Delphinidin, structure

Six anthocyanidins are commonly found in plants and plant-derived foods and beverages pelargonidin, cyanidin, delphinidin, peonidin, petunidin, and malvidin. Their structures differ in the number and position of hydroxyl and methoxyl groups on the flavan nucleus. The most commonly occurring anthocyanidin is cyanidin, and all these anthocyanidins are found in plants as glycosides with or without acylation, leading to around 400 different stractures. 

Figure 9.4 Structure of cis and trans isomers of delphinidin 3-[4-(p-coumaroyl)-L-rhamnosyl-(l-6)-glucopyranoside]-5-glucopyranoside.

The chromophore in hydrangeas is delphinidin (X), which is a member of the anthrocyanidin class of compounds. Compound X reminds us of phenol (VII), indicating that delphinidin is also a weak acid. In fact, all pH indicators are weak acids or weak bases, and the ability to change colour is a visible manifestation of the indicator s ability to undergo reversible changes in structure. In the laboratory, only a tiny amount of the pH indicator is added to the titration solution, so it is really just a probe of the solution pH. It does not participate in the acid-base reaction, except insofar as its own structure changes with the solution pH. 

FIGURE 10.2 The number of anthocyanins based on the various anthocyanidins. The upper dark part of each bar represents the anthocyanins reported later than 1992. Pg, pelargonidin Cy, cyanidin Pn, peonidin, Dp, delphinidin Pt, petunidin Mv, malvidin RMS, rare methylated structures 60H, 6-hydroxy- Des, desoxy- Pyr, pyrano- Sp, sphagnorubins. See Table 10.1 for structures. 

The reports on anthocyanins from Anemone coronaria Pulsatilla cernua and Ranunculus asiaticus reveal structures that are very different from the anthocyanins described above. Many of these latter pigments are based on lathyroside or sambubioside residues located at the 3-hydroxyl of delphinidin, pelargonidin, or cyanidin (Table 10.2). Among these, four unusual anthocyanins containing the acyl moiety tartaryl have been isolated from Anemone coronaria Six of the anthocyanins in this plant have a glucur-onoside in the 3 -position. 

There are five anthocyanidins in the grape delphinidin 1, petunidin 2, malvidin 3, cyanidin 4, and peonidin 5. These aglycones exist in different heterosidic forms or as anthocyanins 3-monoglucosides, 3,5-diglucosides, and acylated heterosides, whose structures, in the case of malvidin, are represented by formulas 6, 7, and 8. In the acylated anthocyanins, one molecule of cinnamic acid, more generally p-coumaric acid, is esterified with the —OH group in the sixth position of a glucose molecule (12). 

Figure F1.4.4 The structure of the acylated anthocyanin delphinidin 3-0-(6"-malonyl-p-glu-copyranoside)-5-p-glucopyranoside, isolated from Aster novi belgii. Structure number 19 corresponds to 1H NMR data in Table F1.4.4.

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