ABSTRACT
The chemical structure of carotenoids is usually based on a C40-tetraterpenoid structure with a centrally located, extended conjugated double-bond system that acts as the light-absorbing chromophore and is related to the color shown. Taking into account their chemical structure, these compounds can be divided into two different groups: hydrocarbon carotenoids, generally named carotenes, and oxygenated carotenoids, commonly known as xanthophylls. Carotenoids are biosynthesized in plants, algae, fungi, and bacteria, whereas animals cannot biosynthesize carotenoids (Poulter and Rilling, 1981). More than 700 different carotenoids have been isolated from nature and their biosynthesis occurs in the so-called acetate-mevalonate pathway (Britton, 1998). Initially, the “head-to-tail” condensation, through the action of the enzyme prenyl transferase, of the C5-isoprenoid precursors IDP (isopentenyl diphosphate) and DMADP (dimethylallyl diphosphate) leads to the formation of the C10-monoterpene GDP (geranyl diphosphate). A second addition of the IDP unit to the GDP leads to the formation of the C15-sesquiterpene FDP (farnesyl diphosphate). A further addition of the IDP to the FDP leads to the formation of the C20diterpene GGDD (geranylgeranyl diphosphate). Successively, the phytoene synthase complex regulates the “tail-to-tail” condensation of two molecules of the C20-diterpene GGDD to form the C40-tetraterpenoid structure, which is a characteristic of the symmetrical carotenoid skeleton. This last condensation forms the phytoene molecule, which has a short triene chromophore and which undergoes a series of sequential desaturation (dehydrogenation) reactions, regulated by the phytoene desaturase enzymes, with the ‚nal formation of the lycopene molecule, which has an acyclic structure with an extended conjugated doublebond system, usually with the (all-E)-con‚guration. The polyene chain may possibly have up to 15 conjugated double bonds in the chromophore of the C40-carotenoid, although 7-11 conjugated double bonds
14.1 Oxycarotenoids: Structure and Functions .................................................................................... 267 14.2 Oxycarotenoids in Food ............................................................................................................... 272 14.3 Oxycarotenoid Analysis ............................................................................................................... 273
14.3.1 General Precautions......................................................................................................... 273 14.3.2 Extraction ........................................................................................................................ 273 14.3.3 Saponi‚cation ...................................................................................................................274 14.3.4 Chromatography ...............................................................................................................274
