Carotenoid reactions

Carotenoid reactions with ROS or RNS (reactive nitrogen species) would generate prooxidative products. ... 

Carotenoid Reactions.—Zeaxanthin (45) has previously been synthesized by Ci9 + C2 + Cl9 and C14 + C12 + C14 routes. A new versatile route has now been developed using a Cl5 + Cjo + Ci5 principle. Details are outlined in Scheme 3. Possibly the most notable features are the directed reduction of the acetylene to give a trans-double-bond, and the use of 1,2-epoxybutane for the... 

Haila, K.M., Nielsen, B.R., Heinonen, M.I., and Skibsted, L.H. 1997. Carotenoid reaction with free radicals in acetone and toluene at different oxygen partial pressure. Food Res. Technol. 204, 81-87. 

FIGURE 3o Effect of TPB on the flash-induced absorbance change of carotenoid. Reaction mixture contained 50 mM Tris buffer (pH 7,8), 50 mM NaCl, 2 1M antimycin A and 15 yiM bacteriochlorophyll,... 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

P-Carotene is prescribed in the treatment of the inherited skin disorder erythropoietic protoporphyria (EPP) to reduce the severity of photosensitivity reactions in such patients. The essential theoretical background relevant to the role of carotenoids as photoconductors has been reviewed (211). P-Carotene has also been used as a photoconductor in recording-media film. 

The AE reactions on 2,5,5-trisubstituted allyl alcohols have received little attention, due in part the limited utility of the product epoxides. Selective ring opening of tetrasubstituted epoxides are difficult to achieve. Epoxide 39 was prepared using stoichiometric AE conditions and were subsequently elaborated to Darvon alcohol. Epoxides 40 and 41 were both prepared in good selectivity and subsequently utilized in the preparation of (-)-cuparene and the polyfunctoinal carotenoid peridinin, respectively. Scheme 1.6.12... 

The fundamental chemistry of carotenoid radicals and the reactions with oxidizing agents, peroxy radicals, etc., is important for evaluating the proposed actions of... 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

In the carotenoid radicals, the unpaired electron is highly delocalized over the conjugated polyene chromophore. This has a stabilizing effect and also allows subsequent reactions. The cation and anion radicals can be detected by their characteristic spectral properties, with intense absorption in the near-infrared region. 

All carotenoids are bound to the light harvesting complexes or reaction centers in membranal systems of bacterial cells. 

In vivo, one of the main groups of carotenoids are the snlfates of eritoxanthin sulfate and of the caloxanthin sulfates. The sulfates of carotenoids are not associated with pigment-protein complexes, for example, they are neither part of the fight harvesting complexes nor of the reaction centers. In nonphotosynthetic bacteria, carotenoids appear sporadically and when present, they have unique characteristics. Some Staphylococci accumulate C30 carotenoids, flavobacteria C45 and C50, while some mycobacteria accumulate C40 carotenoid glycosides. ... 

Carotenoids protect photosynthetic organisms against potentially harmful photooxidative processes and are essential structural components of the photosynthetic antenna and reaction center complexes. Plant carotenoids play fundamental roles as accessory pigments for photosynthesis, as protection against photooxidation, and... 

The photoprotective role of carotenoids is demonstrated in plant mutants that cannot synthesize essential leaf carotenoids. These mutants are lethal in nature since without carotenoids, chlorophylls degrade, their leaves are white in color, and photosynthesis cannot occur. Generally, the carotenoids are effective for visible light but have no effects in ultraviolet, gamma, or x-radiation. The reactions are listed as follows ... 

As can be seen in these reactions, carotenoids may protect photosynthetic bacteria at various levels by quenching the singlet-excited state of O2 or the tiiplet-excited state of chlorophyll. The ground states of oxygen would be 3O2 and for CHL the triplet state. The carotenoids may be the preferred substrates for oxidation or may act in quenching reactive species. ... 

Lafferty, J., Truscott, T.C., and Land, E.J., Electron transfer reactions involving chlorophylls a and b and carotenoids, J. Chem. Soc. Farad. Trans., lA, 2760, 1978. Burri, B.J., Clifford, A.J., and Dixon, Z.R., Beta-carotene depletion and oxidative damage in women, in Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease, Kumulainen, J.T. and Salonen, J.T., Eds., Royal Society of Chemistry, Stockholm, 1999, 231. 

In intestinal cells, carotenoids can be incorporated into CMs as intact molecules or metabolized into mainly retinol (or vitamin A), but also in retinoic acid and apoc-arotenals (see below for carotenoid cleavage reactions). These polar metabolites are directly secreted into the blood stream via the portal vein (Figure 3.2.2). Within intestinal cells, retinol can be also esterified into retinyl esters. 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

Carotenoids are known to be sensitive to oxygen, and are said to be unstable in presence of air. Because of their polyene structures, carotenoids are susceptible to reactions with the so-called ROS that may be radicals (02", HO ) or non-radicals (H2O2, O2). 

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

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