Glycosides and Their Hydrolysis

Glycosides

• Glycosides are non-reducing natural products that hydrolyze (using acids or enzymes) into a glycone (one or more sugars) and an aglycone.
• Hydrolysis breaks the glycosidic link.
• Glycosides can contain reducing or non-reducing sugars.

Hydrolysis of Glycosides

• Acid Hydrolysis: Uses strong acids with heating, or weak acids at room temperature. 2-deoxy sugars (e.g., cardiac glycosides) may require strong acids. C-glycosides need oxidizing agents like FeCl3 or H2O2.
• Alkaline Hydrolysis: Alkali has no effect on glycosidic linkages. May break aglycone or sugar linkages. Acetyl-ester linkages (or any ester linkage) can be broken with mild alkali. Lactone rings (cardiac glycosides) need strong alkali.
• Enzymatic Hydrolysis: Specific enzymes act on specific glycosides. Emulsin breaks β-glycosides, Myrosin breaks sulfur-containing glycosides, and Rhamnase breaks rhamnose-containing glycosides.

Nomenclature of Glycosides

• Trivial Names: Include the source of the glycoside, e.g., digitoxin from Digitalis. Salicin from Salix.
• Oside Ending: Indicate the simple sugar produced after hydrolysis, e.g., glucoside, fructoside, glycoside.
• Systematic Names: For heterosides, e.g., O-hydroxymethylphenyl β-D-glucopyranoside.

Properties of Glycosides

• Physical: Colorless, crystalline, non-volatile solids. Bitter, but exceptions (e.g., populin is sweet). Soluble in water and hydro-alcohol. Insoluble in ether, except steroid glycosides. Fehling's reagent is negative before hydrolysis but positive after.
• Chemical: Reactions are mainly due to the aglycone. Glycosides can reduce Fehling's solution after hydrolysis due to the sugar moiety.

Biosynthesis of Glycosides

• General Scheme: Starts with formation of phosphorylated sugars.
• Activation: Phosphorylated sugar combines with Uridine triphosphate (UTP) → activated sugar UDPG (Uridine diphosphate glucose).
• Reactions of UDPG: UDPG plus a holoside leads to the formation of sucrose with UDP. UDPG and aglycone lead to the formation of glucoside with UDP.

Types and Classification of Glycosides

• Based on Aglycone: Phenol, alcohol, lactone, flavone, anthraquinone, steroid and saponin.
• Based on Specific Groups: Cyanogenetic glycosides (contain a cyano group), Thioglycosides (contain sulfur).
• Based on Physiological Activity: Cardiac glycosides, laxative glycosides.

Role of Glycosides in Plants

• Source of reserved energy (sugar)
• Regulatory role in plant physiology
• Detoxifying role
• Defense mechanism against microorganisms and insects

Extraction of Glycosides

• Common Solvents: Water and alcohol.
• Precautions: Inactivating enzymes (boiling with alcohol or acetone, drying at 100°C for 15 minutes, extraction with liquid CO2, freeze drying). Neutralizing conditions (calcium carbonate in acidic plants). Removing other constituents (e.g., tannins and resins, using lead acetate). Chromatography for isolation (e.g. silica, cellulose, polyamide).

Explore the fascinating world of glycosides, their structure, and the various hydrolysis methods they undergo. This quiz delves into acid, alkaline, and enzymatic hydrolysis, along with the nomenclature of glycosides. Test your knowledge and understanding of these important natural products.