Alkaloids (word formation from Arabic القلية, DMG al-qalya 'the plant ash ' and Greek -oides : "similar") are naturally occurring, chemically heterogeneous, mostly alkaline , nitrogen-containing organic compounds of the secondary metabolism that affect the animal, including the human organism.
Over 10,000 different plant, animal or microorganism-produced substances are assigned to this group of substances. The term was first used in 1819 for "alkali-like" plant substances such as morphine , strychnine and solanine . However, a satisfactory and generally accepted definition for the term “alkaloid” does not yet exist.
The term alkaloid was originally a general term for basic substances isolated from plants. Alkaloids were later isolated from other organisms such as fungi (e.g. ergot alkaloids ) and animals (e.g. the toad alkaloid bufotenin ), so the definition was expanded. Furthermore, the term alkaloid was partly extended to include some non-basic natural substances , such as the autumn crocus alkaloid colchicine , the capsaicin of cayenne pepper or the purine alkaloids caffeine , theophylline and theobromine .
All alkaloids have a characteristic effect on the animal and human organism. Most are poisonous and have a bitter taste. The basicity is a common but not essential characteristic of this group of substances. Basic antibiotics , such as streptomycin , which act on microorganisms , are usually not referred to as alkaloids. In addition, all alkaloids are products of secondary metabolism. They are not required for the basic metabolism of the organisms that make them, but often serve as protection against predators. Basic amino acids or peptides , which are primary metabolic products, are therefore not assigned to the alkaloids.
Some strict definitions require that nitrogen be part of a heterocyclic system that is biogenetically derived from amino acids. This definition separates, for example, the protoalkaloids ephedrine and mescaline and the pseudoalkaloids coniine and caffeine from the family of alkaloids. As an artificial racemization product of ( S ) -hyoscyamine, atropine is also not an alkaloid according to a strict interpretation.
The first isolated alkaloid was morphine , which Friedrich Wilhelm Sertürner extracted in pure form from opium poppies in 1804 . The artificial word "alkaloids" was introduced in 1819 by the Halle pharmacist Carl Friedrich Wilhelm Meißner . Emanuel Merck began the industrial production of alkaloids in Darmstadt in 1827 . In 1885 the chemist August Wilhelm von Hofmann succeeded for the first time in the structure elucidation and synthesis of an alkaloid, coniin.
Alkaloids can be divided according to different criteria. Categorizations according to chemical structure, origin, biogenesis and pharmacological effect are common.
Classification according to chemical structure
The most frequently used classification of alkaloids in the literature is the categorization according to their chemical structure. It is named after the part of the molecule that contains the nitrogen atom.
- Alkaloids with heterocyclic nitrogen
- Pyrrolidine alkaloids : e.g. B. Hygrin
- Pyrrolizidine alkaloids : e.g. B. Senezionine
- Steroid alkaloids : e.g. B. Solanine
- Pyridine alkaloids : e.g. B. nicotine , anabasine
- Piperidine alkaloids : e.g. B. piperine
- Tropane alkaloids : e.g. B. hyoscyamine , scopolamine , cocaine
- Quinoline alkaloids : e.g. B. quinine , quinidine
- Isoquinoline alkaloids : e.g. B. morphine , codeine , papaverine , berberine , tubocurarine
- Indole alkaloids : e.g. B. Ajmaline , Ergotamine , Yohimbine , Reserpine , Strychnine
- Purine alkaloids : e.g. B. caffeine , theophylline , theobromine
- Imidazole alkaloids : e.g. B. pilocarpine , cynodine , cynometrine , odiline
- Acridone alkaloids : e.g. B. acronycin , melicopicin , rutacridone
- Quinolizidine alkaloids : e.g. B. Lupine , Sparteine
- Benzylisoquinoline alkaloids : e.g. B. papaverine , reticulin , laudanosine
- Alkaloids with acyclic nitrogen: e.g. B. ephedrine , mescaline
Classification according to origin
Alkaloids can also be named and classified according to characteristic organisms that produce them or the drugs in which they are contained.
