Naphthoquinone colorants from Arnebia nobilis Rech.f

The roots of Arnebia nobilis have traditionally been used as a colorant in food and cosmetic preparations. The deep red colour obtained is attributed to the presence of shikonin and its isomer alkannin and their derivatives. In this study, five colouring components were extracted from the roots of A. nobilis . These were separated and purified chromatographically and characterised using various spectrophotometric techniques. Three of the five components were identified. The major component was found to be alkannin β, β‐dimethylacrylate [5,8‐dihydroxy‐2‐(1′‐β, β‐dimethylacryloxy‐4′‐methylpent‐3′‐enyl)‐1,4‐naphthoquinone], accounting for nearly 25% of the total colouring matter. Alkannin acetate [2‐(1′‐acetoxy‐4′‐methylpent‐3′‐enyl)‐5,8‐dihydroxy‐1,4‐naphthoquinone] made up ca . 8% and shikonin [(5,8‐dihydroxy‐4′‐methylpent‐3′‐enyl)‐1,4‐naphthoquinone] contributed ca . 6% of the colouring matter. Polyester was dyed pink, nylon was dyed blue and all other substrates acquired a purple hue under similar dyeing conditions. The dyed fabrics showed excellent wash, rub and perspiration fastness; however, light fastness was found to be poor.