Terpenes

If you walk into a garden in bloom and breathe deeply, you are likely to encounter great smells. In many cases, the molecules that bring those scents to your nose are terpenes. Terpenes are a class of molecules that typically contain either ten or fifteen carbon atoms built from a five-carbon building block called isoprene . It is important to note these molecules are volatile —they tend to evaporate relatively easily—which allows them to reach one's nose and trigger the olfactory senses. Figure 1 shows the structure of several molecules in this category and identifies the plant from which they are derived.

Because they share the common building block of isoprene, terpenes can be categorized based on how many terpene units they include. The "base" of this system is a two-isoprene unit, so monoterpenes have ten carbons. This category includes molecules such as camphor, menthol, and pinene. If three isoprenes are present, as in farnesol (shown in Figure 1), the category is called sesquiterpenes.

In plants, terpenes are typically found as a component of the essential oils. The name of this fluid suggests an important characteristic of this class of molecules. If humans have found these oils important enough to refer to them as essential, there is a reasonable chance that they have useful properties. Many terpenes such as menthol (shown in Figure 1) and camphor have medicinal values. You may have used a throat lozenge with menthol to help clear your sinuses the last time you had a cold.

Another use of terpenes may be found in coatings such as varnish. Varnish for musical instruments like violins can be made largely from the terpene pinene. The key to generating a coating such as varnish is to allow polymerization to occur. The fact that terpenes are built up from isoprene building blocks, much like any other polymer, suggests that polymerization is a possibility. When pinene (present in pine tree and made into turpentine) is exposed to air and sunlight, it will slowly polymerize and make a fine finish for wood.

β- carotene is a tetraterpene (structure shown in Figure 1); it has four isoprene units. This molecule, found in abundance in carrots, is useful in

human diets because it can be converted into vitamin A in the body. It also plays an important role in plants because it is among the pigments that absorb light in photosynthesis . When leaves change color in the fall, β- carotene is often responsible for this effect.

see also Lipids; Membrane.

Thomas A. Holme

Bibliography

Gimelli, Salvatore Paul (2001). Aroma Science. Port Washington, NY: Micelle Press.

Maarse, Henk, ed. (1991). Volatile Compounds in Foods and Beverages. New York: Marcel Dekker.

Terpenes

Terpenes (terpenoids) are a very large family of plant compounds that play a variety of roles in many different plants. All terpenes are constructed from isoprenoid units by biochemically unusual pathways involving highly reactive intermediates. The hemiterpene isoprene, which contains five carbons (one isoprene unit), is a gas emitted into the atmosphere by many plant species, where it plays a role in the chemistry of ozone production. A monoterpene (monoterpenoid) contains ten carbons (two isoprene units); a sesquiterpene, fifteen carbons (three isoprene units); a diterpene, twenty carbons (four isoprene units). Triterpenes (thirty carbons) are important structural components of plant cell membranes. Many plant pigments , including the yellow and red carotenoids , are tetraterpenes (forty carbons). Natural rubber is a polyterpene containing many isoprene units. The monoterpenes and sesquiterpenes are common components of the essential oils of herbs and spices (peppermint, lavender), of flower scents (rose), and of turpentine derived from the resin of evergreen trees.

These compounds have important uses as flavorings and perfumes, as well as intermediates in the production of other commercial products like solvents and adhesives. Many terpenes play roles as plant hormones and in the chemical defenses of plants against microbial diseases and insect herbivores ; many others have important medicinal properties. Artemisinin is a sesquiterpene drug derived from traditional Chinese herbal medicine that is useful for treating malaria, and taxol obtained from yew trees is a diterpenoid that is highly effective in treating cancer. Recent advances in molecular biology have made it possible to genetically engineer terpene metabolism in plants for agricultural, industrial, and pharmaceutical purposes.

see also Atmosphere and Plants; Flavor and Fragrance Chemist; Hormones; Medicinal Plants; Oils, Plant-Derived; Pigments; Poisonous Plants.

Rodney Croteau

Bibliography

Cane, David E., ed. Comprehensive Natural Products Chemistry, Vol. 2: Isoprenoids Including Carotenoids and Steroids. Oxford: Elsevier, 1999.

Croteau, Rodney B. "The Discovery of Terpenes." In Discoveries in Plant Biology, Vol. I, eds. Shain-Dow Kung and Shang-Fa Yang. Singapore: World Scientific, 1998.

Langenheim, Jean H. "Higher Plant Terpenoids: A Phytocentric Overview of Their Ecological Roles." Journal of Chemical Ecology 20 (1994): 1223-80.

terpenes A group of unsaturated hydrocarbons present in plants (see essential oil). Terpenes consist of isoprene units, CH₂:C(CH₃)CH:CH₂. Monoterpenes have two units, C₁₀H₁₆, sesquiterpenes three units, C₁₅H₂₄, diterpenes four units, C₂₀H₃₂, etc. Derivatives of terpenes, known as terpenoids, include the plant growth substances abscisic acid and gibberellin, and the carotenoid and chlorophyll pigments used in photosynthesis.

terpenes Major components of the essential oils of citrus fruits; however, they are not responsible for the characteristic flavour, and, since they readily oxidize and polymerize to produce unpleasant flavours, they are removed from citrus oils by distillation or solvent extraction, leaving the so‐called terpeneless oils for flavouring foods and drinks.