Terpenes (/ˈtɜːrpiːn/) are a class of
natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated
hydrocarbons are produced predominantly by
plants, particularly
conifers.[1][2][3] In plants, terpenes and terpenoids are important mediators of ecological
interactions, while some insects use some terpenes as a form of defense. Other functions of terpenoids include cell growth modulation and plant elongation, light harvesting and photoprotection, and membrane permeability and fluidity control.
The one terpene that has major applications is
natural rubber (i.e.,
polyisoprene). The possibility that other terpenes could be used as precursors to produce synthetic
polymers has been investigated. Many terpenes have been shown to have pharmacological effects. Terpenes are also components of some traditional medicines, such as
aromatherapy, and as active ingredients of
pesticides in agriculture. [4]
History and terminology
The term terpene was coined in 1866 by the German chemist
August Kekulé to denote all hydrocarbons having the empirical formula C10H16, of which
camphene was one. Previously, many hydrocarbons having the empirical formula C10H16 had been called "camphene", but many other hydrocarbons of the same composition had had different names. Kekulé coined the term "terpene" in order to reduce the confusion.[5][6] The name "terpene" is a shortened form of "terpentine", an obsolete spelling of "
turpentine".[7]
Although sometimes used interchangeably with "terpenes",
terpenoids (or
isoprenoids) are modified terpenes that contain additional
functional groups, usually oxygen-containing.[8] The terms terpenes and terpenoids are often used interchangeably, however. Furthermore, terpenes are produced from terpenoids and many terpenoids are produced from terpenes. Both have strong and often pleasant odors, which may protect their hosts or attract pollinators. The number of terpenes and terpenoids is estimated at 55,000 chemical entities.[9]
Terpenes are major biosynthetic building blocks.
Steroids, for example, are derivatives of the triterpene
squalene. Terpenes and terpenoids are also the primary constituents of the
essential oils of many types of plants and flowers.[13] In plants, terpenes and terpenoids are important mediators of ecological
interactions. For example, they play a role in
plant defense against herbivory,
disease resistance, attraction of
mutualists such as
pollinators, as well as potentially plant-
plant communication.[14][15] They appear to play roles as
antifeedants.[2] Other functions of terpenoids include cell growth modulation and plant elongation, light harvesting and photoprotection, and membrane permeability and fluidity control.[16]
Higher amounts of terpenes are released by trees in warmer weather,[17] where they may function as a natural mechanism of
cloud seeding. The clouds reflect sunlight, allowing the forest temperature to regulate.[18]
The one terpene that has major applications is
natural rubber (i.e.,
polyisoprene). The possibility that other terpenes could be used as precursors to produce synthetic
polymers has been investigated as an alternative to the use of petroleum-based feedstocks. However, few of these applications have been commercialized.[20] Many other terpenes, however, have smaller scale commercial and industrial applications. For example,
turpentine, a mixture of terpenes (e.g.,
pinene), obtained from the distillation of pine tree
resin, is used as an organic
solvent and as a chemical feedstock (mainly for the production of other terpenoids).[7]Rosin, another by-product of conifer tree resin, is widely used as an ingredient in a variety of industrial products, such as
inks,
varnishes and
adhesives. Rosin is also used by violinists (and players of similar
bowed instruments) to increase friction on the
bow hair.[21] Terpenes are widely used as fragrances and flavors in consumer products such as
perfumes,
cosmetics and
cleaning products, as well as food and drink products. For example, the aroma and flavor of
hops comes, in part, from
sesquiterpenes (mainly
α-humulene and
β-caryophyllene), which affect
beer quality.[22] Some form hydroperoxides that are valued as catalysts in the production of polymers.
Many terpenes have been shown to have pharmacological effects, although most studies are from laboratory research, and
clinical research in humans is preliminary.[23] Terpenes are also components of some traditional medicines, such as
aromatherapy.[24]
Reflecting their defensive role in plants, terpenes are used as active ingredients of
pesticides in agriculture.[25]
Physical and chemical properties
Terpenes are colorless, although impure samples are often yellow. Boiling points scale with molecular size: terpenes, sesquiterpenes, and diterpenes respectively at 110, 160, and 220 °C. Being highly non-polar, they are insoluble in water. Being hydrocarbons, they are highly flammable and have low specific gravity (float on water). They are tactilely light oils considerably less
viscous than familiar vegetable oils like corn oil (28
cP), with viscosity ranging from 1 cP (à la water) to 6 cP. Terpenes are local irritants and can cause gastrointestinal disturbances if ingested.
Terpenoids (mono-, sesqui-, di-, etc.) have similar physical properties but tend to be more polar and hence slightly more soluble in water and somewhat less volatile than their terpene analogues. Highly polar derivatives of terpenoids are the
glycosides, which are linked to sugars. These are water-soluble solids.
