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

Kumi Yoshida,Yoshiki Ichikawa,Masaru Hashimoto,Satoru Iuchi,Tadao Kondo,Miki Goto,Yoshiaki Takaya,Kin-Ichi Oyama,Kazushi Koga,Kohei Kazuma,Nobukazu Nagai

doi:10.1038/s41598-018-37641-0

Kumi Yoshida, Yoshiki Ichikawa + Show 9 more

Open Access

https://doi.org/10.1038/s41598-018-37641-0

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Journal: Scientific Reports	Publication Date: Feb 6, 2019
Citations: 12	License type: open-access

Affiliation: Nagoya University, Meijo University

Abstract

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.

Highlights

The small red bean, Vigna angularis, is primarily used to produce the “an-paste” component of Japanese sweets
In Japan, the adzuki bean occupies a very special position among beans in terms of food culture and agricultural history; food products prepared with adzuki beans, including red rice, and Japanese sweets, are eaten at special ceremonial or celebratory occasions such as weddings, the birth of a child, festivals, and New Year’s Day, because the red color of the seed coat is associated with good fortune[4,5,6]
In 1934, Kuroda and Wada reported that water-insoluble pigment and brown tannin components co-existed in red adzuki beans, but the pigment was very difficult to purify[10]

Summary

Introduction

The small red bean, Vigna angularis, is primarily used to produce the “an-paste” component of Japanese sweets. Many studies on the seed-coat pigments of red adzuki beans have been reported, none have clarified their structures or chemical properties. By extraction of the peeled seed-coat of the red adzuki bean with a 50% aqueous acetonitrile (CH3CN) containing 3% trifluoroacetic acid (TFA) followed by chromatographic purification, we obtained cyanidin 3,5-di-O-glucoside (cyanin, 1, Fig. 1e)[13].

Results

Conclusion