Anthocyanins aldehydes-mediated condensation

A.2.3 Anthocyanin-Flavanol and Anthocyanin-Anthocyanin Aldehyde-Mediated Condensation Reactions... 

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. 

In red wines, flavanols may also react directly with anthocyanins, leading to the formation of anthocyanin-flavanol and flavanol-anthocyanin adducts [283-285], The condensation may also occur mediated by aldehydes, such as acetaldehyde, furfural, HMF, benzaldehyde, and others, leading to the formation of alkyl-methine bridged adducts. These latter display the same /Lav in the visible region at around 540 nm, which is bathochromically shifted when compared with the original anthocyanin (/Lax 525 nm) [279, 280, 286-289], thereby contributing to the color change in red wine. 

One of the first reactions described in red wines was the polymerization reaction between anthocyanins and flavanols (catechins and condensed tannins) mediated by acetaldehyde (Timberlake Bridle, 1976). This reaction involves the protonation of the aldehyde, followed by addition of the respective carbocation to the nucleophilic position C-8 and less likely C-6 of the phloroglucinol ring of the flavanol (see Fig. 3.6). Further dehydration and a nucleophilic attack from the anthocyanin give rise to the formation of two new pigments that differ in the stereochemistry of the asymmetric carbon of the interflavonoid linkage [catechin-HC (CH3)-anthocyanin], as demonstrated by several authors (Rivas-Gonzalo etal, 1995 Escribano-Bailonefa/., 1996 Lee etal, 2004). 

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