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

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 sigma value to employ (sigma(R) or sigma(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 pKap 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. Copyright (C) 2011 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.