Isoprene
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{{Chembox new
| Name = Isoprene
| ImageFile = Isoprene.svg
| ImageFile1 = Isoprene-3d.png
| IUPACName = 2-Methyl-buta-1,3-diene
| OtherNames = isoprene
| Section1 = {{Chembox Identifiers
| CASNo = 78-79-5
| SMILES = C=C(C)C=C
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>5</sub>H<sub>8</sub>
| MolarMass = 68.11 g/mol
| Density = 0.681 g/cm³
| MeltingPt = -145.95 °C
| BoilingPt = 34.067 °C
}}
}}
'''Isoprene''' is a common synonym for the chemical compound 2-methylbuta-1,3-diene. It is commonly used in industry, is an important biological material, and can be a harmful environmental pollutant and toxicant when present in excess quantities.
At room temperature, isoprene is a colorless liquid which is highly flammable and easily ignited. It can form explosive mixtures in air and is highly reactive, capable of polymerizing explosively when heated. The [[United States Department of Transportation]] considers isoprene a hazardous material and requires special marking, labeling, and transportation for it.
It is most readily available industrially as a by-product of the thermal [[Cracking (chemistry)|cracking]] of [[naphtha]] or oil. About 95% of isoprene production is used to produce cis-1,4-polyisoprene - a [[Synthetic rubber|synthetic]] version of natural [[rubber]].
Natural [[rubber]] is a [[polymer]] of isoprene - most often cis-1,4-polyisoprene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials are found in high quality natural rubber.
Some natural rubber sources called [[gutta percha]] are composed of trans-1,4-polyisoprene, a structural [[isomer]] which has similar, but not identical properties.
==Biological roles and effects==
Isoprene is formed naturally in animals and plants and is generally the most common [[hydrocarbon]] found in the human body. The estimated production rate of isoprene in the human body is .15 [[Mole (unit)|µmol]]/kg/h, equivalent to approximately 17 mg/day for a 70 kg person. Isoprene is also common in low concentrations in many foods. Isoprene is produced in the chloroplasts of leaves of certain tree species through the [[DMAPP]] pathway; the enzyme isoprene synthase is responsible for its biosynthesis. The amount of isoprene released from isoprene-emitting vegetation depends on leaf mass, leaf area, light (particularly photosynthetic photon flux density), and leaf temperature. Thus, during the night, little isoprene is emitted from tree leaves while daytime emissions are expected to be substantial (~5-20 mg/m<sup>2</sup>/h) during hot and sunny days.
With a global biogenic production in the range of 400–600 Tg of carbon/year, isoprene has a large impact on atmospheric processes and is thus an important compound in the field of [[Atmospheric chemistry|Atmospheric Chemistry]]. Isoprene affects the oxidative state of large air masses, is an important precursor for [[ozone]], and is a pollutant in the lower atmosphere. Furthermore, isoprene forms secondary organic [[Particulate|aerosols]] through photooxidation with OH radicals, however this has been questioned by some researchers due to the atmospherically irrelevant isoprene and OH concentrations used in these experiments. Therefore, further work is needed due to the possible wide-ranging health effects, particularly for the respiratory tract, and reduce visibility due to light scattering effects. Because of its atmospheric importance, much work has been devoted to emission studies from isoprene-emitting vegetation, and, [[Chemical kinetics|kinetic]] and [[Reaction mechanism|mechanistic]] studies of isoprene oxidation via OH radicals, ozone, and NO<sub>3</sub> radicals.
It is a common structural motif in biological systems. The [[terpene]]s (for example, the [[carotene]]s are tetraterpenes) are derived from isoprene, as are the [[terpenoid]]s and [[coenzyme Q]]. Also derived from isoprene are [[phytol]], [[retinol]] ([[vitamin A]]), [[tocopherol]] ([[vitamin E]]), [[dolichol]]s, 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 terpenes and lanosterol derivatives.
In virtually all organisms, isoprene derivatives are synthetised by the [[HMG-CoA reductase pathway]]. Addition of these chains to proteins is termed [[isoprenylation]].
