Alkannin

Alkannin is a natural dye that is obtained from the extracts of Alkanna tinctoria which is found in the south of France. The dye is used as a food coloring and in cosmetics; the European E number schedule, it is numbered E103. It is used as a red-brown food additive in regions such as Australia. Alkannin is deep red in an acid and blue in an alkaline environment. The chemical structure as a naphthoquinone derivative was first determined by Brockmann in 1936. The R-enantiomer of alkannin is known as shikonin, and the racemic mixture of the two is known as shikalkin.

Biosynthesis
The enzyme 4-hydroxybenzoate geranyltransferase utilizes geranyl diphosphate and 4-hydroxybenzoate to produce 3-geranyl-4-hydroxybenzoate and diphosphate. These compounds are then used to form alkannin.

Research
Because the root bark (cork layers) of Alkanna tinctoria contains large amounts of red naphthoquinone pigments, including alkannin, the roots of these plants are red-purple. When extracted from fresh tissues, the pigment gradually darkens over several days, finally forming black precipitates, which are thought to be polymers.