Conceptually derived from
isoprenes, the structures and formulas of terpenes follow the biogenetic isoprene rule or the C5 rule, as described in 1953 by
Leopold Ružička[26] and colleagues.[27] The C5 isoprene units are provided in the form of
dimethylallyl pyrophosphate (DMAPP) and
isopentenyl pyrophosphate (IPP). DMAPP and IPP are
structural isomers to each other. This pair of building blocks are produced by two distinct
metabolic pathways: the
mevalonate (MVA) pathway and the
non-mevalonate (MEP) pathway. These two pathways are mutually exclusive in most organisms, except for some bacteria and land plants.[citation needed] In general, most archaea and eukaryotes use the MVA pathway, while bacteria mostly have the MEP pathway. IPP and DMAPP are final products of both MVA and MEP pathways and the relative abundance of these two isoprene units is enzymatically regulated in host organisms.
This pathway conjugates three molecules of
acetyl CoA.
The mevalonate (MVA) pathway is distributed in all three domains of life; archaea, bacteria and eukaryotes. The MVA pathway is universally distributed in archaea and non-photosynthetic eukaryotes, while the pathway is sparse in bacteria. In photosynthetic eukaryotes, some species possess the MVA pathway, while others have the MEP pathway or both MVA and MEP pathways. This is due to the acquisition of the MEP pathway by a common ancestor of
Archaeplastida (algae + land plants) through the
endosymbiosis of ancestral
cyanobacteria that possessed the MEP pathway. The MVA and MEP pathways were selectively lost in individual photosynthetic lineages.
Also, the archaeal MVA pathway is not completely homologous to the eukaryotic MVA pathway.[28] Instead, the eukaryotic MVA pathway is closer to the bacterial MVA pathway.
The non-mevalonate pathway or the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway starts with
pyruvate and
glyceraldehyde 3-phosphate (G3P) as the carbon source.
In both MVA and MEP pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase. IPP and DMAPP condense to give
geranyl pyrophosphate, the precursor to monoterpenes and monoterpenoids.
The genomes of many plant species contain genes that encode terpenoid synthase enzymes imparting terpenes with their basic structure, and
cytochrome P450s that modify this basic structure.[2][31]
Structure
Terpenes can be visualized as the result of linking
isoprene (C5H8) units "head to tail" to form chains and rings.[32] A few terpenes are linked “tail to tail”, and larger branched terpenes may be linked “tail to mid”.
Formula
Strictly speaking all monoterpenes have the same chemical formula C10H16. Similarly all sesquiterpenes and diterpenes have formulas of C15H24 and C20H32 respectively. The structural diversity of mono-, sesqui-, and diterpenes is a consequence of isomerism.
Chirality
Terpenes and terpenoids are usually
chiral. Chiral compounds can exist as non-superposable mirror images, which exhibit distinct
physical properties such as odor or toxicity.
Unsaturation
Most terpenes and terpenoids feature C=C groups, i.e. they exhibit unsaturation. Since they carry no functional groups aside from their unsaturation, terpenes are structurally distinctive. The unsaturation is associated with di- and trisubstituted
alkenes. Di- and trisubstituted alkenes resist polymerization (low
ceiling temperatures) but are susceptible to acid-induced
carbocation formation.
Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of isoprene pairs needed to assemble the molecule. Commonly, terpenes contain 2, 3, 4 or 6 isoprene units; the tetraterpenes (8 isoprene units) form a separate class of compounds called carotenoids; the others are rare.
The basic unit isoprene itself is a
hemiterpene. It may form oxygen-containing derivatives such as
prenol and
isovaleric acid analogous to terpenoids.
Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples of sesquiterpenes and sesquiterpenoids include
humulene,
farnesenes,
farnesol,
geosmin.[34] (The sesqui- prefix means one and a half.)
Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. (The sester- prefix means two and a half.) An example of a sesterterpenoid is
geranylfarnesol.
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 either
lanosterol or
cycloartenol, the structural precursors to all the
steroids.
Sesquarterpenes are composed of seven isoprene units and have the molecular formula C35H56. Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.
Tetraterpenes contain eight isoprene units and have the molecular formula C40H64. Biologically important tetraterpenoids include the acyclic
lycopene, the monocyclic
gamma-carotene, and the bicyclic
alpha- and
beta-carotenes.
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.
Norisoprenoids, characterized by the shortening of a chain or ring by the removal of a methylene group or substitution of one or more methyl side chains by hydrogen atoms. These include the C13-norisoprenoid 3-oxo-α-ionol present in
Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in
Shiraz leaves (both grapes in the species Vitis vinifera)[35] or
wine[36][37] (responsible for some of the
spice notes in
Chardonnay), can be produced by fungal
peroxidases[38] or
glycosidases.[39]
Industrial syntheses
While terpenes and terpenoids occur widely, their extraction from natural sources is often problematic. Consequently, they are produced by chemical synthesis, usually from
petrochemicals. In one route,
acetone and
acetylene are condensed to give
2-Methylbut-3-yn-2-ol, which is extended with
acetoacetic ester to give
geranyl alcohol. Others are prepared from those terpenes and terpenoids that are readily isolated in quantity, say from the paper and
tall oil industries. For example,
α-pinene, which is readily obtainable from natural sources, is converted to
citronellal and
camphor. Citronellal is also converted to
rose oxide and
menthol.[1]
^
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