According to the [[United States Department of Health and Human Services]] Eleventh Edition Report on Carcinogens, isoprene is reasonably expected to be a human [[carcinogen]]. Tumors have been observed in multiple locations in multiple test species exposed to isoprene vapor. No adequate human studies of the relationship between isoprene exposure and human cancer have been reported.
==Biosynthesis and its inhibition by statins==
HMG-CoA reductase inhibitors, also known as the group of cholesterol-lowering drugs called [[statins]], inhibit the synthesis of [[mevalonate]]. Mevalonate is a precursor to [[isopentenyl pyrophosphate]], which combines with its isomer, [[dimethylallyl pyrophosphate]], in repeating alternations to form isoprene (or polyprenyl) chains.
[[Statins]] are used to lower [[cholesterol]], which is synthesized from the 15-carbon isoprenoid, [[farnesyl pyrophosphate]], but also inhibit all other isoprenes, including [[Ubiquinone|coenzyme Q10]]. This [http://www.cholesterol-and-health.com/Synthesis-Of-Cholesterol.html flow chart]shows the biosynthesis of isoprenes, and the point at which statins act to inhibit this process.
==References==
* [[Merck Index]]: an encyclopedia of chemicals, drugs, and biologicals, Susan Budavari (ed.), 11th Edition, Rahway, NJ : Merck, 1989, ISBN 0-911910-28-X
*{{cite journal|last=Poisson|first= N.|coauthors=M. Kanakidou, and [[Paul J. Crutzen|P. J. Crutzen]]|year=2000|title=Impact of nonmethanehydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modelling results|url=|journal=Journal of Atmospheric Chemistry|issn=0167-7764|volume=36|issue=2|pages=157–230|doi=10.1023/A:1006300616544}}
*{{cite journal|last=Monson|first=R. K.|coauthors=and E. A. Holland|year=2001|title=Biospheric trace gas fluxes and their control over tropospheric chemistry|url=|journal=Annual Review of Ecology and Systematics|issn=|volume=32|issue=|pages=547–576|doi=10.1146/annurev.ecolsys.32.081501.114136}}
*{{cite journal|last=Claeys|first=M.|coauthors=B. Graham, G. Vas, W. Wang, R. Vermeylen, V. Pashynska, J. Cafmeyer, P. Guyon, M. O. Andreae, P. Artaxo, and W. Maenhaut|year=2004|title=Formation of secondary organic aerosols through photooxidation of isoprene|url=|journal=Science|issn=0036-8075|volume=303|issue=5661|pages=1173–1176|doi=10.1126/science.1092805|pmid=14976309}}
*{{cite journal|last=Pier|first=P. A.|coauthors=and C. McDuffie|year=1997|title=Seasonal isoprene emission rates and model comparisons using whole-tree emissions from white oak|url=|journal=Journal of Geophysical Research|issn=0148-0227|volume=102|issue=D20|pages=23,963–23,971|doi=
}}
*{{cite journal|last=Poschl|first=U.|coauthors=R. von Kuhlmann, N. Poisson, and [[Paul J. Crutzen|P. J. Crutzen]]|year=2000|title=Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modeling|url=|journal=Journal of Atmospheric Chemistry|issn=0167-7764|volume=37|issue=1|pages=29–52|doi=10.1023/A:1006391009798}}
*{{cite journal|last=Guenther|first=A.|coauthors=T. Karl, P. Harley, C. Wiedinmyer, P. I. Palmer and C. Geron|year=2006|title=Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)|url=http://www.copernicus.org/EGU/acp/acp/6/3181/acp-6-3181.htm
|journal=Atmos. Chem. Phys.|issn=|volume=6|issue=|pages=3181–3210|doi=
}}
==External links==
*[http://www.cholesterol-and-health.com/Synthesis-Of-Cholesterol.html Flow Chart Showing the Biosynthesis of Isoprenes]
*[http://ntp.niehs.nih.gov/ntp/roc/toc11.html Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program]
[[Category:Dienes]]
[[Category:Monomers]]
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