Characterization of Artemisia arborescens L. (Asteraceae) leaf-derived essential oil from Southern Italy

Journal of Essential Oil Research: JEOR, May/Jun 2007 by Presti, M Lo, Crupi, M L, Zellner, B d'A, Dugo, G, Et al

Abstract

Leaf-derived essential oils isolated from Artemisia arborescens L. (Asteraceae) were analyzed by using GC/MS and GC-FID (for compound quantification); a total of 82 components were determined. The oils were derived from plants collected at the vegetative stage and originating from three locations in southern parts of Italy: Calabria, Sicily and the island of Lipari (Eolian Islands). In general, the chemical compositions were rather similar with some major differences regarding, in particular, the amounts of oxygenated compounds, such as camphor, chamazulene and α- and β-thujones. The research work was extended to the determination of a series of chiral compounds, as also the olfactive analysis of the aroma-active compounds, with the aim of acquiring an enlargened view on this specific sample-type.

Key Word Index

Artemisia arborescens L., Asteraceae, essential oil composition, camphor, chamazulene, gas chromatography-olfactometry, chiral analysis.

Introduction

Artemisia L. (Asteracae) is a large genus of small herbs or shrubs found in arid zones, notably in the western USA, Asiatic steppes, South Africa, South America and on the Mediterranean coast (1). It comprises some 280 species found in the northern hemisphere (2). Artemisia arborescens, which is commonly known in the Mediterranean area with the denominations of "arboreous absinth" or "arborent mugwort" is a perennial plant of about 60-150 cm in height with woody, erect and branched stems. The leaves have a whitish silvery color in the youngest parts of the plant (3). Artemisia essential oils (wormwood and mugwort) have been used for various purposes such as flavorings, fragrances, rodent and mite repellents and as folk medicine for antispasmodic, anti-pyretic, anti-inflammatory and abortifacient activities (4). In previous investigations (5-7), the main compounds found in A. arborescens oil were camphene, limonene, camphor, terpinen-4-ol, linalool, sabinene, α-cubebene, β-caryophyllene, germacrene D and chamazulene. At present, it is well-known that geographic origin has an influence on the essential oil chemical composition; Sacco et al. showed that the main difference between oils of plants that derived from the Liguria and Sardinia regions was the complete absence of camphor in the Sardinia product (5). An aspect worthy of attention, regards the presence of thujones in A. arborescens oils of different geographical origins. Thujone isomers occur in nature as a mixture consisting α-thujone and β-thujone. Thujone is a component of essential oils such as wormwood (A. absinthium), mugwort (A. vulgaris) and of others derived from different species of plants. With regards to the content in thujones, the information reported in the literature is rather variable; β-thujone has been identified as the most abundant isomer ranging from 0.17% (6) to 68.9% (8) and α- thujone often contained in traces or below 1% (6).

Since the use of thujone-containing plants and products is limited by food regulations, the chemical differentiation of arborescent mugwort (A. arborescens) from other thujone-containing plants would be certainly useful. Through investigations on the biological activities of this isomeric couple have been carried out by the National Institute of Health (9); thujone, expressed as an isomeric mixture, is banned as a food additive in the US and its presence in foods and beverages is regulated in several countries (the EU limit is of 10 mg/L). Both α- and β-thujones were found in 24 food additives listed in the PrioritBased Assessment of Food Additives (PAFA) database (FDA, 1997). It must be emphasized that the restrictions upon the use of thujone-containing plants and derivatives is due mainly to the OC-isomer. The latter is a modulator of GABA (γ-aminobutyrric acid) type A receptors (9,10); GABA moderates the firing of neural synapses; α-thujone antagonizes mildly such an inhibition. The GABA modulating activity of α-thujone leads to convulsant effects at high doses, while it may produce a psycho-stimulant action at lower doses.

Considering that the essential oil of A. arborescens elicits a fresh, slightly sweet, minty, woody, earthy scent and it is well-known that only a small fraction of the large number of volatiles present in an essential oil contributes to its odor, the importance of the distinction between odor active compounds and the entire range of volatiles present in this oil has been investigated. Gas chromatography-olfactometry (GC-O) is the technique of choice for finding key volatile constituents of complex aromas (11,12), comprising the separation of odoriferous components on a GC column, followed by the division of the column effluent between a chemical detector and a transfer line (sniffing port). This technique can be regarded as a screening method, and several techniques have been developed over the last decades to collect and process GC-O data, estimating so the sensory contribution of single odor active compounds. Despite previous direct olfactive description of the oil of A. arborescens, there has been no identification of the key aroma compounds of this oil through Aroma Extraction Dilution Analysis - AEDA (13-15). This technique is based on the subsequent dilution of a sample, respecting a serial dilution, and each one of these is sniffed through GC-O until no significant odor is detected, enabling the investigation of the fragrant components responsible for the global aroma of the matrix, discriminating the odor active components according to their impact. In AEDA the results are expressed as flavor dilution (FD) factors, defined as the ratio between the concentration of odorants in the initial extract and the concentration in the most diluted extract able to be detected through GC-O [12]. Consequently, the dilution factor is the relative measure of odor potency of a compound in the isolate. Several analyses were required in order to reach a dilution of the aroma extract in which odorous regions were no longer detected and the maximum dilution value was determined. Following this, the detected odors were compared with the compounds identified by GC/MS in order to determine key odorants.