alkaloid

alkaloid, any of a class of organic compounds composed of carbon, hydrogen, nitrogen, and usually oxygen that are often derived from plants. Although the name means alkalilike, some alkaloids do not exhibit alkaline properties. Many alkaloids, though poisons, have physiological effects that render them valuable as medicines. For example, curarine, found in the deadly extract curare, is a powerful muscle relaxant; atropine is used to dilate the pupils of the eyes; and physostigmine is a specific for certain muscular diseases. Narcotic alkaloids used in medicine include morphine and codeine for the relief of pain and cocaine as a local anesthetic. Other common alkaloids include quinine, caffeine, nicotine, strychnine, serotonin, and LSD. Aconitine is the alkaloid of aconite. Cinchonine and quinine are derived from cinchona, coniine is found in poison hemlock, and reserpine is an extract of rauwolfia roots. Emetine is an alkaloid of ipecac.

Basic (see base) organic compounds of plant origin, containing combined nitrogen. Alkaloids are amines, so their names usually end in “ine” (e.g., caffeine, nicotine, morphine, quinine). Most have complex chemical structures of multiple ring systems. They have diverse, important physiological effects on humans and other animals, but their functions in the plants that produce them are poorly understood. Some plants (e.g., opium poppy, ergot fungus) produce many different alkaloids, but most produce only one or a few. Certain plant families, including the poppy family (Papaveraceae) and the nightshade family (Solanaceae), are particularly rich in them. Alkaloids are extracted by dissolving the plant in dilute acid.

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''This article is about the chemical compounds alkaloids. For the pharmaceutical company in the Republic of Macedonia see Alkaloid (company).

Alkaloids are naturally occurring chemical compounds containing basic nitrogen atoms. The name derives from the word alkaline and was used to describe any nitrogen-containing base. Alkaloids are produced by a large variety of organisms, including bacteria, fungi, plants, and animals and are part of the group of natural products (also called secondary metabolites). Many alkaloids can be purified from crude extracts by acid-base extraction. Many alkaloids are toxic to other organisms. They often have pharmacological effects and are used as medications and recreational drugs. Examples are the local anesthetic and stimulant cocaine, the stimulant caffeine, nicotine, the analgesic morphine, or the antimalarial drug quinine. Some alkaloids have a bitter taste.

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, arecoline, arecaidine, guvacine, cytisine, lobeline, nicotine, anabasine, 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: opium alkaloids (papaverine, narcotine, narceine), sanguinarine, hydrastine, berberine, emetine, berbamine, oxyacanthine
• Phenanthrene alkaloids: The opium alkaloids (morphine, codeine, thebaine)
• Phenethylamine group: mescaline, ephedrine, dopamine
• Indole group:
• Tryptamines: serotonin, DMT, 5-MeO-DMT, bufotenine, psilocybin
• Ergolines (the ergot alkaloids): ergine, ergotamine, lysergic acid, LSD
• Beta-carbolines: harmine, harmaline, tetrahydroharmine
• Yohimbans: reserpine, yohimbine
• Vinca alkaloids: vinblastine, vincristine
• Mitragyna speciosa alkaloids: mitragynine, 7-hydroxymitragynine
• Tabernanthe iboga alkaloids: ibogaine, voacangine, coronaridine, 18-methoxycoronaridine
• Strychnos nux-vomica alkaloids: strychnine, brucine
• Purine group:
• Xanthines: caffeine, theobromine, theophylline
• Terpenoid group:
• Aconite alkaloids: aconitine
• Steroid alkaloids (containing a steroid skeleton in a nitrogen containing structure):
• solanum (e.g. potato and tomato) alkaloids (solanidine, solanine, chaconine)
• veratrum alkaloids (veratramine, cyclopamine, cycloposine, jervine, muldamine)
• newt alkaloids (samandarin)
• others conessine
• Quaternary ammonium compounds: muscarine, choline, neurine
• Miscellaneous: capsaicin, cynarin, phytolaccine, phytolaccotoxin

Physicochemical properties

Low-molecular weight alkaloids without hydrogen bond donors such as hydroxy groups are often liquid at room temperature, examples are nicotine, sparteine, coniine, and phenethylamine.

The basicity of alkaloids depends on the lone pairs of electrons on their nitrogen atoms. As organic bases, alkaloids form salts with mineral acids such as hydrochloric acid and sulfuric acid and organic acids such as tartaric acid or maleic acid. These salts are usually more water-soluble than their free base form.

See also

• Amine
• Base (chemistry)
• Natural products
• Secondary metabolite

References