On the Limits of Anthocyanins Co-Pigmentation Models and Respective Equations

Anthocyanins co-pigmentation models with application on 1:1 complexes were revisited, and their limitations were critically commented. The flavylium multistate of species is dramatically simplified to a single acid-base equilibrium between flavylium cation and its conjugated base CB, equal to the sum of quinoidal base, hemiketal, and cis and trans-chalcones. Bearing this, a new equation that simultaneously allows calculation of the co-pigmentation constant with flavylium cation (KAH+CP) and with its conjugated base CB (KCBCP) was deduced. This equation can be used at a fixed co-pigment concentration with pH as a variable or at fixed pH and co-pigment concentration variable. A global fitting of all data allows us to calculate both association constants with good accuracy. The model was applied to the co-pigmentation of malvidin-3-glucoside with caffeine and pentagalloyl glucose (PGG). Caffeine gives rise to complexes not only with flavylium cation KAH+CP = 125 ± 7 M-1 but also with CB with KCBCP = 23 ± 3 M-1. PGG complexes exclusively with flavylium cation, KAH+CP = 914 ± 10 M-1, and the possible interaction with quinoidal base is lower than the detection limits that the inherent experimental error permits.