Isoprene, or 2-methyl-1,3-butadiene, is a common

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...

with the formula CH₂=C(CH₃)−CH=CH₂. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals (including humans) and its polymers are the main component of

natural rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...

. C. G. Williams named the compound in 1860 after obtaining it from thermal decomposition (

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

) of natural rubber; he correctly deduced the empirical formula C₅H₈.

Natural occurrences

Isoprene is produced and emitted by many species of trees (major producers are

oak An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...

poplars ''Populus'' is a genus of 25–30 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar (), aspen, and cottonwood. The we ...

eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as euca ...

, and some legumes). Yearly production of isoprene emissions by vegetation is around 600 million

metric ton The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States c ...

s, half from tropical broadleaf trees and the remainder primarily from

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...

s. This is about equivalent to

methane emissions Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...

and accounts for around one-third of all

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

released into the atmosphere. In deciduous forests, isoprene makes up approximately 80% of hydrocarbon emissions. While their input is small compared to trees, microscopic and macroscopic

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

also produce isoprene.

Plants

Isoprene is made through the methyl-erythritol 4-phosphate pathway (MEP pathway, also called the non-

mevalonate pathway The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria. The pathway produces two five-carbon building blocks called isopentenyl ...

) in the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s of plants. One of the two end-products of MEP pathway,

dimethylallyl pyrophosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...

(DMAPP), is cleaved by the enzyme

isoprene synthase The enzyme isoprene synthase (EC 4.2.3.27) catalysis, catalyzes the chemical reaction : prenyl pyrophosphate \rightleftharpoons isoprene + diphosphate This enzyme belongs to the family of lyases, specifically those carbon-oxygen lyases acting on ...

to form isoprene and diphosphate. Therefore, inhibitors that block the MEP pathway, such as

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 ...

, 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.

Regulation

Isoprene emission in plants is controlled both by the availability of the substrate (DMAPP) and by

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

(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. Many species of soil and marine bacteria, such as

Actinomycetota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soi ...

, are capable of degrading isoprene and using it as a fuel source. PolyIsopreneCorrected

Biological roles

Isoprene emission appears to be a mechanism that trees use to combat

abiotic stress Abiotic stress is the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performan ...

es. 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. 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.

Isoprenoids

The isoprene skeleton can be found in naturally occurring compounds called

terpenes Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ar ...

(also known as isoprenoids), but these compounds do not arise from isoprene itself. Instead, the precursor to isoprene units in biological systems is

(DMAPP) and its isomer

isopentenyl pyrophosphate Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of ...

(IPP). The plural 'isoprenes' is sometimes used to refer to

terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ar ...

s in general. Examples of isoprenoids include

carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the exc ...

phytol Phytol (florasol, phytosol) is an acyclic hydrogenated 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 ...

retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophtha ...

(

vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably bet ...

tocopherol Tocopherols (; TCP) are a class of organic chemical compounds (more precisely, various methylated phenols), many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rats, i ...

(

vitamin E Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vitami ...

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 Dolich ...

s, and

squalene Squalene is an organic compound. It is a triterpenoid with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as ''Squalus'' is a genus of sharks). A ...

Heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consisti ...

A has an isoprenoid tail, and

lanosterol Lanosterol is a tetracyclic triterpenoid and is the compound from which all animal and fungal steroids are derived. By contrast plant steroids are produced via cycloartenol. Role in biosynthesis of other steroids Elaboration of lanosterol under e ...

, the sterol precursor in animals, is derived from squalene and hence from isoprene. The functional isoprene units in biological systems are

(DMAPP) and its isomer

(IPP), which are used in the biosynthesis of naturally occurring isoprenoids such as

carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...

quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds

uch as benzene or naphthalene Uch ( pa, ; ur, ), frequently referred to as Uch Sharīf ( pa, ; ur, ; ''"Noble Uch"''), is a historic city in the southern part of Pakistan's Punjab province. Uch may have been founded as Alexandria on the Indus, a town founded by Alexand ...

by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...

s, lanosterol derivatives (e.g. steroids) and the

prenyl Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to ...

chains of certain compounds (e.g. phytol chain of chlorophyll). Isoprenes are used in the cell membrane monolayer of many

Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

, 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. Similarly,

is composed of linear

polyisoprene Polyisoprene is strictly speaking a collective name for Polymer, polymers that are produced by polymerization of isoprene. In practice polyisoprene is commonly used to refer to synthetic ''cis''-1,4-polyisoprene, made by the industrial polymerisa ...

chains of very high

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

and other natural molecules.

Impact on aerosols

After release, isoprene is converted into various species, such as

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

hydroperoxide Hydroperoxides or peroxols are Chemical compound, compounds containing the hydroperoxide functional group (ROOH). If the R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have t ...

s, organic nitrates, and

epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for ...

sOrganic Carbon Compounds Emitted By Trees Affect Air Quality
ScienceDaily, Aug. 7, 2009 by short-lived

free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...

(like the

hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...

) and to a lesser extent by

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

.IUPAC Subcommittee on Gas Kinetic Data Evaluation �
Data Sheet Ox_VOC7
2007 These new species can dissolve into water droplets and contribute to

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...

and

haze Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classification ...

formation.A source of haze
ScienceNews, August 6, 2009 Secondary organic aerosols formed from this pathway are a current topic of research and may have atmospheric impacts. 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. As an example the

Blue Ridge Mountains The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. The mountain range is located in the Eastern United States, and extends 550 miles southwest from southern Pennsylvania through Maryland, West Virgin ...

are noted for having a bluish color when seen from a distance. Trees put the "blue" in Blue Ridge, from the isoprene released into the atmosphere. This contributes to the characteristic haze on the mountains and their distinctive color.

Industrial production

Isoprene is most readily available industrially as a byproduct of the thermal cracking of

naphtha Naphtha ( or ) is a flammable liquid hydrocarbon mixture. Mixtures labelled ''naphtha'' have been produced from natural gas condensates, petroleum distillates, and the distillation of coal tar and peat. In different industries and regions ''n ...

or oil, as a side product in the production of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...

. About 800,000 metric tons are produced annually. About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a synthetic version of

. 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, called

gutta percha Gutta-percha is a tree of the genus ''Palaquium'' in the family Sapotaceae. The name also refers to the rigid, naturally biologically inert, resilient, electrically nonconductive, thermoplastic latex derived from the tree, particularly from ' ...

, are composed of trans-1,4-polyisoprene, a structural

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

that has similar, but not identical, properties.

References

External links

Report on Carcinogens, Fourteenth 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
{{Authority control Alkadienes Hemiterpenes IARC Group 2B carcinogens Monomers Conjugated dienes