Anthocyanidins glycosylation

Four cyanidins with 4 -glucosidation were recently isolated in minor amounts from pigmented scales of red onion. Two were the only anthocyanins with B ring sugar substitutions and no glycosyl moieties in anthocyanidin position 3, whereas the other two were also glycosilated at the usual position 3. ... 

The pKj, value (2.43) of the hydration constant of the cyanidin was found to be lower than the pKj, values of glycosylated and acylated cyanidins, meaning lower resistance of the anthocyanidin to hydration. The stability of nonacylated 3,5-diglucosides was lower compared to the 3-glucoside because the 5 position markedly lowered the hydration constant due to decreased electron density of the pyrilium ring that favors nucleophilic attack by water, enhancing hemiacetal formation. ... 

Peel, G Pang, Y Modolo, LV, Dixon, RA. (2009) The LAPl MYB transcription factor orchestrates anthocyanidin biosynthesis and glycosylation in Medicago Plant J 59 136-149. 

FIGURE 10.6 The number of anthocyanins with glycosyl moieties linked to the various anthocyanidin positions. The upper dark part of each bar represents the anthocyanins reported later than 1992. See Table 10.1 for structures. 

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. 

Figure 3-8. Biosynthesis of anthocyanins and condensed tannins. The enzymes involved in this pathway are (a) anthocyanidin synthase (E.C. 1.14.11.19), (b) anthocyanin 3-glycosyl transferase, and (c) BANYULS.

The branch pathway for anthocyanin biosynthesis starts with the enzymatic reduction of dihydrofiavonols to their corresponding flavan 3,4-diols (leucoanthocyanidins) by substrate-specific dihydroflavonol 4-reductases (DFR). Flavan 3,4-diols are the immediate precursors for the synthesis of catechins and proanthocyanidins. Catechins are formed by enzymatic reduction of the flavan 3,4-diols in the presence of NADPH to leucoanthocyanidins, which are subsequently converted to anthocyanidins by the 2-oxoglutarate-dependant dioxygenase, anthocyanidin synthase. Further glycosylation, methylation, and/or acylation of the latter lead to the formation of the more stable, colored anthocyanins (Scheme 1.1). The details of the individual steps involved in flavonoid and isoflavonoid biosynthesis, including the biochemistry and molecular biology of the enzymes involved, have recently appeared in two excellent reviews.7,8... 

Figure 2.21 Structures of betanldln and a typical anthocyanidin, cyanidin, accumulating as various glycosylated structures and their acylated forms. Both compounds exhibit similar max values. Occurrence of these two classes of pigments mutually excludes each other. The betalains are exclusively found in most families of the plant order Caryophyllales, whereas the anthocyanins are ubiquitously distributed in the other families of the Angiosperms.

---