Carotenoid anions radicals anion

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. 

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. 

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]. 

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. 

The C +-P-Q state in triads such as 35 eventually recombines to the groimd state, unless it is harvested by subsequent reactions. The simplest possible recombination pathway involves electron transfer from the quinone radical anion directly to the carotenoid radical cation. However, this pathway can be very slow, even if thermodynamics are very favorable, because of the weak electronic coupling between the radical ions. In some cases, charge recombination has been found to fol-... 

In step , a Qs near the outer surface accepts an electron from Q of Q -P-C. In step , the radical anion Qs " accepts a proton from the nearby aqueous phase, forming a semiquinone Qs H. Step indicates diffusion ofQs H aaoss the membrane toward the interior aqueous phase where the oxidizing carotenoid moiety is located, and there becomes reoxidized to form Qs H [step ]. Evidence for this reaction is provided by the diminished lifetime ofthe radical cation in the presence ofQs, as mentioned above. The protonated quinone shuttle releases a proton [step ] and then diffuses toward the exterior region [step ] and completes the cycle. 

Radicals are species with an odd electron, and may or may not carry a formal charge. Thus, radicals of a carotenoid CAR are most simply obtained by adding or removing an electron to generate the radical anion and cation respectively, (CAR and CAR " ). For example a process involving aperoxyl radical (ROO ) can be written as ... 

The absorption spectra of a wide range of carotenoid radical cations and anions were first established by nanosecond pulse radiolysis under conditions of mono-electronic processes (Dawe and Land, 1975 Laffertyetal., 1977). This workreported the spectra in hexane for the radical cations and in hexane and methanol for the radical anions. Subsequently, such studies for the radical cations have been extended to other solvents (Hill et al., 1995 Edge et al., 1998). Table 1 gives a selection of A, values for carotenoid radical cations in four... 

Another radical species which reacts with carotenoids to give only the carotenoid radical cation is the superoxide radical anion (O "), although there have been few studies and there is some confusion in the literature. y8-Carotene was shown by Conn et al. 

Carotenoid radical anions and dianions have been less extensively studied than the corresponding cations, and appear to have no biological relevance. A brief review on the topic is given below. 

In principle the uptake of one electron by a carotenoid molecule leads to an anion radical. Uptake of two electrons gives a dianion. 

NIR absorption spectra of radical anions of several carotenoids including phytoene (6), P-carotene (1), lycopene (24) and canthaxanthin (16) were calculated by Hiickel and PPP methods and were in broad agreement with experimental findings [151]. 

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. 

Zeaxanthin (C ) has been incorporated in DMPC and egg lecithin vesicles. This a,(D-bipolar carotenoid reinforces the DMPC vesicle with respect to mechanical stability and water permeability but has no effect on fluid egg lecithin membranes (Lazrak et al.,1987). Electron-poor derivatives with electron-withdrawing carboxyl or pyridinium end groups should reversibly take up electrons in a type of reversible Michael reaction and then act as organic wires. There are, however, no reports on stable anion radicals of such chro-mophores in the literature. Claims of electron transport through vesicle membranes are very probably erroneous. It has been shown by reduction of an entrapped indigo dye that bixin derivatives in DPPC vesicle membranes favor the transport of borohydride and dithionite ions through the membrane rather... 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

Moore, Gust, and coworkers synthesized the quinone-porphyrin-carotenoid (Figme 5) triad molecule. Upon excitation of the porphyrin moiety, initial charge separation occurred between porphyrin and quinone. Hole shift from porphyrin to carotenoid formed the final charge-separated state, that is, quinone radical anion and carotene radical cation, with a lifetime of 170 ns. These processes were confirmed by means of the picosecond and nanosecond laser flash photolysis. Their covalent bonding system was extended to tetrad and pentad using similar chromophores. 

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