The carotenoids are an important group of pigments in bacteria, algae and higher plants, where they function as accessory light-harvesting pigments covering regions of the visible spectrum not utilized by (bacterio)chlorophylls. The carotenoid pigments exhibit strong light absorption in the blue portion of the visible spectrum. For example, lutein has its maximum absorption at 450 nm, cryptoxanthin at 453 nm, and zeaxanthin at 454 nm.
Beta-carotene is made up of eight isoprene units, which are cyclized at each end.

As accessory pigments, carotenoids participate in photoinduced electron transfer processes – they cannot transfer sunlight energy directly to the photosynthetic pathway, but pass their absorbed energy to (bacterio)chlorophylls. Carotenoids also protect against excessive light by quenching both singlet and triplet states of (bacterio)chlorophylls. The carotenoids are brightly colored in the portion of the visible spectrum where their absorbency is low and wavelengths are transmitted or reflected (red to yellow). The yellow colour of many flowers is due to carotenoid-containing chromoplasts, which are usually devoid of chlorophyll.

Ball-stick model of beta-carotene (above).

The numerous compounds in the carotenoid group are tetraterpenes, containing 40 C-atoms in eight isoprene residues. Within the carotenoids are carotenes (pure carbohydrates without additional groups) and the xanthophylls (carotenoids containing oxygen). Oxyfunctionalization of various carotenoids leads to a large number of xanthophylls in which the function may be a carbonyl, epoxy, formyl, hydroxyl, methoxyl, or oxo group. The backbone of carotenoid chains comprises conjugated double bonds – alternating single and double carbon bonds – that forms a conjugated p-electron system. Beta-carotene is made up of eight isoprene units, which are cyclized at each end.

Carotenoids exhibit a diversity of function that is unmatched by any other classes of natural pigments. This is directly related to their unique spectroscopic properties, which result from the structure of the carotenoid molecule. Absorption in the blue portion of the electromagnetic spectrum results from transition to the second excited state S2. The fundamental laws of photophysics ensure that after being promoted to the S2, or other higher energy states, a carotenoid molecule rapidly relaxes to the lowest singlet state. The lifetime of the lowest singlet state is determined by the conjugation length of carotenoids and varies from 300 ps for short conjugated chains to ~1 ps for the longest.

The carotenoids spheroidene and rhodopin glucoside are found in the light-harvesting complexes 1 and 2 (LH1 and LH2) of purple bacteria, and the LHCII light-harvesting complex of plants. LHCII complexes also contain carotenoid species such as violaxanthin, lutein, neoxanthin, and zeaxanthin. The carbonyl carotenoids contribute a substantial part of Earth's photosynthetic CO2 fixation. Carbonyl carotenoids such as peridinin, fucoxanthin or siphonaxanthin, occur in various taxonomic groups of oceanic photosynthetic organisms, including the light-harvesting antennae of algae. Fucoxanthin confers a brown color to kelp and other brown algae, and to the diatoms.

The carotenoids provide a source of vitamin A Retinoids are a group of natural and synthetic analogues of retinal (vitamin A), and their activity is important during the development of the embryo and in postnatal life. The macula of the human retina contains high amounts of the xanthophyll carotenoids lutein and zeaxanthin. It is generally believed that these two carotenoids provide protection against age-related macular degeneration (AMD), which is the leading cause of blindness among the elderly. Many studies point to the health benefits of a diet rich in beta-carotene and other carotenoids, which are found in fruits and vegetables that are yellow, orange, or red in color. However, contrary to earlier expectations, not only do beta-carotene supplements not prevent lung cancer in people at high risk for the disease, they appear to increase rates of the disease, particularly among smokers.

External link Carotenoids

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