Sesquiterpenes are a class of
terpenes that consist of three
isoprene units and often have the
molecular formula C
15H
24. Like
monoterpenes, sesquiterpenes may be cyclic or contain rings, including many unique combinations. Biochemical modifications such as
oxidation or
rearrangement produce the related sesquiterpenoids.
Sesquiterpenes are found naturally in plants and insects, as
semiochemicals, e.g. defensive agents or pheromones.
Biosynthesis and examples
The reaction of
geranyl pyrophosphate with
isopentenyl pyrophosphate
Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of ...
results in the 15-carbon
farnesyl pyrophosphate
Farnesyl pyrophosphate (FPP), also known as farnesyl diphosphate (FDP), is an intermediate in the biosynthesis of terpenes and terpenoids such as sterols and carotenoids. It is also used in the synthesis of CoQ (part of the electron transport cha ...
(FPP), which is an intermediate in the biosynthesis of sesquiterpenes such as
farnesene.
Cyclic sesquiterpenes are more common than cyclic monoterpenes because of the increased chain length and additional double bond in the sesquiterpene precursors. In addition to common six-membered ring systems such as the ones found in
zingiberene
Zingiberene is a monocyclic sesquiterpene that is the predominant constituent of the oil of ginger (''Zingiber officinale''), from which it gets its name. It can contribute up to 30% of the essential oils in ginger rhizomes. This is the compound ...
and
bisacurone
Bisacurone is a chemical compound with the molecular formula C15H24O3 which has been isolated from turmeric (''Curcuma longa''). ''In vitro'', it has several effects including anti-inflammatory, anti-oxidant
Antioxidants are compounds that i ...
, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as
humulene
Humulene, also known as α-humulene or α-caryophyllene, is a naturally occurring monocyclic sesquiterpene (C15H24), containing an 11-membered ring and consisting of 3 isoprene units containing three nonconjugated C=C double bonds, two of them be ...
.
The
cadinene
Cadinenes are a group of isomeric hydrocarbons that occur in a wide variety of essential oil-producing plants. The name is derived from that of the Cade juniper (''Juniperus oxycedrus'' L.), the wood of which yields an oil from which cadinene is ...
s contain two fused six-membered rings.
Caryophyllene
Caryophyllene (), more formally (−)-β-caryophyllene, (BCP), is a natural bicyclic sesquiterpene that is a constituent of many essential oils, especially clove oil, the oil from the stems and flowers of '' Syzygium aromaticum'' (cloves), the ess ...
, a component of many
essential oils such as clove oil, contains a nine-membered ring fused to a
cyclobutane
Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commerci ...
ring. Rishitin is another example of a cadinene, which is found in potatoes and tomatoes.
Vetivazulene
Vetivazulene is an azulene derivate obtained from vetiver oil. It is a bicyclic sesquiterpene and an isomer of guaiazulene
Guaiazulene, also azulon or 1,4-dimethyl-7-isopropylazulene, is a dark blue crystalline hydrocarbon. A derivative of azul ...
and
guaiazulene
Guaiazulene, also azulon or 1,4-dimethyl-7-isopropylazulene, is a dark blue crystalline hydrocarbon. A derivative of azulene, guaiazulene is a bicyclic sesquiterpene that is a constituent of some essential oils, mainly oil of guaiac and chamomile ...
are aromatic bicyclic sesquiterpenoids.
With the addition of a third ring, the possible structures become increasingly varied. Examples include
longifolene,
copaene
Copaene, or more precisely, α-copaene, is the common (or trivial) chemical name of an oily liquid hydrocarbon that is found in a number of essential oil-producing plants. The name is derived from that of the resin-producing tropical copaiba tree, ...
and the alcohol
patchoulol
Patchoulol or patchouli alcohol (C15H26O) is a sesquiterpene alcohol found in patchouli. Patchouli oil is an important material in perfumery. The (−)-optical isomer is one of the organic compounds responsible for the typical patchouli scent. Pat ...
.
Sesquiterpenoids
The FPP backbone can be rearranged in several different ways and further decorated with different functional groups, hence the large variety of sesquiterpenoids.
Geosmin
Geosmin ( ) is an irregular sesquiterpenoid, produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step -dependent reaction. Geosmin, along with the irregular monoterpene 2-me ...
, the volatile compound that gives an earthy taste and musty odor in drinking water and the characteristic odor on a rainy day, is a sesquiterpenoid, produced by bacteria, especially
cyanobacteria, that are present in the soils and water supplies. Oxidation of farnesene then provides the sesquiterpenoid
farnesol
Farnesol is a natural 15-carbon organic compound which is an acyclic sesquiterpene alcohol. Under standard conditions, it is a colorless liquid. It is hydrophobic, and thus insoluble in water, but miscible with oils.
Farnesol is produced from 5- ...
.
Sesquiterpene lactone
Sesquiterpene lactones (SLs) are a class of sesquiterpenoids that contain a lactone ring. They are most often found in plants of the family Asteraceae (daisies, asters). Other plant families with SLs are Umbelliferae (celery, parsley, carrots) ...
s are a common class of sesquiterpenoids that contain a
lactone ring, hence the name. They are found in many plants and can cause allergic reactions and toxicity if consumed excessively, particularly in grazing livestock.
The term merosesquiterpenoids was coined in 1968 to describe molecules of this class that have a mixed biosynthetic origin, meaning isoprenoid precursors like isopentenyl pyrophosphate are derived from both the
mevalonate
Mevalonic acid (MVA) is a key organic compound in biochemistry; the name is a contraction of dihydroxymethylvalerolactone. The carboxylate anion of mevalonic acid, which is the predominant form in biological environments, is known as ''mevalonate ...
and
non-mevalonate pathway
The non-mevalonate pathway—also appearing as the mevalonate-independent pathway and the 2-''C''-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate (MEP/DOXP) pathway—is an alternative metabolic pathway for the biosynthesis of the is ...
s.
References
External links
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{{Terpenoids