Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries.[1]

Biosynthesis[edit]

Monoterpenes are derived biosynthetically from units of isopentenyl pyrophosphate, which is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, 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.

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

Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids).[2] Biosynthesis is mediated by monoterpene synthases.[3][4]

Elimination of the pyrophosphate group from geranyl pyrophosphate leads to the formation of acyclic monoterpenes such as ocimene and the myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others. Many monoterpenes found in marine organisms are halogenated, such as halomon.

In addition to linear attachments, the isoprene units can make connections to form rings. The most common ring size in monoterpenes is a six-membered ring. A classic example is the cyclization of geranyl pyrophosphate to form limonene.

LimoneneBiosynthesis.svg

The terpinenes, phellandrenes, and terpinolene are formed similarly. Hydroxylation of any of these compounds followed by dehydration can lead to the aromatic p-cymene. Important terpenoids derived from monocyclic terpenes are menthol, thymol, hinokitiol, carvacrol, grapefruit mercaptan and many others.

Geranyl pyrophosphate can also undergo two sequential cyclization reactions to form bicyclic monoterpenes, such as pinene which is the primary constituent of pine resin.

Pinene biosynthesis en.svg

Other bicyclic monoterpenes include carene, sabinene, camphene, and thujene. Camphor, borneol, eucalyptol and ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging peroxide functional groups, respectively.[5][6] Umbellulone is another example of bicyclic monoterpene ketone.

Natural roles[edit]

Monoterpenes are found in many parts of different plants, such as barks, heartwood, softwood of trees, in vegetables, fruits and herbs.[7] Essential oils are very rich in montorerpenes. Several monoterpenes produced by trees, such as linalool, hinokitiol, and ocimene have fungicidal and antibacterial activities and participate in wound healing.[8] Some of these compounds are produced to protect the trees from insect attacks.

Monoterpenes are emitted by forests and form aerosols that are proposed to serve as cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate.[9]

Many monoterpenes have unique smell and flavor. For example, sabinene contributes to the spicy taste of black pepper, 3-carene gives cannabis an earthy taste and smell, citral has a lemon-like pleasant odor and contributes to the distinctive smell of citrus fruits, and thujene and carvacrol are responsible for the pungent flavors of summer savory and oregano, respectively.[10][11][12]

Monoterpenes are considered allelochemicals.[13]

Uses[edit]

Many monoterpenes are volatile compounds and some of them are well-known fragrants found in the essential oils of many plants.[14] For example, camphor, citral, citronellol, geraniol, grapefruit mercaptan, eucalyptol, ocimene, myrcene, limonene, linalool, menthol, camphene and pinenes are used in perfumes and cosmetic products. Limonene and perillyl alcohol are used in cleaning products.[15][16]

Many monoterpenes are used as food flavors and food additives, such as bornyl acetate, citral, eucalyptol, menthol, hinokitiol, camphene and limonene.[17][18] Menthol, hinokitiol and thymol are also used in oral hygiene products. Thymol also has antiseptic and disinfectant properties.[19]

Volatile monoterpenes produced by plants can attract or repel insects, thus some of them are used in insect repellents, such as citronellol, eucalyptol, limonene, linalool, hinokitiol, menthol and thymol.[18]

Ascaridole, camphor and eucalyptol are monoterpenes that have pharmaceutical use.[20][21]

See also[edit]

References[edit]

