Spheroidene

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. 

Fujii, R., K. Onaka, M. Kuki, Y. Koyama, and Y. Watanabe. 1998. The 2A energies of all-/rans-neurosporcnc and spheroidene as determined by fluorescence spectroscopy. Chem. Phys. Lett. 288 847-853. 

Sashima, T., M. Shiba, H. Hashimoto, H. Nagae, and Y Koyama. 1999. The 2A energy of crystalline aR-trans-spheroidene as determined by resonance-Raman excitation profiles. Chem. Phys. Lett. 290 36 42. 

Gebhard and coworkers42 reported a synthesis and spectroscopy of chemically modified spheroidenes. The structure and numbering of the system is shown in 66. The syntheses and spectroscopic properties of the all-E isomers of 1112 dihydrospheroidene (67),... 

Spheroidenes 67-71 have the same overall shape as native all-/raw.v spheroidene (66), which is the carotenoid bound in the photosynthetic reaction center of Rhodobacter... 

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. 

Structures of /3-carotene, a major carotenoid in many types of plants, and spheroidene, a common carotenoid of photosynthetic bacteria. About 350 different natural carotenoids exist. These vary widely in color, depending principally on the number of conjugated double bonds. 

The structure of the reaction centre from Rps. viridis is similar to that of Rb. sphaeroides. The principal differences are that the Rps. viridis reaction centre contains BChl b and BPhe b in place of BChl a and BPhe a, menaquinone-9 in place of UQio at the Qa site, and the carotenoid hydroneurosporene in place of spheroidene/spheroidenone. The other major difference is the presence of a fourth extra-membrane subunit in the Rps. viridis reaction centre that consists of a tetra-heme cytochrome that is attached to the periplasmic faces of the L and M subunits (Deisenhofer et ah, 1995 Deisenhofer and Michel, 1989a,b Michel et ah, 1986 Deisenhofer et al., 1985 Deisenhofer et al., 1984). 

UO. Cyts h and c BChl a or b Intracyloplasmic membrane Spirilloxanthin, okenone, (/3-carotene), spheroidene, rhodopin, lycopene, neurosporene... 

These observations led to the prediction that accessory carotenoid pigments would be found in van der Waals contact with bacteriochlorophylls in the reaction centers of photosynthetic bacteria [58]. Indeed, the crystal structure of wild-type Rb. sphaeroides clearly shows spheroidene to be in contact with the adjacent monomer bacteriochlorophyll (Figure 1) [8]. 

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. 

B Arnoux, JF Gaucher, A Ducruix and F Reiss-Husson (1995) Structure of the photochemical centre of a spheroidene-containingpurple bacterium, Rhodobacterspheroides Y, at 3 resolution. Acta Crystallog, Section D, 51 368-379... 

The third example is the light-harvesting complex B800-850 obtained from the wild-type Rb. sphaeroides 2.4.1. This complex contains three BChls and one carotenoid (spheroidene) per protein subunit. The fluorescence excitation was obtained by monitoring the emission at 850 nm. Fig. 3 (C) shows the absorption and fluorescence excitation spectra in the 400-620 nm region, with the two spectra normalized at 590 nm (marked with ). The excellent match ofthe absorption and excitation spectra indicates that photoexcited spheroidene transfers energy to bacteriochlorophyll with a high efficiency. 

Whether carotenoid fluorescence originates from the S2- So or S ->So transition appears to depend on the number of conjugated double bonds, i.e., the chain length, and possibly other molecular-structural factors. For instance, the fluorescence from both (3-carotene and spheroidene, each ofwhich has 10 conjugated double bonds, originates from the S2->So transition, while that of fucoxanthin, which has 8 conjugated double bonds, displays the Stokes-shifted Si- Socmission. 

DeCoster, Christensen, Gebhard, Lugtenburg, Farhoosh and Frank " performed a series of elegant experiments in which they examined the fluorescence spectra of four, all-trans spheroidenes containing 7 to 10 conjugated double bonds, and found that as the extent of it-electron conjugation increases, there was... 

Transient absorption changes of spheroidene in cyclohexane at 540-nm induced by 480-nm pulses showed the ground-state recovery, Sj Sq, atroomtemperature tobe9.1 ps. However, similar scans made with a higher time resolution showed an initial bleaching (attributable to the Sq->S2 transition) which rapidly decayed in 0.34ps to a longer-lived state. The 0.34-ps lifetime of this S2 state is consistent with the lifetime indirectly estimated from the quantum yield of fluorescence, 0. ... 

Fig. 12. Model for energy transfer from the carotenoid spheroidene to BChl in the B800-850 antenna complex of Rhodobacter sphaeroides derived from femtosecond absorbance changes. Figure source Shreve, Trautman, Frank, Owens and Albrecht (1991) Femtosecond energy-transfer processes in the B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1. Biochim Biophys Acta 1058 285.

X-ray crystallographic studies have established the location ofthe carotenoid in the reaction centers of Rp. viridis [cf. Fig. 1 (B)] and two wild-type strains of Rb. sphaeroides. The lone carotenoid molecule (1,2-dihydroneurosporene in Rp. viridis, spheroidene in sphaeroides) is located on the M-subunit side,... 

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. 

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