Isoprene is a common synonym for the chemical compound 2-methyl-1,3-butadiene. It is commonly used in industry, is an important biological material, and can be a harmful environmental pollutant and toxicant when present in excess quantities.
At room temperature, isoprene is a colorless liquid which is highly flammable and easily ignited. It can form explosive mixtures in air and is highly reactive, capable of polymerizing explosively when heated. The United States Department of Transportation considers isoprene a hazardous material and requires special marking, labeling, and transportation for it.
It is most readily available industrially as a by-product of the thermal cracking of naphtha or oil. About 95% of isoprene production is used to produce cis-1,4-polyisoprene - a synthetic version of natural rubber.
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 are composed of trans-1,4-polyisoprene, a structural isomer which has similar, but not identical properties.
Biological roles and effects
Isoprene is formed naturally in plants and animals and is generally the most common hydrocarbon found in the human body. The estimated production rate of isoprene in the human body is 15 µmol/kg/h, equivalent to approximately 17 mg/day for a 70 kg person. It is widely present in the natural environment at low concentrations and is also common in low concentrations in many foods.
It 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. 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 synthetised by the HMG-CoA reductase pathway. Addition of these chains to proteins is termed isoprenylation.
According to the United States Department of Health and Human Services Eleventh Edition Report on Carcinogens, isoprene is reasonably expected to be a human carcinogen. Tumors have been observed in multiple locations in multiple test species exposed to isoprene vapor. No adequate human studies of the relationship between isoprene exposure and human cancer have been reported.
According to research in 2004 by the UK Natural Environment Research Council (NERC), isoprene is released by vascular plants in extreme hot weather, catalysing the conversion of nitrogen oxide, an atmospheric pollutant, to ozone, a gas that irritates the lungs, particularly in asthma sufferers.
Reference
Merck Index, Eleventh Edition, ISBN 911910-28-X.
External link
Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program