A UV-B resistant polyacylated anthocyanin, HBA, from blue petals of morning glory

Prevention of E,Z-isomerization of caffeoyl residues of a tri-(E)-caffeoyl anthocyanin, heavenly blue anthocyanin (HBA), and its stability under UV-B irradiation conditions were studied. We isolated four photoproducts from irradiated HBA and their structures were determined to be mono- or di-Z-caffeoyl isomers of HBA and mono-deglucosylated HBA. Under such conditions one caffeoyl residue, the innermost one, never isomerized to the Z-form, suggesting that intramolecular stacking must prevent photoisomerization. In general, anthocyanins are considered to be more stable in strong acidic than neutral aqueous media. However, with UV-B irradiation HBA was most stable in aqueous solution at pH 7.5 and most unstable in acidic methanol solutions. It was found to emit strong fluorescence on excitation with UV-B, possibly resulting from intramolecular association of caffeoyl moieties with the anthocyanidin nucleus. The finding that pigment in petals is more tolerant of UV-irradiation may be rational in the context of the necessity to resist strong solar radiation.