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]