Glycosides and their Constitution

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Mohammad Ovais Dar


Introduction: A Glycoside 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 . T he first glycoside ever identified was ‘ amygdalin’ , by the French chemists Pierre Robiquet and Antoine Boutron-Charlard in 1830.

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These are the compounds derived by the replacement of the hydrogen atom of the cyclic hemiacetal by an organic radical. The formation of glycoside is comparable to the formation of acetals in which the simple hemiacetals react with another molecule of alcohol e.g.

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(acts as an hemiacetal) Glycoside (acetal)

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Glycosides which are derived from glucose are known as glucosides , from fructose as fructosides , etc. Only the hydroxyl group derived from carbonyl group by ring formation (i.e., C 1 in aldoses and C 2 in ketoses) take part in glycoside formation and hence hydroxyl group is known as glycosidic hydroxyl group.

Natural Glycosides::

Natural Glycosides: Natural glycosides are colourless, crystalline, poisonous compounds, bitter in taste, soluble in water, optically active and do not reduce Fehling’s solution. In nature they are usually accompained by the specific enzymes (usually emulsin and maltase) which are capable of effecting their hydrolysis – Emulsin hydrolyses only the β -glycosides , while maltase hydrolyses only the α -glycosides and hence enzymatic hydrolysis is used to ascertain the nature of linkages in the glycosides.

Classification of glycosides::

Classification of glycosides: I) On the basis of nature of atom to which the sugar is linked: O-Glycosides : When the sugar portion is attached to the oxygen atom e.g. amygdalin, indican, arbutin, etc C-Glycosides : When the sugar moiety is linked to the carbon atom. S-Glycosides : When the sugar moiety is linked to the sulphur atom e.g. sinigrin. N-Glycosides : When the sugar portion of the glycoside is attached to the nitrogen atom.

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II) According to the configuration of the glycosidic linkage: i ) β -glycosides : Here the sugar involved has the  configuration at the hemiacetal carbon The majority of plant glycosides isolated are  -glycosides

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α -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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According to the sugar moiety: Glucosides Rhibosides Rhamnosides IV) According to the aglycone : Phenolic glycosides Flavonoid glycosides Anthraquinone glycosides Coumarin glycosides, etc

Extraction of glycosides::

Extraction of glycosides: The plant material is macerated with water and then treated with lead acetate to precipitate proteins and tannins. It is then filtered and lead is removed as lead sulphide by a current of hydrogen sulphide. The solution is then shaken with ethanol or water in a Soxhlet extractor .

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In the extract so obtained the enzyme is destroyed by heating it to a temperature of about 65 °C. Then, the extract is concentrated to get the glycoside which is then purified by employing usual methods.

Functions of glycosides::

Functions of glycosides: Glycosides may serve as sugar reserves in plants. Glycosides are also considered to be waste products during plant metabolism. In some cases, glycosides are used to remove harmful substances such as phenols by dexofication mechanism.

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It has also been found that anthomycin pigments can influence the rate of photosynthesis. Like alkaloids, many plant glycosides possess physiological activity, e.g. the cardiac glycosides are used medicinally.

Determination of structure::

Determination of structure: The structure of glycosides is first determined by degradation and then by unambiguous synthesis. A. Degradation : The glycoside is hydrolysed with dilute acids and the components i.e., sugar and aglycone parts are separated by paper chromatography. The size of the ring of the sugar is established by common method i.e., complete methylation followed by hydrolysis.

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The configuration of the glycoside linkage is determined by the enzyme hydrolysis; in general emulsin hydrolyses the β -glycosides while maltase hydrolyses α -glycosides. If the sugar unit is some di- or tri- saccharide, specific enzymes are used to effect only partial hydrolysis of the glycoside so as to identify the oligosaccharide.

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Synthesis : The structure of a glycoside established by the above degradation reactions is then confirmed by an unambiguous synthesis. In general, synthesis of a glycoside involves the reactions between O- acetyl α - glycosyl bromide and an alcohol or phenol (i.e., the aglycone) in the presence of silver carbonate in anhydrous benzene, generally during the reaction Walden inversion takes place i.e., β -linked glycosides are produced.

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On the other hand, if α -D-glycoside are to be prepared the reaction must be effected in presence of mercuric acetate or ferric chloride in place of silver carbamate. The required O- acetylglycosyl bromide is prepared from the corresponding sugar. The sugar, e.g. an aldohexose is acetylaed in presence of zinc chloride to give α - or β - penta -acetate which is then treated with glacial acetic acid saturated with hydrogen bromide to form α - acetobromohexose in both the cases i.e., whether the penta-acetate is α or β .


Arbutin: It is a colourless crystalline compound (m.p. 187 °C ). It is bitter in taste and is laevorotatory. It occurs mainly in the leaves of bear-berry .

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Constitution : Its molecular formula is C 12 H 16 O 7 . The colouration with neutral ferric chloride indicates the presence of phenolic group. On hydrolysis with dilute hydrochloride acid or emulsin it gives one molecule of D-glucose and one molecule of hydroquinone, which indicate that the structure of arbutin the D-glucose and hydroquinone are linked with each other by means of β -linkage. Hence arbutin is I which is further confirmed by its synthesis from tetra- O-acetyl- α -D- glucopyranosyl bromide and hydroquinone monobenzoate in presence of silver oxide and quinoline.

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