BRIEF OUTLINE OF OCCURRENCE, DISTRIBUTION, ISOLATION, IDENTIFICATION TESTS, THERAPEUTIC ACTIVITY AND PHARMACEUTICAL APPLICATIONS OF ALKALOIDS, TERPENOIDS, GLYCOSIDES, VOLATILE OILS, TANNINS AND RESINS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacognosists and pharmaceutical scientists!
Secondary metabolites are synthesised from primary metabolites such as carbohydrates, proteins, and lipids. They are present in small quantities and protect plants against microorganisms, insects, and higher herbivorous animals. They have biological or pharmacological action on humans and animals. Their isolation from plants is generally expensive as compared to primary metabolites—for example, quinine from cinchona bark, curcumin from turmeric, etc.
As a pharmacognosy educator with years of experience teaching natural products, I have observed that students often find secondary metabolites challenging due to their complex structures and diverse classifications. Let me tell you: Understanding secondary metabolites is essential for comprehending how natural products exert their therapeutic effects.
In this comprehensive guide, I will walk you through the occurrence, distribution, isolation, identification tests, therapeutic activity, and pharmaceutical applications of six major classes of secondary metabolites—Alkaloids, Terpenoids, Glycosides, Volatile Oils, Tannins, and Resins. By the end of this article, you will have a thorough understanding of these important natural products. Let us begin!
Dpharmguru’s exam insights:
Secondary metabolites are frequently tested in pharmacognosy exams. Remember: Alkaloids are basic nitrogenous compounds; Terpenoids are volatile aromatic substances; Glycosides yield sugar + non-sugar moiety on hydrolysis; Volatile oils evaporate at ordinary temperature; Tannins precipitate proteins; Resins are hydrocarbon secretions. Each class has specific identification tests—Mayer’s, Dragendorff’s for alkaloids; Borntrager’s for anthraquinone glycosides; Keller-Kiliani for cardiac glycosides. These are classic exam questions!
4.1. ALKALOIDS
Alkaloids are heterocyclic nitrogenous compounds. Generally, they are basic in nature due to the presence of amino nitrogen. They can be defined as basic nitrogenous compounds of plant origin which are physiologically active. Chemically, at least one nitrogen atom is present in its cyclic structure.
Ladenburg stated alkaloids as compounds originated from plants with a basic nature, having one or more nitrogen atoms in its ring structure. The basic nature of alkaloids is due to the presence of N-atom in the 5- or 6-membered ring structure.
4.1.1. Classification of Alkaloids
- True Alkaloids: Derived from amino acids; contain nitrogen in heterocyclic ring. Examples: Cocaine, Morphine, Quinine
- Proto Alkaloids: Amino acid nitrogen not in heterocyclic ring; simple amines. Examples: Mescaline, Ephedrine
- Pseudo Alkaloids: Not derived from amino acid precursor; basic in nature. Sub-divided into:
- Steroidal Alkaloids: Conessine
- Purine Bases: Caffeine
4.1.2. Occurrence and Distribution of Alkaloids
McKee in 1962 reported that about 1000 investigated alkaloids belong to almost 100 families, 500 genera, and 1200 species. Alkaloids are absent in algae and in lower groups of plants (except one or two fungi families). Ergot alkaloid is an example of fungal alkaloid.
Alkaloid-rich families: Apocynaceae, Berberidaceae, Papaveraceae, Ranunculaceae, Rubiaceae, Solanaceae.
Around 15% of vascular plants yield alkaloids. Alkaloids are found in any part of the plant. In a species, only one or two specific parts form alkaloids.
