Chemistry and photochemistry of natural plant pigments: the anthocyanins

Abstract

AbstractAnthocyanins are naturally occurring plant pigments responsible for the red, blue, and purple colors of the majority of fruits, flowers, and leaves. The pH‐dependent ground‐state chemistry of anthocyanins is extremely rich. Above about pH 2.5, the colored flavylium cation form typically hydrates to form the colorless hemiacetal, followed by ring‐opening tautomerization to the (E)‐chalcone, which can isomerize to the (Z)‐chalcone. The color of anthocyanins can also be modulated and/or stabilized by complexation with metal ions or with colorless organic molecules (copigments) such as hydroxylated benzoic or cinnamic acids. An understanding of the chemistry that contributes to the loss of color is essential for the development of novel and more effective strategies for the stabilization of the color above pH 3, which would permit the use of anthocyanins as natural pigments in a much wider range of foods or consumer products. In the excited state, uncomplexed anthocyanins undergo ultrafast adiabatic deprotonation (5–20 ps) in aqueous solution to give the corresponding short‐lived (about 200 ps) excited quinonoidal base. Intermolecular and intramolecular complexes of anthocyanins with organic copigments undergo deactivation that is even faster than deprotonation. The colorless hydration products are photoactive in the ultraviolet; thus, the chalcones undergo E–Z photoisomerization, while the hemiacetal form of anthocyanins exhibits photochemistry typical of a chromene. These photoprocesses all potentially contribute to the photostability of anthocyanins in fruit and flowers and are fully consistent with the biological role of anthocyanins in protecting leaves from excess solar radiation. Copyright © 2016 John Wiley & Sons, Ltd.