GLYCOSIDES

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GLYCOSIDES
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• is an organic
  compound, usually of plant origin, that is
  composed of a sugar portion linked to a non-sugar
  moiety. The sugar portion is called glycone,
  while the non-sugar portion is called aglycone or
  genin .

A glycoside
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OH on the anomeric carbon is replaced by an OR
group, such acetal is called glycoside
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• The linkage between the sugar and the aglycone
  is an acetal linkage.
• Types of Glycosides
• I- According to atoms involved in the
  glycosidic linkage
• 1- O-glycosides
• 2- C-glycosides
• 3- S-glycosides
• 4- N-glycosides

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• II- According to the configuration of the
  glycosidic linkage
• 1- ?-glycosides
• Here the sugar involved has the ? configuration
  at the hemiacetal carbon.
• The majority of plant glycosides isolated are
  ?-glycosides.

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2- ? -glycosides

• Here the sugar involved has the ? configuration
  at the hemiacetal carbon the sugar is usually of
  the L-series e.g. L-rhamnose. There are only few
  medicinal ?-glycosides known, especially the
  rhamnosides.

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III- According to the sugar moiety

• 1-Glucosides
• 2- Ribosides
• 3- Rhamnosides
• IV- According to the aglycone
• 1-Cardiac glycosides
• 2- Flavonoid glycosides
• 3- Anthracene glycosides

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• 4- Saponin glycosides
• 5-Thioglycosides
• 6- Cyanogenic glycosides
• 7- Phenolic glycosides possessing phenolic
  aglycones e.g. arbutin.
• 8- Alcohol glycosides, with alcoholic aglycones
  e.g. salicin.
• 9-Antibiotic glycosides
• e.g, Amino-glycoside antibiotics

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10- Ester glycosides, such as the glycosides of
methyl salicylate e.g gaultherin 11- Aldehyde
glycosides, with an aldehyde aglycone e.g.
glucovanillin . 12- Terpene glycosides a-
Monoterpene glycosides e.g. gentiopicrin and
loganin. b- Triterpene saponin glycoside.
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• 13- Glycoalkaloids
• These are the glycosidal alkaloids i.e. They
  possess a glycosidic
• linkage and a cyclic nitrogen in the aglycone.
  e.g steroidal alkaloids of Solanum species e.g.
• solanine.

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• 14- Complex glycosides
• These include a number of complex secondary
  carbohydrates e.g. gums, mucilage pectin a
  number of glycosidal resins e.g. Jalap resin as
  well as a number of glycosidal tannins e.g.
  glucogallin in Rhubarb.

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Sugars in glycosides

• The sugar portion
• 1- It may be molecule of a monosaccharide e.g.
  D-glucose or L-rhamnose.
• - or composed of 2 or more sugars, forming a
  di-or oligosaccharide chain e.g. purpurea
  glycoside A has a tetrasaccharide attached to
  digitoxigenin e.g. Digitoxigenin
• -O-(digitoxose)3-O-glucose .

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• 2- In certain cases, the oligosaccharide portion
  is branched as in the
• steroidal saponin, digitonin where there is a
  branched pentasaccharide chain.
• 3- Sometimes, two monosaccharide units are
  attached at two different
• positions in the aglycone molecule, so we are
  dealing with a diglycoside e.g.

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The sugars involved

• 1- b-D-glucose is the most common sugar in
  glycosides.
• 2- Other sugars include
• i - Other hexoses e.g. galactose
• ii - Pentoses e.g. ribose, xylose and
  arabinose.
• iii - 6-Deoxyhexoses e.g. rhamnose and
  digitalose.

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• iv - 2,6-Dideoxysugars e.g. sarmentose,
  digitoxose and cymarose (rare sugars)
• The 6-Deoxy sugars and 2,6-Dideoxy sugar are
  common in
• Cardiac glycosides.

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• vi - A uronic acid e.g. glucuronic acid found
  in glycyrrhizin .

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Effect of the sugar portion on solubility of the
glycoside

• Ex. salicin, with one glucose unit, is soluble in
  ether, while glycosides with a larger sugar
  portion are more soluble in alcohol or aqueous
  alcoholic solvents .

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Effect of the sugar on susceptibility of the
glycoside to acid hydrolysis

• Generally 2-deoxy sugars can be hydrolyzed easier
  than 6-deoxysugars, and the latter more easily
  than normal sugars . Thus milder conditions can
  be chosen to hydrolyze the glycoside involving
  2-deoxysugars, without affecting the others .

