Carotenoids are organic compounds found in plants, divided into two categories: oxygen-free carotenes and oxygen-containing xanthophylls. They have 40 carbon atoms and a chromophore that determines their color. Carotenoids are important for photosynthesis and nutrition, with some being converted into vitamin A. They also protect chlorophyll from UV light and can be used in the fragrance industry.
Carotenoids belong to the terpenoid class of organic compounds, specifically the tetraterpenoids. They are phytochemicals, present 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” isoprene molecules, CH2=C(CH3)CH=CH2. Tetraterpene 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 carbon atoms, in theory derivable from the union of isoprene units and composed only of carbon, hydrogen and possibly oxygen. The carotenoid structure also includes a component called a chromophore, which is responsible for the color of the molecule. These organic compounds are biologically and nutritionally important and are inextricably associated with the vital process of photosynthesis.
Polymerization is possible because isoprene has two double bonds. Each molecule of isoprene 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 participating molecule remains unused. There is a great variety of terpenoid structures that can form, because the isoprene molecule is not symmetrical. The merge can be 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, zeaxanthin and lycopene. Most carotenoids possess antioxidant properties. Some, including alpha and beta-carotene and beta-cryptoxanthin, can be converted by the body into the structurally similar retinol, vitamin A. The bright colors of vegetables, particularly the yellow of corn, the orange of carrots, and the red of tomatoes, exists because of carotenoids.
The portion of the molecule that produces the colors present in the carotenoid structure is the chromophore, which means “color buffer”. It is largely determined by the uninterrupted collection of alternating double bonds present in the molecule. This collection of pi electrons absorbs energy that coincides with a part of the visible spectrum. What remains of the unabsorbed colors determines the color of the fruit or vegetable. Therefore, a yellow vegetable absorbs light particularly in the blue part of the spectrum.
Carotenoids are found in the chloroplasts and chromoplasts of plants. They perform two specific functions. The compounds absorb light that can be used in the process of photosynthesis through energy transfer and serve to protect the delicate chlorophyll molecules from exposure to harmful ultraviolet light. In the fall in some parts of the world, as the amount of chlorophyll decreases, carotenoids often reveal themselves in the beautiful colors of many plants that change with the seasons. The decomposition products of many of the carotenoids impart pleasant aromas; some of these compounds are used in the industries of essences, perfumes and aromas.
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