Rationalizing the Color in Heavenly Blue Anthocyanin: A Complete Kinetic and Thermodynamic Study

Johan Mendoza, Nuno Basílio, Fernando Pina, Tadao Kondo, Kumi Yoshida

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

All equilibrium and rate constants of heavenly blue anthocyanin (HBA 1) as well as the derivatives with two (HBA 2) or none (HBA 3) acylated units were determined. The three acylated units of the sugar in position 3 of the peonidin chromophore of HBA 1 are essential to confer the peculiar stability of its purple and blue colors. The sugars generate an efficient protective environment around position 2 (and 4) of the flavylium cation, through an intramolecular sandwich-type stacking that retards 35-fold the hydration reaction (kh) and increases 8.8-fold the dehydration reaction (k-h), when compared with the peonidin chromophore HBA 3. The conjugation of these two rates lowers 308-fold the hydration equilibrium constant (Kh), corresponding to a raise of the energy level of the hemiketal by 14.2 kJ mol-1. Conversely, the pKa of the quinoidal base in HBA 1 is only slightly stabilized in comparison with that of HBA 2 and HBA 3. The energy level of hemiketal increases with the number of acylated units, but the inversion of energies between hemiketal and quinoidal base takes place exclusively for HBA 1 (three acylated units), permitting in moderately acidic solutions the stabilization of the purple quinoidal base. Identical inversion of energy was observed for the corresponding ionized species, allowing the stabilization of the blue ionized quinoidal base in slightly basic solutions. At pH values higher than 8, the hydroxyl groups of the hydroxycinnamic acid units start to deprotonate disrupting the intramolecular sandwich-type stacking and the more or less slow degradation of the anthocyanin is observed.

Original languageEnglish
Pages (from-to)4982-4992
Number of pages11
JournalJournal of Physical Chemistry B
Volume122
Issue number19
DOIs
Publication statusPublished - 17 May 2018

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