Pharmacognosy Chapter 5

Lecture 5: Pharmacognosy I (PHCG 111)

• Secondary Plant Metabolites (Active Constituents)
  • Secondary metabolites act as protective agents, waste products of metabolism, and aid in producer survival.
  • They are energy producers and physiologically active.
  • These metabolites are various organic compounds.

Carbohydrates and Related Compounds

• Compounds composed of carbon, hydrogen, and oxygen as polyhydroxy aldehydes or ketone alcohols, such as sucrose, lactose, starch, gums, mucilage, and pectin.
• Gums
  • Polysaccharides or salts of Polysaccharides.
  • Produced in higher plants as protective agents after injury.
  • On hydrolysis, gums yield arabinose, galactose, glucose, mannose, and various uronic acids (which can form salts with Ca, Mg, and other cations).
  • Examples: gum acacia and gum tragacanth.

Mucilage

• Polysaccharide complexes.
• Insoluble in alcohol, forming viscous, non-adhesive colloidal solutions in water, which do not form jelly.
• Types include:
  • Pectose type (red color with ruthenium red, eg. Senna, Buchu, and Henna leaves)
  • Callose type (red color with corallin soda, eg. squill)
  • Neutral type (blue color with methylene blue, eg. Foenugreek)

Pectin

• Purified carbohydrate (CHO) extracted from dilute acid extracts of the inner rind of citrus fruits or apple pomace.
• Soluble in 20 parts water, forming a stiff gel.
• Used as a protective and suspending agent, and an ingredient in antidiarrheal preparations.

Glycosides

• Non-reducing organic compounds that yield one or more sugars upon hydrolysis using dilute acids, alkalis, or enzymes.
• The non-sugar part is called the aglycone (genin), and the sugar component is the glycone.
• Chemically, glycosides are considered sugar ethers, formed by water loss and linkage formation (O-, C-, S-, CN-).
• Glycosides are generally soluble in water and hydro-alcoholic solvents due to the hydrophilic nature of the sugar. The higher the sugar content, the lower the solubility in organic solvents. The aglycone is more soluble in organic solvent.

Types of Glycosides

• A. Phenolic Glycosides
  • i. Simple Phenolic Glycosides: e.g., Arbutin (a hydroquinone glycoside) from Uva ursi leaves, used in urethritis and cystitis.
  • ii. Flavonoid Glycosides: Largest group of naturally occurring phenols occurring in plants as free aglycones, or as glycosides. Examples: Diosmin (Buchu), Hesperidin (Citrus species). They are yellow compounds, dissolving in aqueous alkali and giving a canary-yellow color.
  • iii. Anthocyanin Glycosides: Structurally related to flavonoids, producing pigment in colored flower petals. Their color changes (red-violet to blue) are pH-dependent (e.g., Karkadeh).
  • iv. Anthraquinones Glycosides: Pharmacologically active constituents in laxatives and purgatives (e.g., Senna). Give positive Borntrager's and modified Borntrager's tests.
• B. Alcoholic Glycosides (Cardiac Glycosides):
  • Exert powerful action on cardiac muscles.
  • Small amounts can stimulate a diseased heart, but large doses cause death.
  • Used to increase cardiac muscle tone, excitability, and contractility, and as diuretics due to increased renal circulation.
  • Example: Digitalis.
  • SAR (Structure-Activity Relationship) for Cardiac Glycosides: Steroidal nucleus and lactone ring (5- or 6-membered) with a deoxy sugar (e.g. Digitoxose). Detection of cardiac glycosides: chemical tests for steroidal nucleus and deoxy sugars and unique lactone rings.
• ii. Saponin Glycosides:
  • Characterized by foaming in water, causing red blood cell lysis (hemolysis), sternutatory effects, and irritation of mucous membranes (eyes/nose).
  • Used as emulsifiers in industrial cleaning and for certain resins, fats, and fixed oils.
• C. Cyanophore Glycosides:
  • Yield HCN upon hydrolysis.
  • Limited use in pharmacy due to HCN's toxicity.
  • Detected by reaction with sodium picrate paper (Grignard Reaction).
• D. Thioglycosides:
  • Contain thiocyanate (S-C≡N) or sulphur.
  • Examples: Sinigrin (from black mustard seeds, mustard irritant gas) and Sinalbin (from white mustard seeds, non-volatile).

Tannins

• Large group of complex substances, non-crystalline, with a sharp astringent taste.
  • Form colloidal solutions in water.
  • Precipitate proteins and alkaloids.
• Uses: Astringents, hemostatics, protective coating for burns, antidotes for metal and alkaloid poisoning, tanning of leather, and ink production.
• Types:
  • Hydrolysable Tannins: Hydrolyzed by acids or enzymes (e.g., tannase), composed of phenolic acids (gallic acid), giving a blue-black color with FeCl3 and no precipitate with bromine water. Examples: clove and galls.
  • Condensed Tannins: Not readily hydrolyzed, do not contain sugar moieties, yield catechins on hydrolysis, green color with FeCl3, and precipitate with bromine water. Examples: tea and henna leaves.

Alkaloids

• Basic nitrogenous compounds of biological origin.
• Physiologically active.
• Not all nitrogen-containing compounds are alkaloids.
• Generally, alkaloidal bases are insoluble in water but soluble in organic solvents. Alkaloidal salts are soluble in water or sparingly soluble in organic solvents.
• Detection:
  • Alkaloidal precipitants – e.g. Mayer's reagent (potassium mercuric iodide solution) giving a creamy white precipitate with most alkaloids, except for caffeine.
  • Alkaloidal color reagents – e.g. Dragendorff's reagent gives an orange color.

5-Oils

• Volatile Oils (Essential Oils):
  • Mix of hydrocarbons (mostly terpenes) and oxygenated compounds (e.g., buchu leaves, lavender flowers)
  • Volatile, steam-distilled.
  • Used as flavoring, in perfumery, and as spices, antiseptics, and carminatives.
  • Do not leave a permanent stain on filter paper
  • On standing, they become more resinous.
• Fixed Oils:
  • Triglycerides of fatty acids (e.g., linseed seed).
  • Nonvolatile.
  • Saponifiable
  • Leaves a permanent stain on filter paper
  • On standing, they become rancid.
  • Used in food and paint industries.