|Jmol interactive 3D||Image|
|Molar mass||68.12 g/mol|
|Melting point||−143.95 °C (−227.11 °F; 129.20 K)|
|Boiling point||34.067 °C (93.321 °F; 307.217 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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 volatile liquid. Isoprene is produced by many plants.
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 tonnes, half from tropical broadleaf trees and the remainder primarily from shrubs. This is about equivalent to methane emissions and accounts for ~1/3 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 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 DMAPP.
Isoprene emission in plants is controlled both by the availability of 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 activity.
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.
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. Isoprene is incorporated into and helps stabilize cell membranes in response to heat stress.
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 
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 Archaea are found.
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. 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 is dimethylallyl pyrophosphate (DMAPP) and its isomer isopentenyl pyrophosphate (IPP). 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.
Impact on aerosols
After release, isoprene is converted by 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 and haze.
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 activities. Isoprene can have the opposite effect and quench ozone formation under low levels of NOx.
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 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 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).
- Guenther, A.; T. Karl; P. Harley; C. Wiedinmyer; P. I. Palmer; C. Geron (2006). "Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)" (PDF). Atmos. Chem. Phys. 6 (11): 3181–3210. doi:10.5194/acp-6-3181-2006.
- Gelmont, D; R.A. Stein; J.F. Mead (1981). "Isoprene- the main hydrocarbon in human breath". Biochem. Biophys. Res. Commun. 99 (4): 1456–1460. doi:10.1016/0006-291X(81)90782-8. PMID 7259787.
- King, Julian; Helin Koc; Karl Unterkofler; Pawel Mochalski; Alexander Kupferthaler; Gerald Teschl; Susanne Teschl; Hartmann Hinterhuber; Anton Amann (2010). "Physiological modeling of isoprene dynamics in exhaled breath". J. Theor. Biol. 267 (4): 626–637. doi:10.1016/j.jtbi.2010.09.028.
- Sharkey, TD; AE Wiberley; AR Donohue (2007). "Isoprene Emission from Plants: Why and How". Annals of Botany 101 (1): 5–18. doi:10.1093/aob/mcm240. PMC 2701830. PMID 17921528.
- Vickers, CE; Vickers CE, Possell M, Cojocariu CI, Velikova VB, Laothawornkitkul J, Ryan A, Mullineaux PM, Nicholas Hewitt C. (2009). "Isoprene synthesis protects transgenic tobacco plants from oxidative stress". Plant, Cell & Environment 32 (5): 520–31. doi:10.1111/j.1365-3040.2009.01946.x. PMID 19183288. Cite uses deprecated parameter
- Benjamin, M.T.; Sudol, M.; Bloch, L.; Winer, A.M. (1996). "Low-emitting urban forests: A taxonomic methodology for assigning isoprene and monoterpene emission rates". Atmospheric Environment 30 (9): 1437–1452. doi:10.1016/1352-2310(95)00439-4.
- Heinz-Hermann Greve "Rubber, 2. Natural" in Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_225
- IUPAC Subcommittee on Gas Kinetic Data Evaluation – Data Sheet Ox_VOC7, 2007
- Organic Carbon Compounds Emitted By Trees Affect Air Quality, ScienceDaily, Aug. 7, 2009
- A source of haze, ScienceNews, August 6th, 2009
- C. G. Williams, Proceedings of the Royal Society (1860) 10.
- Merck Index: an encyclopedia of chemicals, drugs, and biologicals, Susan Budavari (ed.), 11th Edition, Rahway, NJ : Merck, 1989, ISBN 0-911910-28-X
- Poisson, N.; M. Kanakidou; P. J. Crutzen (2000). "Impact of nonmethanehydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modelling results". Journal of Atmospheric Chemistry 36 (2): 157–230. doi:10.1023/A:1006300616544. ISSN 0167-7764.
- Claeys, M.; B. Graham; G. Vas; W. Wang; R. Vermeylen; V. Pashynska; J. Cafmeyer; P. Guyon; M. O. Andreae; P. Artaxo; W. Maenhaut (2004). "Formation of secondary organic aerosols through photooxidation of isoprene". Science 303 (5661): 1173–1176. Bibcode:2004Sci...303.1173C. doi:10.1126/science.1092805. ISSN 0036-8075. PMID 14976309.
- Pier, P. A.; C. McDuffie (1997). "Seasonal isoprene emission rates and model comparisons using whole-tree emissions from white oak". Journal of Geophysical Research 102 (D20): 23,963–23,971. Bibcode:1997JGR...10223963P. doi:10.1029/96JD03786. ISSN 0148-0227.
- Poschl, U.; R. von Kuhlmann, N. Poisson, and P. J. Crutzen (2000). "Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modeling". Journal of Atmospheric Chemistry 37 (1): 29–52. doi:10.1023/A:1006391009798. ISSN 0167-7764. Cite uses deprecated parameter
- Monson, R. K.; E. A. Holland (2001). "Biospheric trace gas fluxes and their control over tropospheric chemistry". Annual Review of Ecology and Systematics 32: 547–576. doi:10.1146/annurev.ecolsys.32.081501.114136.
|Wikimedia Commons has media related to Isoprene.|
- Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program
- Science News article describing how isoprene released by plants is converted to light-scattering aerosols