A review on usnic acid, an interesting natural compound

Abstract

Lichens are a world-widespread consortium of fungal and photosynthetic partners. Usnic acid is one of the most common and abundant lichen metabolites, well known as an antibiotic, but also endowed with several other interesting properties. This review summarises the most relevant studies on usnic acid, focusing on a number of biological activities in different fields. On the basis of the existing literature, usnic acid seems to be an exclusive lichen product. No synthetic derivatives more effective than the natural form are known. Both the (+) and (-) enantiomers of usnic acid are effective against a large variety of Gram-positive (G+) bacterial strains, including strains from clinical isolates, irrespective of their resistant phenotype. Of particular relevance is the inhibition of growth of multi-resistant strains of Streptococcus aureus, enterococci and mycobacteria. The (+)-usnic acid enantiomer appears to be selective against Streptococcus mutans without inducing perturbing side effects on the oral saprophyte flora. On the other hand, the (-)-usnic acid enantiomer is a selective natural herbicide because of its blocking action against a specific key plant enzyme. Other recognised characteristics of usnic acid are ultraviolet absorption and preserving properties. The toxicology, the in vitro anti-inflammatory effects and the mechanism of action of usnic acid need to be investigated in greater detail in order to reach clinical trials and to allow further applications. Furthermore, more research is needed to make possible intensive lichen culture, in order to produce large quantities of lichen substances for pharmaceutical, cosmetic and agricultural purposes. Some biological aspects, i.e. the possible biological roles of usnic acid, are discussed.

Solid State Electrochemical Behavior of Usnic Acid at a Glassy Carbon Electrode

“…For pH > 8.8, the oxidation process was pH independent. The value found for pK a $8.8 is in agreement with the literature (Cochietto et al 2002). The oxidation peaks P1 and P3 correspond to the oxidation of the hydroxyl groups in positions 1 and 8 (Scheme 1).…”

“…Among known secondary lichen metabolites usnic acid (UA) [2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-1,3(2H,9bH)-dibenzo-furan-dione], Scheme 1, is the most extensively studied (Cochietto et al 2002;Ingó lfsdó ttir 2002). It is an exclusive lichen product and no synthetic derivatives are known that are more effective than the natural form.…”

Lichen extracts as raw materials in perfumery. Part 1: oakmoss

“…Isolated usnic acid can find applications as an antioxidant in cosmetic preparations [52] and other applications of this interesting natural products have been reviewed recently. [53] …”

Effects of some terricolous lichens [Cladonia rangiformis Hoffm., Peltigera neckerii Hepp ex Müll. Arg., Peltigera rufescens (Weiss) Humb.] on soil bacteria in natural conditions

“…Cladonia, Ramalina, Usnea and others. The biological activities of usnic acid such as antiviral (Scirpa et al 1999), antifungal (Lauterwein et al 1995, Lawrey 1995, Cocchietto et al 2002, antiprotozoal (Fournet et al 1997), antiproliferative, antiherbivoral (Huneck 1999), analgesic (Vijayakumar et al 2000), hepatotoxic (Pramyothin et al 2004) have been reviewed by Ingolfsdottir (2002). Rankovic et al (2007) determined that usnic asid has the strongest antimicrobial activity.…”