Terpenes (//) are a class of natural products consisting of compounds with the formula (C5H8)n. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes are further classified by the number of carbons: monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), etc. A well known monoterpene is alpha-pinene, a major component of turpentine.
Still more numerous than terpenes is a class of compounds called "terpenoids". Terpenoids are terpenes that have been modified with (usually oxygen-containing) functional groups. The terms terpenes and terpenoids are used interchangeably. Both have strong and often pleasant odors, which may protect their hosts or attract pollinators. The inventory of terpenes and terpenoids is estimated at 55,000 chemical entities.
Terpenes are have no large scale commercial applications. In the form of essential oils, terpenes are used widely as fragrances in perfumery and traditional medicine, such as aromatherapy. Some form hydroperoxides that are valued as catalysts in the production of polymers. Terpenes, especially pinenes, are components of turpentine, a solvent obtained from pine trees. The possibility that terpenes could be used as precursors to polymers has been investigated as an alternative to petroleum-based feedstocks. Few applications have been commercialized.
Terpenes are also major biosynthetic building blocks. Steroids, for example, are derivatives of the triterpene squalene. Synthetic variations and derivatives of natural terpenes also greatly expand the variety of aromas used in perfumery and flavors used in food additives. Terpenes and terpenoids help protect the host plant by deterring herbivores and by attracting predators and parasites of herbivores. They appear to play roles as antifeedants and wound repair.
Terpenes are useful active ingredients as part of natural agricultural pesticides. Terpenes are used by termites of the subfamily Nasutitermitinae to ward off predatory insects, through the use of a specialized mechanism called a fontanellar gun.
Higher amounts of terpenes are released by trees in warmer weather, and may be a natural form of cloud seeding. The clouds reflect sunlight, allowing the forest temperature to regulate. The aroma and flavor of hops comes, in part, from sesquiterpenes (mainly α-humulene and β-caryophyllene), which affect beer quality.
Physical and chemical properties
Terpenes are typically aromatic oils, except camphene which is a crystalline solid. They are colorless, although impure samples are often yellow. Boiling points range from 120 - 220 °C. Being highly non-polar, they are insoluble in water. (But notably glycosides of terpenes are soluble in water). Most terpenes have low specific gravity, and therefore float on water. They are tactilely light oils considerably less viscous than familiar vegetable oils like corn oil (28 cP), with viscosity ranging from 1 cP(ala water) to 6 cP. Like other hydrocarbons, they are highly flammable. Terpenes are local irritants and can cause gastrointestinal disturbances if ingested.
Although terpenes are derived from achiral precursors (isopentenyl or dimethylallyl phosphates), they are often chiral and generally available in high optical purity. Thus, they are important components of the chiral pool. Since they carry no functional groups aside from their unsaturation, terpenes are distinctive. The unsaturation in terpenes is usually associated with disubstituted alkenes. Disubstituted alkenes resist polymerization (low ceiling temperatures) but are highly susceptible to acid-induced carbocation formation.
The term "terpene" was coined in 1866 by the German chemist August Kekulé.Although sometimes used interchangeably with "terpenes", terpenoids (or isoprenoids) are modified terpenes that contain additional functional groups, usually oxygen-containing.The name "terpene" is a shortened form of "terpentine", an obsolete spelling of "turpentine".
Conceptually derived from isoprenes, the structures and formulas of terpenes follow the biogenetic isoprene rule or the C5 rule, as described in 1953 by Leopold Ružička and coworkers. The isoprene units are provided from isoprenyl pyrophosphate (aka dimethylallyl pyrophosphate) and isopentenyl pyrophosphate, which exist in equilibrium. This pair of building blocks are produced by two distinct metabolic pathways: the mevalonic acid pathway and the MEP/DOXP pathway.
Mevalonic acid pathway
The 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP pathway), also known as non-mevalonate pathway or mevalonic acid-independent pathway, starts with pyruvate as the carbon source.
Pyruvate and glyceraldehyde 3-phosphate are converted by DOXP synthase (Dxs) to 1-deoxy-D-xylulose 5-phosphate, and by DOXP reductase (Dxr, IspC) to 2-C-methyl-D-erythritol 4-phosphate (MEP). The subsequent three reaction steps catalyzed by 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase (YgbP, IspD), 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (YchB, IspE), and 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (YgbB, IspF) mediate the formation of 2-C-methyl-D-erythritol 2,4-cyclopyrophosphate (MEcPP). Finally, MEcPP is converted to (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP) by HMB-PP synthase (GcpE, IspG), and HMB-PP is converted to isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) by HMB-PP reductase (LytB, IspH).
