Anthocyanins

Abstract

Anthocyanins, the largest group of water-soluble pigments in the plant kingdom, are responsible for most of the red, blue, and purple colors of fruits, vegetables, flowers, and other plant tissues (Harborne, 1967). The anthocyanins are all based chemically on a single aromatic structure, cyanidin, and all are derived from this compound by the addition or subtraction of hydroxyl groups, by the degree of methylation of these hydroxyl groups, and by the nature and number of sugars and their position on the aglycon (Harborne, 1998). Additionally, sugars can be esterified with aliphatic or aromatic acids leading to mono-and polyacylated anthocyanins. There are six common anthocyanins whose structures are shown in Figure 5.1. With a few exceptions anthocyanins are always glycosylated at C-3. The most common sugar is glucose, but arabinose, galactose, rhamnose, or xylose may also be present. Many di-and trisaccharides are known to occur with combinations of the previously mentioned sugars. Besides the C-3 position, other sugars can also be attached at any one of the hydroxyls at C-5, C-7, C-3', C-5', and even C-4'(Brouillard, 1988). Sugars may be acylated with acetic, malonic, malic, oxalic, succinic, p-hydroxybenzoic, or hydroxycinnamic (p-coumaric, caffeic, ferulic, or sinapic) acids, with acylation most commonly occurring with the sugar moiety at the C-3 position. Acylation with aliphatic dicarboxylic acids such as malic, malonic, oxalic, and succinic acids renders the cationic anthocyanin a zwitterion that allows the distinction between these pigments and other anthocyanins by paper electrophoresis in a weakly acidic buffer (Harborne and Grayer, 1988). Another important feature of anthocyanins acylated with aliphatic acids is the labile nature of the ester bond. When malonated anthocyanins are subjected to standard extraction procedures using methanol acidified with 0.1 to 1.0% HCl, there is an intermediate methyl ester formation at the free carboxyl group, while the main reaction is the loss of the malonyl group in a short time (Harborne, 1988).