Quinonoidal bases

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

An anthocyanin occurs in solution as a mixture of different secondary structures, a quinonoidal base, a carbinol pseudobase, and a chalcone pseudobase. ° hi addition, different mechanisms for the stabilization of anthocyanins lead to the formation of tertiary structures such as self-association, inter-, and intra-molecular co-pigmentation. ... 

Anthocyanins are usually represented as the red flavylium cations (Figure 5.1, left). However, this form is predominant only in very acidic solvents (pH < 2) such as those used for HPLC analysis. In mildly acidic media, the flavylium cations undergo proton transfer and hydration reactions, respectively, generating the quinonoidal base and the hemiketal syn carbinol) form (Figure 5.1, right) that can tautomerize to the chalcone. Thus, at wine pH, malvidin 3-glucoside occurs mostly as the colorless hemiketal (75%), the red flavylium cation, yellow chalcone, and blue quinonoidal base being only minor species. 

The hydration and protonation reactions of the methylmethine-linked malvidin 3-glucoside dimer can be summarized as follows, with AH ", AOH, and A representing the flavylium, hemiketal, and quinonoidal base forms, respectively ... 

Fig. 13 Equilibrium distribution of four anthocyanin forms of malvidin-3-glucoside as a function of pH The red flavylium cation (AH+), the blue quinonoidal base (A), the colorless carbinol pseudobase (B), and the colorless chalcone (C). (From Ref. 138.)...

It involves, on one hand an anthocyanin under its flavylium or quinonoidal base form, and on the other hand another planar hydrophobic structure that can be another anthocyanin unit (self-association), or another colorless species (copigment), covalently bound (intramolecular copigmentation) or not (intermolecular copigmentation) to the anthocyanin pigment. 

Copigmentation is driven by hydrophobic vertical stacking between the anthocyanin and the copigment to form tt-tt complexes from which water is excluded. The flavylium cation as well as the quinonoidal base are planar hydrophobic structures and can be involved in such complexes whereas the hemiketal form cannot. The association thus results in displacement of the anthocyanin hydration equilibrium from the colorless hemiketal to the red flavylium form that can be easily measured by spectrophotometry. 

The anthocyanins are structurally dependent on the conditions and composition of the media where they are dissolved and suffer interactions among them and with other compounds that influence their structural equilibria and modify their color. Anthocyanins are usually represented as their red flavylium cation, but in aqueous media this form undergoes rapid proton transfer reactions, leading to blue quinonoidal bases, and hydration, generating colorless hemiketals in equilibrium with chalcone structures. The proportion of each form is determined by the pH... 

Fig. 9D.1 Equilibrium distribution at 25 °C among structural forms of malvidin 3-glucoside as a function of pH. AH+, flavyUum cation A, quinonoidal base B, hemiketal C, chalcone (from Brouillard 1982, with permission from Elsevier)...

The pH of cell sap of most flowers is such that many antho-cyanin-containing flowers should be colorless, yet the presence in nature of colorless anthocyanins is rare. Anthocy-anidins undergo a series of complex color changes in water at varying pH (Brouillard, 1988). The flavylium cation of natural anthocyanins behaves as a weak diacid, whereas a neutral quinonoid base is at the same time a weak acid and a weak base. The pH of crude extracts of flowers varies from 2,8 to 6.2. 

Fig. 1.3 Anthocyanin equilibria in aqueous solution and the corresponding structural transformations. AH2 represents the flavylium cation that predominates at acidic pH values AH represents the two tautomeric quinonoid bases A depicts the anionic quinonoid bases that appears in alkaline solutions BH2 is the colorless hemiketal adduct and and are isomeric retrochalcones.

FIGURE 5.2 Structural transformations of anthocyanins in aqueous solution at varying pH. 1 = flavylium cation (red to orange color), 2-4 = neutral quinonoidal bases (purple to violet color), 5-7 = ionized quinonoidal bases (blue color), 8 = carbinol pseudobase (colorless), 9 = chalcone pseudobase (colorless). (Adapted from Brouillard, R., The Flavo-noids Advances in Research Since 1980, Chapman and Hall, London, United Kingdom, 1988, 525-538.)... 

---