Pharmacognosy Lec 4 PDF

# Glycosides - Definition of Glycosides - Hydrolysis of Glycosides - Nomenclature of Glycosides - Properties of Glycosides - Biosynthesis of Glycosides - Types & Classification of Glycosides - Role of Glycosides in Plants - Extraction of Glycosides ## Definition Glycosides: - Non-reducing natural...

# Glycosides - Definition of Glycosides - Hydrolysis of Glycosides - Nomenclature of Glycosides - Properties of Glycosides - Biosynthesis of Glycosides - Types & Classification of Glycosides - Role of Glycosides in Plants - Extraction of Glycosides ## Definition Glycosides: - Non-reducing natural products which on hydrolysis (acids or enzymes) - Glycone (one or more sugar) (reducing) + Aglycone. A diagram shows a reversible reaction between a Glycoside and an aglycone and a glycone. - The Glycoside is labeled as non-reducing, while the aglycone is labeled as Non-sugar and the glycone is labeled as reducing. ## Hydrolysis - Acid Hydrolysis - Strong acids with heating. - 2-deoxy sugars (eg. cardiac glycosides) require weak acids at room temperature - C-glycosides require an oxidizing agent eg., FeCl3 or H2O2 - Alkaline Hydrolysis - Alkali has no action on the glycosidic linkages - May cause breakage in aglycone or sugar 1) Acetyl-ester group (or any ester linkage): mild alkali. 2) Lactone ring (cardiac glycosides): strong alkali. - Enzymatic Hydrolysis - Specific in action. - Emulsin → β-glycosides - Myrosin → Sulphur-containing glycosides. - Rhamnase → Rhamnose-containing glycosides. ## Nomenclature - 1-Trivial names: *in*-ending → Source of the glycoside. - eg., digitoxin from Digitalis - salicin from Salix - 2- *oside*-ending → simple sugar produced on hydrolysis - eg., glucoside, fructoside, glycoside. - 3-Systematic names: For Heteroside: - *O*-hydroxymethylphenyl *β*-D-glucopyranoside. - For Holoside - 4-O-(α-D-Glucopyranosyl)-D-glucopyranose. (*osyl* can be replaced by *osido* ) ## Properties ### Physical Properties of Glycosides - Colorless, crystalline, non-volatile solids. - Bitter taste. - Exceptions, eg., populin is sweet. - Fehling's reagent -Ve, after hydrolysis +Ve. - Soluble in water and hydro-alcohol. - Insol. in ether; exception: Steroid-glycoside. ### Chemical Properties - Reactions due to aglycone: Most characteristic chemical tests of glycosides. - Reactions due to sugar: Reduce Fehling's solution after hydrolysis. ### Pharmacological Action - Is due to aglycone. #### Role of Sugar - Solubilization - Enhance the pharmacodynamic properties. - Carry the aglycone to the site of its action. ## Biosynthesis - General Scheme: - Formation of phosphorylated sugar: - Activation of phosphorylated sugar with Uridine triphosphate nucleotide (UTP) into the activated sugar UDPG (Uridine diphosphate glucose). ## Types & Classification ### Types of Linkages - Glycosidic linkage - O-glycosides (OH) group. - S-glycosides - N-glycoside - C-glycosides (or C-C-glycosides) ### Classification of Glycosides (Heterosides) - 1) Chemical nature of aglycone - Phenol - Alcohol - Lactone - Flavone - Anthraquinone - Steroid - Saponin - 2) Specific group in the molecule: - Cyano-group (CN) → Cyanogenetic glycosides (Amygdalin). - Sulphur (S) → Thioglycoside (Sinigrin). - 3) Physiological activity: - Cardiac glycoside. - Laxative Glycosides. - 4) Nature of glycoside: - Two main groups according to nature. - Primary glycosides: The unchanged from, occurring in the fresh plant e.g., Purpurea glycoside A. - Secondary glycosides: The result from the hydrolysis of other glycoside by enzymatic activity with splitting of one or more sugar unit, e.g., Digitoxin. ## Role in plants - Source of reserved energy (sugar -content). - Regulatory role in plant physiology. - Detoxifying role. - Defense role against micro-organisms and insects. ## Extraction - Extraction and isolation of glycosides: - Common Solvents are water and alcohol. - Precautions in extraction procedure: - Inactivation of enzymes by: - Boiling with alcohol or acetone. - Drying at 100°C for 15 minutes. - Extraction by liquid CO2. - Freeze drying. - Neutral condition: Calcium carbonate in case of plant rich in acid. - Defatting: By petroleum ether in case of plants rich in lipids. - Removal of other constituents: e.g., tannin and resin ...etc. by treating with lead acetate. - Use of chromatography for isolation: Silica, cellulose, polyamide, etc.