Quinoidal base

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

In 1967, Timberlake and Bridle proposed that copigmentation complex formation reactions between cyanidin and quercetin in aqueous buffered solutions took place between the colored forms of the flavylium cation (AH+) cyanidin at pH 3.0 and the quinoidal base (A) at pH 5.0. 

Absorption spectra have also been used in the reexamination of pH-dependent color and structural transformations in aqueous solutions of some nonacylated anthocyanins and synthetic flavylium salts." ° In a recent study, the UV-Vis spectra of flower extracts of Hibiscus rosasinensis have been measured between 240 and 748 nm at pH values ranging from 1.1 to 13.0." Deconvolution of these spectra using the parallel factor analysis (PARAFAC) model permitted the study of anthocyanin systems without isolation and purification of the individual species (Figure 2.21). The model allowed identification of seven anthocyanin equilibrium forms, namely the flavylium cation, carbinol, quinoidal base, and E- and Z-chalcone and their ionized forms, as well as their relative concentrations as a function of pH. The spectral profiles recovered were in agreement with previous models of equilibrium forms reported in literature, based on studies of pure pigments. 

In flavylium compounds that bear OH substituents in their 4 - and/or 7-positions, deprotonation of the OH group can result in other forms being obtained, not seen in the case of the 4 -methoxyflavylium compound discussed above. Figure 8 illustrates this for the 4 -hydroxyflavylium ion.1171 The new species are the quinoidal base A, obtained by simple deprotonation of the AH+ flavylium cation, and the dianionic Cc2- and Ct2- forms, obtained by second deprotonations of Cc and Ct. The roles played by these forms depend on the specific compound and the pH conditions. For... 

In solution, the anthocyanins actually might exist in equilibrium with essentially four molecular forms - the flavylium cation, the quinoidal base, the hemiacetal base and chalcone [19]. The relative amounts of the four structure forms depends on both the pH and the difference in structure of the anthocyanins [20-22], Generally, anthocyanins exist primarily as the stable flavylium cation above pH 2. This uniqueness in the chemical structure is one of the important key factors affecting their absorption, metabolism, bioavailability and, consequently, the biological responses of the human body to anthocyanins. 

Unlike other subgroups of flavonoids with a similar C6-C3-C6 skeleton, anthocyanins have a positive charge in their structure at acidic pH. In solution, the anthocyanin actually occurs in equilibrium with essentially four forms the flavylium cation, the quinoidal base, the hemiacetal base, and chalcone (Cooke et al., 2005) (Figure 5.3). The relative amounts of these four forms vary with both pH and structure of the anthocyanins (Brouillard et al., 1977 Brouillard 1982 Mazza and Miniati, 1993). Anthocyanins exist primarily as the stable flavylium cation only when the pH is less than 2. 

Figure 5.3 Simplified diagram of anthocyanin structural transformation in solution. A flavylium cation, B the quinoidal base, C the hemiacetal base, D chalcone. (Gly glycoside) (Modified from Mazza and Miniati, 1993).

Hoshino, T. (1991). An approximate estimate of self-association constants the self-stacking conformation of malvin quinoidal bases studied by IH NMR. Phytochemistry, 30, 2049-2055. 

Grapes, wine and other berry products are acidic, and the anthocyanins are in the flavylium form. A hypothesis that the anthocyanins may be pH-transformed into their carbinol pseudo-base and quinoidal base, or the chalcone, in the intestine and blood system during digestion has been proposed by Lapidot et al. (1999). These compounds have been shown to have antioxidant activity and are also most likely absorbed from the gut into the blood system. The pseudo-base and the quinoidal base of malvidin and malvidin-3-glucoside remained as very effective antioxidants, both when tested by a linoleate-oxidizing assay and by a microsomal lipid peroxidation assay. 

The pH of the medium plays a particularly important role in the equilibrium between these different anthocyanin forms, and consequently in color modification. In strongly acid solution, at a pH below 2, the red cation AH+ is the dominant form. As the pH is increased, a rapid proton loss occurs to yield the red or blue quinoidal base A, usually existing in two forms (Figure 9.3). On standing, a further reaction occurs, i.e., hydration of flavylium cation AH+, to give colorless carbinol pseudobase B. Relative amounts of forms AH+, A, B, and C at equilibrium vary with both pH and the structure of anthocyanins. For the common anthocyanin 3-glycosides... 

The A. are glycosides, the actual chromophores of which are their aglycones, the anthocyanidins. A common feature of all known anthocyanidins is the skeleton of a C-4 -hydroxylated 2-phenylchromene, existing primarily as a flavylium cation. Secondary structures such as quinoid bases, and carbinol or chalcone pseudobases occur in aqueous solution in dependence on pH value. 

