Chemical Overview And Chemotaxonomy

Several secondary metabolites characterise the chemical composition of the genus Artemisia. A survey of the literature indicates that almost all classes of compounds are present in the genus, with particular reference to terpenoids and flavonoids. However, wax constituents, polyacetylenes and, to a lesser extent, nitrogen containing molecules have also been found in several species. The wide array of molecules present in the genus and the distribution of plants in several different habitats provides the opportunity for the study of genotypic and phenotypic variations as well as chemotaxonomic relationships among species. In this section we will give an overview of the chemical variability and taxonomic relationships within the genus Artemisia, by considering the main classes of secondary metabolites mentioned above. Owing to the introductory nature of this chapter the discussion will be more like a journey in the field of secondary metabolites of Artemisia. Let us start from the most studied class of metabolites, the terpenoids.

Terpenoids

The terpenoids present in Artemisia species are representative of all classes of compounds, from monoterpenes up to triterpenes. Most of the species are characterized by the typical fragrance of lower terpenoids, such as monoterpenes and sesquiterpenes. These volatile molecules are present in the essential oils, which impart strong aromatic odours to the plants. Among the various compounds, lower terpenoids such as camphor, thujone, borneol and 1,8-cineole are the most representative (Fig. 1). Recently, analytical methods to determine the oil components have been improved by the use of capillary electrophoresis, whilst the separation of enan-tiomers has been achieved by the use of /3-cyclodextrin coated chiral capillary columns (Ravid et al., 1992). As for many aromatic plants, the oil content of Artemisia is affected by environmental factors. The monoterpenoid content of some A. tridentata ssp. tridentata plants varied seasonally with the highest content reached in July (4.18%) and the lowest during May (0.97%) (Cedarleaf et al, 1983).

The huge variability in the essential oil composition within Artemisia can be summarized by taking into consideration the data published on the most important oil crops. The variability in the monoterpenoids of the genus ranges from the artemisyl derivatives (such as artemisia alcohol, artemisia ketone, artemisia acetate, artemisia triene and yomogi alcohol/ketone) to santolinyl and lavandulyl skeleton derivatives. Furthermore, the presence of many irregular monoterpenes seems to be a particular characteristic of the tribe Anthemideae (Fig. 1) (Greger 1977).

A. annua oil composition has been a matter of several investigations. The essential oils from the inflorescence of A. annua of Chinese as well as Dutch origin were particularly rich in the monoterpene artemisia ketone (63%; Liu et al., 1988; Woerdenbag et al., 1992, 1993), while in Mongolian chemotypes the phenols thymol and carvacrol were the main compounds (Satar 1986). The leaf oil composition of a population of A. annua grown in a greenhouse in our department showed a high content of the monoterpene hydrocarbons thujene (a+/3), a-terpinene and limonene, followed by good percentages of the oxygenated monoterpenes camphor and a-terpineol (Table 3). A few sesquiterpenes were identified, a-guaiene being the most abundant. In the same table, the percentage of oil components is flanked by the corresponding coefficient of variation. This statistical parameter indicates the variability within the population for every single compound. As shown in Table 3, a very low variability was evident for the main monoterpene hydrocarbons (values below 15%) as well as for the monoterpene alcohol a-terpineol. On the other hand, a great variability was observed for the pinenes, y-terpinene, c/s-sabinol (values above 80%) as well as for borneol and the sesquiterpenes.

The chemical composition of Artemisia oils has been investigated in many other species for chemotaxonomic reasons. However, the continuous search for new active and/or flavouring molecules is the driving force for the improvement and development of extracting and purifying techniques. Camphor, 1,8-cineole, camphene, terpinen-4-ol and a-terpineol are the main oil components of A. sieberi (Weyerstahl et al., 1993), A. hololeuca, A. gelinii and A. pontica (Bodrug et al., 1987), whereas artemisia ketone made up 94% of the total oil of A. alba (Bodrug et al., 1987).

(-)-limonene (+)-piperitone a-terpinene a-terpineol a-terpineol

terpinen-4-ol a-thujene a-thujone p-thujone

1,8-cineole thymol a-thujene a-thujone p-thujone

1,8-cineole thymol

OH HO, carvacrol

|i-pinene

|i-pinene ascaridole carvacrol ascaridole santolinyl santolinyl lavandulyl lavandulyl artemisia ketone artemisia ketone

Figure 1 Structural formulae of some representative monoterpenoids found in the genus Artemisia.

However, the use of biotechnology, as in the case of micropropagation, does not always gives the same products as in vivo cultures. For example, micropropagation of A. alba afforded mainly isopinocamphone and camphor, but not artemisia ketone or a-thujone (Ronse and De Pooter 1990).

Table 3 Leaf oil chemical composition of a population of Artemisia annua grown in a greenhouse. C.V. = coefficient of variation.

Monoterpenes

%

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