Carotenoid anions

Radical anions of all-trans P-carotene (1) and its 15-c/s isomer and of all-trans lycopene (24) were produced using pulse radiolysis techniques [118]. Pulse radiolysis was also employed for the preparation of radical anions of a series of carotenoids including phytoene (6) and canthaxanthin (16) [151]. More recently [152] the radical anion of P-carotene (1) was studied by pulse radiolysis, which appears to be the method of choice for the generation of carotenoid anion radicals. 

The application of NMR in carbanion chemistry has been reviewed [158]. No NMR data seems to have been reported for carotenoid anions. 

Reduction — The addition of one electron to the carotenoid molecule would give the radical anion CAR -H e- — CAR". 

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. 

Concentration of Hydrophilic Carotenoids in Water for Almost Complete Inhibition of Aqueous Superoxide Anion (02 )... 

EPR techniques were used to show (Polyakov et al. 2001a) that one-electron transfer reactions occur between carotenoids and the quinones, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), and tetrachlorobenzoquinone (CA). A charge-transfer complex (CTC) is formed with a -values of 2.0066 and exists in equilibrium with an ion-radical pair (Car Q ). Increasing the temperature from 77 K gave rise to a new five-line signal with g=2.0052 and hyperfine couplings of 0.6 G due to the DDQ radical anions. At room temperature a stable radical with y=2.0049 was detected, its... 

The radical anions of a variety of carotenoids have been shown to absorb in the infrared (like the radical cations). The anions typically absorb at wavelengths around 120nm shorter than then-respective radical cations in nonpolar solvents, such as benzene and hexane. However, for carotenoids containing carbonyl groups on the rings, the order is switched and it is the anions that absorb farthest to the red (Dawe and Land 1975, Lafferty et al. 1977, Hill 1994). 

Carotenoid radical anions contrast with radical cations in that they have been shown to react with oxygen at diffusion-controlled rates (Conn et al. 1992) whereas the radical cations do not react with oxygen (Dawe and Land 1975) at all. 

Interaction with Other Carotenoids 14.4.2.1 Radical Anions... 

El-Agamey, A., Edge, R., Navaratnam, S., Land, E.J., and Truscott, T.G. 2006. Carotenoid radical anions and their protonated derivatives. Org. Lett. 8 4255 4258. 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (Cl), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

Electronic Absorption Spectroscopy. Absorption spectra have been obtained for radical cations and anions generated from a number of carotenoids [phytoene (7,8,ll,12,7, 8, ll, 12 -octahydro-i//,i/ -carotene) (135) and canthaxanthin ()3,/3-carotene-4,4 -dione) (130)] and related polyenes [7,7 -dihydro- -carotene (131),... 

Saffron extract contains many carotenoids such as crocetin, crocetin di-glucose ester, crocetin gentiobiose glucose ester, and crocin (crocetin di-gentiobiose ester), whose chemical structures are shown in Figure 58.1. These carotenoids scavenge free radicals, especially superoxide anions, and so may protect cells from oxidative stress. Indeed, it has been demonstrated that these carotenoids are useful in sperm cryoconservation and in protecting heptocytes from toxins. 

Polivka had investigated the co-adsorption of carotenoid and pheophytin (111) on the surface of TiC>2 electrode and the photophysical properties of pheophytin in this film. The results demonstrated that the fluorescence of 111 was efficiently reductive quenched by carotenoid in this co-assembled film, suggesting similar mechanisms to that in the natural photosynthetic systems. The radical anion of 111 formed during the electron transfer recovered to the neutral state quickly before the charge recombination between carotenoid cation and pheophytin anion took place. It is suspected that the electron injection from the pheophytin anion to the conduction band of Ti02 was responsible for this quick recovery. This result indicated that such a self-assembling strategy may be also considered for novel DSSC constructions [108]. 

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

A study of a we o-tetraphenylporphyrin bearing four negatively charged, bixin-based carotenoid substituents has shown that in water at pH 9, unilamellar vesicles made up of monolayer membranes are formed [169]. In the presence of guanidi-nium porphyrin counter ions, excitation at wavelengths absorbed by porphyrins leads to photoinduced electron transfer. Spectroscopic evidence for the bixin radical cations and porphyrin radical anions was obtained. Presumably, photoinduced electron transfer from the bixin to porphyrin first excited singlet states is involved in the formation of the radical ions. 

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