Hydrophobic effects are thought to position the anthocyanin chromophore and the copigment to form a ti-ti complex [244], and by this way, the most efficient overlap would occur with planar flavonols when compared with hydroxyciimamoyl, galloyl esters, or with the nonplanar flavan-3-ols [245]. Usually, it is thought that the flavylium ion is the major colored species that contributes to the copigmentatirm phenomenon [244, 246]. However, some authors have suggested that the neutral quinoidal base is the main species involved [247, 248]. 

In aqueous media, most of the natural anthocyanins behave like pH indicators, being red at low pH, bluish at intermediate pH and colorless at high pH. It has been demonstrated that in acidic or neutral media, four anthocyanin structures exist in equilibrium the fiavylium cation (AH-I-), the quinoidal base, the carbinol pseudo base and the chalcone. In general, anthocyanins are present in the fiavylium form at... 

A colored anionic quinoidal bases unstable forms... 

The color of anthocyanins depends essentially on the different structural forms in which they can be found, these structures being strongly influenced by the pH value. At pH values between 4 and 6, the typical value for fresh and processed fruits, an equilibrium between four structural forms the red flavylium cation (I), the blue anhydrous quinoidal base (IV), the colorless carbinol pseudobase (II) and the yellow chalcone (III) occurs for all naturally anthocyanins (Figure 3). 

In solutions of pH 1.0 and lower, anthocyanins exist solely as red-coloured flavylium salts. When increasing pH, the equilibrium shifts in favour of colourless carbinol pseudo base and the red colour fades. Around the range of pH values of 4.0 to 4.5, anthocyanins are completely colourless. Another increase in pH is manifested by the purplish-red colour, which is caused by formation of a neutral quinoid base that requires the presence of free hydroxyl groups on one of C-5, C-7 or C-4 carbons. In solutions of pH 7 a blue coloured quinoid base is formed. After some time or following an increase in pH value, a gradual decrease of blue colour intensity occurs as a result of yellow chalcone formation. If the solution is acidified to around pH 1.0, the blue quinoid and colourless carbinol bases are converted back into red flavyhum cations. The transformation of chalcones is slower and not quantitative. 

In processed fruit and vegetables, the situation is more complex. Anthocyanins in plants (pH of from 2.5 to 7.5) occur as a purplish-red neutral quinoid base, but in food products they may be in media of different pH. However, they are mostly stabilised by inter co-pigmentation (interactions with other flavonoids) or intra copigmentation (acylated forms), or by interactions with other food components. Many products therefore retain their original colour... 

The photochromic behaviour of synthetic flavylium compounds, more properly the one of the trans-chalcone spiecies, only recently was perceived as a new photochromic system with many p>otentiahties.[7] In spite of its ap>p)arent complexity is a simple sequence of three well known elementary steps i) photo induced isomerisation ii) cyclization and iii) dehydration to give the coloured product, the flavylium cation, (or the very coloured quinoidal base, depending on pH), Scheme 2.7. 

The system is equivalent to two competitive parallel readions with the particularity that one of them is dependent on pH. The best pH value to observe the colour contrast is thus the result of two opposite effects that follow immediately the irradiation by one hand higher proton concentration favours the appearance of the flavylium cation, by the other hand the proton concentration should not be excessive otherwise flavylium cation is the most stable specie (colour exists prior to the irradiation). Once the coloured species are formed, flavylium or quinoidal base, the system reverts completely back to the equilibrium according to eq.(8).[7] In other words, the bleaching of Ct due to the irradiation is recovered in two steps i) faster one from Cc in competition with the hydration that leads to the coloured species, ii) slower one from the coloured species via hydration followed by ring opening and isomerisation, eq.(8). 

At pH=l the yellow flavylium, AH+, is the dominant species and the solution is yellow. When the pH is raised, (direct pH jump) the orange quinoidal base. A, is immediately formed around pH=4-6 and the pink ionized base, A-, at higher pH values. However these are transient sp>ecies when A is formed disappears in several minutes (depends on pH) to give the most stable species and only slightly coloured frnns-chalcone, Ct when A is formed reverts to the ionized chalcones Cb or CP-, depending on pH with a rate which is also dependent on pH At pH>12 the A- is transformed in CP- in a few minutes. If now the stable CP- at pH=12 suffers a pH jump to acidic (reverse pH jump), the protonated chalcones are... 

In the case of the compoimd 7 4 -dihydroxyflavylium in the same biphasic system water/[BMIM] [PFe] the situation is similar but due to the existence of one more hydroxyl substituent an ionized quinoidal base, A-, can be formed, and in water no cis-trans isomerisation barrier exists. [13]... 

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