  1. ^ Eberhard Breitmaier (2006). "Hemi‐ and Monoterpenes". Terpenes: Flavors, Fragrances, Pharmaca, Pheromones. doi:10.1002/9783527609949.ch2.
  2. ^ Davis, Edward M.; Croteau, Rodney (2000). "Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes". Topics in Current Chemistry. 209: 53–95. doi:10.1007/3-540-48146-X_2.CS1 maint: uses authors parameter (link)
  3. ^ Kumari, I.; Ahmed, M.; Akhter, Y. (2017). "Evolution of catalytic microenvironment governs substrate and product diversity in trichodiene synthase and other terpene fold enzymes". Biochimie. 144: 9. doi:10.1016/j.biochi.2017.10.003. PMID 29017925.
  4. ^ Pazouki, L.; Niinemets, Ü. (2016). "Multi-Substrate Terpene Synthases: Their Occurrence and Physiological Significance". Frontiers in Plant Science. 7: 1019. doi:10.3389/fpls.2016.01019. PMC 4940680. PMID 27462341.
  5. ^ Brown, R.T. (1975). "Bicyclic Monoterpenoids". Supplements to the 2nd Edition of Rodd's Chemistry of Carbon Compounds: 53–93. doi:10.1016/B978-044453346-3.50098-6.
  6. ^ Vil’, Vera; Yaremenko, Ivan; Ilovaisky, Alexey; Terent’ev, Alexander (2 November 2017). "Peroxides with Anthelmintic, Antiprotozoal, Fungicidal and Antiviral Bioactivity: Properties, Synthesis and Reactions". Molecules. 22 (11): 1881. doi:10.3390/molecules22111881.
  7. ^ Sjöström, Eero. "Chapter 5: Extractives". Wood Chemistry: Fundamentals and Applications (Second ed.). San Diego. ISBN 978-0-08-092589-9.
  8. ^ Rowell, Roger M. (2013). "Chater 3: Cell Wall Chemistry". Handbook of Wood Chemistry and Wood Composites (2nd ed.). Boca Raton: Taylor & Francis. ISBN 9781439853801.
  9. ^ D. V. Spracklen; B. Bonn; K. S. Carslaw (2008). "Boreal forests, aerosols and the impacts on clouds and climate" (PDF). Philosophical Transactions of the Royal Society A. 366 (1885): 4613–26. Bibcode:2008RSPTA.366.4613S. doi:10.1098/rsta.2008.0201. PMID 18826917.
  10. ^ "14 - Toxicological Aspects of Ingredients Used in Nonalcoholic Beverages". Non-alcoholic beverages. Volume 6: The Science of Beverages. Duxford, United Kingdom: Woodhead Publishing. 2019. ISBN 978-0-12-815270-6.
  11. ^ PDR for herbal medicines (4th ed.). Montvale, NJ: Thomson. 2007. p. 802. ISBN 1-56363-512-7.
  12. ^ "Herbs of the Labiatae". Encyclopedia of food sciences and nutrition (2nd ed.). Amsterdam: Academic Press. 2003. ISBN 978-0-12-227055-0.
  13. ^ Saeidnia, Soodabeh; Gohari, Ahmad Reza (2012). "Trypanocidal Monoterpenes". Studies in Natural Products Chemistry. 37: 173–190. doi:10.1016/B978-0-444-59514-0.00006-7.
  14. ^ Loza-Tavera, Herminia (1999). "Monoterpenes in Essential Oils". Chemicals via Higher Plant Bioengineering. 464: 49–62. doi:10.1007/978-1-4615-4729-7_5.
  15. ^ "Limonene". pubchem.ncbi.nlm.nih.gov.
  16. ^ Laszlo, Pierre (2007). Citrus: A history. Chicago: University of Chicago Press. ISBN 9780226470283.
  17. ^ Caputi, Lorenzo; Aprea, Eugenio (1 January 2011). "Use of Terpenoids as Natural Flavouring Compounds in Food Industry". Recent Patents on Food, Nutrition & Agriculturee. 3 (1): 9–16. doi:10.2174/2212798411103010009.
  18. ^ a b Comprehensive natural products chemistry (1st ed.). Amsterdam: Elsevier. 1999. p. 306. ISBN 978-0-08-091283-7.
  19. ^ "R.E.D. FACTS: Thymol" (PDF). United States Environmental Protection Agency.
  20. ^ "Camphor Cream and Ointment Information". Drugs.com.
  21. ^ Tisserand, Robert (2014). "Chapter 13: Essential oil profiles". Essential oil safety : a guide for health care professionals (Second ed.). Edinburgh: Churchill Livingstone. ISBN 978-0-443-06241-4.