Xanthophylls

oxygen-containing carotenoids, the principal components of the yellow pigments in the leaves, flowers, fruits, and buds of higher plants, as well as in many algae and microorganisms. Xanthophylls are found less frequently in the animal world, for example, in the yolk of chicken eggs and in the liver and fatty tissue of mammals. These carotenoids are contained in the chloroplasts of the green parts of plants, in the chromoplasts of flowers and fruits, and in the lipophil sections of bacterial cells. Together with flavonoids, xanthophylls are responsible for the autumnal coloration of leaves.

The biological importance of xanthophylls is related to their ability to absorb sunlight from the short-wave range of the visible spectrum (380-520 nanometers). All photosynthetizing organs of green plants and of photosynthetic microorganisms contain xanthophylls. In these organs the absorbed light energy is transformed into chlorophyll or similar complexes. In this way, xanthophylls participate as auxiliary pigments in the photosynthetic process. Apparently, they also play the role of filters, protecting sensitive enzymes from destruction by light.

There are more than 50 known xanthophylls; they fall into various functional groups, including alcohols, ketones, aldehydes, acids, and simple and complex ethers. They belong to acyclic, monocyclic, and bicyclic carotenoids, containing 40 atoms of carbon. A typical representative is zeaxanthin (C₄₀H₅₆O₂), the yellow crystals (melting point, 207–215°C), and its isomer—xanthophyl (lutein)—whose crystals are violet (melting point, 190–193°C). The biosynthesis of xanthophylls from colorless carotenoid hydrocarbons is catalyzed by atmospheric oxygen in sunlight.