Isoprenoid

isoprenoid, any of a class of organic compounds composed of two or more units of hydrocarbons, with each unit consisting of five carbon atoms arranged in a specific pattern. Isoprenoids play widely varying roles in the physiological processes of plants and animals. They also have a number of commercial uses.

Isoprenoids in living organisms range in function from pigments and fragrances to vitamins and precursors of sex hormones. One of the most familiar natural substances, rubber, is a polyisoprene. Other commercially valuable isoprenoids are those used as flavourings, solvents, and raw materials for chemicals.

The five-carbon unit that constitutes the basic building block of isoprenoids is a hydrocarbon called isoprene. Isoprene (2-methyl-1,3-butadiene) is a branched-chain unsaturated hydrocarbon, unsaturated meaning it contains one or more double bonds between carbon atoms. Isoprene has in fact two carbon-carbon double bonds. Isoprenoids contain from two to many thousands of isoprene units. The carbon backbone of an isoprenoid can have one or more functional chemical groups, such as hydroxyl and carbonyl, attached to it. These contribute to the diversity of isoprenoids.

The name terpene specifically refers to naturally occurring compounds that are derivatives of a single isoprene unit, but modern usage has become less precise, and the terms terpenes and isoprenoids are used interchangeably. The word terpene comes from an older spelling of turpentine (terpentine), a cleaner that is a mixture of isoprenoids (originally the resin of the terebinth plant).

The smallest terpene molecules—those containing 10 carbon atoms—are called monoterpenes. The larger molecules, increased by one isoprene unit at a time, are called sesquiterpenes, diterpenes, triterpenes, and tetraterpenes, respectively (see below Isoprenoid compounds). The monoterpenes are mostly volatile, which accounts for their fragrances. Terpenes of higher molecular weight are less volatile, although sesquiterpenes contribute to the flavours of some foods.