Alkaloids: Definition, Characteristics, Classification
Alkaloids are defined as a group of naturally occurring chemical compounds that mostly contain basic nitrogen atoms. The term "alkaloid" historically refers to substances with properties resembling alkalis. However, not
By Dr. Khalid Mehmood, MD1 min read
Reviewed by Medical Review Team ·
Alkaloids are defined as a group of naturally occurring chemical compounds that mostly contain basic nitrogen atoms. mostly found in plants, that usually have a marked physiological (pharmacological) effect on humans and animals, even in small doses. They are typically basic in nature (because of the nitrogen), often bitter-tasting, and usually derived from amino acids.. They have a unique property – an ability to work as either hydrogen-acceptor or hydrogen-donor for hydrogen bonding, depending on the type of amine functionality present in alkaloids.
Examples of alkaloids
- Morphine
- Quinine
- Atropine
- Scopolamine
- Nicotine
- Caffeine
- Cocaine
- Reserpine
General Characteristics of Alkaloids
Most alkaloids have the following characteristics:
- They contain nitrogen.
- Many have a heterocyclic nitrogen-containing ring.
- They are commonly produced by plants as secondary metabolites.
- Many have a pronounced effect on the human body.
- Many alkaloids are basic, although important exceptions exist.
- Their free bases are often soluble in organic solvents, while their salts are generally more soluble in water.
- They may occur in plants as salts of organic acids.
- Many are pharmacologically active even in relatively small quantities.
- They can be identified by chemical and chromatographic methods.
Important point:
Do not write that all alkaloids are strongly basic. Purine alkaloids such as caffeine are weakly basic or essentially neutral in ordinary acid-base behavior.
| Class | Biosynthetic Precursor | Examples |
|---|---|---|
| Pyrrolidine alkaloids | Ornithine | Hygrine |
| Tropane alkaloids | Ornithine | Atropine, Hyoscine, Cocaine |
| Pyridine–Piperidine alkaloids | Lysine | Nicotine, Lobeline |
| Quinoline alkaloids | Tryptophan | Quinine, Quinidine |
| Isoquinoline alkaloids | Tyrosine | Morphine, Codeine, Papaverine |
| Indole alkaloids | Tryptophan | Reserpine, Vincristine, Ergot alkaloids |
| Imidazole alkaloids | Histidine | Pilocarpine |
| Steroidal alkaloids | Cholesterol (not amino acid) | Solanidine |
| Purine alkaloids | Purine nucleotide pathway (not amino acid) | Caffeine, Theobromine, Theophylline |
