FIGURE 8.1 The major phenolic compounds in the medicinal Echinacea species.
Soicke et al., 1988), the main ones being cichoric acid (also called chicoric acid) and caftaric acid (Figure 8.1). Apart from its contribution to the immunostimulatory action of Echinacea extracts (Bauer, 1999a), cichoric acid has shown activity against HIV (Lin et al., 1999).
The main caffeoyl phenols (Figure 8.1) are useful chemical markers to differentiate between the three medicinal Echinacea species (Bauer and Wagner, 1990). Cynarin (1,5-0-dicaffeoylquinic acid) is found in E. angustifolia roots but not in roots of E. pallida or E. purpurea, and echinacoside in the roots of E. pallida and E. angustifolia but not in E. purpurea (Bauer, 1999a).
These phenolics all contain several hydroxyl groups, rendering them polar and requiring alcohol:water mixtures to extract them. The caffeate group present in all these compounds provides a characteristic UV absorption, enabling selective detection during HPLC analyses (see below). The caffeate group contains a catechol substructure, which is important for biological activity (Lin et al., 1999), but is also susceptible to polyphenol oxidases, which can lead to instability of these compounds under some conditions (see below).
The first report of alkamides (also called alkylamides) of Echinacea was by Jacobson (1967) who reported that roots of E. angustifolia contained echinacein, assigned the structure dodeca-2E,6Z,8E,10E-tetraenoic acid isobutylamide. A wide variety of alkamides have since been identified from Echinacea (Figure 8.2), but none with the structure of echinacein. This structure was probably wrongly assigned by the techniques available at that time. The alkamides are pungent (Jacobson, 1967), being responsible for the tongue tingle caused by some Echinacea preparations.
Bauer and Remiger (1989) conducted a comprehensive study of alkamides to determine the difference between Echinacea species. They found that E. purpurea and E. angustifolia have several root alkamides in common, especially the dodeca-2E,4E,8Z,10E and 10Z-tetraenoic acid isobutyla-mides 8 + 9 (Figure 8.2), which are the most abundant alkamides in both species. Among the other alkamides, the 2,4-dienoic acid unit is present in E. purpurea (e.g., 1 to 5) (Figure 8.2), whereas E. angustifolia is characterized by the 2-monoenoic acid unit (e.g., 12 to 14) (Figure 8.2). The chro-mophore of the dienoic acid exhibits a UV absorption maximum at 259 nm, while the monoene maximum is at 210 nm, which is important for HPLC detection by UV (see below). E. pallida roots are differentiated from E. purpurea and E. angustifolia by the presence of 2-ketoalkenes and 2-ketoalkynes (e.g., 20) (Figure 8.2). The aerial parts of the three species show considerable similarity as all contain undeca-2E,4Z-diene-8,10-diynoic acid isobutylamide 1, dodeca-2E,4E,8Z,10E/Z-tet-raenoic acid isobutylamides 8 + 9, and dodeca-2E,4E-dienoic acid isobutylamide 11. E. pallida has been shown to also contain hexadeca-2E,9Z-diene-12,14-diynoic acid isobutylamide 19 (Bauer and Remiger, 1989). It is important to note that some authors give levels of tetraene alkamides 8 + 9, whereas others report total alkamides. This shows the most difference for E. purpurea roots, which contain high proportions of other alkamides (Perry et al., 1997).
The long hydrocarbon chain in all these compounds renders them lipophilic (low polarity), so they do not dissolve in solvent mixtures containing high concentrations of water. The double bonds are susceptible to the same autoxidation processes that lead to rancidity of fatty acids, which may cause instability of the alkamides (see below).
High-performance liquid chromatography (HPLC) is now the analytical method of choice for the quantitative and qualitative determination of both alkamides and phenolics in the medicinal Echina-cea species (some recent methods are summarized in Table 8.1). The first step is extraction, and the more polar phenolics are generally extracted with some water present, which is not necessary for the lipophilic alkamides.
undeca-2E,4Z-diene-8,10-diynoic acid isobutylamide undeca-2Zl4E-diene-8,10-diynoic acid isobutylamide
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