4.1.3. Isolation of Alkaloids
- Stas-Otto Process: Based on basicity and solubility; drug treated with alkali to free base, then extracted with organic solvent
- Manske’s Process: Uses methanol extraction and acidification
- Kippenberger’s Process: Digestion with tannin in glycerol at 40°C; coagulation of proteinaceous substances
- Steam Distillation: For volatile liquid alkaloids like coniine, nicotine
4.1.4. Identification Tests for Alkaloids
- Mayer’s Reagent: White or pale yellow precipitate
- Dragendorff’s Reagent: Orange-red precipitate
- Wagner’s Reagent: Brown or reddish-brown precipitate
- Hager’s Reagent: Characteristic crystalline precipitate
- Tannic Acid (5% w/v): Buff-coloured precipitate
- Ammonium Reineckate: Pink flocculent precipitate
4.1.5. Therapeutic Activity of Alkaloids
- Morphine & Codeine: Analgesics (pain relievers)
- Atropine: Mydriatic (eye dilation)
- Quinine: Antimalarial
- Vincristine & Vinblastine: Anti-cancer agents (leukemia treatment)
4.1.6. Pharmaceutical Applications of Alkaloids
| Alkaloid | Pharmacological Action |
|---|---|
| Morphine | Narcotic and analgesic |
| Codeine | Expectorant and analgesic |
| Brucine | CNS stimulant |
| Strychnine | CNS stimulant |
| Ergotamine | Uterine muscle contraction |
| Atropine | Mydriatic |
| Ephedrine | Hypertensive |
| Reserpine | Hypotensive |
| Quinine | Antimalarial |
| Caffeine | CNS stimulant |
4.2. TERPENOIDS
Terpenoids are volatile and aromatic substances present in plants that give their characteristic fragrance or odour. Most recently, terpenoids have been defined as the hydrocarbons of plant origin of the general formula (C₅H₈)n as well as their oxygenated, hydrogenated, and dehydrogenated derivatives.
4.2.1. Classification of Terpenoids
| Class | Value of n | Formula |
|---|---|---|
| Monoterpenoids | 2 | C₁₀H₁₆ |
| Sesquiterpenoids | 3 | C₁₅H₂₄ |
| Diterpenoids | 4 | C₂₀H₃₂ |
| Sesterpenoids | 5 | C₂₅H₄₀ |
| Triterpenoids | 6 | C₃₀H₄₈ |
| Tetraterpenoids | 8 | C₄₀H₆₄ |
| Polyterpenoids | >8 | (C₅H₈)n |
4.2.2. Occurrence and Distribution of Terpenoids
Terpenes occur universally in both plants and animals. Essential oils contain mono- and sesquiterpenes; balsams and resins contain sesqui-, di-, and triterpenes; tetraterpenes are obtained from pigments; and polyterpenes from latexes. Terpenes can be obtained naturally from trees such as conifers and citrus plants.
4.2.3. Isolation of Terpenoids
- Expression Method: Chopping and crushing plant material
- Steam Distillation: Most widely used method
- Extraction by Volatile Solvents: Using light petrol at 50°C
- Adsorption in Purified Fats (Enfleurage): French method for jasmine and rose
4.2.4. Identification Tests for Terpenoids
- Noller’s Test: Pink colour with tin and thionyl chloride (triterpenoids)
- Test for Terpenoids: Reddish-brown colour at junction of chloroform and conc. H₂SO₄
4.2.5. Pharmaceutical Applications of Terpenoids
- Perilla alcohol: Antineoplastic agent
- Artemisinin: Antimalarial property
- Glycyrrhizin: Antiulcer, antimicrobial, diuretic
- Taxol (diterpenoid): Anticancer property
4.3. GLYCOSIDES
Glycosides may be defined as organic compounds of plant or animal origin which on enzymatic or acid hydrolysis give one or more sugar moieties (glycone) along with a non-sugar moiety (aglycone or genin).
4.3.1. Classification of Glycosides
- Based on Glycone: Glucoside, Fructoside, Rhamnoside, Pentoside
- Based on Aglycone: Anthraquinone, Sterol/Cardiac, Saponin, Cyanogenetic, Thiocyanate, Flavone, Aldehyde, Phenol, Steroidal, Bitter glycosides
- Based on Glycosidal Linkage: O-Glycosides, S-Glycosides, N-Glycosides, C-Glycosides
4.3.2. Occurrence and Distribution of Glycosides
Glycosides widely occur in various parts of plants like roots, barks, fruits, seeds, and to a small extent in leaves. Pharmaceutically important glycosides are obtained from vegetable sources. Most commonly occurring sugars from hydrolysis are glucose, mannose, and galactose. Glycosides are colorless, crystalline, non-reducing, optically active compounds, usually levorotatory.
