The Chemical Reactivity of Anthocyanins and Its Consequences in Food Science and Nutrition

Abstract

Owing to their specific pyrylium nucleus (C-ring), anthocyanins express a much richer chemical reactivity than the other flavonoid classes. For instance, anthocyanins are weak diacids, hard and soft electrophiles, nucleophiles, prone to developing π-stacking interactions, and bind hard metal ions. They also display the usual chemical properties of polyphenols, such as electron donation and affinity for proteins. In this review, these properties are revisited through a variety of examples and discussed in relation to their consequences in food and in nutrition with an emphasis on the transformations occurring upon storage or thermal treatment and on the catabolism of anthocyanins in humans, which is of critical importance for interpreting their effects on health.

Keywords: anthocyanin; chemistry; flavylium; interactions.

Figure 1

Flavylium ions are weak diacids.

Figure 2

(I) Absorption spectra of Cat-Mv3Glc: pH jump from pH = 1.0 (100% flavylium) to pH 3.00, 3.59, 4.50, 5.70, 5.96, 6.25, and 7.15, respectively. Spectra recorded 10 ms after mixing (negligible water addition). (II) Spectra of the components obtained by mathematical decomposition. From [4] with permission of the American Chemical Society.

Figure 3

Flavylium ions are hard electrophiles reacting at C2 with O-centered nucleophiles, such as water (water addition followed by formation of minor concentrations of chalcones).

Figure 4

Simulations of the pH dependence of the apparent rate constant (A) and relative magnitude (B) of color loss. Selected values for parameters: pK_a1 = 4, pK_a1 = 7, pK′_h = 2.5, k_h = 0.1 s⁻¹, k′_−h ≈ k_h/K′_h.

Figure 5

(I) Spectral changes of Cat-Mv3Glc between 10 ms and 9 s following a pH jump from pH = 1 to pH = 2.45; half-life of flavylium = 2.4 s. (II) pH jump from pH = 1 to pH = 4.5; half-life of quinonoid bases = 53.3 s. At pH = 6, the half-life of quinonoid bases ≈ 30 min. From reference [4] with permission of the American Chemical Society.

Figure 6

Flavylium ions are soft electrophiles that react at C4 with S- and C-centered nucleophiles, such as bisulfite and 4-vinylphenols.

Figure 7

Anthocyanin hemiketals are nucleophiles reacting with carbocations (Ar = catechol ring).

Figure 8

Pathways of anthocyanin degradation.

Figure 9

Possible mechanisms of anthocyanin degradation with pre-formed hydrogen peroxide.

Figure 10

Possible mechanisms of anthocyanin degradation without pre-formed hydrogen peroxide.

Figure 11

Possible mechanisms for the antioxidant activity of anthocyanins in food and in the gastro-intestinal tract.

Figure 12

Co-pigmentation of malvin (malvidin 3,5-diglucoside, 50 µM) by rutin bis(hydrogensuccinate) (mixture of 3 regioisomers, 200 equiv.). (A) pH = 3.5, malvin + co-pigment at T = 15.5 (1), 25.0 (2), 35.0 (3), 44.2 (4) °C, malvin alone at T = 25.3 °C (5). (B) pH = 0.9, T = 25.0 °C, malvin alone (1), malvin + co-pigment (2). Adapted from reference [35].

Figure 13

Acylated anthocyanins: discrimination of intramolecular co-pigmentation (type 1) and self-association (types 2 and 3) by circular dichroism (pink or blue CD spectra depending on the chirality of the stacks). From [34] with permission of the Royal Society of Chemistry.

Figure 14

Triacylated (B) vs. non-acylated (A) Morning glory (Pharbitis nil) anthocyanins: equilibrium distribution of anthocyanin species in aqueous solution. Red solid line: flavylium ion, blue solid line: neutral base, dotted green line: total colorless forms. Parameters for plots are pK’_h = 2.30, pK_a1 = 4.21 (A); pK’_h = 4.01, pK_a1 = 4.32 (B). From [36,37].

Figure 14

Triacylated (B) vs. non-acylated (A) Morning glory (Pharbitis nil) anthocyanins: equilibrium distribution of anthocyanin species in aqueous solution. Red solid line: flavylium ion, blue solid line: neutral base, dotted green line: total colorless forms. Parameters for plots are pK’_h = 2.30, pK_a1 = 4.21 (A); pK’_h = 4.01, pK_a1 = 4.32 (B). From [36,37].

Figure 15

(A) 3′,4′-Dihydroxy-7-O-β-d-glucopyranosyloxyflavylium (50 µM) in a pH 4 buffer (0.1 M acetate), red spectrum: before hydration, blue spectrum: 10 min after addition of Al³⁺ (4 equiv.); (B) equilibrium distribution of species in aqueous solution. Red solid line: flavylium ion, blue dotted line: neutral base, dotted green line: total colorless forms, blue solid line: Al³⁺ complex. Parameters for plots are pK’_h = 3.42, pK_a1 = 4.72, K_M = 2 × 10⁻⁴. From [39].

Figure 16

(A) Frontier MOs of the flavylium ion of cyanidin (from reference [55]) and its most representative mesomeric forms in the ground state (left) and first excited state (right). (B) The fate of free anthocyanins in the excited state (from references [54,56]).

Figure 16

(A) Frontier MOs of the flavylium ion of cyanidin (from reference [55]) and its most representative mesomeric forms in the ground state (left) and first excited state (right). (B) The fate of free anthocyanins in the excited state (from references [54,56]).

Figure 17

The influence of co-pigmentation on the fate of anthocyanins in the excited state. (A) Intermolecular co-pigmentation (from reference [57]). (B) Intramolecular co-pigmentation (from reference [58]).