Monoterpene Explained

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

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.

Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids).[2] 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.

Main examples

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.[3] [4] Umbellulone is another example of bicyclic monoterpene ketone.

Natural roles

Monoterpenes are found in many parts of different plants, such as barks, heartwood, bark and leaves of coniferous trees, in vegetables, fruits and herbs.[5] Essential oils are very rich in monoterpenes. Several monoterpenes produced by trees, such as linalool, hinokitiol, and ocimene have fungicidal and antibacterial activities and participate in wound healing.[6] 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.[7]

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.[8] [9] [10]

Monoterpenes are considered allelochemicals.[11]

Uses

Many monoterpenes are volatile compounds and some of them are well-known fragrants found in the essential oils of many plants.[12] 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.[13] [14]

Many monoterpenes are used as food flavors and food additives, such as bornyl acetate, citral, eucalyptol, menthol, hinokitiol, camphene and limonene.[15] [16] Menthol, hinokitiol and thymol are also used in oral hygiene products. Thymol also has antiseptic and disinfectant properties.[17]

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.

Ascaridole, camphor and eucalyptol are monoterpenes that have pharmaceutical use.[18] [19]

Health effects

A study suggests that a range of floor cleaners with certain monoterpenes may cause indoor air pollution equivalent or exceeding the harm to respiratory tracts when the time is spent near a busy road. This is due to ozonolysis of monoterpenes like Limonene leading to the production of atmospheric SOA.[20] [21] Another study suggests monoterpenes substantially affect ambient organic aerosol with uncertainties regarding environmental impacts.[22] In a review, scientists concluded that they hope that these "substances will be extensively studied and used in more and more in medicine".[23] A 2013 study found that "Based on adverse effects and risk assessments, d-limonene may be regarded as a safe ingredient. However, the potential occurrence of skin irritation necessitates regulation of this chemical as an ingredient in cosmetics."[24] According to a review, several studies showed "that some monoterpenes (e.g., pulegone, menthofuran, camphor, and limonene) and sesquiterpenes (e.g., zederone, germacrone) exhibited liver toxicity" and that i.a. intensive research on terpenes toxicity is needed.[25]

