NOMENCLATURE AND STRUCTURE

Carotenoids are a class of hydrocarbons (carotenes) and their oxygenated derivatives (xanthophylls). They consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position relationship and the remaining nonterminal methyl groups are in a 1,5-position relationship. All carotenoids may be formally derived from the acyclic C40H56 structure (I) (Fig. 1), having a long central chain of conjugated double bonds, by (I) hydrogenation, (2) dehydrogenation, (3) cyclization, or (4) oxidation, or any combination of these processes. The class also includes compounds that arise from certain rearrangements or degradations of the carbon skeleton (I), provided that the two central methyl groups are retained. About 600 carotenoids have been isolated from natural sources; they are listed with their trivial and semisystematic names in Key to Carotenoids (Pfander, 1987) and in the Appendix of Carotenoids, Volume 1A (Kull & Pfander 1995) which also includes literature references for their spectroscopic and other properties. It must be pointed out, however, that for many of the carotenoids listed the structure (this term includes the stereochemistry) is still uncertain and in all these cases a reisolation, followed by structural elucidation with all the modern spectroscopic methods (especially high resolution nuclear magnetic resonance (NMR) spectroscopy) is absolutely necessary. About 370 of the naturally occurring carotenoids are chiral, bearing from one to five asymmetric carbon atoms, and in most cases one carotenoid occurs only in one configuration in Nature.   Figure 1 Rules for the nomenclature of carotenoids (semisystematic names) have been published by the International Union of Pure and Applied Chemistry (IUPAC) and IUPAC-International Union of Biochemists (IUB) Commissions on Nomenclature (1975). For the most common carotenoids trivial names are normally used. If these trivial names are used in a paper, the semisystematic name should always be given, in parentheses or in a footnote, at the first mention. All specific names are based on the stem name carotene, which corresponds to the structure and numbering in II (Fig. 2).     Figure 2 The name of a specific compound is constructed by adding two Greek letters as prefixes (Figure 3) to the stem name carotene; the Greek letter prefixes are cited in alphabetical order.   Figure 3 The oxygenated carotenoids (xanthophylls) are named according to the usual rules of organic chemical nomenclature. The functions most frequently observed are hydroxy, methoxy, carboxy, oxo, and epoxy. In addition, carotenoids with triple bonds are also known. Important and characteristic carotenoids (Fig. 4) are lycopene (gamma,gamma-carotene) (I), beta-carotene (beta,beta-carotene) (III), alpha-carotene ((6’R)-beta,epsilon-carotene) (IV), beta-cryptoxanthin ((3R)-beta,beta-caroten-3-ol) (V), zeaxanthin ((3R,3'R)-beta,beta carotene-3,3'-diol) (VI), lutein ("xanthophyll", (3R,3'R,6'R)-beta,epsilon -carotene-3,3'-diol) (VII), neoxanthin ((3S,5R,6R,3'S,5'R,6'S)-5',6'-epoxy-6,7-didehydro-5,6,S',6'-tetrahydro-beta,beta-carotene-3,5,3'-triol) (VIII), violaxanthin ((3S,5R,6R,3’S,5'R,6'S)-5,6,5',6'-diepoxy-5,6,5',6'-tetrahydro-beta,beta-carotene-3,3'-diol) (IX), fucoxanthin ((3S,5R,6S,3'S,5'R,6'R)-5,6-epoxy-3,3',5'-trihydroxy-6',7'-didehydro-5,6,7,8,5',6'-hexahydro-beta,beta-caroten-8-one 3'-acetate) (X), canthaxanthin (beta,beta-carotene-4,4'-dione) (XI), and astaxanthin ((3S,3'S)-3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) (XII).   Figure 4  Derivatives in which the carbon skeleton has been shortened by the formal removal of fragments from one or both ends of a carotenoid are named apo- and diapocarotenoids, respectively, e.g. beta-apo-8'-carotenal (8'-apo-beta-caroten-8'-al) (XIII). Other structural variations are encountered in the norcarotenoids, in which one or more carbon atoms have been eliminated from within the typical C40-skeleton. A prominent example is the C37-skeleton of peridinin ((3S,5R,6R,3’S,5’R,6’R)-epoxy-3,5,3’-trihydroxy-6,7-didehydro-5,6,5’,6’-tetrahydro-10,11,20-trinor-beta,beta-caroten-19’,11’-olide 3-acetate) (XIV) characteristic of diatoms.   This cis-trans or (E/Z)-isomerism of the carbon-carbon double bonds is another interesting feature of the stereochemistry of the carotenoids, because it was demonstrated that the (E/Z)-isomers may have different biological properties. The literature in this field is extensive: the first comprehensive review of the cis-trans isomerism of carotenoids and vitamin A was published in 1962 (Zechmeister, 1962). According to the number of double bonds a great number of (E/Z)-isomers exist for each carotenoid, e.g. 1056 for lycopene (I) and 272 for b-carotene (III). In view of the (E/Z)-isomerism the double bonds of the polyene chain can be divided into two groups: (I) double bonds with no steric hindrance of the (Z)-isomer (central 15,15'-double bond and the double bonds bearing a methyl group, such as the 9-, 9'-, 13-, and 13'-double bonds) and (2) double bonds with steric hindrance (7-, 7'-, 11-, and 11’-double bonds). Although isomers with sterically hindered (Z)-double bonds are known ((11Z)-retinal) the number of possible (Z)-isomers is in practice reduced considerably, e.g., for lycopene (I) to 72. Normally carotenoids occur in Nature as the (all-E)-isomer; however, exceptions are known, such as the (15Z)-phytoene isolated from carrots, tomatoes, and other organisms. On the other hand, some carotenoids undergo isomerization very easily during workup; therefore many (Z)-isomers that are described in the literature as natural products are artifacts. For experimental work it must be kept in mind that (E/Z)-isomerization may occur when a carotenoid is kept in solution. Normally the percentage of the (Z)-isomers is rather low, but it is enhanced at higher temperature. Furthermore, the formation of (Z)-isomers is increased by exposure to light.

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