Usnic acid

July 26, 2021
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Image of Usnic acid 3D Image of Usnic acid

Usnic acid is a natural antibacterial compound found in lichens. It is not a carboxylic acid; instead, it gets acidity from the phenolic hydroxyl groups. Its pKa is 4.4.

In the 1840s, German and Austrian scientists isolated yellow-colored usnic acid from several lichen genera, such as Usnea, from which it derives its name. The bitter-tasting acid exists naturally as its (R)- and (S)-enantiomers as well as the racemate. (R)-Usnic acid is shown in the 3-D image.

More than 90 years after its was isolated, usnic acid’s structure was elucidated by Frank H. Curd and Alexander Robertson* at the University of Liverpool (UK). In a long series of articles, these authors reported the structure and laboratory synthesis of usnic acid and many of its derivatives.

As shown in the hazard information table, usnic acid is hazardous to health and the environment. Nevertheless, it is an ingredient in some over-the-counter dietary supplements. The Memorial Sloan Kettering Cancer Center, among other reputable organizations, warns against using it.

Even with its hazards, usnic acid is being studied for medical applications, especially in cancer research. In 2012, Helga M. Ogmundsdottir and colleagues at the University of Iceland (Reykjavik) reported that it affects mitochondrial and lysosomal function in cancer cells and has “implications for therapeutic manipulation of autophagy and pH-determined drug distribution.”

More recently, in 2018, Wensheng Pan and co-workers at Zhejiang University and People’s Hospital of Hangzhou Medical College (both in Hangzhou, China) found that usnic acid induces cycle arrest, apoptosis, and autophagy in gastric cancer cells.

Usnic acid hazard information

Hazard class**GHS code and hazard statement
Acute toxicity, oral, category 3H301—Toxic if swallowedChemical Safety Warning
Skin corrosion/irritation, category 2H315—Causes skin irritationChemical Safety Warning
Skin sensitization, category 1H317—May cause an allergic skin reactionChemical Safety Warning
Serious eye damage/eye irritation, category 2AH319—Causes serious eye irritationChemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3H335—May cause respiratory irritationChemical Safety Warning
Short-term (acute) aquatic hazard, category 1H400—Very toxic to aquatic lifeChemical Safety Warning
Long-term (chronic) aquatic hazard, category 1H410—Very toxic to aquatic life with long-lasting effectsChemical Safety Warning

*Compilation of multiple safety data sheets.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. 
Explanation of pictograms. 

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Usnic acid fast facts

CAS Reg. No.125-46-2
1,3(2H,9bH)-Dibenzofurandione, 2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-
Empirical formulaC18H16O7
Molar mass344.32 g/mol
AppearanceYellow crystals or powder
Melting point204 °C
Water solubility≈1 g/L
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