Carotenoids triplet

Rodgers, M.A.J. and Bates A.L. 1980. Kinetic and spectroscopic features of some carotenoid triplet states Sensitization by singlet oxygen. Photochem. Photobiol. 31 533-537. 

Table II. Attempts to Detect Carotenoid Triplet-state by ESE ...

Then, the carotenoid triplet state decays via non-radiative intersystem crossing ... 

The decay of the carotenoid radical cation absorption of C +-P-C6o occurs on the micro second time scale in the frozen glass. It is accompanied by the rise of C-P-Ceo generated by charge recombination of the C -P-Ceo biradical, which is formed with a quantum yield of 0.07. The major component of the decay of the - C-P-Ceo transient has a time constant of 10 ps, which is a typical lifetime for a carotenoid triplet state. The absorption of C -P-Ceo " at 77 K does not decay exponentially, but an average decay rate of 7.5 x 10 s may be calculated from the data [155]. Time-resolved experiments have allowed detection of the EPR resonances of the C +-P-C6o biradical and C-P-Ceo- The spin-polarization of the carotenoid triplet spectrum verifies formation of this state by the radical pair... 

The low-temperature formation of C +-P-C6o and its recombination to a triplet state allow some interesting and potentially useful magnetic field effects. In the presence of a small (20 mT) static magnetic field, the lifetime of the C +-P-C6o charge-separated state in 43 is increased by 50 % [155]. This is ascribed to the effect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initially formed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four states have comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. In the presence... 

There was a marked difference in the rate of triplet energy transfer for 24 and 26. In a benzene solution of 24, the carotenoid triplet species had a rise time of ca. 2/is and decayed in ca. lO s. Concomitant with the rise of the carotenoid triplet absorbance at 550 nm, the porphyrin triplet absorbance at 440 nm decayed with a time constant of 2/js. There was no appreciable change in these parameters when 24 was dissolved in a rigid plastic matrix [73]. For 26 the triplet energy transfer was much faster. In 1981, we reported it as faster than 30 ns, which was the limit of our instrumentation [91]. Measurements with greater time resolution were made in 1983, but it remained difficult to separate the carotenoid triplet rise time from the instrument response time [73]. In any case, under conditions ranging from solution to solid plastic to a glass at 10 K, the rise time of triplet carotene in 26 was < 5 ns. 

As expected, carotenoporphyrin species in which the carotenoid is linked to the porphyrin meso aryl ring by an amide linkage in which there is no methylene group, such as 3 and most of the other compounds discussed in the section on electron transfer, exhibit extremely fast triplet-triplet energy transfer. The rise time for the carotenoid triplet is < 4 ns, which was the time response of the instrument employed for the measurement [29b],... 

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 Photoprotective Role of Carotenoids Triplet-Triplet Energy Transfer.245... 

Lous EJ and Hoff AJ (1989) Isotropic and linear dichroitic triplet-minus-singlet absorbance difference spectra of two carotenoid-containing bacterial photosynthetic reaction centers in the temperature range 10-288 K. An analysis of the bacteriochlorophyll-carotenoid triplet-transfer. Biochim Biophys Acta974 88-103... 

Cogdell RJ, Monger TG and Parson WW (1975) Carotenoid triplet states in reaction centers from Rhodopseudomonas sphaeroides and Rhodospirillum rubrum. Biochim Biophys Acta 408 189-199... 

Resonance Raman spectra of the RC-bound carotenoid in its triplet state were firstrecorded using pulsed lasers (Lutz et al, 1983). Later on, it was shown that it was possible to accumulate the carotenoid triplet state in the reaction centers with continuous illumination, by making use of the fact that this triplet state is usually produced in these proteins with a high yield when the electron transfer is blocked (Robert et al, 1989 see Fig. 4). From these experiments it was concluded that the particular, 15-15"-dx, twisted, structure of the carotenoid is conserved when these molecules are in the triplet state. In particular, resonance Raman spectra of the RC-bound carotenoid in their triplet states exhibit an unusuallyintense 4, which likely reveals that the molecule is not planar (Lutz et al., 1983, Robert et al, 1989). 

A. Carotenoid Triplet States in Photosynthetic Antenna Complexes.204... 

This chapter reviews the work ofthe last five to six years on paramagnetic states of carotenoids using electron magnetic resonance. Mainly radical cation and neutral molecular triplet states are treated. Part of this ch te deals with paramagnetic states of carotenoids in model systems. These have been synthesized in order to mimic both electron and energy transfer processes in the natural photosynthetic systems. Consequently, the electron magnetic resonance (EMR) spectroscopy of carotenoid triplet and radical states yields important information about their photochemistry. Finally, the EMR spectroscopy on carotenoid radicals is reviewed. It serves to establish the database on their intrinsic properties which is necessary for the analysis of carotenoid radicals in vivo. 

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