Pyranoanthocyanins

Several works have shown that, in general, pyranoanthocyanins exhibit a higher color intensity and stability up to pH 6 and higher resistance to bisulfite bleaching than anthocyanins. The substitution at the carbon 4 of the pyranic ring C protects the chromophore group from the nucleophilic attack of water (hydration) and bisulfite anion (Bakker  

Timberlake, 1997 Francia-Aricha et al, 1997 Schwarz Winterhalter, 2003 Oliveira et al, 2006,2007). 

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Hillebrand, S., Schwarz, M., and Winterhalter, P, Characterization of anthocyanins and pyranoanthocyanins from blood orange [Citrus sinensis (L.) Osbeck] juice, J. Agric. Food Chem., 52, 7331, 2004. 

CZE is particularly useful for separating anthocyanin dimers or polymeric anthocyanins. Calvo et al. (2004)" separated 13 anthocyanins by CZE including acylated and non-acylated anthocyanins, pyranoanthocyanins, and flavonol derivatives in wine. Saenz-Lopez et al. (2004)" applied CZE to analyze wine aging (1 to 14 yr) as related to monomeric anthocyanins, anthocyanin derivatives, tannins, and fla-vonols. Bicard et al. (1999)" reported the improved detection sensitivity of anthocyanin chemical degradation analysis by CZE. 

Size-exclusion chromatography combined with RP-HPLC-MS was employed for the separation of pyranoanthocyanins from red wine. Wine samples (10 ml) were acidified with 3 M HC1 to pH 1 then sodium bisulphite was added at a concentration of 400 g/1. After 15 min reaction time the treated wine was loaded into a gel column (200 X 15 mm i.d.). Pigments were eluted with 95 per cent ethanol followed with 100 per cent methanol. The various fractions were acidified to pH 1, concentrated and redissolved in water. HPLC-DAD was carried out in an ODS column (150 X 4.6 mm i.d. particle size 5 /nn) at 35°C. Solvents were 0.1 per cent aqueous TFA (A) and ACN (B). The gradient started with 10 per cent B for 5 min to 15 per cent B for 15 min isocratic for 5 min to 18 per cent B for 5 min to 35 per cent B for 20 min. The flow rate was 0.5 ml/min and analytes were detected at 520 nm. MS conditions were sheath and auxiliary gas were a mixture of nitrogen and... 

Fig. 2.102. Structures of all the compounds found in fraction A (a) anthocyanins (b) A-type vitisins (c) pyranoanthocyanins originated by reaction between anthocyanins and vynilphenol, vynilcatechol or vynilguaiacol (d) pyranoanthocyanins originated by reaction between anthocyanins and vynil(epi)catechin. Reprinted with permission from C. Alcalde-Eon et al. [236],...

Fig. 2.105. Formation of pyranoanthocyanins. Reprinted with permission from A. E. Hakansson et al. [239].

Fig. 2.107. The formation of pyranoanthocyanins 6-10 from anthocyanins 1-5 and acetone. Reprinted with permission from Y. Lu et al. [242].

Novel pyranoanthocyanins have also been isolated and identified in blackcurrant (Ribes nigrum) seed using HPLC, 2D NMR and ES-MS. Blackcurrant seeds were extracted with acetone-water (70 30, v/v) and the components of the extract were separated in a polyamide column followed by HPLC-DAD. The new pigments were finally separated in an MCI-HP20 column. The chemical structures of anthocyanins 1-2 and the novel pyranoanthocyanins 3-6 with the pyrano[4,3,2-de]-l-bcn/opyrylium core structure are shown in Fig. 2.110. It was stated that the analytical method developed separated well the novel pyranoanthocyanins [245],... 

C. Alcalde-Eon, M. T. Escribano-Bailon, C. Santos-Buelga and J.C. Rivas-Gonzalo, Separation of pyranoanthocyanins from red wine by column chromatography. Anal. Chim. Acta 513 (2004) 305-318. 

Y. Lu and Y. Foo, Unusual anthocyanin reaction with acetone leading to pyranoanthocyanin formation. Tetrahedron Lett. 42 (2001) 1371-1373. 

