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Skeletal formula of alkannin
Space-filling model of the alkannin molecule
Preferred IUPAC name
Other names
C.I. Natural red 20
Alkanet extract
Anchusaic acid
3D model (JSmol)
ECHA InfoCard 100.007.497 Edit this at Wikidata
E number E103 (colours)
  • InChI=1S/C16H16O5/c1-8(2)3-4-10(17)9-7-13(20)14-11(18)5-6-12(19)15(14)16(9)21/h3,5-7,10,17-19H,4H2,1-2H3/t10-/m0/s1 checkY
  • InChI=1/C16H16O5/c1-8(2)3-4-10(17)9-7-13(20)14-11(18)5-6-12(19)15(14)16(9)21/h3,5-7,10,17-19H,4H2,1-2H3/t10-/m0/s1
  • O=C\2c1c(O)ccc(O)c1C(=O)/C(=C/2)[C@@H](O)CC=C(C)C
Molar mass 288.299 g·mol−1
Appearance Red-brown crystalline prisms
Density 1.15 g/mL
Melting point 149 °C (300 °F; 422 K)
Boiling point 567 °C (1,053 °F; 840 K)
Sparingly soluble
Lethal dose or concentration (LD, LC):
3.0 g/kg (mice)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Alkannin is a natural dye that is obtained from the extracts of plants from the borage family Alkanna tinctoria that are found in the south of France. The dye is used as a food colouring and in cosmetics. 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 utilises 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 this plant contains large amounts of red naphthoquinone pigments, the roots of these plants are red-purple. If shikonin is extracted from fresh tissues, it gradually darkens over several days, finally forming black precipitates, which are thought to be polymers.[7]


  1. ^ The Merck Index, 11th Edition, 243
  2. ^ Additives Archived 2011-04-06 at the Wayback Machine, Food Standards Australia New Zealand
  3. ^ "Alkanet" in Dispensatory of the United States of America, year 1918, edited by Joseph P. Remington and Horatio C. Wood.
  4. ^ H. Brockmann (1936). "Die Konstitution des Alkannins, Shikonins und Alkannans". Justus Liebigs Ann. Chem. 521: 1–47. doi:10.1002/jlac.19365210102.
  5. ^ Shmuel Yannai (2012). Dictionary of Food Compounds. CRC Press. p. 478.
  6. ^ a b Vassilios P. Papageorgiou; Andreana N. Assimopoulou; Elias A. Couladouros; et al. (1999). "The Chemistry and Biology of Alkannin, Shikonin, and Related Naphthazarin Natural Products". Angew. Chem. Int. Ed. 38 (3): 270–300. doi:10.1002/(SICI)1521-3773(19990201)38:3<270::AID-ANIE270>3.0.CO;2-0.
  7. ^ Kazufumi Y. (2017). Lithospermum erythrorhizon cell cultures: Present and future aspects. Plant Biotechnology 34: 131–142.

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