Carotenoids triplet-state energy

Rios, A. de O., A. Z. Mercadante, and C. D. Borsarelli. 2007. Triplet state energy of the carotenoid bixin determined by photoacoustic calorimetry. Dyes Pigments 74 561-565. 

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

Again, in order to establish that conclusions can be transferred from porphyrin to chlorophyll-based systems we measured the triplet-triplet energy transfer rate in carotenopyropheophorbide 28 [58]. The rise time of the carotenoid triplet state was faster than the 50 ns response time of the spectrometer, and is likely limited by k, g. 

Miscellaneous Physical Chemistry. Various aspects of the physical chemistry of /3-carotene and related carotenoids have been reported, including several theoretical calculations related to spectroscopic properties,investigations of carotenoid triplet states and triplet energies,studies of carotenoid radical ions, and examination of electron-transfer reactions between carotenoids and chlorophyll Two reviews offer brief surveys of the year s literature on the photochemistry of... 

The other function of carotenoids is to protect against photooxidation processes that are caused by the excited triplet state of chlorophyll. Carotenoid molecules, with nine or more conjugated carbon-carbon double bonds, can absorb triplet state energy from chlorophyll and thus prevent the formation of harmful singlet oxygen. 

The reaction center contains one carotenoid molecule, except in the carotenoidless strain R-26 of Rb. sphaeroides. Both the spheroidene in Rb. sphaeroides and the 1,2-dihydroneurosporene in Rp. viridis assume the 15,15 -cA configuration and are located near the Bb molecule (see Figs. 9 and 10). The protein environment around the carotenoid consists of a large number of aromatic residues, which probably impose strong steric constraints on the carotenoid, and may account for the red shift in the absorption spectrum of the carotenoid relative to that of the free carotenoid. The proximity of the carotenoid to Bb suggests that the latter could serve as a conduit for the transfer of triplet-state energy from the primary donor to the carotenoid. 

The triplet-state energies of the BChl a and BChl b derived from the phosphorescence spectra are summarized for chromophores of Rb. sphaeroides and Rp. viridis at the appropriate levels in the diagram in Fig. 15 (C), together with those for the respective carotenoids, spheroidene (in Rb. sphaeroides) and l,2-dihydroneurosporene(inRp. viridis). Singlet oxygen is also shown. 

Fig. 15. (A) Absorption, fluorescence and phosphorescence spectra of BChl a in vitro at 77 K spectra scaled for convenient presentation also note break of horizontal scale (B) Phosphorescence spectrum of quinone-depleted (-Q) and quinone-containing (+Q) Rb. sphaeroides reaction centers in polyvinyl-alcohol film at 22 K (C) Energy diagram for the components involved in triplet-triplet energy transfer with carotenoids. (A) and (B) and numerical values for the triplet-state energies of BChls a and b and the primary-donors of Rb. sphaeroides and Rp. viridis, i.e., [BChl a and [BChl bjj, respectively, are taken from Takiff and Boxer (1987) Phosphorescence spectra ofbacteriochlorophylls. J Am Chem Soc 110 4425.

The spin polarization of the anteima carotenoid triplet state has been observed by Frank et al. (1980 1982a 1987) in quite a number of different purple bacterial strains, and under all conditions shows an eae aea pattern (where e means emission and a absorption of microwaves) that can be explained with intersystem crossing in a BChl molecule with subsequent triplet energy transfer to the carotenoid. This seems to contradict the additional triplet formation pathway by hetero-fission of a carotenoid singlet excitation and it would therefore be of great interest to revisit the earlier time-resolved optical... 

Carbonera D, Di Valentin M, Agostini G, Giacometti G, Liddell PA, Gust D, Moore AL and Moore TA (1997b) Energy transfer and spin polarization ofthe carotenoid triplet state in synthetic carotenoporphyrin dyads and in natural antenna complexes. Appl Magn Res 13 487-504... 

Frank HA and Violette CA (1989) MonomericbacteriochlorophyU is required for the triplet energy transfer between the primary donor and the carotenoid in photosynthetic bacterial reaction centers. Biochim Biophys Acta 976 222-232 Frank HA, Bolt JD, de B. Costa SM and Sauer K (1980) Electron paramagnetic resonance detection of carotenoid triplet states. J Am Chem Soc 102 4893 898 Frank HA, Machniki J and Felber M (1982a) Carotenoid triplet states in photosynthetic bacteria. Photochem Photobiol 35 713-718... 

The first demonstration that /3-carotene could inhibit photosensitized oxidation and was, therefore, an efficient quencher of O was reported by Foote and Denny (1968). Wilkinson and Ho (1978) showed that quenching by electron exchange energy transfer to produce the carotenoid triplet state ( CAR) is the principal mechanism of carotenoid photoprotection against O ... 

Although carotenoid triplet states are not formed in appreciable yield by the usual intersystem crossing pathway, they can be produced via triplet-triplet energy transfer from other species or by radical pair recombination processes in artificial reaction centers (Gust et al., 1 2 Liddell et al 1997 Carbonera et al., 1998). Thus, their transient absorption characteristics are well known (Mathis and Kleo, 1973 Bensasson et al., 1976). Typically, they have strong absorption maxima at about 540 nm (Cr - 1 = 10 ... 

Because the carotenoid triplet state is much lower in energy than singlet oxygen, it returns harmlessly to the ground state with the liberation of heat. 

The transient optical spectroscopic data show that the borohydride-treated, spheroidene-reconstituted reaction center sample is less able to form carotenoid triplet states than the native Rb. sphaeroides R26 reaction centers that have been reconstituted with spheroidene to the same extent. However, before these results can be attributed to an involvement of the bacteriochlorophyll monomer in the triplet energy transfer process, it is necessary to provide compelling evidence that spheroidene is bound in a single site, in the same environment and with the same structure in both borohydride-treated and native Rb. sphaeroides R26 reaction center samples. This evidence is provided by the following arguments (1) The absorption spectral features shown in Figs. 1 and 2 for the carotenoid in borohydride-treated and untreated, spheroidene-reconstituted complexes are very similar to each other and very different from the carotenoid in either Triton X-100 detergent or pentane. (See Fig. 3.) (2)... 

The overall process of ()2 quenching simply converts the excess energy of singlet oxygen to heat via the carotenoid [CAR] lowest excited triplet state [3CAR],... 

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