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Carotenoids belong to the terpenoids class of organic compounds, specifically the tetraterpenoids. They are phytochemicals, occurring almost exclusively in plants, and are divided into two categories: oxygen-free carotenes and oxygen-containing xanthophylls. Terpenoids can be derived, at least theoretically, by linking together or “polymerizing” molecules of isoprene, CH2=C(CH3)CH=CH2. Tetraterpenoid skeletons contain four 10-carbon terpene units for a total of 40 carbon atoms. Narrowing the definition, the International Union of Pure and Applied Chemistry refers to the structure of carotenoids as those tetraterpenoids formally derived from the acyclic parent lycopene.
In the structure of carotenoids, there are exactly 40 skeletal carbons, in theory derivable by linking isoprene units and composed of only carbon, hydrogen and possibly oxygen. The structure of carotenoids also includes a component called the chromophore, which is responsible for the molecule's color. These organic compounds are biologically and nutritionally important and are inextricably associated with the life-sustaining process of photosynthesis.
Polymerization is possible because isoprene possesses two double bonds. Each isoprene molecule has five carbon atoms, so combining two molecules results in a single chain of 10 carbon atoms. Growth can continue beyond this length, because the second double bond for each participant molecule remains unused. There is a large variety of terpenoid structures that can form, because the isoprene molecule is not symmetrical. Joining can take place head-to-head, head-to-tail or tail-to-tail; the longer the chain, the greater the number of combinations.
Carotenoids are among the nutritionally beneficial foods found in fruits and vegetables. Among the nutrients are lutein, zeaxanthine and lycopene. Most carotenoids possess antioxidant properties. Some, including alpha- and beta-carotene and beta-cryptoxanthin, can be converted by the body into a structurally similar retinol, vitamin A. The bright colors of vegetables, especially the yellow of corn, the orange of carrots and the red of tomatoes, exists because of the carotenoids.
The portion of the molecule that produces the colors found in the structure of carotenoids is the chromophore, which means “color bearing.” It is largely determined by the uninterrupted collection of alternating double bonds found in the molecule. This collection of pi electrons absorbs energy that coincides with a part of the visible spectrum. What remains by way of unabsorbed colors determine the color of the fruit or vegetable. Thus, a yellow vegetable absorbs light particularly in the blue part of the spectrum.
Carotenoids are found in plant chloroplasts and chromoplasts. They serve two specific functions. The compounds both absorb light that can be used in the process of photosynthesis through energy transfer and serve to shield delicate chlorophyll molecules from exposure to damaging ultraviolet light. In autumn in some parts of the world, as the quantity of chlorophyll decreases, the carotenoids often reveal themselves in the beautiful colors of many plants that change with the seasons. Decomposition products of many of the carotenoids impart pleasant aromas; some of such compounds are used in the essence, perfume and flavoring industries.