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

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 ( k) and increases 8.8-fold the dehydration reaction ( k), when compared with the peonidin chromophore HBA 3. The conjugation of these two rates lowers 308-fold the hydration equilibrium constant ( K), corresponding to a raise of the energy level of the hemiketal by 14.2 kJ mol. Conversely, the p K 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.

Exploring the Applications of the Photoprotective Properties of Anthocyanins in Biological Systems

“…Anthocyanin’s structure is pH-dependent, meaning that they can adapt to different environments, translating not only in different forms but also in different visible colors [ 6 , 17 ]. Such a fact attracted the scientific community to study how these phenomena happen, and nowadays the chemical pH dependence of anthocyanin’s structure is well understood [ 18 , 19 , 20 , 21 , 22 , 23 ].…”

Quantum Chemical Investigation of the Intramolecular Copigmentation Complex of an Acylated Anthocyanin

“…10,[16][17][18][19][20][21][22][23] In the plant vacuoles where anthocyanins concentrate, [24][25][26] the anthocyanin cation AH + can chelate with metal cations 23,27 such as Al 3+ and/or form complexes with colorless organic copigment molecules, such as electron-rich derivatives of hydroxybenzoic or hydroxycinnamic acids, flavones or one of the colorless neutral forms of the anthocyanin itself. 10,[16][17][18][19][20][21][22][23] Metal cation chelation can lead to large changes in the color, primarily from red to blue as, for example, in Hydrangea, 28 but is limited to anthocyanins with two or more free OH groups in the B-ring 23,27,29 (i.e., anthocyanins derived from cyanidin, delphinidin and petunidin). In contrast, copigmentation via complexation with organic molecules results in much smaller red shifts of the absorption, but can increase the pH at which hydration occurs, consistent with steric hindrance to attack of water and charge transfer from the copigment to the anthocyanin as an important contributor to the stability of the anthocyanin-copigment complex.…”

“…In contrast, copigmentation via complexation with organic molecules results in much smaller red shifts of the absorption, but can increase the pH at which hydration occurs, consistent with steric hindrance to attack of water and charge transfer from the copigment to the anthocyanin as an important contributor to the stability of the anthocyanin-copigment complex. 10,[16][17][18][19][20][21][22][23] One limitation of this bimolecular copigmentation is the range of stability constants for the complexation, 21,23 which rarely exceed 10 4 M -1 , requiring mM or greater local copigment concentrations in order to achieve a substantial percentage of complexation of the anthocyanin. Some plants have overcome the entropic limitations of bimolecular copigmentation by covalently attaching one or more copigment molecules to the sugar residues of the anthocyanin (e.g., acyl ester derivatives), [18][19][20]22,23 transforming the copigmentation into an entropically much more favorable intramolecular complexation phenomenon.…”

Impact of a Water‐Soluble Gallic Acid‐Based Dendrimer on the Color‐Stabilizing Mechanisms of Anthocyanins

“…Moreover,t he flavylium cation is more stabilized than the hemiketal form in the presence of the dendrimer owing essentially to af aster dehydration and lower hydration rate constants compared with the absence of dendrimer as observed for other analogous systems. [52] Hence, the hydration constant increasesi no ne pH unit in the presence of the dendrimer.M oreover,f or the cy3glc-dendrimer complex, K t is increased, which reveals ap referential interaction of the cis-chalcone for the host than the hemiketal species. Finally,t he mole fraction distribution of cy3glc ( 19.8 mm) in the absence of the dendrimer (Figure 10 a) was compared with the one of cy3glc ( 19.8 mm) in the presence of dendrimer (26 mm;F igure 10 b).…”