Alkannin

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

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
CAS Number	517-88-4 Y
3D model (JSmol)	Interactive image
ChEMBL	ChEMBL28457 Y
ChemSpider	65430 Y
ECHA InfoCard	100.007.497
E number	E103 (colours)
KEGG	C10292 Y
PubChem CID	72521
UNII	075CRZ9995 Y
CompTox Dashboard (EPA)	DTXSID201029621
InChI 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 Y Key: NEZONWMXZKDMKF-JTQLQIEISA-N Y 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
SMILES O=C\2c1c(O)ccc(O)c1C(=O)/C(=C/2)[C@@H](O)CC=C(C)C
Properties[1]
Chemical formula	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)
Solubility in water	Sparingly soluble
Hazards
Lethal dose or concentration (LD, LC):
LD50 (median dose)	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). N verify (what is YN ?) Infobox references

Biosynthesis

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

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

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. 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.