Lawsone

Lawsone (2-hydroxy-1,4-naphthoquinone), also known as hennotannic acid, is a red-orange dye present in the leaves of the henna plant, for which it is named, as well as in the common walnut and water hyacinth. Humans have used henna extracts containing lawsone as hair and skin dyes for more than 5,000 years. Lawsone reacts chemically with the protein keratin in skin and hair via a Michael addition reaction, resulting in a strong permanent stain that lasts until the skin or hair is shed.

Darker colored staining is due to more lawsone–keratin interactions occurring, which evidently break down as the concentration of lawsone decreases and the tattoo fades.
Lawsone strongly absorbs UV light, and aqueous extracts can be effective sunless tanning agents and sunscreens.
Lawsone is a 1,4-naphthoquinone derivative, an analog of hydroxyquinone containing one additional ring.

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Source: Wikipedia · Image: Wikimedia Commons · Public domain