Alkannin is a natural dye that is obtained from the extracts of the plant dyer's alkanet (Alkanna tinctoria) which is found in the Mediterranean region. The dye is used as a food coloring and in cosmetics; within 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 Hans 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 Edit this at Wikidata
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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Biosynthesis

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The enzyme 4-hydroxybenzoate geranyltransferase utilizes geranyl diphosphate and 4-hydroxybenzoic acid to produce 3-geranyl-4-hydroxybenzoic acid and diphosphate. These compounds are then used to form alkannin.[6]

Research

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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]

References

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  1. ^ The Merck Index, 11th Edition, 243
  2. ^ Additives Archived 2011-04-06 at the Wayback Machine, Food Standards Australia New Zealand
  3. ^ Joseph P. Remington and Horatio C. Wood, ed. (1918). "Alkanet". Dispensatory of the United States of America.
  4. ^ H. Brockmann (1936). "Die Konstitution des Alkannins, Shikonins und Alkannans". Justus Liebigs Ann. Chem. 521 (1): 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. PMID 29711637.
  7. ^ Yazaki, Kazufumi (2017). "Lithospermum erythrorhizon cell cultures: Present and future aspects". Plant Biotechnology. 34 (3): 131–142. doi:10.5511/plantbiotechnology.17.0823a. PMC 6565996. PMID 31275019.