Influence of E,Z-isomerization and stability of acylated anthocyanins under the UV irradiation

Abstract The E,Z-isomerization reaction and stability of acylated anthocyanins under the influence of UV irradiation was studied. Among monoacylated anthocyanins in flower petals the E-configurated cinnamoyl derivative residues are predominant. However, these E-isomers are readily isomerized to Z-forms in acidic methanol solutions with UV irradiation to give a 1:1 mixture of the isomers. Under acidic aqueous conditions the equilibrium ratio of the Z-isomer of malonylawobanin to the E-isomer becomes lower (Z/E: 1:6) than in acidic methanol solutions. Furthermore, the pigment is stabilized, the extent depending on its concentration. In neutral aqueous solutions with 10 eq. of flavocommelin, the ratio of the Z- to E-isomers of malonylawobanin was also 1:6 with UV light, but a significant stabilization effect was observed. In commelinin, a supramolecular metal-complex pigment, malonylawobanin was found to be stacked much more strongly than with a 1:1 mixture of malonylawobanin and flavocommelin. Under such conditions malonylawobanin was more stabilized. With the diacylated anthocyanin, gentiodelphin, UV induced isomerization from E-isomer to Z-isomer was significantly suppressed. In acidic methanol solutions the content of Z-isomer was only a few percent, and in aqueous solutions gentiodelphin did not isomerize. Therefore, molecular associations prevent light induced E,Z-isomerization. Under UV irradiation conditions anthocyanins in neutral aqueous solutions were most stable. These phenomena are in line with biological function of anthocyanins in plant tissue blooming under solar radiation.