Spheroidene triplet states

Independently, a monomeric triplet state was observed in RCs of the carotenoid-less mutant Rb. sphaeroides R26, and later identified with BJby its characteristic microwave induced absorption (MIA) spectrum (Angerhofer and Aust, 1993 Hartwich et al., 1995). The identification was confirmed by the interaction band which is visible in the MIA spectrum of the spheroidene triplet state (Hartwich et al.,... 

Fujii, R. et al., Cis-to-trans isomerization of spheroidene in the triplet state as detected by time-resolved absorption spectroscopy, J. Phys. Chem. A, 106, 2410, 2002. Montenegro, M.A. et al., Model studies on the photosensitized isomerization of bixin, J. Agric. Food Chem., 52, 367, 2004. 

In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation-radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex has been proven. 

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. 

The Trstate CTI in spheroidene was found to be much less efficient than that in /2-carotene, which facilitated the examination of triplet-state dynamics by time-resolved absorption spectroscopy. Scheme 3.5 shows the configurations of the all-trans and cis isomers of spheroidene. The all-trans isomer consists of an open conjugated chain (n= 10) shifted to the left in the entire carbon skeleton, to which a pair of large peripheral groups are attached at both ends. Concerning the position of the ds-bend, the 13 -ds and 9-cis isomers can be classified into peripheral-ds isomers, whereas the 13-ds and 15-ds isomers are central-ds isomers. Concerning the structure of the cis bend, 13 -ds, 9-cis, and 13-ds isomers are methylated cis iso-... 

The Triplet-Excited Region of All-trans-Spheroidene in Solution and the Triplet-State Structure of 15-cis-Spheroidene Bound to the Bacterial Reaction Center Determined by Raman Spectroscopy and Normal Coordinate Analysis [18]... 

However, since the light-induced difference-minus-ground-state spectrum of [P%7o a 1 shows a bleaching of one of the bacteriopheophytins (BPh) near 546 nm, the charge recombination of the primary radical pair would yield a very similar spectral profile and its time constant would be of similar magnitude as the one observed (Shuvalov and Parson, 1981). Thus it seems still premature to make a decisive statement on the fast transient component observed as an increase in absorption at the wavelength of the spheroidene triplet-triplet absorption band at 550 nm. 

Although the main 15,15 "-cis conformation of the spheroidene in RCs of Rb. sphaeroides has been well documented, two slightly distinct triplet states appear at temperatures lower than 60 K (Kolaczkowski et al., 1988 Ullrich, 1988). They may be due todifferent sites that freeze out in slightly different conformations. Possible twists ofthe portion ofthe carotenoid that protrudes from the RC have been discussed in this respect. Possible twisting along the conjugated backbone of the carotenoid was indicated by resonance Raman data (Ohashi et al., 1996). 

Lutz M, Kleo J and Reiss-Husson F (1976) Resonance Raman scattering of bacteriochlorophyll, bacteriopheophytin and spheroidene in reaction centers of Rhodopseudomonas spheroides. Biochem Biophys Res Comm 69 711-717 Lutz M, Chinsky L and Turpin PY (1982) Triplet states of carotenoids bound to reaction centers ofphotosynthetic bacteria Time-resolved resonance Raman spectroscopy. Photochem Photobiol 36 503-515... 

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)... 

Fig. 3.16 Changes in the stretching force constants upon excitation from S0 state (open circles) to the T, state (filled circles), showing the triplet-excited region for all-Jrans-spheroidene in solution (a) and for 15-c/s-spheroidene bound to the reaction center (b). In both peripheral parts, some force constants (double circles) were assumed and could not be determined because of the limited number of deuterio species and the lack of observed Raman lines [18]. Circles with a horizontal line indicate that those force constants are the same in both the S0 and T, states.

The triplet-excited region has been spectroscopically identified in terms of carbon-carbon stretching force constants in all-trans-spheroidene in solution and in 15-ris-spheroidene bound to the bacterial reaction center (Fig. 3.16). Large changes in bond order are actually seen in the central part of the conjugated chain in both cases. The conformation of the reaction center-bound 15-cis-spheroidene in the T, state was determined to be (+45°, -30°, +30°) around the cis C15=C15, trans 03=04 and 01=02 bonds. 

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