Lichens anthraquinones

The objectives of this paper are broad. Our first objective is to describe the primary mechanism of action of usnic acid on plants as ascertained by our laboratory.33 A second objective is to describe the phytotoxic activity of selected lichen anthraquinone analogues. In addition to the phytotoxic activity, we describe the effects of these secondary metabolites on phloem-feeding insects. Finally, we provide a hypothesis to explain the functional roles of these metabolites in the ecosystem. 

Gaskell et al. (1973) analyzed the hydrocarbons from three lichens by GLC and Furuya et al. (1966) some lichen anthraquinones. Ikekawa et al. (1965) reported on the GLC of zeorin and other triterpenes. Zeorin has a retention time of 4.96 min (1.5% SE-30 on Gas Chrom P, 80-lOOmesh, 150 x 4mm, 225°C, 80mlN2/min). Shibata et al. (1965) found that free zeorin gave two peaks, the minor one corresponding to zeorinine. The relative retention times for the acetates of the sterols isolated from Pseud-evernia furfuracea are summarized in Table 17 (Wojciechowski et al. 1973). 

Besides playing a vital role in the oxidation-reduction processes of living organisms, quinones occur widely as natural pigments found mainly in plants, fungi, lichens, marine organisms, and insects (see alizarin, Section 28-4A, as representative of a natural anthraquinone-type dye). 

The simplest quinones are o- and p-benzoquinone [(3) and (4) respectively]. This quinonoid structural feature is widespread in naturally occurring compounds isolated from moulds, fungi, lichens, plants and insects,52 which include not only substituted benzoquinones but also substituted polycyclic quinones [i.e. the substituted analogues of, for example, 1,2-naphthoquinone (5), 9,10-anthraquinone (6), and 9,10-phenanthraquinone (7)]. 

Most of the previously identified 25 chlorinated anthraquinones are found in lichen and fungi (1). The newly discovered examples have a wider range of sources. Studies of the lichen Nephroma laevigatum from the British Columbia coast have identified the new anthraquinone, 7-chloro-l-O-methyl-co-hydroxy-emodin (2157), and the two novel hypericins, 7,7 -dichlorohypericin (2158) and 2,2, 7,7 -tetrachlorohypericin (2159) (1931), as well as 5-chloroemodin (2160), 5-c h I oro -1 - (9 - m e t h v I - o >- h yd ro x ye m od i n (2161), and 5-chloro-co-hydroxyemodin (2162) (1932). In addition to containing several known chlorinated anthraquinones, the Scandinavian fungus Dermocybe sanguinea has afforded the new 5,7-dichloroendocrocin (2163) (1933). The novel tetracyclic anthraquinones... 

Cohen PA, Towers GHN (1996) Biosynthetic Studies on Chlorinated Anthraquinones in the Lichen Nephroma laevigtum. Phytochemistry 42 1325... 

Cohen PA, Towers GHN (1997) Chlorination of Anthraquinones by Lichen and Fungal Enzymes. Phytochemistry 44 271... 

Several anthraquinones with high antimicrobial activity have been isolated and characterized from some species in the lichen genus Xanthoria.30... 

Emodin is a naturally occurring anthraquinone produced in many species of lichen, fungi, and higher plants (e.g., the genus, Rhamnus). Extracts containing emodin have been used in traditional medicine as herbal laxatives. It has also... 

Rhodocladonic acid is an anthraquinone that occurs in several lichen species, especially in the family Roccellaceae).20 Little research has been done documenting bioactivity, particularly phytotoxic activity. Similar to emodin, we tested two sets of analogues (Fig. 1.10). Series 1 consisted of a group of compounds with aliphatic R-groups ending in a terminal hydroxyl. Series 2 had a terminal methyl. The R-group substitutions were identical to those of emodin. 

Manojilovic, N. T., Solujic, S., Sukdolak, S., and Krstic, L. J., 2000. Isolation and antimicrobial activity of anthraquinones from some species of the lichen genus Xanthoria. J. Serb. Chem. Soc. 65, 555-560. 

Remarkably, the same compound can be produced in different ways by different organisms. The best known example is probably the polyketide anthraquinone, chrysophanol 9, which occurs in both eukaryotes (higher plants, lichens, fungi and insects) and prokaryotes, but is produced through different folding modes of polyketide chains.10 Similarly, it has also been demonstrated that the biosynthesis of gibberellins involves different metabolic sequences in fungi and plants.11... 

