Prevention of UV-light induced E,Z-isomerization of caffeoyl residues in the diacylated anthocyanin, gentiodelphin, by intramolecular stacking

Prevention of UV-B light induced E,Z-isomerization of caffeoyl residues in a diacylated anthocyanin, gentiodelphin, by molecular stacking was studied. We first isolated an isomer of gentiodelphin containing a Z-caffeoyl residue from an acidic methanol solution, irradiated with UV-B light. The isomerized pigment had only one Z-configurated caffeoyl residue, attached to the glucose of the 5-position. Under the irradiation conditions the other caffeoyl residue linking to the glucose on B-ring stacked to the anthocyanidin nucleus and did not isomerize. In acidic and neutral aqueous solutions the content of the Z-isomer was very low when the isomerization reaction was at equilibrium, intramolecular stacking of the caffeoyl residues being stronger than in acidic methanol. Therefore, intramolecular stacking may prevent light-isomerization of the α,β double bond of caffeoyl residues. Under physiological conditions the pigment was more tolerant of UV-irradiation, which may play an important role in quenching solar radiation energy in flower petals.