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|Molar mass||68.12 g/mol|
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Isoprene, or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=C(CH3)CH=CH2. It is a colorless volatile liquid. Isoprene is produced by many plants.
Isoprene is produced and emitted by many species of trees into the atmosphere (major producers are oaks, poplars, eucalyptus, and some legumes). The yearly production of isoprene emissions by vegetation is around 600 million tonnes, with half that coming from tropical broadleaf trees and the remainder coming from shrubs. This is about equivalent to methane emission into the atmosphere and accounts for ~1/3 of all hydrocarbons released into the atmosphere. After release, isoprene is converted by free radicals (like the hydroxyl (OH) 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 and haze. While most in the field 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 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 normally not regarded as a pollutant, as it is one of the natural products from plants. Formation of tropospheric ozone is only possible in presence of high levels of NOx, which comes almost exclusively from industrial activities. In fact, isoprene can have the opposite effect and quench ozone formation under low levels of NOx.
Isoprene production from plants
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 diphosphate (DMADP), is catalyzed by the enzyme isoprene synthase to form isoprene. Therefore, inhibitors that block the MEP pathway, such as fosmidomycin, also blocks 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 from DMADP.
Regulation of isoprene emission
Isoprene emission in plants is controlled both by the availability of substrate (DMADP) 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 activity.
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 stresses (~ 40 °C). It was proposed that isoprene emission was specifically used by plants to protect against large fluctations in leaf temperature.
Isoprene is incorporated into and helps stabilize cell membranes in response to heat stress, conferring some tolerance to heat spikes. 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 
Isoprene in other organisms
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 also common in low concentrations in many foods.
Isoprene vs isoprenoids
The isoprene skeleton can be found in naturally occurring compounds called terpenes, but these compounds do not arise from isoprene itself. Terpenes can be viewed as multiples of isoprene subunits, and this perspective is the cornerstone of the "isoprene rule". The precursor to isoprene units in biological systems are dimethylallyl diphosphate (DMADP) and its isomer isopentenyl diphosphate (IDP). The plural “isoprenes” is sometimes used to refer to terpenes in general. Isoprene chains are commonly found in numerous biologically active oligomers such as Vitamin A. Similarly, natural rubber is composed of linear polyisoprene chains of very high molecular weight and other natural molecules.
Isoprene was first isolated by thermal decomposition of natural rubber. It 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 tonnes 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 weight of 100,000 to 1,000,000. 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.
Isoprene as a structural motif
Isoprene is a common structural motif in biological systems. The isoprenoids (for example, the carotenes are tetraterpenes) are derived from isoprene. Also derived from isoprene are 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 diphosphate (DMADP) and its isomer isopentenyl diphosphate (IDP), 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).
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