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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 pK_a 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, .

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 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 .