Anthocyanin transformations

Wang, H., Race, E.J., Shiikhande, A.J. (2003a). Anthocyanin transformation in Cabernet Sauvignon wine during aging. J. Agric. Food Chem. 51, 7989-7994. 

Reaction 9.3 Structural anthocyanin transformations in aqueous media... 

Anthocyanin Transformation in Cabernet Sauvignon Wine during Aging... 

The objective of this work is to establish anthocyanin profiles and individual anthocyanins in Cabernet Sauvignon wines in selected vintages by LC-MS/MS, and to elucidate the mechanisms of anthocyanin transformation. Based on anthocyanin profiles established, the involvement of anthocyanins in wine s reduced astringency during maturation is discussed. 

Over the past decades, several groups of compoimds resulting from anthocyanin transformations have been identified in wines (summarized in Fig. 3.4). In general, these... 

Catechol-O-methyltransferase (COMT EC 2.1.1.6) is located in many tissues and catalyzes the methylation of polyphenols. The methylation is a well-established pathway in the metabolism of flavonoids such as those that undergo 3, 4 -dihydrox-ylation of ring B excreted as 3 -0-methyl ether metabohtes in rat bile. " Recently, the apparent methylation of both cyanidin-3-glucoside and cyanidin-3-sambubioside (cyanidin is an anthocyanin with a 3, 4 -dihydroxylation of ring B) to peonidin-3-glucoside and peonidin-3-sambubioside was reported in humans. In rats, this transformation occurred mainly in the liver and was catalyzed by COMT."°... 

Timberlake, C.F. and Bridle, P., Flavylium salts anthocyanidins and anthocyanins. Structural transformations in acid solutions, J. Sci. Food Agric., 18, 473, 1967. 

For many situations, a simple total anthocyanin determination is inappropriate because of interference from polymeric anthocyanins, anthocyanin degradation products, or melanoidins from browning reactions. In those cases, the approach has been to measure the absorbance at two different pH values. The differential method measures the absorbance at two pH valnes and rehes on structural transformations of the anthocyanin chromophore as a function of pH. Anthocyanins switch from a saturated bright red-bluish color at pH 1 to colorless at pH 4.5. Conversely, polymeric anthocyanins and others retain their color at pH 4.5. Thus, measurement of anthocyanin samples at pH 1 and 4.5 can remove the interference of other materials that may show absorbance at the A is-max-... 

Many methods are currently available for the qualitative analysis of anthocyanins including hydrolysis procedures," evaluation of spectral characteristics, mass spectroscopy (MS), " nuclear magnetic resonance (NMR), and Fourier transform infrared (FTIR) spectroscopy. - Frequently a multi-step procedure will be used for... 

Scheme 9.1 Structural transformation of a generic anthocyanin in aqueous solvent at different pH values.

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. 

Markham, K.R., Novel anthocyanins produced in petals of genetically-transformed lisianthus, Phytochemistry, 42, 1035, 1996. 

Ray, H. et al.. Expression of anthocyanins and proanthocyanidins after transformation of alfalfa with maize Lc. Plant Physiol, 132, 1448, 2003. 

Since the early contributions of Willstatter and Robinson, several alternative approaches following mainly two routes have been considered for synthesis of anthocyanins.One of the routes includes condensation reactions of 2-hydroxybenzaldehydes with acetophenones, while the other uses transformations of anthocyanidin-related compounds like flavonols, flavanones, and dihydroflavonols to yield flavylium salts. The urge for plausible sequences of biosynthetic significance has sometimes motivated this latter approach. In the period of this review, new synthetically approaches in the field have also predominantly been following the same general routes however, some new features have been shown in synthesis of pyranoanthocyanidins. 

Anthocyanin pigments undergo reversible structural transformations with a change in pH manifested by strikingly different absorbance spectra (Fig. FI.2.1). The colored oxonium form predominates at pH 1.0 and the colorless hemiketal form at pH 4.5 (Fig. FI.2.2). The pH-differential method is based on this reaction, and permits accurate and rapid measurement of the total anthocyanins, even in the presence of polymerized degraded pigments and other interfering compounds. 

The differential method (see Basic Protocol 1) measures the absorbance at two different pH values, and relies on the structural transformations of the anthocyanin chromophore as a function of pH (Fig. Fl.2.1 and Fig. FI.2.2). This concept was first introduced by Sondhe-imer and Kertesz in 1948, who used pH values of 2.0 and 3.4 for analyses of strawberry jams (Francis, 1989). Since then, the use of other pH values has been proposed. Fuleki and Francis (1968b) used pH 1.0 and 4.5 buffers to measure anthocyanin content in cranberries, and modifications of this technique have been applied to a wide range of commodities (Wrolstad et al., 1982, 1995). The pH differential method has been described as fast and easy for the quantitation of monomeric anthocyanins (Wrolstad et al., 1995). 

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