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Alkaloids are a group of secondary metabolites that can be isolated mostly from higher plants. Today about 12,000 alkaloids are known. The definition and delimitation is controversial and difficult. Alkaline plant substances were historically known as alkaloids. The Römpp Lexikon Chemie called alkaloids as basic natural substances with one or more nitrogen atoms in the molecule that is mostly heterocylic. (“basische Naturstoffe mit einem oder mehreren, meist heterocyclischen eingebauten Stickstoffatomen im Molekül.”) In the book “Arzneidrogen” by Georg Schneider alkaloids are known as “nitrogen-containing secondary metabolites, which exists mostly as secondary and tertiary but also as primary amines”. (“stickstoffhaltige, sekundäre Naturstoffe, die als primäre, vor allem aber als sekundäre und tertiäre Amine […] vorkommen.”) Also their heterocyclic backbone is mentioned.
In addition, alkaloids are often biosynthesized from amino acids. The division into alkaloids is done with exceptions and is often grown historically.
Alkaloids can be gained from the plant by extraction. In this case, usually, the salts of amines can be isolated, which have improved solubility properties as the “pure” amines have (due to the increased polarity). The extraction works with organic solvents, especially with alcohols, such as ethanol or methanol. For the formation of the salt acetic acid, oxalic acid or lactic acid is added.
Due to the inconsistent classification general properties can always specify only with restrictions. Through their functionalisation as amines, they often act as bases and are easily soluble in water. They are often highly active and toxic substances and act as neurotoxins. Therefore, they are often in very small concentrations in the plants. Thus, it is believed that the alkaloids could serve as natural protection against herbivores and microorganisms. On the other hand, many drugs are from the alkaloids, which served as a lead structure, and the alkaloids helped the science to create and contribute a better understanding of structure-activity relationships.
The alkaloids are further divided into sub-groups. This classification is partly due to their basic structure (eg tropane alkaloids) as well as due to their shared isolation from a drug (eg ergot alkaloids). You can define 10 different main groups, which differ in their basic structure (indole alkaloids, purine-like alkaloids, quinoline, isoquinoline alkaloids, pyrrolizdin alkaloids, tropane alkaloids, chinolizidine alkaloids, pyrrolidine alkaloids, pyridin alkaloids and imidazole alkaloids). In addition, there are three other unsystematic groups of alkaloids (steroid-like alkaloids, diterpene-like alkaloids and alkaloids with exocyclic nitrogen). This classification is generally accepted, but there are various exceptions in particular.
|Ergot (fungus)||ergot alkaloids, indole alkaloids||Lysergic acid|
|Coffee plant||purine-like alkaloids||Coffeine|
|Quina tree||chinoline alkaloids||Chinine|
|Strychnine tree||indole alkaloids||Strychnine|