A person wearing black gloves paints a translucent paste on the face of a person who lies back in a chair. Chemical peels are a popular skin-care treatment with a growing market. Credit: Alamy ...

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. More than 90 years after its ...