Isoprene

Isoprene

Overview
Isoprene or 2-methyl-1,3-butadiene, is a common organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 with the formula CH2=C(CH3)CH=CH2. Under standard conditions it is a colorless liquid. However, this compound is highly volatile because of its low boiling point.

Isoprene (C5H8) is the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 of natural rubber and also a common structure motif to an immense variety of other naturally occurring compounds, collectively termed the isoprenoids.
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Encyclopedia
Isoprene or 2-methyl-1,3-butadiene, is a common organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 with the formula CH2=C(CH3)CH=CH2. Under standard conditions it is a colorless liquid. However, this compound is highly volatile because of its low boiling point.

Isoprene (C5H8) is the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 of natural rubber and also a common structure motif to an immense variety of other naturally occurring compounds, collectively termed the isoprenoids. Molecular formula of isoprenoids are multiples of isoprene in the form of (C5H8)n, and this is termed the isoprene rule. The functional isoprene units in biological systems are dimethylallyl diphosphate
Dimethylallyl pyrophosphate
Dimethylallyl pyrophosphate is an intermediate product of both mevalonic acid pathway and DOXP/MEP pathway. It is an isomer of isopentenyl pyrophosphate and exists in virtually all life forms...

 (DMADP) and its isomer isopentenyl diphosphate
Isopentenyl pyrophosphate
Isopentenyl pyrophosphate is an intermediate in the classical, HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. IPP is formed from acetyl-CoA via mevalonic acid...

 (IDP).

The singular terms “isoprene” and “terpene” are synonymous whereas the plurals “isoprenes” or “terpenes” refer to terpenoid
Terpenoid
The terpenoids , sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways. Most are multicyclic structures that differ from one another not only in...

s (isoprenoids).

Natural occurrences


Isoprene is produced and emitted by many species of trees into the atmosphere (major producers are oaks
Oaks
-Horse races:"Oaks" is generally used to describe a Thoroughbred horse race restricted to 3-year-old fillies. Among the best-known races using the term are:*Epsom Oaks, The Oaks Stakes, at Epsom Downs Racecourse, Surrey, England; the original "Oaks" race...

, poplars, eucalyptus
Eucalyptus
Eucalyptus is a diverse genus of flowering trees in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia...

, and some legumes). The yearly production of isoprene emissions by vegetation is around 600 Tg, with half that coming from tropical broadleaf trees and the remainder coming from shrubs. This is about equivalent to methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

  into various species, such as aldehydes, hydroperoxides, organic nitrates, and epoxides, which mix into water droplets and help create aerosols and haze
Haze
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...

. While most in the field acknowledges that isoprene emission effect 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)
NOx
NOx is a generic term for the mono-nitrogen oxides NO and NO2 . They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures...

 it contributes to the formation of tropospheric (lower atmosphere) ozone
Tropospheric ozone
Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...

, 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
Non-mevalonate pathway
The non-mevalonate pathway or 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway of isoprenoid biosynthesis is an alternative metabolic pathway leading to the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate that has been elucidated only...

 (MEP pathway, also called the non-mevalonate pathway) in the chloroplasts of plants. One of the two end products of MEP pathway, dimethylallyl diphosphate
Dimethylallyl pyrophosphate
Dimethylallyl pyrophosphate is an intermediate product of both mevalonic acid pathway and DOXP/MEP pathway. It is an isomer of isopentenyl pyrophosphate and exists in virtually all life forms...

 (DMADP), is catalyzed by the enzyme isoprene synthase
Isoprene synthase
In enzymology, an isoprene synthase is an enzyme that catalyzes the chemical reactionHence, this enzyme has one substrate, dimethylallyl diphosphate, and two products, isoprene and diphosphate....

 to form isoprene. Therefore, inhibitors that block the MEP pathway, such as fosmidomycin
Fosmidomycin
Fosmidomycin is an antibiotic that was originally isolated from culture broths of bacteria of the genus Streptomyces. It specifically inhibits DXP reductoisomerase, a key enzyme in the non-mevalonate pathway of isoprenoid biosynthesis. It is a structural analogue of 2-C-methyl-D-erythrose...

, 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.

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 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 may also confer some 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
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

/(kg·h), equivalent to approximately 17 mg/day for a person weighing 70 kg. Isoprene is also common in low concentrations in many foods.

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
Cracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...

 of naphtha
Naphtha
Naphtha normally refers to a number of different flammable liquid mixtures of hydrocarbons, i.e., a component of natural gas condensate or a distillation product from petroleum, coal tar or peat boiling in a certain range and containing certain hydrocarbons. It is a broad term covering among the...

 or oil, as a side product in the production of ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

. About 800,000 tonnes are produced annually. About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a synthetic
Synthetic rubber
Synthetic rubber is is any type of artificial elastomer, invariably a polymer. An elastomer is a material with the mechanical property that it can undergo much more elastic deformation under stress than most materials and still return to its previous size without permanent deformation...

 version of natural rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

.

Natural rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

 is a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 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
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

 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 carotene
Carotene
The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...

s are tetraterpene
Tetraterpene
Tetraterpenes are terpenes consisting of eight isoprene units and have the molecular formula C40H64.Biologically important tetraterpenes include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes....

s) are derived from isoprene. Also derived from isoprene are phytol
Phytol
Phytol is an acyclic diterpene alcohol that can be used as a precursor for the manufacture of synthetic forms of vitamin E and vitamin K1. In ruminants, the gut fermentation of ingested plant materials liberates phytol, a constituent of chlorophyll, which is then converted to phytanic acid and...

, retinol
Retinol
Retinol is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals....

 (vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

), tocopherol
Tocopherol
Tocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...

 (vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...

), dolichol
Dolichol
Dolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.-Functions:...

s, and squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...

. Heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

 A has an isoprenoid tail, and lanosterol, the sterol precursor in animals, is derived from squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...

 and hence from isoprene. The functional isoprene units in biological systems are dimethylallyl diphosphate
Dimethylallyl pyrophosphate
Dimethylallyl pyrophosphate is an intermediate product of both mevalonic acid pathway and DOXP/MEP pathway. It is an isomer of isopentenyl pyrophosphate and exists in virtually all life forms...

 (DMADP) and its isomer isopentenyl diphosphate
Isopentenyl pyrophosphate
Isopentenyl pyrophosphate is an intermediate in the classical, HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. IPP is formed from acetyl-CoA via mevalonic acid...

 (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).

Further reading

  • Merck Index
    Merck Index
    The Merck Index is an encyclopedia of chemicals, drugs and biologicals with over 10,000 monographs on single substances or groups of related compounds. It also includes an appendix with monographs on organic name reactions. It is published by the United States pharmaceutical company Merck & Co...

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

External links