Terpene
From Free net encyclopedia
Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, but also by some animals. They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids. Some authors will use the term terpene to include all terpenoids as well.
Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfumery, in aroma therapy, and in traditional and alternative medicines. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives. Image:Closeuppineneedlessm.jpg
Contents |
Structure and biosynthesis
Image:Isoprene structure.png Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8. The basic molecular formulas of terpenes are multiples of it, (C5H8)n where n is the number of linked isoprene units. This is called the isoprene rule or the C5 rule. The isoprene units may be linked together "head to tail" to form linear chains or they may can be arranged to form rings. One can consider the isoprene unit as one of nature's preferred building blocks.
Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. IPP can then be isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes.
Hemiterpenes
Hemiterpenes consist of a single isoprene unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids.
Monoterpenes
Monoterpenes consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings.
Acyclic
IPP and DMAPP can be combined to form geranyl pyrophosphate.
Image:Geranyl pyrophosphate.png
Image:Myrcen.png Elimination of the pyrophosphate group leads to the formation of acyclic monoterpenes such as ocimene and the myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others.
Monocyclic
In addition to linear attachments, the isoprene units can make connections to form rings. The most common ring size in monoterpenes is a six-membered ring. A classic example is the cyclization of geranyl pyrophosphate to form limonene.
Image:LimoneneBiosynthesis.png
The terpinenes, phellandrenes, and terpinolene are formed similarly. Hydroxylation of any of these compounds followed by dehydration can lead to the aromatic p-cymene. Important terpenoids derived from monocyclic terpenes are menthol, thymol, carvacrol and many others.
Bicyclic
Geranyl pyrophosphate can also undergo two sequential cyclization reactions to form bicyclic monoterpenes, such as pinene which is the primary constituent of pine resin.
Other bicyclic monoterpenes include carene, sabinene, camphene, and thujene. Camphor, borneol and eucalyptol are examples of bicyclic monoterpenoids containing ketone, alcohol, and ether functional groups, respectively.
Sesquiterpenes
Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be acyclic or contain rings, including many unique combinations.
Acyclic
When geranyl pyrophosphate reacts with IPP, the result is the 15-carbon farnesyl pyrophosphate which is an intermediate in the biosynthesis of sesquiterpenes such as farnesene. Oxidation can then provide sesquiterpenoids such as farnesol. Image:Farnesyl pyrophosphate.png
Monocyclic
With the increased chain length and additional double bond, the number of possible ways that cyclization can occur is also increased and there exist a wide variety of cyclic sesquiterpenes. In addition to common six-membered ring systems such as is found in zingiberene, a consitituent of the oil from ginger, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as humulene.
Bicyclic
Image:Cadinene.png In addition to common six-membered rings such as in the cadinenes, one classic bicyclic sesquiterpene is caryophyllene, from the oil of cloves, which has a nine-membered ring and cyclobutane ring. Additional unsaturation provides aromatic bicyclic sesquiterpenoids such as vetivazulene and guaiazulene.
Tricylic
With the addition of a third ring, the possible structures become increasingly varied. Examples include longifolene and copaene.
Diterpenes
Diterpenes are composed for four isoprene units and have the molecular formula C20H32. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes are cembrene and taxadiene. Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol. Image:Geranylgeranyl pyrophosphate.png
Sesterterpines
Terpenes having 25 carbons and five isoprene units, sesterterpenes, are rare relative to the other sizes.
Triterpenes
Triterpenes consist of six isoprene units and have the molecular formula C30H48. The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to generate lanosterol, the structural precursor to all the steroids.
Tetraterpenes
Tetraterpenes contain eight isoprene units and have the molecular formula C40H56. Biologically important tetraterpenes include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.
Polyterpenes
Polyterpenes consist of long chains of many isoprene units. Natural rubber consists of polyisoprene in which the double bonds are cis. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha.
External links
es:Terpeno eo:Terpeno fr:Terpène nl:Terpeen ja:テルペノイド pl:Terpeny ru:Терпены zh:萜类