Hemi terpenes

Terpenes and Hemi-terpenes.—The more common lerpene hydrocarbons or terpenes in the narrow sense, such as those obtained from oil of turpentine and lemon oil, have the composition represented by the formula CjoHie. This is considered as the lerpene unit and certain members of the series which have the composition CsHs are termed hemi-terpenes. 

This is a terpene hydrocarbon obtained by the distillation of caoutchouc. It has the formula CsHs and on this account is termed a hemi-terpene, it being one half of CioHie the more general composition. The constitution of isoprene has been established as follows ... 

This a-domain is common to all class I terpene synthases and is the only domain in bacterial and fungal class I enzymes such as the pentalenene synthase from Streptomyces ey oliatus or the trichodiene synthase from Fusarium sporotri-chioides (Fig. 87.14a) [191,194]. Plant hemi-, mono-, and sesquiterpene synthases exhibit a second helical p-domain that resembles a barrel structure. An example of this class is given by the (+)-5-cadinene synthase from G. arboreum (Fig. 87.14b) [195]. In plant diterpene cyclases and in the exceptional case of the ( )-a-bisabolene synthase from A. grandis (Fig. 87.14c), a third helical y-domain with a barrel-like structure is present [197, 200, 202]. The p- and y-domains in the plant enzymes are nonfunctional but are required for correct enzyme folding. 

Terpene compounds have natural occurrence in plants found as major components of most of the plant essential oils. Based on their structural and functional properties, terpene compounds have been classified according to their basic structural unit isoprene containing five carbons, fii the formation of terpenes, prenyldi-phosphate serves as a precursor. The terpene compounds exist in the form of mono-, sesqui-, hemi-, di-, tri-, and tetraterpenes. The mmioterpenes containing two isoprene units are responsible to cmistruct the major portion of all the essential oils. These compounds work as carbure, alcohol, aldehyde, ketone, ester, ether, peroxyde, and phenols [58]. The sesquiterpene compounds contain three isoprene units, and the functional properties are very close to monoterpene compounds. 

Croteau and Ronald (1983) and Coscia (1984) reviewed chromatographic studies on terpinoids. Those authors considered terpinoids as ubiquitous, structurally diverse, lipophilic compounds. Terpinoids exist in various polymeric forms of the unsaturated branch-chain pentane, isoprenol. Terpenes are characterized as hemi-prenes (C5), monoterpenes (Cio), sesquiterpenes (C15). diterpenes (C20), and triterpenes (Cjo)- A representative hemiprene (hemiterpene) is mevalonic acid, a precursor in the biosynthesis of isoprenoids. Monoterpenes contain numerous... 

---