4.3.3. Isolation of Glycosides
Stas-Otto method is the general method of glycosides extraction. The drug is finely powdered and extracted by continuous hot percolation using alcohol as solvent and Soxhlet apparatus. For thermolabile glycosides, extraction should be carried out at temperatures below 45°C.
4.3.4. Identification Tests for Glycosides
- Borntrager’s Test: Pink/red colour in ammonical layer (Anthraquinone glycosides)
- Modified Borntrager’s Test: For C-type anthraquinone glycosides
- Haemolysis Test: RBCs rupture (Saponin glycosides)
- Foam Test: Froth formation (Saponin glycosides)
- Libermann-Burchard Test: Violet to blue ring (Steroid glycosides)
- Salkowaski Test: Yellow ring turning red (Steroid glycosides)
- Keller-Kiliani Test: Reddish-brown layer turning bluish-green (Cardiac glycosides)
- Legal Test: Pink colour (Cardiac glycosides)
- Baljet Test: Yellow to orange (Cardiac glycosides)
- Sodium Picrate Test: Brick red colour (Cyanophoric glycosides)
4.3.5. Pharmaceutical Applications of Glycosides
| Type | Uses |
|---|---|
| Cardiotonic | Improve heart function (digoxin) |
| Sulphureted | Choleretic antibiotic, cholagogue |
| Anthocyanins | Anti-rheumatic, anti-inflammatory |
| Cyanogenic | Forms hydrocyanic acid (powerful poison) |
| Anthraquinone | Laxative and purgative |
| Coumarin | Antispasmodic, antibiotic, venous tonic |
| Flavonoids | Strengthen blood capillaries |
| Saponin | Relaxes bowel, diuretic, analgesic |
4.4. VOLATILE OILS
Volatile or ethereal oils are defined as odorous volatile principles of plant and animal origin which evaporate when exposed to air at ordinary temperature. They are basically a mixture of hydrocarbon terpenes, sesquiterpenes, polyterpenes, and their oxygenated derivatives. Volatile oils carry the plant’s active constituents and hence are also called essential oils.
4.4.1. Classification of Volatile Oils
- Hydrocarbon Volatile Oil: Limonene, α-Pinene
- Esters Volatile Oil: Methyl salicylate
- Ether Volatile Oil: Anethole
- Alcohol Volatile Oils: Citronellol, Menthol
- Aldehyde Volatile Oil: Citral
- Ketone Volatile Oil: Camphor
- Oxide Volatile Oil: Cineole
- Phenol Volatile Oil: Eugenol
4.4.2. Occurrence and Distribution of Volatile Oils
Aromatic plants occur in over 60 families, including Lauraceae, Myrtaceae, Umbelliferae, Labiatae, and Compositae. The oil present in them ranges from a very small amount to as much as 1-2%.
4.4.3. Isolation of Volatile Oils
- Steam Distillation: Most commonly used method
- Scarification: For lemon, orange, bergamot oils (Sponge process, Ecuelle process)
- Extraction by Non-volatile Solvents: Enfleurage, Maceration, Spraying
- Extraction by Volatile Solvents: For thermolabile oils
4.4.4. Identification Tests for Volatile Oils
- Sudan III Test: Red colour (volatile oils)
- Tincture of Alkane Test: Red colour (volatile oils)
4.4.5. Therapeutic Activity of Volatile Oils
- Biochemical: Chemical interaction with hormones and enzymes
- Physiological: Participation in specific physiological functions
- Psychological: Triggers olfactory area of brain; mental and emotional changes
4.4.6. Pharmaceutical Applications of Volatile Oils
- Carminative (umbelliferous fruits)
- Anthelmintic (chenopodium oil)
- Diuretic (juniper)
- Antiseptic (eucalyptus)
- Counter-irritant (oil of wintergreen)
- Local anaesthetic (clove)
- Sedative (jatamansi)
- Insect repellent (citronella)
- Source of vitamin A (lemongrass)
4.5. TANNINS
Tannins are complex substances that usually occur as mixtures of polyphenols that are very difficult to separate since they do not crystallise. The term tannin was first used by Seguin in 1976 to denote substances present in plant extracts which are able to combine with proteins of animal hides, prevent their putrefaction, and convert them into leather.
