Previously, the seed coat pigment of small red beans (Vigna angularis) was presumed to be anthocyanin with brown colored tannins. We isolated anthocyanin pigment, cyanin (1), from small red beans in 1996, but the content in the seed coat was very low, less than 0.01 mg/g dry seed coat. Therefore, the seed-coat color was not due to anthocyanin. We identified two purple pigment in the seed coat of small red beans, which was responsible to the purple color of an-paste, and determined the structure. In this study, we will report the structure and chemical properties of the purple pigments. An aqueous acetonitrile solution containing trifluoroacetic acid was analyzed by PDA detected ODS-HPLC. Two pigment peaks which exhibit purple color in strong acidic condition were found. From 12 Kg of beans we extracted the pigment and purified using repeated column chromatography to be obtained AZ- 1 (2) and AZ-2 (3), respectively. From the various NMR experiments and MS analysis, the partial structure of AZ-1 (2) was determined. However, the structure could not be determined. Then, we degraded AZ-1 (2) to AZ-1d (4) and determined the structure, first. From the structure of AZ-1d (4) we could finally determine the structure of AZ-1 (2) and named catechinopyranocyanidin A. It has a condensed ring structure with catechin and cyanidin.
