terpene - Structure and biosynthesis, Types

A class of natural products, also known as essential oils, based upon the oligomerization of isoprene, having formulae closely related to (C₅H₈)_n. Common examples, whose names show their origins, are pinene, limonene, menthol, and carotene.

Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some insects such as swallowtail butterflies, which emit terpenes from their osmeterium. When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids.

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives.

Structure and biosynthesis

Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8. The basic molecular formulas of terpenes are multiples of that, (C5H8)n where n is the number of linked isoprene units. The isoprene units may be linked together "head to tail" to form linear chains or they may be arranged to form rings. One can consider the isoprene unit as one of nature's preferred building blocks.

Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway.

As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes.

Types

Hemiterpenes consist of a single isoprene unit. Monoterpenes consist of two isoprene units and have the molecular formula C10H16. the geranyl- prefix indicates two isoprene units. Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. the farnesyl- prefix indicates three isoprene units. Diterpenes are composed for four isoprene units and have the molecular formula C20H32. Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. Triterpenes consist of six isoprene units and have the molecular formula C30H48. Tetraterpenes contain eight isoprene units and have the molecular formula C40H56. Polyterpenes consist of long chains of many isoprene units.