See also

Notes and References

  1. Book: Terpenes: Flavors, Fragrances, Pharmaca, Pheromones. Hemi‐ and Monoterpenes. Eberhard Breitmaier. 2006. 10–23. 10.1002/9783527609949.ch2. 9783527609949.
  2. Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes. Davis, Edward M. . Croteau, Rodney . Topics in Current Chemistry. 2000. 209. 53–95. 10.1007/3-540-48146-X_2. 978-3-540-66573-1.
  3. Brown . R.T. . Bicyclic Monoterpenoids . Supplements to the 2nd Edition of Rodd's Chemistry of Carbon Compounds . 1975 . 53–93 . 10.1016/B978-044453346-3.50098-6. 9780444533463 .
  4. Vil’ . Vera . Yaremenko . Ivan . Ilovaisky . Alexey . Terent’ev . Alexander . Peroxides with Anthelmintic, Antiprotozoal, Fungicidal and Antiviral Bioactivity: Properties, Synthesis and Reactions . Molecules . 2 November 2017 . 22 . 11 . 1881 . 10.3390/molecules22111881. 29099089 . 6150334 . free .
  5. Book: Sjöström . Eero . Wood Chemistry: Fundamentals and Applications . San Diego . 978-0-08-092589-9 . Second . Chapter 5: Extractives. 22 October 2013 .
  6. Book: Rowell . Roger M. . Handbook of Wood Chemistry and Wood Composites . 2013 . Taylor & Francis . Boca Raton . 9781439853801 . 2nd . Chater 3: Cell Wall Chemistry.
  7. D. V. Spracklen . B. Bonn . K. S. Carslaw . 2008 . Boreal forests, aerosols and the impacts on clouds and climate . . 10.1098/rsta.2008.0201 . 18826917 . 366 . 1885 . 4613–26 . 2008RSPTA.366.4613S. 206156442 .
  8. Book: Non-alcoholic beverages. Volume 6: The Science of Beverages . Woodhead Publishing . Duxford, United Kingdom . 978-0-12-815270-6 . 2019 . 14 - Toxicological Aspects of Ingredients Used in Nonalcoholic Beverages . 10.1016/B978-0-12-815270-6.00014-1 . https://www.sciencedirect.com/science/article/pii/B9780128152706000141. Tamer . Canan Ece . Suna . Senem . Özcan-Sinir . Gülşah . 441–481 . 187341441 .
  9. Book: PDR for herbal medicines. . 2007 . Thomson . Montvale, NJ . 978-1-56363-512-0 . 802 . 4th.
  10. Book: Encyclopedia of food sciences and nutrition . 2003 . Academic Press . Amsterdam . 978-0-12-227055-0 . 2nd . Herbs of the Labiatae . 3082–3090 . 10.1016/B0-12-227055-X/00593-9 . https://www.sciencedirect.com/science/article/pii/B012227055X005939.
  11. Saeidnia . Soodabeh . Gohari . Ahmad Reza . Trypanocidal Monoterpenes . Studies in Natural Products Chemistry . 2012 . 37 . 173–190 . 10.1016/B978-0-444-59514-0.00006-7. 9780444595140 .
  12. Book: Loza-Tavera . Herminia . Chemicals via Higher Plant Bioengineering . Monoterpenes in Essential Oils . Advances in Experimental Medicine and Biology . 1999 . 464 . 49–62 . 10.1007/978-1-4615-4729-7_5. 10335385 . 978-1-4613-7143-4 .
  13. Web site: Limonene . pubchem.ncbi.nlm.nih.gov . en.
  14. Book: Laszlo . Pierre . Citrus: A history . 2007 . University of Chicago Press . Chicago . 9780226470283.
  15. Caputi . Lorenzo . Aprea . Eugenio . Use of Terpenoids as Natural Flavouring Compounds in Food Industry . Recent Patents on Food, Nutrition & Agriculture . 1 January 2011 . 3 . 1 . 9–16 . 10.2174/2212798411103010009. 21114471 .
  16. Book: Comprehensive natural products chemistry . 1999 . Elsevier . Amsterdam . 978-0-08-091283-7 . 306 . 1st.
  17. Web site: R.E.D. FACTS: Thymol . United States Environmental Protection Agency.
  18. Web site: Camphor Cream and Ointment Information . Drugs.com . en.
  19. Book: Tisserand . Robert . Essential oil safety : a guide for health care professionals . 2014 . Churchill Livingstone . Edinburgh . 978-0-443-06241-4 . Second . Chapter 13: Essential oil profiles . https://www.sciencedirect.com/science/article/pii/B9780443062414000138.
  20. News: Cleaning products cause indoor pollution levels similar to a busy road . 10 March 2022 . New Scientist.
  21. Rosales . Colleen Marciel F. . Jiang . Jinglin . Lahib . Ahmad . Bottorff . Brandon P. . Reidy . Emily K. . Kumar . Vinay . Tasoglou . Antonios . Huber . Heinz . Dusanter . Sebastien . Tomas . Alexandre . Boor . Brandon E. . Stevens . Philip S. . Chemistry and human exposure implications of secondary organic aerosol production from indoor terpene ozonolysis . Science Advances . 25 February 2022 . 8 . 8 . eabj9156 . 10.1126/sciadv.abj9156 . 35213219 . 8880786 . 2022SciA....8J9156R . 2375-2548.
  22. Zhang . Haofei . Yee . Lindsay D. . Lee . Ben H. . Curtis . Michael P. . Worton . David R. . Isaacman-VanWertz . Gabriel . Offenberg . John H. . Lewandowski . Michael . Kleindienst . Tadeusz E. . Beaver . Melinda R. . Holder . Amara L. . Lonneman . William A. . Docherty . Kenneth S. . Jaoui . Mohammed . Pye . Havala O. T. . Hu . Weiwei . Day . Douglas A. . Campuzano-Jost . Pedro . Jimenez . Jose L. . Guo . Hongyu . Weber . Rodney J. . de Gouw . Joost . Koss . Abigail R. . Edgerton . Eric S. . Brune . William . Mohr . Claudia . Lopez-Hilfiker . Felipe D. . Lutz . Anna . Kreisberg . Nathan M. . Spielman . Steve R. . Hering . Susanne V. . Wilson . Kevin R. . Thornton . Joel A. . Goldstein . Allen H. . Monoterpenes are the largest source of summertime organic aerosol in the southeastern United States . Proceedings of the National Academy of Sciences . 12 February 2018 . 115 . 9 . 2038–2043 . 10.1073/pnas.1717513115 . 29440409 . 5834703 . 2018PNAS..115.2038Z . 0027-8424. free .
  23. Wojtunik‐Kulesza . Karolina A. . Kasprzak . Kamila . Oniszczuk . Tomasz . Oniszczuk . Anna . Natural Monoterpenes: Much More than Only a Scent . Chemistry & Biodiversity . 8 November 2019 . 16 . 12 . e1900434 . 10.1002/cbdv.201900434 . 31587473 . 203850556 . 1612-1872.
  24. Kim . Young Woo . Kim . Min Ji . Chung . Bu Young . Bang . Du Yeon . Lim . Seong Kwang . Choi . Seul Min . Lim . Duck Soo . Cho . Myung Chan . Yoon . Kyungsil . Kim . Hyung Sik . Kim . Kyu Bong . Kim . You Sun . Kwack . Seung Jun . Lee . Byung-Mu . Safety Evaluation And Risk Assessment Of d-Limonene . Journal of Toxicology and Environmental Health, Part B . 1 January 2013 . 16 . 1 . 17–38 . 10.1080/10937404.2013.769418 . 23573938 . 40274650 . 1093-7404.
  25. Zárybnický . Tomáš . Boušová . Iva . Ambrož . Martin . Skálová . Lenka . Hepatotoxicity of monoterpenes and sesquiterpenes . Archives of Toxicology . 1 January 2018 . 92 . 1 . 1–13 . 10.1007/s00204-017-2062-2 . 28905185 . 22483285 . en . 1432-0738.

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