Isoprene

Isoprene

Names
IUPAC name 2-methyl-1,3-butadiene
Identifiers
CAS Number	78-79-5 Yd
ChEBI	CHEBI:35194 Y
ChemSpider	6309 Y
Jmol 3D model	Interactive image
KEGG	C16521 Y
PubChem	6557
UNII	0A62964IBU Y
InChI InChI=1S/C5H8/c1-4-5(2)3/h4H,1-2H2,3H3 Y Key: RRHGJUQNOFWUDK-UHFFFAOYSA-N Y InChI=1/C5H8/c1-4-5(2)3/h4H,1-2H2,3H3 Key: RRHGJUQNOFWUDK-UHFFFAOYAS
SMILES CC(=C)C=C
Properties
Chemical formula	C5H8
Molar mass	68.12 g/mol
Density	0.681 g/cm3
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).
Y verify (what is YN ?)
Infobox references

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.

Natural occurrences

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.

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 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.

Regulation

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.

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 common in low concentrations in many foods.

Biological roles

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 protects 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.

Isoprenoids

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.

Chemical structure of cis-polyisoprene, the main constituent of natural rubber.

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 (NO_x) 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 NO_x, which comes almost exclusively from industrial activities. Isoprene can have the opposite effect and quench ozone formation under low levels of NO_x.

Industrial production

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.

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 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).

See also

• Natural rubber
• Neoprene

Further reading

• Merck Index: an encyclopedia of chemicals, drugs, and biologicals, Susan Budavari (ed.), 11th Edition, Rahway, NJ : Merck, 1989, ISBN 0-911910-28-X

External links

• 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