IPP and DMAPP are the end-products in either pathway, and are the precursors of isoprene, monoterpenoids (10-carbon), diterpenoids (20-carbon), carotenoids (40-carbon), chlorophylls, and plastoquinone-9 (45-carbon). Synthesis of all higher terpenoids proceeds via formation of geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP).
The MVA and MEP are mutually exclusive in most organisms.
|Bacteria||MVA or MEP|
|Plants||MVA and MEP|
Geranyl pyrophosphate phase and beyond
In both MVA and MEP pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase. IPP and DMAPP condense to give geranyl pyrophosphate, the precursor to monoterpenes and monoterpenoids.
Geranyl pyrophosphate is also converted to farnesyl pyrophosphate and geranylgeranyl pyrophosphate, respectively C15 and C20 precursors to sesquiterpenes and diterpenes (as well as sesequiterpenoids and diterpenoids). Biosynthesis is mediated by terpene synthase.
Terpenes to terpenoids
Terpenes can be visualized as the result of linking isoprene units "head to tail" to form chains, which can be arranged to form rings. Isoprene is a compound in its own right, a colorless volatile liquid with a petroleum-like odor. A few terpenes are linked “tail to tail”, and larger branched terpenes may be linked “tail to mid”. These are called “irregular” terpenes. The number and types of variations which can occur are large and flexible: at least 20,000 distinct skeletons (characteristic arrangements of carbon atoms and bonds) are known to occur naturally.
Terpenes may exist in different structural forms with the same general name and chemical formula called isomers. Each such form may also occur in chiralities called stereoisomers or enantiomers. Different structural forms do not necessarily share the same physical and chemical properties. Nature usually has a preferred chirality, i.e. selectivity for a right-handed or left-handed version of a molecule, such that one occurs and the other not at all, called homochirality. Such is also true of the terpenes. Even though stereoisomers have the same physical and chemical properties they may (and usually do) have different biological and pharmacological effects. Structural forms differ in the position of carbon-carbon double bonds or functional group substitutions, branch arrangements of chain compounds, or differing functional groups with the same chemical formula.
Limonene, a monoterpene.
Carvone is a monoterpenoid, a modified monoterpene.
Humulene, a sesquiterpene.
Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of isoprene pairs needed to assemble the molecule. Commonly, terpenes contain 2, 3, 4 or 6 isoprene units; the tetraterpenes (8 isoprene units) form a separate class of compounds called carotenoids; the others are rare. The classification is formalistic only; nothing may be inferred about their properties, uses or occurrence.
- 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 and monoterpenoids include geraniol, terpineol (present in lilacs), limonene (present in citrus fruits), myrcene (present in hops), linalool (present in lavender) or pinene (present in pine trees). Iridoids derive from monoterpenes.
- Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesenes, farnesol. (The sesqui- prefix means one and a half.)
- Diterpenes are composed of four isoprene units and have the molecular formula C20H32. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes and diterpenoids are cafestol, kahweol, cembrene and taxadiene (precursor of taxol). 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. (The sester- prefix means two and a half.) 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.
- Sesquarterpenes are composed of seven isoprene units and have the molecular formula C35H56. Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.
- Tetraterpenes contain eight isoprene units and have the molecular formula C40H64. 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.
- Norisoprenoids, such as the C13-norisoprenoids 3-oxo-α-ionol present in Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in Shiraz leaves (both grapes in the species Vitis vinifera) or wine (responsible for some of the spice notes in Chardonnay), can be produced by fungal peroxidases or glycosidases.
While terpenes and terpenoids occur widely, their extraction from natural sources is often problematic. Consequently, they are produced by chemical synthesis, usually from petrochemicals. In one route, acetone and acetylene are condensed to give 2-Methylbut-3-yn-2-ol, which is extended with acetoacetic ester to give geranyl alcohol. Others are prepared from those terpenes and terpenoids that are readily isolated in quantity, say from the paper and tall oil industries. For example, α-pinene, which is readily obtainable from natural sources, is converted to citronellal and camphor. Citronellal is also converted to rose oxide and menthol.
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|Wikimedia Commons has media related to Terpenes.|
- Institute of Chemistry - terpenes
- Structures of alpha pinene and beta pinene
- Terpenes at the US National Library of Medicine Medical Subject Headings (MeSH)
- Pope, Frank George (1911). Encyclopædia Britannica. 26 (11th ed.). Cambridge University Press. pp. 647–652. This contains a detailed survey of the chemistry of many terpenes, which had recently been investigated fully. . In Chisholm, Hugh (ed.).