Isolation and characterization of a cyanidin-catechin pigment from adzuki bean (Vigna angularis).

Adzuki bean is used to prepare many kinds of foods in east Asia, and the seed coat contains water-soluble anthocyanins, catechins, and flavonols. In the present study, ethyl acetate-soluble purplish pigments were isolated from adzuki bean. Pigments of soaked adzuki bean were extracted with 1% HCl in methanol. Ethyl acetate-soluble purple pigments were obtained from the methanol soluble components. Purple pigments 1 and 2 were purified from the ethyl acetate-soluble pigments by Sephadex LH-20 column chromatography and preparative reversed-phase HPLC. NMR and mass spectra suggested that pigment 1 was a condensation product of cyanidin and (+)-catechin, in which 5-hydroxy and C-4 positions of the cyanidin moiety were substituted by the addition of 5-hydroxy and C-6 positions of the (+)-catechin moiety, respectively. Pigment 2 was an isomer of pigment 1. It is suggested that pigments 1 and 2 contribute to the purplish-red colour of foods prepared using adzuki bean.