Substituent effects on the pH-dependent multiequilibria of flavylium salt analogs of anthocyanins

摘要

Substituent effects on the hydration, tautomerization, and isomerization equilibria of flavylium salts can be described by a series of linear free energy relationships (LSER) based on Hammett correlations. The positions on the flavylium rings were classified as either activated (para-like) or nonactivated (meta-like) to decide which σ value to employ (σ_R or σ_m, respectively), while the steric effects of substituents at C-3 were included via the ES parameter. Based on these relationships, we then show that it is possible to predict values of the "apparent pK_a" (pK_(ap)) of flavylium ions that were not included in the original data set, as well as those of several naturally occurring anthocyanins. The value of pK_(ap) provides a measure of the thermodynamic stability of the flavylium cation as a function of pH and is directly related to the pH range in which the color of the flavylium cation form of anthocyanins persists in aqueous solution.