Investigation of color-deepening phenomenon in catechin-(4→8)-dimer as a proanthocyanidin model and structural determination of its derivatives by oxidation.

To elucidate the mechanism of the color-deepening phenomenon in aged red rice samples, a partial structure of proanthocyanidin, (+)-catechin-(4→8)-dimer (1), was synthesized as a model compound and subjected to chemical oxidation. HPLC analysis of oxidized 1 revealed new peaks which were isolated; among these, four compounds (2-5) having formed intramolecular bonds between the D and B rings were determined. These compounds and their derivatives in red rice extracts were identified by multiple reaction monitoring mass spectrometry. The UV-Vis spectra of 1-5 were recorded to clarify the correlation between the color-deepening effect and their chemical structures. Hence, the spectral absorption maxima between 300 and 500nm, corresponding to the oxidation products, increase. Therefore, the color-deepening phenomenon of red rice is thought to proceed via the formation of intramolecular bonds in proanthocyanidin by oxidation.