Do We See the True Color of Anthocyanidins?

Anthocyanins are important as pigments, and not only contribute to the colorful appearance of plant petals, leaves, and fruits, but are also expected to be used as natural dyes. Anthocyanin aqueous solutions show different colors with changes in pH. This color change is due to the structural transformations of anthocyanidins (the aglycones of anthocyanins): the colors of the flavylium cation, quinoidal base, and quinoidal base anion forms are often considered to be red, purple, and blue, respectively. However, there is no experimental way to confirm whether these are true except for the case of the flavylium cation, because various molecular species resulting from structural changes and complex formation coexist in solution due to chemical equilibrium. Therefore, in this study, the color of the quinoidal base monomers was evaluated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Additionally, color changes were predicted when the quinoidal bases or the flavylium cations dimerize, and when they form heterogeneous 1:1 complexes with other molecular species in solution. These calculations revealed that what has been believed to be the color of the quinoidal base monomers is actually that of the solution including the contribution of several molecular species.