The heterocycle of anthocyanidins is a pyrilium kation. Anthocyanidins are typically not found as free aglycones, with the excepton of the following widely distributed, colored compounds: Pelargonidin (orange-red; 1.45), cyanidin (red; 1.46), peonidin (rose-red; 1.47), delphinidin (blue-violet; 1.48), petunidin (blue-purple; 1.49), and malvidin (purple; 1.50). A convenient mnemonic is: PCP-DPM. The most common anthocyanidin is cyanidin. These compounds are present in the vacuoles of colored plant tissues such as leaves or flower petals. The color of the pigment depends on the pH, metal ions present, and the combination of substituted sugars and acylesters. Different colors can also result from the presence of combinations of several anthocyanidins (Figure 1.2).
Note that each of the six common anthocyanidins has the basic structure of the flavylium cation (2-phenyl benzopyrilium; 1.31).
Other anthocyanidins exist, and can be categorized into two groups:
1. Those where either the C5 or C7 position is substituted with a methoxyl group
2. The deoxyanthocyanidins, which do not contain a hydroxyl group at the C3 position
There are five deoxyanthocyanidins: Apigeninidin (1.51), luteolinidin (1.52), 7-methoxyapigeninidin (1.53), 5-methoxy-luteolinidin (1.54), and the caffeic acid ester of arabinosyl 5-O-apigeninidin (1.55).
Anthocyanins are water-soluble glycosides of anthocyanidins. The most common glycoside is the 3-glycoside. If a second sugar is present, it is almost always at the 5-hydroxyl position, and almost always a glucose residue. Such compounds are called 3,5-dimonosides. In addition, there are a few rare 3,7-substitutions. While glucose is the most common sugar, substitutions of other sugars, such as arabinose, are sometimes observed. Anthocyanins can also be acylated. In this case an organic acid - typically p-coumaric acid (1.11), caffeic acid (1.12), or ferulic acid (1.13) - is esterified to the sugar. An example is petanin (3-[6-O-(4-O-E-p-coumaroyl-O-a-rhamnopyranosyl)-P-glucopyranoside] -5-O-P-glucopyranoside; 1.56), a compound found in Solanaceae.
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