Monoterpenes are a class of that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries.
Biosynthesis
Monoterpenes are derived biosynthetically from units of isopentenyl pyrophosphate, which is formed from
acetyl-CoA via the intermediacy of
mevalonic acid in the HMG-CoA reductase pathway. An alternative, unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP-(2-methyl-
D-erythritol-4-phosphate) pathway, which is initiated from
Pentose. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids).
Main examples
Myrcen.png|Myrcene is an acyclic monoterpene, an alkene hydrocarbon
Menthol_skeletal.svg|Menthol is a monocyclic monoterpenoid
Limonene-2D-skeletal.svg|Limonene is a monocyclic monoterpene
Carvone.svg|Carvone is a monoterpenoid
Beta-thujaplicin.png|Hinokitiol is a monoterpenoid, a tropolone derivative
Linalool_skeletal.svg|Linalool is an acyclic monoterpenoid
Bicyclic monoterpenes include
carene,
sabinene,
camphene, and
thujene.
Camphor,
borneol,
eucalyptol and
ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging
peroxide functional groups, respectively.
is another example of bicyclic monoterpene ketone.
Natural roles
Monoterpenes are found in many parts of different plants, such as barks,
heartwood, bark and leaves of coniferous trees, in
,
and
.
are very rich in monoterpenes. Several monoterpenes produced by trees, such as
linalool,
hinokitiol, and
ocimene have
Fungicide and
antibacterial activities and participate in wound healing.
Some of these compounds are produced to protect the trees from
insect attacks.
Monoterpenes are emitted by and form that are proposed to serve as cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate.
Many monoterpenes have unique smell and flavor. For example, sabinene contributes to the Spice taste of black pepper, 3-carene gives cannabis an earthy taste and smell, citral has a lemon-like pleasant odor and contributes to the distinctive smell of citrus fruits, and thujene and carvacrol are responsible for the Pungency flavors of summer savory and oregano, respectively.
Monoterpenes are considered Allelopathy.
Uses
Many monoterpenes are volatile compounds and some of them are well-known
Aroma compound found in the
of many plants.
For example,
camphor,
citral,
citronellol,
geraniol, grapefruit mercaptan,
eucalyptol,
ocimene,
myrcene,
limonene,
linalool,
menthol,
camphene and
are used in
and
Cosmetics products.
Limonene and
perillyl alcohol are used in
Cleaning agent products.
Many monoterpenes are used as food Flavoring and , such as bornyl acetate, citral, eucalyptol, menthol, hinokitiol, camphene and limonene.
Volatile monoterpenes produced by plants can attract or repel insects, thus some of them are used in , such as citronellol, eucalyptol, limonene, linalool, hinokitiol, menthol and thymol.
Ascaridole, camphor and eucalyptol are monoterpenes that have pharmaceutical use.
Health effects
A study suggests that a range of floor cleaners with certain monoterpenes may cause indoor
air pollution equivalent or exceeding the harm to respiratory tracts when the time is spent near a busy road. This is due to
ozonolysis of monoterpenes like
limonene leading to the production of atmospheric SOA.
Another study suggests monoterpenes substantially affect ambient organic aerosol with uncertainties regarding environmental impacts.
In a review, scientists concluded that they hope that these "substances will be extensively studied and used in more and more in medicine".
A 2013 study found that "Based on adverse effects and risk assessments, d-limonene may be regarded as a safe ingredient. However, the potential occurrence of skin irritation necessitates regulation of this chemical as an ingredient in cosmetics."
According to a review, several studies showed "that some monoterpenes (e.g., pulegone, menthofuran, camphor, and limonene) and sesquiterpenes (e.g., zederone, germacrone) exhibited liver toxicity" and that i.a. intensive research on terpenes toxicity is needed.
See also
