Dictionary:

alkaloid

  (ăl'kə-loid')

Any of various organic compounds normally with basic chemical properties and usually containing at least one nitrogen atom in a heterocyclic ring, occurring chiefly in many vascular plants and some fungi. Many alkaloids, such as nicotine, quinine, cocaine, and morphine, are known for their poisonous or medicinal attributes.

[ALKAL(I) + –OID.]

A cyclic organic compound that contains nitrogen in a negative oxidation state and is of limited distribution among living organisms. Over 10,000 alkaloids of many different structural types are known; and no other class of natural products possesses such an enormous variety of structures. Therefore, alkaloids are difficult to differentiate from other types of organic nitrogen-containing compounds.

Simple low-molecular-weight derivatives of ammonia, as well as polyamines and acyclic amides, are not considered alkaloids because they lack a cyclic structure in some part of the molecule. Amines, amine oxides, amides, and quaternary ammonium salts are included in the alkaloid group because their nitrogen is in a negative oxidation state (the oxidation state designates the positive or negative character of atoms in a molecule). Nitro and nitroso compounds are excluded as alkaloids. The almost-ubiquitous nitrogenous compounds, such as amino acids, amino sugars, peptides, proteins, nucleic acids, nucleotides, prophyrins, and vitamins, are not alkaloids. However, compounds that are exceptions to the classical-type definition (that is, a compound containing nitrogen, usually a cyclic amine, and occurring as a secondary metabolite), such as neutral alkaloids (colchicine, piperine), the β-phenyl-ethylanines, and the purine bases (caffeine, theophylline, theobromine), are accepted as alkaloids.

Alkaloids often occur as salts of plant acids such as malic, meconic, and quinic acids. Some plant alkaloids are combined with sugars, for example, solanine in potato (Solanum tuberosum) and tomatine in tomato (Lycopersicum esculentum). Others occur as amides, for example, piperine from black pepper (Piper nigrum), or as esters, for example, cocaine from coca leaves (Erythroxylum coca). Still other alkaloids occur as quaternary salts or tertiary amine oxides.

While most alkaloids have been isolated from plants, a large number have been isolated from animal sources. They occur in mammals, anurans (frogs, toads), salamanders, arthropods (ants, millipedes, ladybugs, beetles, butterflies), marine organisms, mosses, fungi, and certain bacteria.

Many alkaloids exhibit marked pharmacological activity, and some find important uses in medicine. Atropine, the optically inactive form of hyoscyamine, is used widely in medicine as an antidote to cholinesterase inhibitors such as physostigmine and insecticides of the organophosphate type; it is also used in drying cough secretions. Morphine and codeine are narcotic analgesics, and codeine is also an antitussive agent, less toxic and less habit-forming than morphine. Colchicine, from the corms and seeds of the autumn crocus, is used as a gout suppressant. Caffeine, which occurs in coffee, tea, cocoa, and cola, is a central nervous system stimulant; it is used as a cardiac and respiratory stimulant and as an antidote to barbiturate and morphine poisoning. Emetine, the key alkaloid of ipecac root (Cephaelis ipecacuanha), is used in the treatment of amebic dysentery and other protozoal infections. Epinephrine or adrenaline (see structure), produced in most animal species by the adrenal medulla, is used as a

Naturally occurring organic bases which have marked pharmacological actions in man and other animals. Many are found in plant foods, including potatoes and tomatoes (the Solanum alkaloids), or as the products of fungal action (e.g. ergot), although they also occur in animal foods (e.g. tetrodotoxin in puffer fish, tetramine in shellfish).

Any one of the many nitrogen-containing organic bases derived from plants. The alkaloids are bitter and physiologically active. A number are useful therapeutic agents.

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A member of a group of nitrogen-containing compounds that include cocaine, morphine, and nicotine.

One of a large group of small organic compounds, mainly derived from amino acids, and containing nitrogen, found in plants. They are water-soluble, usually bitter in taste and are characterized by powerful physiological activity. Examples are morphine, cocaine, atropine, quinine, nicotine and caffeine. The term is also applied to synthetic substances that have structures similar to plant alkaloids, such as procaine. When treated with acids they are converted to water-soluble salts. In cases of poisoning by alkaloids the recommended antidote is tannic acid, but heavy metal salts and iodine also precipitate them. Includes pyrrolizidine and solanaceous alkaloids.

A plant compound containing nitrogen that, possibly because of its bitter taste, defends plants against predators. Alkaloids also include plant poisons or medicines. Nicotine, cocaine, and quinine are examples of plant alkaloids.

Chemical structure of ephedrine, a phenethylamine alkaloid

An alkaloid is, strictly speaking, a naturally occurring amine produced by a plant, but amines produced by animals and fungi are also called alkaloids^[1]. 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 usually derivatives of amino acids, and many have a bitter taste. They are found as secondary metabolites in plants (e.g. potatoes and tomatoes), animals (e.g. shellfish) and 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.

Alkaloid classifications

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:
  • 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.

Physicochemical Properties of Alkaloids

• 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)
• Most are optically active, 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

1. ^ Carey, Francis A. [1987] (2006). Organic Chemistry, 6th ed., New York: McGraw Hill, 954. ISBN 0-07-282837-4. “(Strictly speaking, these compounds are not classified as alkaloids because they are not isolated from plants.)” 

Major families of biochemicals
Peptides | Amino acids | Nucleic acids | Carbohydrates | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides
Analogues of nucleic acids:	Types of Alkaloids	Analogues of nucleic acids:
General:
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
Pyridine group:	Coniine
Pyrrolidine group:	Nicotine
Quinoline group:	Quinine
Isoquinoline group:	Codeine | Morphine
Tropane group:	Atropine | Cocaine | Hyoscyamine | Scopolamine
Terpenoid group:	Aconitine | Solanine
Betaines:	Choline | Muscarine
Pyrazole group:

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