid rinse with distilled water and
    place in the solution to be tested for 5 min.
    Wash with distilled water and transfer to a 1
    solution of ferrous sulphate. A brown or black
    colour on the skin denotes the presence of
    tannins. Goldbeater's skin is a membrane prepared
    from the intestine of the ox and behaves
    similarly to an untanned hide.

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  • Gelatin test
  • Solutions of tannins (about 0.5-1 ) precipitate
    a 1 solution of gelatin containing 10 sodium
    chloride. Gallic acid and other pseudotannins
    also precipitate gelatin if the solutions are
    sufficiently concentrated.
  • Phenazone test
  • Test for catechin
  • Test for chlorogenic acid

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Medicinal and biological properties
  • The applications of tannin-containing drugs are
    limited, and result from their affinity for
    proteins.
  • Tannin-containing drugs will precipitate protein
    and have been used traditionally as styptics and
    internally for the protection of inflamed
    surfaces of mouth and throat.
  • They act as antidiarrhoeals and have been
    employed as antidotes in poisoning by heavy
    metals, alkaloids and glycosides.

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Gall-oak
sumac
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