- Curare alkaloids : e.g. B. Toxiferin , Tubocurarine , Alcuronium
- Ergot alkaloids : e.g. B. Ergotamine , Ergometrine
- Opiates : e.g. B. morphine , codeine , thebaine , papaverine , noscapine , cryptopine
- Vinca alkaloids : e.g. B. vincristine , vinblastine
- Lobelia alkaloids : e.g. B. lobeline , Lelobanidin , Lobelanidin
- Strychnos alkaloids : e.g. B. akuammicin , brucine , strychnine
- Catharanthus alkaloids : e.g. B. catharanthine , vindoline
- Amaryllidacean alkaloids : e.g. B. Lycorin , Galantamine
- Dendrobates alkaloids : e.g. B. Histrionicotoxin , Pumiliotoxin A , Pumiliotoxin B
- Lupine alkaloids : e.g. B. Lupinine , Lupanine , Sparteine
- China alkaloids : e.g. B. quinine , quinidine
- Coca alkaloids : e.g. B. cocaine , ecgonine , hygrine
Classification according to biogenesis
A further classification of the alkaloids can be made on the basis of similarities and differences in biosynthesis . Alkaloids are classified according to the amino acids that are used as a nitrogen source in biosynthesis.
- Alkaloids derived from aspartic acid or lysine : e.g. B. Nicotine , Lupine
- Alkaloids derived from glycine : e.g. B. caffeine , theophylline , theobromine
- Alkaloids derived from histidine : e.g. B. pilocarpine
- Alkaloids derived from ornithine : e.g. B. hyoscyamine , scopolamine , cocaine
- Alkaloids derived from phenylalanine or tyrosine : e.g. B. colchicine , morphine , codeine , papaverine , tubocurarine , berberine
- Alkaloids derived from tryptophan : e.g. B. ergotamine , ergometrine , ajmaline , reserpine , strychnine
In contrast, the collective term terpene alkaloids, which include, for example, the diterpene alkaloid aconitine and the steroid alkaloids , indicate the biogenetic origin of the carbon skeleton from terpene metabolism .
Classification according to pharmacological effect
- According to pharmacological effect; z. B. Hallucinogens
The in biological material primarily in the form of its acetic , oxalic , lactic , tartaric , aconitic or quinic acid salts, rare in pure form or as esters present alkaloids are in conventional manner by extraction obtained. The method of extraction depends on the physicochemical properties of the alkaloids to be isolated. For most alkaloids, the use of the polar solvent methanol with the addition of acetic acid has proven itself. After evaporation and absorption in an aqueous acidic medium (e.g. dilute hydrochloric acid ), foreign components such as fats and other lipophilic substances can be removed by extraction with the lipophilic solvent ether . For the extraction of the alkaloids from the aqueous phase, chloroform has proven itself after alkalinization with sodium hydroxide solution or another suitable base. After drying and evaporation, the chloroform phase usually gives an alkaloid mixture which can preferably be separated by chromatography . To isolate lipidunlöslicher alkaloids having a quaternary ammonium structure can precipitation with picric or Reinecke acid are used.
In addition, a partial synthesis or chemical total synthesis is possible for numerous alkaloids . These are used in particular when they are cheaper than extraction. An example of alkaloids that are usually synthetically produced is theophylline .
Dragendorff's reagent is used as a classic detection reagent for alkaloids . In the presence of alkaloids it forms an ion pair with the composition [BiI 4 ] - [NHR 3 ] + , which, depending on the alkaloid, can be recognized as a yellow-orange to brown precipitate . Alternatively, Mayer's reagent (tetraiodomercurate), silicotungstic acid , Wagner's reagent (iodine-potassium iodide), Sonnenschein's reagent (phosphomolybdic acid), iodoplatinate and others are used as detection reagents. The selectivity of these detection reagents varies widely and is limited within the group of alkaloids. In order to increase the selectivity, they are usually combined with chromatographic methods such as thin-layer chromatography .
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