Structure of two purple pigments, catechiopyranocyanidins A and B from the seed-coat of the small red bean, Vigna angularis.

The small red bean, Vigna angularis, is primarily used to produce the "an-paste" component of Japanese sweets. Through the manufacturing process, the red seed-coat pigment is transferred to the colorless "an-particles", imparting a purple color. However, the major pigment in the seed coat has not yet been identified, although it is historically presumed to be an anthocyanin. Here, we report the isolation and structural determination of two hydrophobic purple pigments in the seed coat via instrumental analysis and derivatization. The new pigments, catechinopyranocyanidins A and B, contain a novel pyranoanthocyanidin skeleton condensed with a catechin and cyanidin ring system, and no sugar moieties. Catechinopyranocyanidins A and B are diastereomers with a different configuration at the catechin moiety, and both are purple in color in strongly acidic-to-neutral media. Catechinopyranocyanidins A and B are very stable under dark conditions, but, labile to light and decompose to colorless compounds. Thus, these pigments exhibit quite different chemical properties compared to simple anthocyanidins.