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Solubility of glycosides

• The presence or absence of various polarity
  contributing functional groups in the structure
  of the aglycone portion, would contribute to the
  degree of solubility in a given solvent e.g.
  thioglycosides are soluble in water partly
  because of the ionic sulfate residue.

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• The number and kind of monosaccharide units
  present in the sugar portion would contribute a
  certain polar character and thus contributing to
  the degree of solubility in polar solvents.

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Hydrolytic cleavage and stability

• 1-Acid Hydrolysis
• Glycosides can be hydrolyzed by heating with a
  dilute acid, where by the glycosidic linkages
  are cleaved.
• Glycosidic linkages involving different kinds of
  sugars are hydrolyzed with different degrees by
  acid hydrolysis

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• e.g. 2-deoxysugars are hydrolyzed easier than
  6-deoxy-sugars, which is still easier than normal
  sugars.
• - If there is a di-or an oligosaccharide unit in
  the molecule, it is practically impossible to
  cleave one unit of sugar at a time with acid
  hydrolysis.

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When stepwise cleavage of monosaccharide units is
needed, specific enzyme hydrolysis must be used.

• C-glycosides are more resistant to acid
  hydrolysis than O-glycosides
• i.e. they do require more vigorous conditions.

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2- Enzymatic Hydrolysis

• Glycosides can be hydrolyzed by appropriate
  enzymes, which are usually found in the same
  plant, in separate compartments.
• The glycosidic linkages involving b-D-glucose as
  well as most b-glycosides can be hydrolyzed by
  various b-glycosidases.

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• - Stepwise hydrolysis of monosaccharide units is
  possible using specific enzyme.
• a- Digitalidase, in Digitalis purpurea
• b- Lanatosidase present in D. lanata
• c-Strophanthibiase acts upon K-strophanthoside to
  give cymarin.
• It is thus clear that enzymatic hydrolysis is
  specific .

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• N.B
• During extraction or isolation of glycosides
  appropriate measures should be taken to prevent
  the cleavage of glycosides by the
• enzymes that may be present in the same plant.

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3- Effect of Alkali

• - Alkalies do not , generally, cleave glycosidic
  linkages.
• - Ester groups in the glycosides may be cleaved
  e.g. the acetyl-
• group in lanatosides or the sulfate group in
  thioglycosides ... etc.
• - Degradation may occur in the aglycone or in the
  sugar portion by alkali.

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• The lactone ring at C17, in cardiac aglycones is
  opened by strong alkalies, with subsequent loss
  of the cardiotonic activity, this change is
  irreversible on addition of acids.

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Extraction and Isolation

• 1- Acidic conditions should be avoided during
  extraction.
• 2- Inactivation of the enzymes
• a- Boiling alcohol for 10-20 minutes.
• b- Boiling with acetone.
• c- Low temperature i.e. in the cold.
• d- precipitates the enzymes with ammonium
  sulfate.

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• e- Freeze-drying.
• f-A special drying procedure such as a short
  heat treatment.
• Extraction Procedures
• Generally, the glycoside is extracted from the
  crude drug with
• Alcohol.
• -Adding basic lead acetate to the alcoholic
  extract causes the impurities to precipitate.

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After filteration, the alcoholic extract is
concentrated, and the glycosides may
crystallize ( oily seeds should be defatted).

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Pharmacological or Therapeutic Activity

• 1- cardiac glycosides of Digitalis, Strophanthus,
  and squill are cardiotonic agents useful in
  congestive heart failure.
• 2- Anthracene glycosides of cascara, Frangula,
  senna, Rhuharb, Aloes have a laxative effect.

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• 3- Flavonoid glycosides e.g. rutin and hesperidin
  are used to decrease capillary fragility and
  permeability.
• 4- In case of cyanogenic or thioglycosides, that
  the aglycone is the active component used. The
  aglycone from the former (e.g. HCN
  Benzaldehyde) is useful as a sedative and
  flavoring agent

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• aglycone from the latter (e.g. allyliso-
  thiocyanate) is useful as a rubifacient and
  counterirritant.
• Generally the pharmacological action of the
  glycoside is due to the aglycone. However, the
  sugar portion is necessary to carry the aglycone
  to the site of action.

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By - Jaspreet Rekhi www.rhombnet.com
www.ppt.rhombnet.com