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

• Published in: Journal of physical organic chemistry Vol. 24; no. 12; pp. 1201 - 1208
• Main Authors: Freitas, Adilson A., Dias, Luis G., Maçanita, António A.L., Quina, Frank H.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.12.2011
• Subjects: anthocyanins; flavylium salt; hydration reaction; linear free energy relationships; substituent effects
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1847

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 (σR or σm, 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 pKa” (pKap) 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 © 2011 John Wiley & Sons, Ltd. Linear Free Energy Relationships (LSER) based on Hammett correlations were used to describe substituent effects on the hydration, tautomerization, and isomerization equilibria of flavylium salts; these relationships permit prediction of values of the “apparent pKa” (pKap) of naturally‐occurring anthocyanins