Terpene

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

Terpenes (/ˈtɜːrpn/) 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.[1][2] 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.[3]

Applications[edit]

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.[4] 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.[5]

Biological function[edit]

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers.[6]

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.[7][8] They appear to play roles as antifeedants and wound repair.

Terpenes are useful active ingredients as part of natural agricultural pesticides.[9] 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.[10]

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.[11] The aroma and flavor of hops comes, in part, from sesquiterpenes (mainly α-humulene and β-caryophyllene), which affect beer quality.[12]

Physical and chemical properties[edit]

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.

Terminology[edit]

The term "terpene" was coined in 1866 by the German chemist August Kekulé.[13]Although sometimes used interchangeably with "terpenes", terpenoids (or isoprenoids) are modified terpenes that contain additional functional groups, usually oxygen-containing.[14]The name "terpene" is a shortened form of "terpentine", an obsolete spelling of "turpentine".

Biosynthesis[edit]

Bioynthetic conversion of geranylpyrophosphate to the terpenes α-pinene and β-pinene and to the terpinoid α-terpineol.[2]

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.[15] 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[edit]

Most organisms produce terpenes through the HMG-CoA reductase pathway, known as the Mevalonate pathway, named for the intermediates mevalonic acid. This pathway begins with acetyl CoA.

MEP/DOXP pathway[edit]

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.

Organism Pathways
Bacteria MVA or MEP
Archaea MVA
Green Algae MEP
Plants MVA and MEP
Animals MVA
Fungi MVA

Geranyl pyrophosphate phase and beyond[edit]

Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) condense to produce geranyl pyrophosphate, precursor to all terpenes and terpenoids.

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).[2] Biosynthesis is mediated by terpene synthase.[16][17]

Terpenes to terpenoids[edit]

The genomes of 17 plant species contain genes that encode terpenoid synthase enzymes imparting terpenes with their basic structure, and cytochrome P450s that modify this basic structure.[2][18]

Structure[edit]

Terpenes can be visualized as the result of linking isoprene units "head to tail" to form chains, which can be arranged to form rings.[19] 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.

Classification[edit]

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.

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

Industrial syntheses[edit]

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.[1]

Summary of an industrial route to geranyl alcohol from simple reagents.

References[edit]

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  2. ^ a b c d Davis, Edward M.; Croteau, Rodney (2000). "Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes". Biosynthesis. Topics in Current Chemistry. 209. pp. 53–95. doi:10.1007/3-540-48146-X_2. ISBN 978-3-540-66573-1.CS1 maint: uses authors parameter (link)
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  13. ^ Kekulé coined the term "terpene" to denote all hydrocarbons having the empirical formula C10H16, of which camphene was one. Previously, many hydrocarbons having the empirical formula C10H16 had been called "camphene", but many other hydrocarbons of the same composition had had different names. Hence Kekulé coined the term "terpene" in order to reduce the confusion. See:
    • Kekulé, August (1866). Lehrbuch der organischen Chemie [Textbook of Organic Chemistry] (in German). vol. 2. Erlangen, (Germany): Ferdinand Enke. pp. 464–465. From pp. 464–465: "Mit dem Namen Terpene bezeichnen wir … unter verschiedenen Namen aufgeführt werden." (By the name "terpene" we designate in general the hydrocarbons composed according to the [empirical] formula C10H16 (see §. 1540). Many chemists subsume the hydrocarbons of the formula C10H16 under the general name "camphene". That name seems unsuitable, because a specific substance of this group has been designated as "camphene". In general, a great confusion prevails in the designation of substances belonging here [i.e., to the terpene group]. Many, obviously different hydrocarbons have not been distinguished for a long time and have been assigned the same names, while on the other hand, probably identical substances from different sources have often been specified by different names.)
    • Dev, Sukh (1989). "Chapter 8. Isoprenoids: 8.1. Terpenoids.". In Rowe, John W. (ed.). Natural Products of Woody Plants: Chemicals Extraneous to the Lignocellulosic Cell Wall. Berlin and Heidelberg, Germany: Springer-Verlag. pp. 691–807. ; see p. 691.
  14. ^ "IUPAC Gold Book - terpenoids".
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