Schwartz, M. and Winterhalter, P., A novel synthetic route to substituted pyranoanthocyanins with unique colour properties. Tetrahedron Lett., 44, 7583, 2003. 

Another reaction of the flavylium cation has recently been demonstrated. " It involves concerted addition of compounds possessing a polarizable double bond on the electron-deficient site C-4 and the oxygen of the 5-hydroxyl group of the anthocyanin. The new pigments thus formed, showing a second pyran ring, have been referred to as vitisins, but the term pyranoanthocyanins proposed by Lu and Foo is preferred. 

Structure/ Reaction of grape anthocyanin 3-glucosides with acetaldehyde yielded the corresponding series of pyranoanthocyanins that were also formed after yeast fermentation of a synthetic must containing anthocyanins/ ... 

Shortly after the discovery of phenyl pyranoanthocyanins, another series of anthocyanin derivatives showing similar UV-visible spectra, suggesting that they were also derived from a pyranoanthocyanin chromophore, and masses differing from those of grape anthocyanins by... 

This mechanism can be extrapolated to other enolizable precursors potentially present in wine, including yeast metabolites such as a-ketoglutatic acid and 2-hydroxybutan-2-one, but also to acetone, which can react with anthocyanins during solvent extraction proced-ures. The resulting products are pyranoanthocyanins as presented in Figure 5.11, with Rs = CH2-COOH, R4 = COOH R3 = H, R4 = CHOH CH3 R3 = H, R4 = CH3. 

Formation of pyranoanthocyanins through reaction of flavylium cations with compounds possessing a polarizable double bond, namely vinylphenol derivatives (including vinylflavanols and hydroxycinnamic acids) and enolizable aldehydes and ketones (e.g., acetaldehyde and pyruvic acid). 

Finally, reactions of flavonoid and nonflavonoid precursors are affected by other parameters like pH, temperature, presence of metal catalysts, etc. In particular, pH values determine the relative nucleophilic and electrophilic characters of both anthocyanins and flavanols. Studies performed in model solutions showed that acetaldehyde-mediated condensation is faster at pH 2.2 than at pH 4 and limited by the rate of aldehyde protonation. The formation of flavanol-anthocyanin adducts was also limited by the rate of proanthocyanidin cleavage, which was shown to take place at pH 3.2, but not at pH 3.8. Nucleophilic addition of anthocyanins was faster at pH 3.4 than at pH 1.7, but still took place at pH values much lower than those encountered in wine, as evidenced by the formation of anthocyanin-caffeoyltartaric acid adducts, methylmethine anthocyanin-flavanol adducts,and flavanol-anthocyanin adducts. The formation of pyranoanthocyanins requiring the flavylium cation was faster under more acidic conditions, as expected, but took place in the whole wine pH range. Thus, the availability of either the flavylium or the hemiketal form does not seem to limit any of the anthocyanin reactions. 

Pyranoanthocyanins are extremely stable compared to anthocyanins and methylmethine-... 

Two classes of dimeric anthocyanins isolated from plants (section 10.2.6) have been identified in plants for the first time. One class includes pigments where an anthocyanin and a flavone or flavonol are linked to each end of a dicarboxylic acyl unit. The other class includes four different catechins linked covalently to pelargonidin 3-glucoside. During the last decade, seven new desoxyanthocyanidins and a novel type of anthocyanidin called P)Tanoanthocyanidins have been reported (Section 10.2.2). Toward the end of the 20th century, several color-stable 4-substituted anthocyanins, pyranoanthocyanins, were discovered in small amounts in red wine and grape pomace.Recently, similar compounds have been isolated from extracts of petals of Rosa hybrida cv. M me Violet, scales of red onion, and strawberries. About 94% of the new anthocyanins in the period of this review are based on only six anthocyanidins (Table 10.2). 

The Structures of Naturally Occurring Anthocyanidin. The Numbering of the Structure on the Left is Used for all Anthocyanins the Numbering for the Pyranoanthocyanins is Given in the Structure on the Right... 

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