Alkaloids –
Complete Notes
for D.Pharmacy
Definition · Types · Isolation · Chemical Tests · Therapeutic Uses
Simple language, exam-ready!
📋 Table of Contents
Definition of Alkaloids
- They are basic (alkaline) in nature
- Nitrogen is present in a heterocyclic ring
- Derived from plants (mainly)
- Biosynthesised from amino acids
- Physiologically active (affect body systems)
Deviations from Definition
Not all alkaloids follow the exact definition. These exceptions are very important for exams!
| Property | Deviation | Examples |
|---|---|---|
| Basicity | Some are NOT basic | Colchicine, Piperine, Quaternary alkaloids |
| Nitrogen | N not in heterocyclic ring | Ephedrine, Colchicine, Mescaline |
| Plant Origin | From non-plant sources | Bacteria, Fungi, Insects, Frogs, Animals |
| Biosynthesis | Not from amino acids | Purine alkaloids, Steroidal alkaloids |
Basicity · Nitrogen · Plant origin · Biosynthesis
These are the 4 deviations — learn one example each!
Types of Alkaloids 🌟 Most Imp
Alkaloids are divided into 3 main types based on their structure and biosynthetic origin:
Proto → from amino acid but N NOT in ring (e.g., Ephedrine)
Pseudo → NOT from amino acid but N in ring (e.g., Caffeine)
| Type | From Amino Acid? | N in Ring? | Example |
|---|---|---|---|
| True | ✅ Yes | ✅ Yes | Atropine |
| Proto | ✅ Yes | ❌ No | Ephedrine |
| Pseudo | ❌ No | ✅ Yes | Caffeine |
Functions of Alkaloids in Plants
- Protection: Act as protective agents against insects and herbivores due to their bitterness and toxicity.
- Nitrogen Source: May act as a source of nitrogen during nitrogen deficiency in plants.
- Growth Regulators: Sometimes act as growth regulators in certain metabolic systems of the plant.
- Energy Source: May be utilized as a source of energy during deficiency in CO₂ assimilation.
Occurrence of Alkaloids
- Occur in bacteria — e.g., Pseudomonas aeruginosa
- Rarely in fungi — e.g., Psilocin from hallucinogenic mushrooms
- Rarely occur in lower plants (mosses, ferns)
- Dicots are more rich in alkaloids than monocots
- Rarely do plants contain more than 1 type of alkaloid
- All alkaloids of one plant have a common biogenetic origin
| Category | Plant Families |
|---|---|
| Rich in Alkaloids | Apocynaceae, Rubiaceae, Solanaceae, Papaveraceae |
| Free from Alkaloids | Rosaceae, Labiatae |
Distribution of Alkaloids
Alkaloids occur in all plant parts but are usually localized in one organ (bark, seeds, leaves, etc.).
| Plant Part | Example Drug / Plant |
|---|---|
| All parts | Datura |
| Bark | Cinchona |
| Seeds | Nux vomica |
| Roots | Aconite |
| Fruits | Black pepper |
| Leaves | Tobacco |
| Latex | Opium |
Isolation of Alkaloids
General method involves 4 steps:
Crushing / Powdering
Sample is crushed into very fine powder. Finer particles = greater surface area = better extraction.
Defatting — Remove Fats & Oils
Fats, terpenes and oils are removed using hexane or petroleum ether (non-polar solvents). These are then discarded.
Extraction of Alkaloids
Alkaloids are extracted using methanol or ethanol. Multiple extractions done. Solvent evaporated using Rotary Evaporator → crude alkaloid mixture obtained.
Purification
Methods used:
(i) Direct crystallisation from solvent
(ii) Steam distillation
(iii) Chromatographic techniques
(iv) Gradient pH techniques
Defatting solvent = Hexane / Petroleum ether (non-polar)
Extraction solvent = Methanol / Ethanol
Chemical Tests for Alkaloids 🌟 Most Imp
These 4 general tests are answered by ALL alkaloids. Very important for viva & practical exams!
Reagent: Potassium bismuth iodide solution
Procedure: Add few drops of Dragendorff's reagent to 2–3 mL of alkaloid solution.
Result: Orange-Brown Precipitate ✅Reagent: Potassium mercuric iodide solution
Procedure: Add few drops of Mayer's reagent to 2–3 mL of alkaloid solution.
Result: White / Cream Precipitate ✅Reagent: Saturated solution of Picric acid
Procedure: Add few drops of Hager's reagent to 2–3 mL of alkaloid solution.
Result: Yellow Precipitate ✅Reagent: Iodine–Potassium iodide solution
Procedure: Add few drops of Wagner's reagent to 2–3 mL of alkaloid solution.
Result: Reddish-Brown Precipitate ✅Mayer → White / Cream
Hager → Yellow
Wagner → Reddish-Brown
| Test | Reagent | Result / Colour |
|---|---|---|
| Dragendorff's | K bismuth iodide | Orange-Brown ppt |
| Mayer's | K mercuric iodide | White / Cream ppt |
| Hager's | Picric acid (sat.) | Yellow ppt |
| Wagner's | I₂–KI solution | Reddish-Brown ppt |
Therapeutic Activity & Pharmaceutical Uses
Important alkaloids with their pharmacological actions — frequently asked in exams!
Quinine = Antimalarial | Morphine = Analgesic
Colchicine = Gout | Atropine = Anticholinergic
Vinblastine = Anticancer
| Alkaloid | Therapeutic Action |
|---|---|
| Ajmaline | Antiarrhythmic |
| Atropine, Hyoscyamine, Scopolamine | Anticholinergic |
| Caffeine | Adenosine receptor antagonist, CNS Stimulant |
| Codeine | Analgesic, Antitussive (cough suppressant) |
| Colchicine | Remedy for Gout |
| Emetine | Emesis, Antiprotozoal |
| Ergot alkaloids | Vasoconstriction, Uterotonic, Hallucinogenic |
| Morphine | Strong Analgesic (narcotic) |
| Nicotine | Nicotinic ACh receptor agonist, Stimulant |
| Physostigmine | Acetylcholinesterase inhibitor |
| Quinidine | Antiarrhythmic |
| Quinine | Antimalarial, Antipyretic |
| Reserpine | Antihypertensive |
| Tubocurarine | Muscle relaxant |
| Vinblastine, Vincristine | Antitumour (Anticancer) |
| Vincamine | Antihypertensive, Vasodilating |
| Yohimbine | Aphrodisiac, Stimulant |
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