Chemistry Sources and Physiology

B K Ishida and G E Bartley, Agricultural Research Service, Albany, CA, USA

Published by Elsevier Ltd.

Chemistry Structure

Most carotenoids are 40-carbon isoprenoid compounds called tetraterpenes. Isoprenoids are formed from the basic five-carbon building block, isoprene (Figure 1). In nature, carotenoids are synthesized through the stepwise addition of isopentenyl dipho-sphate (IPP) units to dimethylallyl diphosphate (DMAPP) to form the 20-carbon precursor geranylger-anyl diphosphate (GGPP). Two molecules of GGPP are combined to form the first carotenoid in the bio-synthetic pathway, phytoene, which is then desatu-rated, producing 11 conjugated double bonds to form lycopene, the red pigment in ripe tomato fruit (Figure 1). Nearly all other carotenoids can be derived from lycopene. Lycopene can be cyclized on either or both ends to form a- or 0-carotene, and these in turn can be oxygenated to form xanthophylls such as 0-cryptoxanthin, zeaxanthin, or lutein (Figure 1 and Figure 2). Carotenoids having fewer than 40 carbons can result from loss of carbons within the chain (nor-carotenoids) or loss of carbons from the end of the molecule (apocarotenoids). Longer carotenoids, homocarotenoids (C45-C50), are found in some bacterial species. The alternating double bonds along the backbone of carotenoid molecules form a polyene chain, which imparts unique qualities to this group of compounds. This alternation of single and double bonds also allows a number of geometrical isomers to exist for each carotenoid (Figure 1). For lycopene, the theoretical number of steric forms is 1056; however, when steric hindrance is considered, that number is reduced to 72. In nature most carotenoids are found in the all-trans form although mutants are known in plants, e.g., Lycopersicon esculentum (Mill.) var. Tangerine tomato, and eukaryotic algae that produce poly-cis forms of carotenoids. The mutant plant is missing an enzyme, carotenoid isomerase (CRTISO), which catalyzes the isomerization of the cis isomers of lycopene and its precursors to the all-trans form during biosynthesis. Light can also cause cis to trans isomeri-zation of these carotenoids depending upon the surrounding environment. The isomeric form determines the shape of the molecule and can thus change the properties of the carotenoid affecting solubility and absorbability. Trans forms of carotenoids are more rigid and have a greater tendency to crystallize or aggregate than the cis forms. Therefore, Cis forms may be more easily absorbed and transported. End groups such as the 0 or e rings of a-carotene and 0-carotene and the amount of oxygenation will also affect carotenoid properties.

Chemical Properties

In general, carotenoids are hydrophobic molecules and thus are soluble only in organic solvents, having only limited solubility in water. Addition of hydro-xyl groups to the end groups causes the carotenoid to become more polar, affecting its solubility in various organic solvents. Alternatively, carotenoids can solubilize in aqueous environments by prior integration into liposomes or into cyclic oligosac-charides such as cyclodextrins.

In general, carotenoid molecules are very sensitive to elevated temperatures and the presence of acid, oxygen, and light when in solution, and are subject to oxidative degradation.

Electronic Properties

What sets carotenoids apart from other molecules and gives them their electrochemical properties is the conjugated double bond system. In this alternating double and single bond system, the ^-electrons are delocalized over the length of the polyene chain. This polyene chain or chromo-phore imparts the characteristic electronic spectra and photophysical and photochemical properties to this group of molecules. The highly delocalized ^-electrons require little energy to reach an excited state so that light energy can cause a transition. The length of the conjugated polyene or

trans-Lycopene

2,6-Cyclolycopene-1,5-diol A

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