Alkaloid

Chemical structure of ephedrine, a phenethylamine alkaloid

An alkaloid is a nitrogen-containing naturally occurring compound, produced by a large variety of organisms, including fungi, plants, animals, and bacteria. Many alkaloids have pharmacological effects on humans and other animals. The name derives from the word alkaline; originally, the term was used to describe any nitrogen-containing base. Alkaloids are part of the group of natural products (also called secondary metabolites) and are chemical derivatives of amino acids or other nitrogen-containing compounds, such as polyamines. Some alkaloids may have a bitter taste. Produced in dedicated biosynthetic pathways consisting of multiple enzyme -catalysed steps, such as prenylation, methylation/demethylation, and various oxidation/reduction reactions, alkaloids are found in plants (e.g. potatoes and tomatoes), animals (e.g. shellfish) and many fungi. Many alkaloids can be purified from crude extracts by acid-base extraction. While many alkaloids are poisonous, some are used medicinally as analgesics (pain relievers) or anaesthetics, particularly morphine and codeine, and for other uses.

Caffeine

Alkaloid classifications

Alkaloids are usually classified by their common molecular precursors, 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), emetine, berbamine, oxyacanthine
• Phenethylamine group: mescaline, ephedrine, dopamine, amphetamine
• Indole group:
  • Tryptamines: DMT, N-methyltryptamine, psilocybin, serotonin
  • Ergolines: the ergot alkaloids (ergine, ergotamine, lysergic acid, LSD etc.)
  • Beta-carbolines: harmine, harmaline, yohimbine, reserpine
  • Rauwolfia alkaloids: Reserpine
• Purine group:
  • Xanthines: caffeine, theobromine, theophylline
• Terpenoid group:
  • Aconite alkaloids: aconitine
  • Steroids: solanine, samandaris (quaternary ammonium compounds): muscarine, choline, neurine
• Vinca alkaloids: vinblastine, vincristine. They are antineoplastic and binds free tubulin dimers thereby disrupting balance between microtuble polymerization and delpolymerization resulting in arrest of cells in metaphase.
• Miscellaneous: capsaicin, cynarin, phytolaccine, phytolaccotoxin

Physicochemical Properties of Alkaloids

• colourless,crystalline,non-volatile solids that are insoluble in water but soluble in organic solvents such as ethanol,ether,chloroform,etc.
• Molecular weight 100-900
• Those without oxygen atoms in the structure are usually liquid at ordinary temperature (e.g. nicotine, sparteine, coniine)
• Those with oxygen atoms are crystalline (e.g. berberine -> yellow)
• Most are optically active (usually laevorotatory), but not those from the purine group
• Basicity depends on the availability of lone pairs of electrons on the N atom & the type of heterocycle and substituents.
• Ability to form salts with mineral acids (e.g. with HCl, H₂SO₄, HNO₃) or organic acids (e.g. tartrates, sulfamates, maleates)

References

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Alkaloid

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