Alkannin

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]

Alkannin
Skeletal formula of alkannin
Space-filling model of the alkannin molecule
Names
Preferred IUPAC name
5,8-Dihydroxy-2-[(1S)-1-hydroxy-4-methylpent-3-en-1-yl]naphthalene-1,4-dione
Other names
C.I. Natural red 20
Alkanet extract
Anchusaic acid
Anchusin
Identifiers
3D model (JSmol)
ChEMBL
ChemSpider
ECHA InfoCard 100.007.497
E number E103 (colours)
KEGG
UNII
  • 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
    Key: NEZONWMXZKDMKF-JTQLQIEISA-N 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
Properties[1]
C16H16O5
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
Hazards
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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Infobox references

Biosynthesis

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]

Research

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]

References
  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. 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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