Acyclic Carotenes

Carotene 7,8,7 8 -Tetrahydro-t/, -carotene Acyclic carotene Inactive... 

There are basically two types of carotenoids those that contain one or more oxygen atoms are known as xanthophylls those that contain hydrocarbons are known as carotenes. Common oxygen substituents are the hydroxy (as in p-cryptoxanthin), keto (as in canthaxanthin), epoxy (as in violaxanthin), and aldehyde (as in p-citraurin) groups. Both types of carotenoids may be acyclic (no ring, e.g., lycopene), monocyclic (one ring, e.g., y-carotene), or dicyclic (two rings, e.g., a- and p-carotene). In nature, carotenoids exist primarily in the more stable all-trans (or all-E) forms, but small amounts of cis (or Z) isomers do occur. - ... 

Figure 6.2.2 shows the separations of mixtures of standards on a monomeric C,g column and also on a polymeric C30 column. The elution order on the monomeric C18 column is, as expected, first the dihydroxy xanthophyUs (lutein and zeaxanthin), followed by the monohydroxy compounds (rubixanthin and P-cryptoxanthin), and finally by the carotenes (y-, a-, and p-carotene). However, on the C30 column, rubixanthin and y-carotene, both with 1 acyclic /-end group, eluted after a- and P-carotene, with two cyclic end groups. 

Of the acyclic carotenes, lycopene and -carotene are the most common. Lycopene is the principal pigment of some red-fleshed fruits and fruit vegetables, such as tomato, watermelon, red-fleshed papaya and guava, and red or pink grapefruit (see Table 7.3). -Carotene is more ubiquitous, but it is usually present at low levels except in Brazilian passion fruit (Mercadante and others 1998) and in carambola (Gross and others... 

Structurally, vitamin A (retinol) is essentially one half of the molecule of (3-carotene. Thus, (3-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted (3 ring with a Cn polyene chain is the minimum requirement for vitamin A activity, y -Carotene, a-carotene, (3-cryptoxanthin, a-cryptoxanthin, and (3-carotene 5,6-epoxide, all having one unsubstituted ring, have about half the bioactivity of (3-carotene (Table7.4) On the other hand, the acyclic carotenoids, devoid of (3-rings, and the xanthophylls, in which the (3-rings have hydroxy, epoxy, and carbonyl substituents, are not provitamin A-active for humans. 

Acyclic Carotenoids. Two pigments from Rhodopseudomonas sphaeroides have been identified as methoxyspheroidene [l,T-dimethoxy-3,4-didehydro-l,2,T,2, 7, 8 -hexahydro-i/f,i/f-carotene (1)] and methoxyspheroidenone [1,T-dimethoxy-3,4-didehydro-1,2,1, 2, 7, 8 -hexahydro-(/f, />-caroten-2-one (2)]. Ano-... 

Scheme 7.5 Formation of some aroma compounds after oxidative cleavage of a acyclic carotenoids (e.g., lycopene, phytofluene and phytoene) and b cyclic carotenoids (e.g. a-carotene and / -caro-tene)...

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

Oxidation of tetraterpenes yields many compounds, and the cyclic compounds with a trimethylcyclohexane ring are easily associated with degradation of monocyclic and bicyclic carotenes, but allylic compounds are now as conspicuous in their biogenetlc origin. Many such compounds are derived from lycopene, phytoene and phyto-fluene, three very common acyclic carotenes. These allylic compounds are often identified only as terpene aldehydes and ketones and not as carotene degradation products (10). 

Distributions of carotenoids can be characteristic for various groups of photosynthetic organisms. Fucoxan-thin is characteristic of diatoms (BaciUariophyceae) and peridinin is found in many dinoflagellates (Dinophyceae Fig. 2.23). In contrast, diatoxanthin and diadinoxanthin occur in many phytoplanktonic classes due to their xanthophyll cycle role (Fig. 2.25).Although it is a less specific marker compound, (3-carotene is abundant in cyanobacteria. Photosynthetic bacteria produce acyclic and aromatic carotenoids (e.g. lycopene and okenone, respectively Fig. 2.23). Astaxanthin and its esters are major constituents of marine zooplankton (Fig. 2.23). 

The formation of the normally present cyclic carotenoids is Inhibited. The transformation of the acyclic lycopene to the monocyclic gamma-carotene Is partially inhibited and further cyclizatlon to the bicyclic beta-carotene is totally Inhibited. However, when the treatment of the entire tree which Is sprayed with 2,000 ppm MPTA Is conducted preharvest at the fully mature stage of fruit development [2], the carotenoid composition of the endocarp reflects a net synthesis of the cyclic pigments and differs from the pattern seen in the peel as shown in Table II. 

Further demonstration of these two distinct and separate properties is evident in the carotene response pattern observed in further studies of the bioregulation of carotene synthesis in the carotenogenic mold Phvcomvces blakesleeanus. Mycelia of P. blakesleeanus cultured on media containing 10 ppm DCPTA show the acyclic lycopene as the main pigment constituent as shown in Table III. 

A 5 mm diameter circular mycelial mat from the 10 ppm DCPTA media [1st transfer] was transferred to media containing 0 ppm DCPTA and allowed to further develop for 3 days to about 35 cm in diameter. Examination of the latter mycelial mat clearly show no evidence of any carryover of DCPTA from the initial media. The carotenoid composition of the first transfer show the usually present bicyclic beta-carotene, not the acyclic lycopene, as the main pigment, similar to the pattern normally found in the mycelia of the mold. However, there was a ten-fold increase in the amount of beta- and... 

There is observed an increased production of the normal pigment constituents and of both the acyclic lycopene and the bicyclic beta-carotene in the fruits from treated plants with no indication of cyclic inhibition. At low concentrations DCPTA has been shown also to affect biomass development in tomato and other crops [8]. 

Fig. 7.2 Classification and structure of carotenoids (a) lycopene - acyclic hydrocarbon (b) 7-carotene - monocyclic hydrocarbon (c) /3-carotene - bicyclic hydrocarbon (d) lutein - bicyclic xanthophyll.

Carotenoids are tetraterpenes derived from a symmetrical C40 skeleton. Carotenoids can be classified into two great groups carotenes, which are strictly hydrocarbons, and xanthophylls, derived from the former that contain oxygenated functions. Stracturally, the carotenoids may be acyclic (e.g., lycopene) or contain a ring of five or six carbons at one or both ends of the molecule (e.g., /3-carotene). Figure 10.1 shows the stmcture and the system of numbering using lycopene and /3-carotene as models of acyclic and bicyclic carotenoids respectively. Stractures of some representative carotenes and xanthophylls, commonly found in fruits are also illustrated. 

Inhibitor studies with the photosynthetic bacterium Rhodomicrobium vannielii showed that both nicotine and CPTA [2-(4-chlorophenylthio)triethylammonium chloride] block the formation of both /3-carotene and the major acyclic carotenoids such as rhodopin [l,2-dihydro- /r,i/f-caroten-l-ol (184)] and spirillo-xanthin [l,T-dimethoxy-3,4,3, 4 -tetradehydro-1,2,1, 2 -tetrahydro-i/, t/r-caro-... 

The Heliobacteriaceae only have Cj acyclic carotenes, 4,4"-diapocarotenes, instead of the usual carotenoids (Fig. 11 Takaichi et al., 1997b). 4,4"-Diaponeurosporene is the major carotene, and diapophytoene, diapophytofluene, diapo-C-carotene and diapolycopene are also found as minor components. Two genes encoding enzymes in the early steps of diapocarotene biosynthesis have been... 

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