Terpene
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Many terpenes are derived from conifer resins, here a pine. |
Terpenes are a large and varied class of
hydrocarbons, produced primarily by a wide variety of plants, particularly
conifers, though also by some insects such as swallowtail butterflies, which emit terpenes from their
osmeterium. 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.
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.
Terpenes are derived biosynthetically from units of
isoprene, which has the molecular formula C
5H
8. The basic molecular formulas of terpenes are multiples of that, (C
5H
8)
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.
| Dimethylallyl pyrophosphate |
| | Isopentenyl pyrophosphate |
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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.
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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.
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Monoterpenes consist of
two isoprene units and have the molecular formula C
10H
16. Monoterpene alcohol is also known as
geraniol; the geranyl- prefix indicates two isoprene units.
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Sesquiterpenes consist of
three isoprene units and have the molecular formula C
15H
24. Sequiterpene alcohol is also known as
farnesol; the farnesyl- prefix indicates three isoprene units.
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Diterpenes are composed for
four isoprene units and have the molecular formula C
20H
32. 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.
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Sesterterpenes, terpenes having 25 carbons and
five isoprene units, are rare relative to the other sizes.
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Triterpenes consist of
six isoprene units and have the molecular formula C
30H
48. 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.
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Tetraterpenes contain
eight isoprene units and have the molecular formula C
40H
56. Biologically important tetraterpenes include the acyclic
lycopene, the monocyclic
gamma-carotene, and the bicyclic
alpha- and
beta-carotenes.
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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.
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Institute of Chemistry - terpenes*
Structures of alpha pinene and beta pinene
