Alkaloid: Wikis

  
  

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From Wikipedia, the free encyclopedia

Chemical structure of ephedrine, a phenethylamine alkaloid

Alkaloids are naturally occurring chemical compounds containing basic nitrogen atoms.[1] The name derives from the word alkaline and is used to describe any nitrogen-containing base and organic compounds with one or more of the following features: a heterocyclic compound containing nitrogen, with an alkaline pH and a marked physiological action on animal physiology. However, There are exceptions to each of these criteria. 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, as recreational drugs, or in entheogenic rituals. Examples are the local anesthetic and stimulant cocaine, the stimulant caffeine, nicotine, the analgesic morphine, or the antimalarial drug quinine. Most alkaloids have a bitter taste.

Caffeine, a Purine alkaloid
Chemical structure of Vinblastine, a Chemotheraputic alkaloid isolated from the Madagascar periwinkle

Contents

Alkaloid classifications

The classification of the alkaloids is complex and may be guided by a set of rules that take into account the structure and other chemical features of the alkaloid molecule, its biological origin, as well as the biogenetic origin where known.[2][3] For example, where the biosynthesis pathway of an alkaloid is unknown, it may be grouped based on structural similarities with known compounds, including non-nitrogenous compounds, or by the organism(s) from which the alkaloid was isolated.[3]

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.

With the exception of caffeine, alkaloids are one of several classes of compound which are generally precipitated from aqueous solutions by tannins, a fact which is important in herbalism.

See also

References

  1. ^ International Union of Pure and Applied Chemistry (1995). "alkaloids". Compendium of Chemical Terminology Internet edition.
  2. ^ Rogers MF, Wink M. (1998). Alkaloids: biochemistry, ecology, and medicinal applications. Plenum Press. pp. 2–3. ISBN 0-306-45465-3.  
  3. ^ a b Hesse M. (2005). Alkaloids: nature's curse or blessing?. Plenum Press. pp. 11–114. ISBN 0-306-45465-3.  
  4. ^ http://www.ansci.cornell.edu/plants/toxicagents/steroid.html,

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ALKALOID, in chemistry, a term originally applied to any organic base, i.e. a nitrogenous substance which forms salts with acids; now, however, it is usual to restrict the term to bases of vegetable origin and characterized by remarkable toxicological effects. Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids. They may be extracted by exhausting the plant-tissues with a dilute acid, and precipitating the bases with potash, soda, lime or magnesia. The separation of the mixed bases so obtained is effected by repeated fractional crystallization, or by taking advantage of certain properties of the constituents.

A chemical classification of alkaloids is difficult on account of their complex constitution. I. A. Wyschnegradsky, and afterwards W. Konigs, expressed the opinion that the alkaloids were derivatives of pyridine or quinoline. This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known. The purely chemical literature on the alkaloids is especially voluminous; and from the assiduity with which the constitutions of these substances have been and are still being attacked, we may conclude that their synthesis is but a question of time. Piperine, conine, atropine, belladonine, cocaine, hyoscyamine and nicotine have been already synthesized; the constitution of several others requires confirmation, while there remain many important alkaloids - quinine, morphine, strychnine, &c. - whose constitution remains unknown.

The following classification is simple and convenient; the list of alkaloids makes no pretence at being exhaustive.

(1) Pyridine group. Piperine; conine; trigonelline; arecaidine; guvacine; pilocarpine; cytisine; nicotine; sparteine.

(2) Tropine group. Alkaloids characterized by containing the tropine nucleus. Atropine; cocaine; hygrine; ecgonine; pelletierine.

(3) Quinoline group. The alkaloids of the quina-barks: quinine, &c.; the strychnos bases: strychnine, brucine; and the veratrum alkaloids: veratrine, cevadine, &c.

(4) Isoquinoline group. The opium alkaloids: morphine, codeine, thebaine, papaverine, narcotine, narceine, &c.; and the complicated substances hydrastine and berberine.

In addition to the above series there are a considerable number of compounds derived from purin which are by some writers classed with the alkaloids. These are treated in the article Purin. There are also reasons for including such compounds as muscarine, choline, neurine and betaine in this group.

The greater number of these substances are of considerable medicinal value; this aspect is treated generally in the article Pharmacology. Reference should also be made to the articles on the individual alkaloids for further details as to their medicinal and chemical properties.

The chemistry of the alkaloids is treated in detail by Ame Pictet in his La Constitution chimique des alcaloides vegetaux (Paris, 1897); enlarged and translated by H. C. Biddle with the title The Vegetable Alkaloids (New York, 1904); and by J. W. Bruhl, E. Hjelt, and O. Aschan: Die Pflanzen-Alkaloide (two). A pamphlet, Die Alkaloidchemie in den Jahren 1900-1904, by Julius Schmidt, may also be consulted.


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Simple English

[[File:|thumb|Chemical structure of ephedrine, a phenethylamine alkaloid]] [[File:|180px|thumb|Caffeine]]

Alkaloids are chemical compounds that can be made naturally. They contain basic nitrogen atoms.[1] The name comes from the word alkaline and was used to describe any alkaline containing nitrogen. Alkaloids are made by a large variety of organisms, such as bacteria, fungi, plants, and animals and are part of the group of natural products (also called secondary metabolites). Many alkaloids can be purified from basic substances by acid-base extraction. Many alkaloids are toxic to other organisms. Some alkaloids have a bitter taste.

Well-known alkaloids

References

  1. International Union of Pure and Applied Chemistry (1995). "alkaloids". Compendium of Chemical Terminology Internet edition.









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