Terpene

Many terpenes are derived commercially from conifer resins, such as those made by this pine.

Terpenes ( /ˈtɜrpiːn/ tur-peen) are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers,^[1] though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective function.

They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". 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 natural flavor additives for food, as fragrances in perfumery, and in traditional 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 an example of a terpene.

Terpenes are released by trees more actively in warmer weather, acting as a natural form of cloud seeding. The clouds reflect sunlight, allowing the forest to regulate its temperature.^[2]

The aroma and flavor of hops, highly desirable in some beers, comes from terpenes. Of the terpenes in hops myrcene, b-pinene, b-caryophyllene, and a-humulene are found in the largest quantities.^[3]

Structure and biosynthesis

Isoprene

Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C₅H₈. The basic molecular formulae 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.

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. An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.

Dimethylallyl pyrophosphate

Isopentenyl pyrophosphate

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

Second or third instar caterpillars of Papilio glaucus emit terpenes from their osmeterium

Terpenes may be classified by the number of terpene 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 of monoterpenes are: geraniol, limonene and terpineol.
• Sesquiterpenes consist of three isoprene units and have the molecular formula C₁₅H₂₄. Examples of sesquiterpenes are: farnesenes, farnesol. (The sesqui- prefix means one and a half.)
• Diterpenes are composed for four isoprene units and have the molecular formula C₂₀H₃₂. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes are cafestol, kahweol, cembrene and taxadiene (precursor of taxol). Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol. They are known to be antimicrobial and antiinflammatory.
• Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. (The sester- prefix means half to three, i.e. two and a half.) An example of a sesterterpene 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 C₄₀H₆₄. 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.

Other uses

Research into terpenes has found that many of them possess qualities that make them ideal active ingredients as part of natural agricultural pesticides.^{[citation needed]}

Terpin hydrate is a derivative of turpentine. An expectorant and humectant, it is commonly used in the treatment of acute or chronic bronchitis and related conditions.

Terpenes are used by termites of the Nasutitermitinae family to attack enemy insects, through the use of a specialized mechanism called a Fontanellar gun.^[4]

References

1. ^ Merriam-Webster Online Dictionary
2. ^ http://www.guardian.co.uk/environment/2008/oct/31/forests-climatechange
3. ^ Glenn Tinseth, "Hop Aroma and Flavor", January/February 1993, Brewing Techniques. <http://realbeer.com/hops/aroma.html> Accessed July 21, 2010.
4. ^ W. L. Nutting, M. S. Blum, and H. M. Fales. (1974). "Behavior of the North American Termite, Tenuirostritermes tenuirostris, with Special Reference to the Soldier Frontal Gland Secretion, Its Chemical Composition, and Use in Defense". Psyche (Hindawi Publishing Corporation) 81 (1): 167–177. doi:10.1155/1974/13854. http://psyche.entclub.org/81/81-167.html. Retrieved July 22, 2011. 

External links

• Institute of Chemistry - terpenes
• Structures of alpha pinene and beta pinene
• MeSH Terpenes