Binapthoquinones include the phototoxic phytotoxin cercosporin from the fungus Cercospora (two Phe Q moieties linked by two Phe—Phe links and an MD link). Hypericin (two anthraquinones linked by three Phe—Phe linkages) is a bianthraquinone from Hypericum species (Hypericaceae). Hypericin is a phototoxic protein kinase inhibitor that causes light-dependent ovine facial eczema. Benzonaphthoquinones include the der-matitic cypripedin (Phe Phe QJ. Lichen 7-chloroemodin is a novel chloroanthraquinone and the fused tricyclic pyrano-a-naphthoquinone (3-lapachone (Phe oQJ C50) is a reverse transcriptase inhibitor with antimicrobial and cytotoxic activity. 

A second large group of polyketide metabolites that have been isolated from lichens are heptaketide xanthones. Their structures are mainly based on norli-chexanthone (7.54). An interesting feature of these compounds is the presence of chlorine in several of the metabolites. Lichens also produce some anthraquinone pigments, exemplified by physcion (7.6) from Xanthora species. 

Like the fungal and lichen xanthones, anthraquinones, which are also produced both by lichens and fungi, are derived from extended polyketides by cyclization. Several chlorinated compounds were described. 

Anthraquinones (from fungi). Over 40 monomeric A. derivatives are known from ascomycetes and imperfect fungi and there are several dimeric compounds (e.g., luteoskyrin, rugulosin, skyrin). Many A. have also been isolated from plants, lichens, basidio-mycetes, bacteria (streptomycetes), and insects ( an-thranoids). Some A. occur both in fungi and in lichens, e.g. chiysophanol, emodin, erythroglaucin, and physcion. 

C21H20O7, Mr 384.39, orange-red cryst., mp. 201 °C. An anthraquinone from the foliose lichen Solorina crocea, the undersides of the thalli are colored orange. S. is used as a reference substance for HPLC analysis of lichen substances. 

Anthraquinones derived from acetate-malonate pathways are particularly common in fungi and lichens, but are often found in higher plants as well. Acetate-malonate-de-rived anthraquinones usually can be distinguished by their structures because they possess substituents in both benze-noid rings of the anthraquinone nucleus (also see Chapter 5), although there are some exceptions to this generalization. 

Anthraquinones derived from acetate-malonate (polyke-tide) pathways are widespread in fungi and lichens, but less common in higher plants. Emodin (11) is among the compounds frequently found in all of these groups. 

Anthracenes of polyketide origin are built in molds, e.g., Aspergillus and PeniciU lium lichens, Basidiomycetes and higher plants, e.g., Polygonaceae and Rhamna-ceae (see however the formation of anthraquinone derivatives from shikimic acid, D 8.1). Ergochromes have been found in molds and lichens. 

Bohman G (1970) Chemical studies on lichens. 25. A new anthraquinone from Mycoblastus sanguinarius. Tetrahedron Lett 445-446... 

Mishchenko NP, Stepanenko LS, Kriovshchekova OE, Maximov OB (1980) The anthraquinones from the lichen Asahinea chrysantha. Chim Prir Soedin 160-165 Mishchenko NP, Maximov OB, Krivoshchekova OE, Stepanenko LS (1984) Depsidones and fatty acids of Parmelia stygia. Phytochemistry 23 180-181 Miyagawa H, Hamada N, Sato M, LFeno T (1993) Hypostrepsilic acid, a new dibenzofuran from the cul-... 

Nakano H, Komiya T, Shibata S (1972) Anthraquinones of the lichens of Xanthoria and Caloplaca and their cultivated mycobionts. Phytochemistry 11 3505-3508... 

Gas chromatography and mass spectrometry Xanthones, anthraquinones, dibenzofurans, terpenes and pulvinic acid derivates which lack thermolabile ester groups can be studied by gas chromatography with mass spectrometry (GCMS). Xanthones in hchens were studied by injecting a lichen extract directly into a mass spectrometer. More recently, the main terpenoid comptments of the lichens of the family Pyxinaceae have been studied by GMCS (Nash 1996 Karunaratne et al. 2005). 

The secondary metabolites of the lichen are active substances against pathogenic microorganisms. Most known lichen substances with antimicrobial activity are usnic acid, phenolic compounds, triterpenes, steroids, anthraquinones, depsides, depsidones, and dapsones, and most of them are known mechanisms of their antibiotic action. 

Anthraquinones and xanthones are also important constituents of many lichens. Anthraquinmies such as parietin, parietinic acid, emodin, fallacinol, and fallacinal were shown to have a high antimicrobial effect (Manojlovic et al. 2002). Manojlovic et al. (2010) found that the antimicrobial activity of Uchen Laurera benguelensis is mainly related to the presence of Uchexanthone. 

Manojlovic et al. (2005) reported antifungal activity of the anthraquinone parietin isolated from Caloplaca cerina. A potent fungitoxic compotmd, lecanoric acid, was isolated from Parmotrema tinctorum lichen and tested against the fungus... 

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