4.5.1. Classification of Tannins
- True Tannins: High molecular weight (1000-5000); precipitated by gelatin in 1% solution
- Hydrolysable Tannins: Gallitannins (gallic acid), Ellagitannins (hexahydroxydiphenic acid)
- Condensed Tannins: Resistant to hydrolysis; derived from flavanol, catechins
- Complex Tannins: Biosynthesised from both hydrolysable and condensed tannins
- Pseudotannins: Lower molecular weight; do not respond to Goldbeater’s skin test
4.5.2. Occurrence and Distribution of Tannins
Gallitannins: Rhubarb, red rose petals, bearberry leaves, Chinese galls, Turkish gall
Ellagitannins: Pomegranate rind, myrobalans, eucalyptus leaves, oak bark
Condensed Tannins: Barks (cinnamon, cinchona, willow, acacia), roots (krameria, male fern), flowers (lime, hawthorn), seeds (cocoa, guarana), leaves (hamamelis, tea)
4.5.3. Identification Tests for Tannins
- Goldbeater’s Skin Test: Brown or black colour (tannins)
- Gelatin Test: White buff-coloured precipitate
- Phenazone Test: Bulky coloured precipitate
- Catechin Test (Matchstick Test): Pink or red colour (phloroglucinol)
- Chlorogenic Acid Test: Green colour with aqueous ammonia
- Vanillin-Hydrochloric Acid Test: Pink or red colour
4.5.4. Therapeutic Activity of Tannins
- Styptics: Precipitate proteins; protect inflamed surfaces
- Anti-diarrhoeal: Antidotes in poisoning by heavy metals, alkaloids, glycosides
- Anti-tumour: Ellagitannins exhibit strong anti-tumour activity
- Antidote: Form insoluble tannates with alkaloids (non-toxic)
4.5.5. Pharmaceutical Applications of Tannins
- Myrobalan (Harde): Astringent, stomachic, purgative; ingredient of Triphala Churna
- Bahera: Astringent; ingredient of Triphala Churna
- Arjuna: Cardiotonic and hypotensive
- Ashoka Bark: Uterine tonic and oxytocic
- Black Catechu (Cutch): Astringent for boils, skin eruptions
4.6. RESINS
Resins can be defined as the hydrocarbon secretion of various plants (especially the coniferous trees). They are considered valuable because of their chemical constituents and various uses—adhesives, varnishes, perfumes, incense, nail polish, etc. Fossilised resins act as a source of amber.
4.6.1. Classification of Resins
- Balsams: Contain esters of cinnamic and benzoic acids (Tolu balsam, Benzoin, Peru balsam, Storax)
- Oleoresins: Occur with volatile oils (Turpentine, Capsicum, Ginger, Male fern)
- Gum Resins: Occur with gums (Asafoetida, Gambage, Myrrh)
- Oleo-Gum Resins: Occur with gums and volatile oils (Myrrh, Frankincense)
- Glycoresins (Glucoresins): Naturally combined with glycosides (Ipomoea, Scammony, Jalap, Podophyllum)
4.6.2. Identification Tests for Resins
- Colophony Resin: Purple violet colour with acetic anhydride + sulphuric acid; Emerald green colour with copper acetate
- Guaiacum Resin: Deep blue colour with ferric chloride
4.6.3. Therapeutic Activity of Resins
- Podophyllum: Purgative
- Colocynth, Gamboge, Ipomoea: Cathartic
- Asafoetida: Laxative
- Jalap: Hydragogue
- Cannabis: Sedative
- Capsicum and Turpentine: Counter-irritant
- Aspidium: Anthelmintic
- White Pine, Copaiba, Storax, Tolu Balsam, Benzoin: Expectorants
Dpharmguru’s exam insights:
Identification tests for secondary metabolites are frequently tested. Remember: Alkaloids use Mayer’s, Dragendorff’s, Wagner’s reagents. Glycosides use Borntrager’s (anthraquinone), Keller-Kiliani (cardiac), Libermann-Burchard (steroids). Tannins use Goldbeater’s skin test, Gelatin test. Volatile oils use Sudan III test. Resins use acetic anhydride + sulphuric acid for colophony. These are classic exam questions!
