Isoprene

Isoprene, also known as 2-methyl-1,3-butadiene, is a common volatile organic compound. In its pure form, it is a colorless liquid that evaporates very easily. This substance is highly significant because its polymers form the main component of natural rubber.

Beyond rubber, isoprene plays a vital role in the biological processes of many living organisms. It is produced by various plants, animals, and even humans. Understanding isoprene helps scientists learn how life manages heat and builds complex molecules.

In plants, isoprene is created through a specific chemical process called the methyl-erythritol 4-phosphate pathway, or the MEP pathway. This process occurs inside the chloroplasts, which are the parts of plant cells that handle light energy. One of the end-products of this pathway is a substance called dimethylallyl pyrophosphate, or DMAPP.

An enzyme known as isoprene synthase then acts on the DMAPP. This enzyme cleaves the molecule to produce isoprene and diphosphate. Because this enzyme is required, any substance that blocks the MEP pathway will also stop isoprene from forming.

Plants release isoprene into the atmosphere in massive quantities. Vegetation produces roughly 600 million metric tons of isoprene emissions every year. About half of this comes from tropical broadleaf trees, while the rest comes mostly from shrubs. In deciduous forests, isoprene accounts for about 80% of all hydrocarbon emissions. This amount is nearly equivalent to the amount of methane released into the air.

These emissions are not constant. They change based on leaf mass, leaf area, light levels, and temperature. During the night, plants release very little isoprene. On hot, sunny days, emissions can reach up to 25 micrograms per gram of dry leaf weight every hour.

Scientists believe isoprene serves a protective biological role for plants. This is known as the thermotolerance hypothesis. Isoprene emission increases dramatically as temperatures rise, reaching a maximum at around 40 °C. It is thought that isoprene helps plants combat abiotic stresses, such as moderate heat. It may also help plants handle large changes in leaf temperature. Specifically, isoprene is incorporated into cell membranes to help stabilize them against heat. It also provides resistance against reactive oxygen species.

The history of isoprene research began in the mid-19th century. In 1860, Charles Greville Williams named the compound. He obtained it through the pyrolysis, or heating, of natural rubber. Williams correctly identified the mass shares of carbon and hydrogen in the molecule. In 1879, researchers first observed the recombination of isoprene into a rubber-like substance. Five years later, William A. Tilden successfully identified the chemical structure of the molecule.

Isoprene is also a natural part of animal and human biology. In humans, it is the most abundant hydrocarbon that can be measured in our breath. A person weighing 70 kg produces approximately 17 mg of isoprene per day. This production comes from the metabolism of cholesterol within the peroxisomes of skeletal muscles. This process is determined by the IDI2 gene. Interestingly, animals like pigs and bottle-nose dolphins do not exhale isoprene because they lack this specific gene.

Isoprene is a fundamental building block for a larger group of molecules called isoprenoids. These include terpenes and terpenoids, which are oxygenated versions of terpenes. While isoprene itself is not the direct source, its biological precursors, DMAPP and IPP, are used to build them.

Common isoprenoids include carotene, retinol (vitamin A), and tocopherol (vitamin E). Even complex molecules like squalene and lanosterol are derived from these isoprene-based pathways. This connectivity shows how a single chemical structure can support a vast array of life-sustaining substances.

Finally, isoprene is important for modern industry. Humans produce about 800,000 metric tons of isoprene annually through industrial methods. It is often obtained as a byproduct of the thermal cracking of petroleum naphtha or oil. Some processes also produce it by the dehydrogenation of isopentane. About 95% of this industrial isoprene is used to create cis-1,4-polyisoprene. This is a synthetic version of natural rubber used in many manufactured goods.