The flavonoids are fifteen-carbon compounds that are generally distributed throughout the plant kingdom.
The most common basic flavonoid skeletron, shown below, is usually modified in such a way that more double bonds are present, causing the compounds to absorb visible light and thus giving them colour.
The two carbon rings at the left and right ends of the molecule are designed the A and B rings respectively.
Three widely distributed groups of flavonoids are: anthocyanins, flavonols, and flavones. The anthocyanins are coloured pigments most commonly seen in the red, purple and blue flowers. They are also present in various other plant parts, such as certain fruits, stems, leaves, and even roots.
Most fruits and flowers owe their colours to anthocyanins, although some, such as tomato fruits and several yellow flowers, are coloured by carotenoids.
Several different anthocyanins exist in plants, and often more than one is present in a particular flower. These molecules differ only in the number of hydroxy1 groups attached to the B ring of the basic flavonoid structure.
The exact colour of the anthocyanins depends first upon the substituent groups present on the B ring.
When methy1 groups are present they cause a reddening effect. Secondly, the anthocyanins are sometimes associated with other phenoic types of compounds, and this seems to cause them to become bluer. Finally, the pH of the cell sap has a strong controlling influence upon their colour.
The flavonols and flavones are closely related to the anthocyanins, except that they differ in the central oxygen-containing ring structure of the flavonoid. Naturally occurring flavonols and flavones are hydroxylated in various positions on both A and B rings.
Most of the flavones and flavonols are yellowish or ivory coloured pigments and, like the anthocyanins, they often contribute to the yellow, cream, ivory and white colour of flowers.
Sometimes they do not appear coloured to the human eye, but they are apparent to bees or other insects that are attracted to flowers containing them. This is because the eyes of the insects are sensitive t ultraviolet wavelengths that give these compounds their colours.