FREQUENTLY ASKED QUESTIONS (FAQs)
1. What are secondary metabolites?
Secondary metabolites are organic compounds synthesised from primary metabolites. They are present in small quantities and protect plants against microorganisms and herbivores. Examples include alkaloids, terpenoids, glycosides, volatile oils, tannins, and resins.
2. What is the difference between true, proto, and pseudo alkaloids?
True alkaloids are derived from amino acids and contain nitrogen in a heterocyclic ring (e.g., morphine). Proto alkaloids have nitrogen not in a heterocyclic ring (e.g., ephedrine). Pseudo alkaloids are not derived from amino acids (e.g., caffeine).
3. What is the difference between hydrolysable and condensed tannins?
Hydrolysable tannins can be hydrolysed by acids or enzymes and are composed of gallic acid or hexahydroxydiphenic acid units. Condensed tannins are resistant to hydrolysis and are derived from flavanol and catechins.
4. What is the Keller-Kiliani test used for?
The Keller-Kiliani test is used to detect cardiac glycosides. A reddish-brown layer is formed which upon standing changes to bluish-green, confirming the presence of digitoxose.
5. What is the difference between volatile oils and fixed oils?
Volatile oils evaporate at ordinary temperature and have a characteristic odour (e.g., clove oil, peppermint oil). Fixed oils do not evaporate and leave a permanent stain (e.g., olive oil, castor oil).
6. What are the different types of resins?
Resins are classified as balsams, oleoresins, gum resins, oleo-gum resins, and glycoresins based on their occurrence in combination with other compounds.
SUMMARY
Secondary metabolites are essential natural products with significant therapeutic and pharmaceutical applications. This guide covered:
- Alkaloids: Basic nitrogenous compounds; true, proto, and pseudo types; identified by Mayer’s, Dragendorff’s, Wagner’s reagents
- Terpenoids: Volatile aromatic substances; classified by number of carbon atoms (mono-, sesqui-, di-, tri-, tetra-, polyterpenoids)
- Glycosides: Sugar + non-sugar moiety; classified by glycone, aglycone, and glycosidal linkage; identified by Borntrager’s, Keller-Kiliani, Libermann-Burchard tests
- Volatile Oils: Evaporate at ordinary temperature; classified as hydrocarbon, ester, ether, alcohol, aldehyde, ketone, oxide, phenol types
- Tannins: Polyphenols that precipitate proteins; hydrolysable, condensed, and complex types; identified by Goldbeater’s skin test, Gelatin test
- Resins: Hydrocarbon secretions; balsams, oleoresins, gum resins, oleo-gum resins, glycoresins
As I always tell my students: “Secondary metabolites are nature’s gift to medicine. Understanding their chemistry, isolation, and therapeutic applications is the key to unlocking the healing power of plants.”
REFERENCES AND FURTHER READING
- Pharmacy Council of India (PCI). (2022). Pharmacognosy Syllabus. New Delhi: PCI.
- Tyler, V. E., Brady, L. R., & Robbers, J. E. (2020). Pharmacognosy and Pharmacobiotechnology (9th ed.). Wolters Kluwer.
- Trease, G. E., & Evans, W. C. (2019). Trease and Evans’ Pharmacognosy (16th ed.). Elsevier.
- Kokate, C. K., Purohit, A. P., & Gokhale, S. B. (2020). Pharmacognosy (50th ed.). Nirali Prakashan.
- Bruneton, J. (2020). Pharmacognosy, Phytochemistry, Medicinal Plants (3rd ed.). Lavoisier.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns. Pharmaceutical regulations and guidelines may vary by region—always refer to your local regulatory authorities for specific requirements.
written by:
Dr. N. Sujith Kumar
For More Study Materials, Visit: www.dpharmguru.com
