isoprene

Isoprene
IUPAC name	2-methyl-1,3-butadiene
Other names	isoprene
Identifiers
CAS number	[78-79-5]
SMILES	C=C(C)C=C
Properties
Molecular formula	C5H8
Molar mass	68.12 g/mol
Density	0.681 g/cm³
Melting point	−145.95 °C
Boiling point	34.067 °C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Isoprene (short for isoterpene) or 2-methyl-1,3-butadiene is a common organic compound with the formula CH₂=C(CH₃)CH=CH₂. It is present under standard conditions as a colorless liquid. It is the monomer of natural rubber and is a precursor to an immense variety of other naturally occurring compounds.

Occurrence and production

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 that has similar, but not identical properties.^[1]

Isoprene was first isolated by thermal decomposition of natural rubber.^[2] 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 20M kg are produced annually.^[1] About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a synthetic version of natural rubber.

Biological roles and effects

It is generally the most common hydrocarbon found in the human body.^{[citation needed]} The estimated production rate of isoprene in the human body is 0.15 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. Isoprene is incorporated into and helps stabilize cell membranes in response to heat stress, conferring some tolerance to heat spikes. Isoprene may also confer some resistance to reactive oxygen species.^[3] 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 (~5–20 mg/m²/h)^{[citation needed]} during hot and sunny days.

Isoprene is a common structural motif in biological systems. The terpenes (for example, the carotenes are tetraterpenes) are derived from isoprene, as are the terpenoids and coenzyme Q.^{[citation needed]} 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 terpenes and lanosterol derivatives.

In virtually all organisms, isoprene derivatives are synthesized by the HMG-CoA reductase pathway. Addition of these chains to proteins is termed isoprenylation.

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 coenzyme Q10. This flow chartshows the biosynthesis of isoprenes, and the point at which statins act to inhibit this process.

Environmental impact

Isoprene may be the source of the pollution referred to by U.S. President Ronald Reagan, when in 1981 he stated "Trees cause more pollution than automobiles do." 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. 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 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 reduced visibility due to light scattering effects. Because of its atmospheric importance, much work has been devoted to emission studies from isoprene-emitting vegetation, and, kinetic and mechanistic studies of isoprene oxidation via OH radicals, ozone, and NO₃ radicals.

See also

• 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, and 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, and W. Maenhaut (2004). "Formation of secondary organic aerosols through photooxidation of isoprene". Science 303 (5661): 1173–1176. doi:10.1126/science.1092805. ISSN 0036-8075. PMID 14976309. 
• Pier, P. A.; and 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. 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. 
• Guenther, A.; T. Karl, P. Harley, C. Wiedinmyer, P. I. Palmer and C. Geron (2006). "Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)". Atmos. Chem. Phys. 6: 3181–3210. http://www.copernicus.org/EGU/acp/acp/6/3181/acp-6-3181.htm. 
• Monson, R. K.; and 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. 

References

External links

• Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program