Photoprotection and the Photophysics of Acylated Anthocyanins

• Published in: Chemistry : a European journal 2012-03, Vol.18 (12), p.3736-3744
• Main Authors: Ferreira da Silva, Palmira, Paulo, Luísa, Barbafina, Arianna, Elisei, Fausto, Quina, Frank H., Maçanita, António L.
• Format: Article
• Language: English
• Subjects: Acylation; Adiabatic flow; Anthocyanins; Anthocyanins - chemistry; Chemistry; Damage; Energy dissipation; Energy transfer; Excitation; Light; Pathways; Photochemical; Photochemistry; photophysics; plant chemistry; Plants - chemistry; reaction mechanisms; Ultraviolet Rays; UV/Vis spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201102247

The proposed role of anthocyanins in protecting plants against excess solar radiation is consistent with the occurrence of ultrafast (5–25 ps) excited‐state proton transfer as the major de‐excitation pathway of these molecules. However, because natural anthocyanins absorb mainly in the visible region of the spectra, with only a narrow absorption band in the UV‐B region, this highly efficient deactivation mechanism would essentially only protect the plant from visible light. On the other hand, ground‐state charge‐transfer complexes of anthocyanins with naturally occurring electron‐donor co‐pigments, such as hydroxylated flavones, flavonoids, and hydroxycinnamic or benzoic acids, do exhibit high UV‐B absorptivities that complement that of the anthocyanins. In this work, we report a comparative study of the photophysics of the naturally occurring anthocyanin cyanin, intermolecular cyanin–coumaric acid complexes, and an acylated anthocyanin, that is, cyanin with a pendant coumaric ester co‐pigment. Both inter‐ and intramolecular anthocyanin–co‐pigment complexes are shown to have ultrafast energy dissipation pathways comparable to those of model flavylium cation–co‐pigment complexes. However, from the standpoint of photoprotection, the results indicate that the covalent attachment of co‐pigment molecules to the anthocyanin represents a much more efficient strategy by providing the plant with significant UV‐B absorption capacity and at the same time coupling this absorption to efficient energy dissipation pathways (ultrafast internal conversion of the complexed form and fast energy transfer from the excited co‐pigment to the anthocyanin followed by adiabatic proton transfer) that avoid net photochemical damage. Photoprotection in plants: The acylation of anthocyanins provides UV protection against photochemical damage by fast energy transfer (ET) from the excited co‐pigment (Coum) to the anthocyanin (Cy) followed by ultrafast adiabatic proton transfer (PT) to water or internal conversion (IC) of the complex (see figure).