Juglone

Juglone (5-hydroxy-1,4-naphthalenedione) is a naturally occurring naphthoquinone derivative that has been known for more than a century. In 1905, French scientists M. Brissemoret and R. Combes reported that juglone exists in the green portions of walnut trees (leaves, stems, and husks). Their report cites articles from 1884. In 1887, German chemists August Bernthsen and August Semper described juglone’s structure and its synthesis from naphthalene.

Juglone is found in many species of the walnut family (Juglandaceae), particularly in eastern American black walnut (Juglans nigra). The compound serves as a defense mechanism against surrounding plants: It can stunt their growth, a process known as allelopathy. Since its discovery, juglone has occasionally been used as an herbicide and an anthelmintic (a group of antiparasitic drugs). It is also ichthyotoxic, and some cultures catch fish by throwing walnut husks into the water.

Juglone is more frequently used as a dye. In its pure form, it has a yellow color; but when it is exposed to oxygen, it turns orange, red, or, mainly, brown. As a dye, it is known as C.I. Natural Brown 7, a classification of the database Colour Index International. It is used to dye fabrics, cosmetics, and even foods, despite its properties listed in the hazard information table. It is chemically related to henna.

To learn how the naphthalene-like structure of juglone is biosynthesized, read the 2018 Horticulture Research article by Joshua R. Widhalm and co-workers at Purdue University.