Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, and by some insects. They often have a strong odor and may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores. Although sometimes used interchangeably with "terpenoids", the two terms have different meanings. Terpenes are hydrocarbons, whereas terpenoids contain additional functional groups.

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 be arranged to form rings. One can consider the isoprene unit as one of nature's common building blocks.

Types

Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.

• 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. Examples include geraniol and limonene.
• Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples include farnesol and artemisinin.
• Diterpenes are composed of four isoprene units and have the molecular formula C20H32. They derive from geranylgeranyl pyrophosphate. Examples include taxol and gibberellin.
• Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. An example of a sesterterpenoid is geranylfarnesol.
• 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 either lanosterol or cycloartenol, the structural precursors to all the steroids.
• Tetraterpenes contain eight isoprene units and have the molecular formula C40H56. Biologically important tetraterpenoids 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.

Role in plants and animals

In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature. Steroids, for example, are derivatives of the triterpene squalene.

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. Terpenoids are also known as isoprenoids.

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as fragrances in perfumery, and in medicine and alternative medicines such as aromatherapy. 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. Vitamin A is a terpenoid.

See also

• List of beneficial terpenes
• Terpenoid
• Isoprene
• Essential oil
• Aromatherapy
• Perfumery
• Food additives

References

Terpene Resources
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