Anthocyanins

Anthocyanin pigments, although sometimes unmasked by loss of chlorophyll, are frequently synthesized from small precursors during the final stages of fruit maturation. Their appearance is subject to control by many environmental factors. The biosynthetic sequences for the different anthocyanidins are similar and represent one endpoint of the flavonoid biosynthetic pathway, starting with the condensation of p-coumaryl-CoA with three malonyl-CoA units to form tetrahydroxychalcone that is then enzymatically isomerized to the flavanone naringenin. In the sequence to cyanidin, naringenin is converted to dihydrokaempferol, which is hydroxylated to produce dihy-droquercetin. Further changes are less well documented, but it is thought that dihydroquercetin is reduced to a flavan-3,4-diol, and then oxidation, dehydration, and appropriate glycosylation produce the different cyanidin glycosides. The biosynthetic sequence is unique to higher plants and derived from the plant-specific production of the aromatic amino acids, phenylalanine and tryrosine.

Plants can metabolize anthocyanin molecules. A good example of this is chicory, in which new blue flowers open each morning and the anthocyanin is gone by early afternoon, leaving white flowers. In most fruit, synthesis and catabolism occur at the same time, and the concentration of pigment is a function of the synthesis rate and the catabolic rate.

The anthocyanin color of a fruit can be due to a single pigment (rare) or to mixtures of anthocyanins. In flower petals, it has been shown that two cultivars which can be distinguished by eye have identical chemical composition. Findings indicate that the pigments in one petal are mixed in the same cells, while in the other they are in different cell layers, causing unique light reflections. In many fruit, the depth of the cell layer containing the pigments is important to the final color. Some fruit make pigments in all their cells and others only in the external cells. Anthocyanin pigments are found in vacuoles and are greatly influenced by vacuolar pH.

Considering the large number of anthocyanin pigments identified by chemists, the major anthocyanidin of tree fruit is cyanidin. Apples and pears accumulate the 3-galactoside; sweet cherries, plums, and peaches, the 3-glucoside; and cherries, the 3 rutinoside. These are the major anthocyanin pigments, but apparently all anthocyanin-produc-ing plants make minor pigments as well, and the number known is dependent on how extensively they have been sought. Species-specific chemistry is a significant tool in plant identification, and an entire field of specialists in chemotaxonomy exist. The chemotaxonomy of processed fruit can be employed to expose adulteration.

Chemicals other than pigments affect the final perceived color of anthocyanins. These are known as copigments and include metal ions (magnesium, iron, aluminum), hydroxycinnamoyl esters, galloyl esters, and flavone and flavonol glycosides. The three classes of pigments are only synthesized in plants. Animals cannot produce chlorophylls, carotenoids, or anthocyanins.

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