Isoprene, or2-methyl-1,3-butadiene, is a commonvolatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals[1] (including humans) and its polymers are the main component ofnatural rubber.
Charles Greville Williams named the compound in 1860 after obtaining it from thepyrolysis of natural rubber. He correctly deduced the mass shares of carbon and hydrogen[2] (but arrived at an incorrect formula C10H8 because the modern atomic weight of carbon was not adopted until theKarlsruhe Congress held later that year). He did not specify the reasons for the name, but it is hypothesized that it came from "propylene" with which isoprene shares some physical and chemical properties. The first one to observe recombination of isoprene into rubber-like substance wasGustave Bouchardat [de] in 1879, andWilliam A. Tilden identified its structure five years later.[3]
Isoprene is produced and emitted by many species of trees (major producers areoaks,poplars,eucalyptus,phytoplankton, and some legumes). Yearly production of isoprene emissions by vegetation is around 600 millionmetric tons, half from tropical broadleaf trees and the remainder primarily fromshrubs.[4] This is about equivalent tomethane emissions and accounts for around one-third of allhydrocarbons released into the atmosphere. Indeciduous forests, isoprene makes up approximately 80% of hydrocarbon emissions. While their contribution is small compared to trees, microscopic and macroscopicalgae also produce isoprene.[5]
Isoprene is made through themethyl-erythritol 4-phosphate pathway (MEP pathway, also called the non-mevalonate pathway) in thechloroplasts of plants. One of the two end-products of MEP pathway,dimethylallyl pyrophosphate (DMAPP), is cleaved by the enzymeisoprene synthase to form isoprene and diphosphate. Therefore, inhibitors that block the MEP pathway, such asfosmidomycin, also block 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. Global emission of isoprene by plants is estimated at around 350 million tons per year.[6]
Isoprene emission in plants is controlled both by the availability of the substrate (DMAPP) and byenzyme (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.[7][8][9] The estimated production rate of isoprene in the human body is 0.15μmol/(kg·h), equivalent to approximately 17 mg/day for a person weighing 70 kg. Human breath isoprene originates from lipolytic cholesterol metabolism within the skeletal muscular peroxisomes andIDI2 gene acts as the production determinant.[10] Due to the absence ofIDI2 gene, animals such as pigs and bottle-nose dolphins do not exhale isoprene.
Isoprene is common in low concentrations in many foods. Many species of soil and marine bacteria, such asActinomycetota, are capable of degrading isoprene and using it as a fuel source.
Chemical structure ofcis-polyisoprene, the main constituent of natural rubber
Isoprene emission appears to be a mechanism that trees use to combatabiotic stresses.[11] In particular, isoprene has been shown to protect against moderate heat stress (around 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.[12] 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.[13]
The isoprene skeleton can be found in naturally occurring compounds calledterpenes andterpenoid (oxygenated terpenes), collectively called isoprenoids. These compounds do not arise from isoprene itself. Instead, the precursor to isoprene units in biological systems isdimethylallyl pyrophosphate (DMAPP) and its isomerisopentenyl pyrophosphate (IPP). The plural 'isoprenes' is sometimes used to refer toterpenes in general.
Examples of isoprenoids includecarotene,phytol,retinol (vitamin A),tocopherol (vitamin E),dolichols, andsqualene.Heme A has an isoprenoid tail, andlanosterol, the sterol precursor in animals, is derived from squalene and hence from isoprene. The functional isoprene units in biological systems aredimethylallyl pyrophosphate (DMAPP) and its isomerisopentenyl pyrophosphate (IPP), which are used in the biosynthesis of naturally occurring isoprenoids such ascarotenoids,quinones, lanosterol derivatives (e.g. steroids) and theprenyl chains of certain compounds (e.g. phytol chain of chlorophyll). Isoprenes are used in the cell membrane monolayer of manyArchaea, 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.
Isoprene is most readily available industrially as a byproduct of the thermalcracking ofpetroleum naphtha or oil, as a side product in the production ofethylene. Where thermal cracking of oil is less common, isoprene can be produced by dehydrogenation ofisopentane. Isoprene can be synthesized in two steps fromisobutylene, starting with itsene reaction withformaldehyde to give isopentenol, which can be dehydrated to isoprene:[15]
Production of isoprene from isobutene viaene reaction
Where cheapacetylene is produced from coal-derivedcalcium carbide, it may be combined withacetone to make 3-methylbutynol which is then hydrogenated and dehydrated to isoprene.[16]
About 800,000 metric tons are produced annually. About 95% of isoprene production is used to produce cis-1,4-polyisoprene—asynthetic version ofnatural rubber.[14]
Natural rubber consists mainly of poly-cis-isoprene with a molecular mass of 100,000 to 1,000,000 g/mol. Typically natural rubber contains a few percent of other materials, such as proteins, fatty acids, resins, and inorganic materials. Some natural rubber sources, calledgutta percha, are composed of trans-1,4-polyisoprene, a structuralisomer that has similar, but not identical, properties.[14]
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^Vickers CE, Possell M, Cojocariu CI, Velikova VB, Laothawornkitkul J, Ryan A, Mullineaux PM, Nicholas Hewitt C (May 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.PMID19183288.
^Benjamin MT, Sudol M, Bloch L, Winer AM (1996). "Low-emitting urban forests: A taxonomic methodology for assigning isoprene and monoterpene emission rates".Atmospheric Environment.30 (9):1437–1452.Bibcode:1996AtmEn..30.1437B.doi:10.1016/1352-2310(95)00439-4.
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