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.

Phylogenomics, biogeography, and evolution of the blue‐ or white‐fruited dogwoods (Cornus)—Insights into morphological and ecological niche divergence following intercontinental geographic isolation

“…Previous studies on Vaccinium suggest that fruit color is associated with elevation, such that darker fruits occur at higher altitudes, whereas lighter fruits occur at lower ones (Lätti et al, 2008;Zoratti et al, 2015). This association can be attributed to the increased exposure to solar radiation at higher elevations (Madronich et al, 1995;McKenzie et al, 2001;Dahlback et al, 2007) and the role of anthocyanins as a protectant against excessive light (Winkel-Shirley, 2002;Ferreira da Silva et al, 2012;Burns, 2015). This same association between elevation and fruit color is also observed in the BW disjunct species.…”

The Chemical Reactivity of Anthocyanins and Its Consequences in Food Science and Nutrition

“…In the presence of a co-pigment, other mechanisms ( Figure 17 ) supersede the fast flavylium deprotonation observed with free anthocyanins [ 57 ] in the following ways: (a) within the complex in the excited state, through ultrafast internal conversion (<1 ps) via a low-energy co-pigment-to-pigment charge transfer state, resulting in static fluorescence quenching; and (b) for the fraction of free anthocyanin, diffusion-controlled electron transfer from the co-pigment to the flavylium ion in the excited state, resulting in dynamic fluorescence quenching. The mechanism of energy dissipation by ultrafast internal conversion has been confirmed for the folded conformation of a cyanidin glycoside acylated by p -coumaric acid [ 58 ]. In addition, fast energy transfer to the chromophore following absorption of UV light by the acyl residue operates ( Figure 17 ), thereby conferring acylated anthocyanins to have an important role in plant photoprotection.…”

Exploring the Applications of the Photoprotective Properties of Anthocyanins in Biological Systems

“…Purple sweet potato anthocyanins have particular properties regarding pH and heat resistance, photosensitivity, and overall stability due to the acylation with phenolic acids such as caffeic acid, p -coumaric acid, and ferulic acid, which could be advantageous to preserve their natural colors and desired effects in a broad range of applications, including the cosmeceutical industry [ 87 ]. Besides, it has been reported that the presence of these aromatic organic acids in the structure of anthocyanins enhances their natural UV absorption capacity, thus representing a more efficient mean of photoprotection [ 88 , 89 ]. In fact, there has been a growing interest in exploring the use of anthocyanins in UV-blocking formulations, especially due to raising awareness of the safety and toxicity issues associated with the synthetic compounds conventionally used in sunscreens.…”