Structural characterization of proanthocyanidins from adzuki seed coat

Adzuki bean seed coats contain a variety of pigments with potential bioactivity, including polyphenols, such as proanthocyanidin. To clarify the structure of the pigments from adzuki bean seed coats, the MeOH extract of the seed coats and the extraction residue were subjected to various instrumental analyses and degradation by thiolysis. The MeOH extract was found to contain two types of polymeric red pigments. One of these pigments was assumed to be a simple proanthocyanidin, whereas the other was a complicated polyphenol produced by several modifications and conversions. Direct thiolysis of adzuki seed coat yielded three products as extension subunits. From their chemical structures, a glyoxylic acid-bridged catechin skeleton was suggested as a partial structure of adzuki pigments.