title: Photoprotection and the photophysics of acylated anthocyanins.
abstract: 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.
authors.affiliation: Centro de Química Estrutural, Instituto Superior Técnico, IST/UTL, Lisboa, Portugal. palmira@ist.utl.pt
authors.forename: Palmira Ferreira
authors.initials: PF
authors.lastname: da Silva
authors.forename: Luísa
authors.initials: L
authors.lastname: Paulo
authors.forename: Adrianna
authors.initials: A
authors.lastname: Barbafina
authors.forename: Fausto
authors.initials: F
authors.lastname: Eisei
authors.forename: Frank H
authors.initials: FH
authors.lastname: Quina
authors.forename: António L
authors.initials: AL
authors.lastname: Maçanita
mesh_terms: D000215:Acylation; D000872:Anthocyanins / Q000737:chemistry*; D008027:Light; D010777:Photochemistry; D010944:Plants / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D000872:Anthocyanins