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.[2] Alkannin is deep red in an acid and blue in an alkaline environment.[3] The chemical structure as a naphthoquinone derivative was first determined by Brockmann in 1936.[4] The R-enantiomer of alkannin is known as shikonin, and the racemic mixture of the two is known as shikalkin.[5][6]
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.[6]
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.[7]