BP105T • Unit 5Topic 3 of 3⏱ 5 Min Read
3. Diagnostic Phytochemical Screening of Secondary Metabolites
💡 Core Concept & Clinical Context:
Secondary metabolites are detected through standardized chemical color and precipitation tests conducted on plant extracts. The major classes include Alkaloids (basic organic nitrogen-containing compounds), Glycosides (organic compounds yielding a sugar and non-sugar aglycone upon hydrolysis), Tannins (high molecular weight phenolic astringents that precipitate proteins), and Volatile Oils (steam-distillable aromatic terpenoids).
Clinical Scenarios & Pharmacopoeial Examples:
1Official Pharmacopoeial Colorimetric Screening Test Suite
Reagents and characteristic positive color reactions.
Step-by-Step Breakdown:
•A. ALKALOID CONFIRMATORY TESTS:
•1. Mayer's Reagent (Potassium mercuric iodide) -> Cream / Pale yellow precipitate.
•2. Dragendorff's Reagent (Potassium bismuth iodide) -> Orange-red / Reddish-brown precipitate.
•3. Wagner's Reagent (Iodine in potassium iodide) -> Reddish-brown precipitate.
•4. Hager's Reagent (Saturated aqueous picric acid) -> Crystalline yellow precipitate.
•B. GLYCOSIDE CONFIRMATORY TESTS:
•1. Borntrager's Test (Anthraquinone glycosides - Senna/Aloe):
•- Boil extract with dil. H2SO4, filter, extract with organic solvent (chloroform/benzene).
•- Separate organic layer and add dilute Ammonia solution -> Rose-pink to intense cherry-red color in ammoniacal layer.
•2. Keller-Kiliani Test (Deoxy-sugars in Cardiac glycosides - Digitalis):
•- Extract in glacial acetic acid + 1 drop FeCl3 + underlay with Conc. H2SO4 -> Reddish-brown ring at interface; upper acetic acid layer turns greenish-blue.
•3. Foam / Froth Test (Saponin glycosides):
•- Shake aqueous extract vigorously in test tube -> Stable, persistent honeycomb froth lasting > 15 minutes.
•4. Shinoda Test (Flavonoids):
•- Alcoholic extract + Magnesium ribbon turnings + Conc. HCl -> Intense pink, scarlet, or crimson red color.
•C. TANNIN CONFIRMATORY TESTS:
•1. Ferric Chloride (FeCl3) Test:
•- Hydrolyzable Tannins -> Blue-black color / precipitate.
•- Condensed Tannins (Catechol) -> Greenish-brown color.
•2. Goldbeater's Skin Test:
•- Ox-intestine membrane treated with 2% HCl, rinsed, soaked in tannin extract, rinsed, and treated with 1% FeSO4 -> True tannins stain membrane deep black.
•3. Gelatin Test: 1% Gelatin solution containing 10% NaCl added to tannin extract -> White buff-colored precipitate.
Comparison & Reference Table:
| Secondary Metabolite | Diagnostic Reagent / Test | Observation / Positive Reaction | Phytochemical Class |
|---|---|---|---|
| Alkaloids | Dragendorff's Reagent | Prominent Orange-Red Precipitate | Nitrogenous base (e.g. Quinine, Atropine) |
| Anthraquinones | Borntrager's Test | Cherry-Red color in upper Ammonia layer | Senna, Aloe, Rhubarb |
| Cardiac Glycosides | Keller-Kiliani Test | Greenish-Blue layer above Reddish-Brown ring | Digitalis purpurea (Digitoxose sugars) |
| Saponins | Froth / Foam Test | Persistent honeycomb foam (> 15 min) | Dioscorea, Glycyrrhiza, Senega |
| Flavonoids | Shinoda Test (Mg + HCl) | Crimson / Scarlet Red color | Rutin, Quercetin, Ginkgo biloba |
| Tannins | Goldbeater's Skin / Gelatin | Black staining / Gelatin precipitate | Catechu, Ashoka, Arjuna bark |
📋Key Takeaways & Exam Highlights
- Mnemonic for Alkaloid Reagents: MDWH -> Mayer (Cream), Dragendorff (Orange-red), Wagner (Reddish-brown), Hager (Yellow).
- Cardiac Glycosides: Aglycone (Steroid nucleus with unsaturated lactone ring) + Glycone (Digitoxose deoxy-sugar).
- Hydrolyzable Tannins (Gallotannins/Ellagitannins) yield gallic acid upon heating with mineral acids; Condensed Tannins (Proanthocyanidins) polymerize into insoluble red Phlobaphenes.
🇮🇳 हिन्दी व्याख्या (3. Diagnostic Phytochemical Screening of Secondary Metabolites)
अल्कलॉइड की जांच के लिए Dragendorff (नारंगी-लाल अवक्षेप) और Mayer (हल्का पीला) टेस्ट होते हैं। सेन्ना के लिए Borntrager टेस्ट (गुलाबी-लाल रंग), डिजिटलिस के लिए Keller-Kiliani टेस्ट, और सैपोनिन के लिए झाग (Froth Test) किया जाता है।