Terpene

Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some insects such as swallowtail butterflies, which emit terpenes from their osmeterium. They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids. Some authors will use the term terpene to include all terpenoids.

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfumery, in aroma therapy, and in traditional and alternative medicines. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives.

Structure and biosynthesis

Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C₅H₈. The basic molecular formulas of terpenes are multiples of that, (C₅H₈)_n where n is the number of linked isoprene units. This is called the isoprene rule or the C5 rule. The isoprene units may be linked together "head to tail" to form linear chains or they may can be arranged to form rings. One can consider the isoprene unit as one of nature's preferred building blocks.

Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. IPP can then be isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.

As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes.

Types

Hemiterpenes consist of a single isoprene unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids.

Monoterpenes consist of two isoprene units and have the molecular formula C₁₀H₁₆.

Sesquiterpenes consist of three isoprene units and have the molecular formula C₁₅H₂₄.

Diterpenes are composed for four isoprene units and have the molecular formula C₂₀H₃₂. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes are cembrene and taxadiene. Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol.

Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes.

Triterpenes consist of six isoprene units and have the molecular formula C₃₀H₄₈. The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to generate lanosterol, the structural precursor to all the steroids.

Tetraterpenes contain eight isoprene units and have the molecular formula C₄₀H₅₆. Biologically important tetraterpenes include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.

Polyterpenes consist of long chains of many isoprene units. Natural rubber consists of polyisoprene in which the double bonds are cis. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha.

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Structure and biosynthesis

Types

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