Theterpenoids, also known asisoprenoids, are a class of naturally occurringorganic chemicals derived from the 5-carbon compoundisoprene and its derivatives calledterpenes,diterpenes, etc. While sometimes used interchangeably with "terpenes", terpenoids contain additionalfunctional groups, usually containing oxygen.[1] When combined with the hydrocarbon terpenes, terpenoids comprise about 80,000 compounds.[2] They are the largest class of plantsecondary metabolites, representing about 60% of knownnatural products.[3] Many terpenoids have substantialpharmacological bioactivity and are therefore of interest to medicinal chemists.[4]
Plant terpenoids are used for their aromatic qualities and play a role in traditional herbal remedies. Terpenoids contribute to the scent ofeucalyptus, the flavors ofcinnamon,cloves, andginger, the yellow color insunflowers, and the red color intomatoes.[5] Well-known terpenoids includecitral,menthol,camphor,salvinorin A in the plantSalvia divinorum,ginkgolide andbilobalide found inGinkgo biloba and thecannabinoids found in cannabis. The provitaminbeta carotene is a terpene derivative called acarotenoid.
Thesteroids andsterols in animals are biologically produced from terpenoid precursors. Sometimes terpenoids are added toproteins, e.g., to enhance their attachment to thecell membrane; this is known asisoprenylation. Terpenoids play a role in plant defense as prophylaxis against pathogens andattractants for the predators of herbivores.[6]
Terpenoids are modifiedterpenes,[7] whereinmethyl groups have been moved or removed, oroxygen atoms added. Some authors use the term "terpene" more broadly, to include the terpenoids. Just like terpenes, the terpenoids can be classified according to the number ofisoprene units that comprise the parent terpene:
| Terpenoids | Analogueterpenes | Number ofisoprene units | Number of carbon atoms | General formula | Examples[8] |
|---|---|---|---|---|---|
| Hemiterpenoids | Isoprene | 1 | 5 | C5H8 | DMAPP,isopentenyl pyrophosphate,isoprenol,isovaleramide,isovaleric acid,HMBPP,prenol |
| Monoterpenoids | Monoterpenes | 2 | 10 | C10H16 | Bornyl acetate,camphor,carvone,citral,citronellal,citronellol,geraniol,eucalyptol,hinokitiol,iridoids,linalool,menthol,thymol |
| Sesquiterpenoids | Sesquiterpenes | 3 | 15 | C15H24 | Farnesol,geosmin,humulone |
| Diterpenoids | Diterpenes | 4 | 20 | C20H32 | Abietic acid,ginkgolides,paclitaxel,retinol,salvinorin A,sclareol,steviol |
| Sesterterpenoids | Sesterterpenes | 5 | 25 | C25H40 | Andrastin A,manoalide |
| Triterpenoids | Triterpenes | 6 | 30 | C30H48 | Amyrin,betulinic acid,limonoids,oleanolic acid,sterols,squalene,ursolic acid |
| Tetraterpenoids | Tetraterpenes | 8 | 40 | C40H64 | Carotenoids |
| Polyterpenoid | Polyterpenes | >8 | >40 | (C5H8)n | Gutta-percha,natural rubber |
Terpenoids can also be classified according to the type and number of cyclic structures they contain: linear, acyclic, monocyclic, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic.[8] TheSalkowski test can be used to identify the presence of terpenoids.[9]
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Terpenoids, at least those containing an alcohol functional group, often arise by hydrolysis of carbocationic intermediates produced fromgeranyl pyrophosphate. Analogously hydrolysis of intermediates fromfarnesyl pyrophosphate givessesquiterpenoids, and hydrolysis of intermediates fromgeranylgeranyl pyrophosphate givesditerpenoids, etc.[10]
In air, terpenoids are converted into various species, such asaldehydes,hydroperoxides, organic nitrates, andepoxides[11] by short-livedfree radicals (like thehydroxyl radical) and to a lesser extent byozone.[12] These new species can dissolve into water droplets and contribute toaerosol andhaze formation.[13]Secondary organic aerosols formed from this pathway may have atmospheric impacts.[14]
As an example theBlue Ridge Mountains in the U.S. andBlue Mountains of New South Wales in Australia are noted for having a bluish color when seen from a distance. Trees put the "blue" in Blue Ridge, from their terpenoids released into the atmosphere.[15][16][17]
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