An alkaloid is, strictly speaking, an amine that is produced by a plant. However, the term has been extended to amines produced by animals and fungi as well. The name derives from the word alkaline; originally, the term was used to describe any nitrogen-containing base.
Alkaloids are usually derivatives of amino acids, and many have a bitter taste. They are found as secondary metabolites in plants (such as potatoes and tomatoes), animals (such as shellfish), and fungi. In many cases, they can be purified from crude extracts by acid-base extraction.
Various alkaloids have pharmacological effects on humans and animals. Many are poisonous, but some are used medicinally as analgesics (pain relievers) or anesthetics, particularly morphine and codeine. Some, such as vinblastine, are used to treat certain types of cancer.
Alkaloids are usually classified by their common molecular feat, based on the metabolic pathway used to construct the molecule. When not much was known about the biosynthesis of alkaloids, they were grouped under the names of known compounds, even some non-nitrogenous ones (since those molecules' structures appear in the finished product; the opium alkaloids are sometimes called "phenanthrenes," for example), or by the plants or animals they were isolated from. When more is learned about a certain alkaloid, the grouping is changed to reflect the new knowledge, usually taking the name of a biologically-important amine that stands out in the synthesis process.
- Pyridine group: piperine, coniine, trigonelline, arecaidine, guvacine, pilocarpine, cytisine, nicotine, sparteine, pelletierine
- Pyrrolidine group: hygrine, cuscohygrine, nicotine
- Tropane group: atropine, cocaine, ecgonine, scopolamine, catuabine
- Quinoline group: quinine, quinidine, dihydroquinine, dihydroquinidine, strychnine, brucine, veratrine, cevadine
- Isoquinoline group: The opium alkaloids (morphine, codeine, thebaine, Isopapa-dimethoxy-aniline, papaverine, narcotine, sanguinarine, narceine, hydrastine, berberine)
- Phenethylamine group: mescaline, ephedrine, dopamine, amphetamine
- Indole group:
- Tryptamines: DMT, N-methyltryptamine, psilocybin, serotonin
- Ergolines: the ergot alkaloids (ergine, ergotamine, lysergic acid, etc.)
- Beta-carbolines: harmine, harmaline, yohimbine, reserpine, emetine
- Rauwolfia alkaloids: Reserpine
- Purine group:
- Xanthines: caffeine, theobromine, theophylline
- Terpenoid group:
- Vinca alkaloids: vinblastine, vincristine. They are antineoplastic and binds free tubulin dimers thereby disrupting the balance between microtubule polymerization and depolymerization resulting in arrest of cells in metaphase.
Properties of alkaloids
- Their molecular weight ranges from 100 to 900 daltons.
- Those without oxygen atoms in their structure are usually liquid at ordinary temperature. Examples are nicotine, sparteine, and coniine.
- Those with oxygen atoms are crystalline, such as berberine.
- Most alkaloids are optically active, except for those from the purine group.
- Basicity depends on the availability of lone pairs of electrons on the N atom and the type of heterocycle and substituents in the molecule.
- Alkaloids have the ability to form salts with mineral acids (such as with HCl, H2SO4, HNO3) or organic acids (such as with tartaric acid and sulfamic acid).
- Francis A. Carey, Organic Chemistry, 6th ed. (New York: McGraw Hill,  2006, ISBN 0-07-282837-4), p. 954. (Strictly speaking, these compounds are not classified as alkaloids because they are not isolated from plants.)
- Barton, Derek, Koji Nakanishi, and Otto Meth-Cohn, (eds.). 1999. Comprehensive Natural Products Chemistry. Vol. 4. Amsterdam: Elsevier. ISBN 0080431569
- Fattorusso, Ernesto, and Orazio Taglialatela-Scafati. 2008. Modern Alkaloids: Structure, Isolation, Synthesis and Biology. Weinheim: Wiley-VCH. ISBN 978-3527315215
- Hesse, Manfred. 2002. Alkaloids: Nature's Curse or Blessing? Zurich: Verlag Helvetica Chimica Acta. ISBN 3906390241
|Major families of biochemicals|
|Peptides | Amino acids | Nucleic acids | Carbohydrates | Nucleotide sugars | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides|
|Analogues of nucleic acids:||Types of Alkaloids||Analogues of nucleic acids:|
|Indole group:||5-MeO-DMT | Dimethyltryptamine | Harmala alkaloids | Psilocin | Psilocybin | Reserpine | Serotonin | Tryptamine | Yohimbine|
|Phenethylamine group:||Amphetamine | Cathinone | Ephedrine | Mescaline | Methamphetamine | Phenethylamine | Tyramine|
|Purine group:||Caffeine | Theobromine | Theophylline|
|Isoquinoline group:||Codeine | Morphine|
|Tropane group:||Atropine | Cocaine | Hyoscyamine | Scopolamine|
|Terpenoid group:||Aconitine | Solanine|
|Betaines:||Choline | Muscarine|
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