Isoprene, or 2-methyl-1,3-butadiene, is a common organic compound with
the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless
Isoprene is produced by many plants, and its polymers
are the main component of natural rubber. C. G. Williams named the
compound in 1860 after obtaining it from thermal decomposition
(pyrolysis) of natural rubber; he correctly deduced the empirical
1 Natural occurrences
1.2 Other organisms
2 Biological roles
3 Impact on aerosols
4 Industrial production
5 See also
7 Further reading
8 External links
Isoprene is produced and emitted by many species of trees (major
producers are oaks, poplars, eucalyptus, and some legumes). Yearly
production of isoprene emissions by vegetation is around 600 million
metric tons, half from tropical broadleaf trees and the remainder
primarily from shrubs. This is about equivalent to methane
emissions and accounts for around one-third of all hydrocarbons
released into the atmosphere.
Isoprene is made through the methyl-erythritol 4-phosphate pathway
(MEP pathway, also called the non-mevalonate pathway) in the
chloroplasts of plants. One of the two end products of MEP pathway,
dimethylallyl pyrophosphate (DMAPP), is catalyzed by the enzyme
isoprene synthase to form isoprene. Therefore, inhibitors that block
the MEP pathway, such as fosmidomycin, also block isoprene formation.
Isoprene emission increases dramatically with temperature and
maximizes at around 40 °C. This has led to the hypothesis that
isoprene may protect plants against heat stress (thermotolerance
hypothesis, see below). Emission of isoprene is also observed in some
bacteria and this is thought to come from non-enzymatic degradations
Isoprene emission in plants is controlled both by the availability of
the substrate (DMAPP) and by enzyme (isoprene synthase) activity. In
particular, light, CO2 and O2 dependencies of isoprene emission are
controlled by substrate availability, whereas temperature dependency
of isoprene emission is regulated both by substrate level and enzyme
Isoprene is the most abundant hydrocarbon measurable in the breath of
humans. The estimated production rate of isoprene in the human
body is 0.15 µmol/(kg·h), equivalent to approximately 17 mg/day
for a person weighing 70 kg.
Isoprene is common in low
concentrations in many foods.
Chemical structure of cis-polyisoprene, the main constituent of
Isoprene emission appears to be a mechanism that trees use to combat
abiotic stresses. In particular, isoprene has been shown to protect
against moderate heat stress (~40 °C). It may also protect
plants against large fluctuations in leaf temperature.
incorporated into and helps stabilize cell membranes in response to
Isoprene also confers resistance to reactive oxygen species. The
amount of isoprene released from isoprene-emitting vegetation depends
on leaf mass, leaf area, light (particularly photosynthetic photon
flux density, or PPFD) and leaf temperature. Thus, during the night,
little isoprene is emitted from tree leaves, whereas daytime emissions
are expected to be substantial during hot and sunny days, up to 25
μg/(g dry-leaf-weight)/hour in many oak species 
The isoprene skeleton can be found in naturally occurring compounds
called terpenes (also known as isoprenoids), but these compounds do
not arise from isoprene itself. Instead, the precursor to isoprene
units in biological systems is dimethylallyl pyrophosphate (DMAPP) and
its isomer isopentenyl pyrophosphate (IPP). The plural 'isoprenes' is
sometimes used to refer to terpenes in general.
Examples of isoprenoids include carotene, phytol, retinol (vitamin A),
tocopherol (vitamin E), dolichols, and squalene.
Heme A has an
isoprenoid tail, and lanosterol, the sterol precursor in animals, is
derived from squalene and hence from isoprene. The functional isoprene
units in biological systems are dimethylallyl pyrophosphate (DMAPP)
and its isomer isopentenyl pyrophosphate (IPP), which are used in the
biosynthesis of naturally occurring isoprenoids such as carotenoids,
quinones, lanosterol derivatives (e.g. steroids) and the prenyl chains
of certain compounds (e.g. phytol chain of chlorophyll). Isoprenes are
used in the cell membrane monolayer of many Archaea, filling the space
between the diglycerol tetraether head groups. This is thought to add
structural resistance to harsh environments in which many
Similarly, natural rubber is composed of linear polyisoprene chains of
very high molecular weight and other natural molecules.
Simplified version of the steroid synthesis pathway with the
intermediates isopentenyl pyrophosphate (IPP), dimethylallyl
pyrophosphate (DMAPP), geranyl pyrophosphate (GPP) and squalene shown.
Some intermediates are omitted.
Impact on aerosols
After release, isoprene is converted by short-lived free radicals
(like the hydroxyl radical) and to a lesser extent by ozone into
various species, such as aldehydes, hydroperoxides, organic nitrates,
and epoxides, which mix into water droplets and help create aerosols
While most experts acknowledge that isoprene emission affects aerosol
formation, whether isoprene increases or decreases aerosol formation
is debated. A second major effect of isoprene on the atmosphere is
that in the presence of nitric oxides (NOx) it contributes to the
formation of tropospheric (lower atmosphere) ozone, which is one of
the leading air pollutants in many countries.
Isoprene itself is not
normally regarded as a pollutant, as it is a natural plant product.
Formation of tropospheric ozone is only possible in presence of high
levels of NOx, which comes almost exclusively from industrial
Isoprene can have the opposite effect and quench ozone
formation under low levels of NOx.
Isoprene is most readily available industrially as a byproduct of the
thermal cracking of naphtha or oil, as a side product in the
production of ethylene. About 800,000 metric tons are produced
annually. About 95% of isoprene production is used to produce
cis-1,4-polyisoprene—a synthetic version of natural rubber.
Natural rubber consists mainly of poly-cis-isoprene with a molecular
mass of 100,000 to 1,000,000 g/mol. Typically natural rubber contains
a few percent of other materials, such as proteins, fatty acids,
resins, and inorganic materials. Some natural rubber sources, called
gutta percha, are composed of trans-1,4-polyisoprene, a structural
isomer that has similar, but not identical, properties.
^ C. G. Williams, Proceedings of the Royal Society (1860) 10.
^ M. J. Loadman (2012-12-06). "Analysis of
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^ A source of haze, ScienceNews, August 6th, 2009
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Wikimedia Commons has media related to Isoprene.
Report on Carcinogens, Eleventh Edition; U.S. Department of Health and
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Science News article describing how isoprene released by plants is
converted to light-scattering aerosols