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 3 H301—Toxic if swallowed Chemical Safety Warning
Skin corrosion/irritation, category 2 H315—Causes skin irritation Chemical Safety Warning
Skin sensitization, category 1 H317—May cause an allergic skin reaction Chemical Safety Warning
Serious eye damage/eye irritation, category 2A H319—Causes serious eye irritation Chemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3 H335—May cause respiratory irritation Chemical Safety Warning
Short-term (acute) aquatic hazard, category 1 H400—Very toxic to aquatic life Chemical Safety Warning
Long-term (chronic) aquatic hazard, category 1 H410—Very toxic to aquatic life with long-lasting effects Chemical 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 formula C18H16O7
Molar mass 344.32 g/mol
Appearance Yellow crystals or powder
Melting point 204 °C
Water solubility ≈1 g/L
Chemical Abstract Service - a division of ACS

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