ABSTRACT
References ............................................................................................................. 270
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 C3. 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 C3 position, other sugars can also be attached at any one of the hydroxyls at C5, C7, C30, C50, and even C40
(Brouillard, 1998). 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 C3 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
OHO
OH OR
OH
+ OHO
OH OR
OH OH
Pelargonidin Cyanidin
OHO
OH OR
OH OCH3
Peonidin
OHO
OH OR
OH OH
OH +
Delphinidin
OHO
OH OR
OH OCH3
OH +
Petunidin
OHO
OH OR
OH OCH3
Malvidin
FIGURE 5.1 Structures of six common anthocyanins. R¼ sugar moiety. This is often glucose but may also be galactose, rhamnose, xylose, or arabinose.
