Anthocyanidins common, 24

There are six anthocyanidins commonly occurring in grapes and berry fruits. The anthocyanidins are classified according to the number and position of their hydroxyl and methoxyl groups on the flavan nucleus and are named pelargonidin, cyanidin, delphinidin, peonidin, petunidin and malvidin (Figure 3.2). 

Anthocyanins are glycosides of anthocyanidins, the latter being polyhydroxyl and methoxyl derivatives of flavylium. The arrangement of the hydroxyl and methoxyl groups around the flavylium ion in six anthocyanidins common in foods is shown in Fig. 4. 

FIGURE 4 Six anthocyanidins common in foods. The electric charge shown at position 1 is delocalized over the entire structure by resonance. 

Glycosides, particularly of phenoHc compounds, are widely distributed in plant tissues (2,10). Glycosides of anthocyanidins, flavones, flavanols, flavanones, flavanonols, stilbenes and saponins, gaUic acid derivatives, and condensed tannins are all common. 

As we have seen above, anthocyanins comprise an aglycone fraction commonly known as anthocyanidin and a frequently acylated osidic substituent. This characteristic leads to two different approaches for the analysis of these pigments (1) a direct anthocyanin analysis without a hydrolysis stage requiring identification of a number of molecules (several hundreds in the plant kingdom) or (2) an analysis of the anthocyanidin fraction only after hydrolysis of the anthocyanins present in the medium. 

Anthocyanins are the deeply colored glycosides of the aglycone anthocyanidins. Only the six aglycones shown in Figure 13.6 are common, although others occur sporadically [11,12]. 

The role of anthocyanidin synthase (ANS) in the biosynthetic pathway is to catalyze reduction of the leucoanthocyanidins to the corresponding anthocyanidins. However, in vivo it is anthocyanidins in pseudobase form that are formed, as is described below. In this chapter, use of anthocyanidin should be taken to include the pseudobase form. Furthermore, although the name ANS is commonly used, the enzyme is also referred to in the literature as leucoantho-cyanidin dioxygenase (LDOX), reflecting the reaction type. 

In addition to the 18 anthocyanidins listed previously, Table 10.1 contains seven new desoxyanthocyanidins and a novel type of anthocyanidin called pyranoanthocyanidins. While 31 monomeric anthocyanidins have been properly identified, most of the anthocyanins are based on cyanidin (30%i), delphinidin (22%i), and pelargonidin (18%), respectively (Figure 10.2). Altogether 20%i of the anthocyanins are based on the three common anthocyanidins (peonidin, malvidin, and petunidin) that are methylated. Around 3, 3, and 2% of the anthocyanins or anthocyanidins are labeled as 3-desoxyanthocyanidins, rare methylated anthocyanidins, and 6-hydroxyanthocyanidins, respectively. 

Compared to the more common flavylium salts, these latter pigments showed large bathochro-mic shifts (from about 100 to 200 nm), thus providing blue-violet and green colors, which are difficult to obtain with anthocyanidins in general. 

On the other hand, anthocyanins, i.e. the glycosylated anthocyanidins, are an important group of water-soluble pigments occuring in 27 families of food plants (mainly red fruits and vegetables), Fig. (11) [53]. Table 15 shows the most common anthocyanidins and anthocyanins. 

Table 15. Structure of the most common anthocyanidins and anthocyanins...

Table F1.2.2 Molecular Weights of Anthocyanidins, Anthocyanins, and Acylating Groups Commonly Found in Nature0... 

Table F1.4.6 Typical 1H-1H Coupling Constants of the Most Common Anthocyanidins ... 

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