Spheroidene absorption spectra

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. 

Petek H, Bell AJ, Choi YS, Yoshihara K, Tounge BA and Christensen RL (1995) One- and two-photon fluorescence excitation spectra of the 2 Ag states of linear tetraenes in free jet expansions. J Chem Phys 102 4726 739 Ramasesha S and Zoos ZG (1984) Correlated states in linear polyenes, radicals, and ions Exact PPP transition moments and spin densities. J Chem Phys 80 3278-3287 Raubach RA and Guzzo AV (1973) Singlet-triplet absorption spectrum of uH-trans-retinal. J Phys Chem 75 983-984 Ricci M, Bradforth SE, Jimenez R and Fleming G (1996) Internal conversion and energy transfer dynamics of spheroidene in solution and in the LH-1 and LH-2 light-harvesting complexes. Chem Phys Lett 259 381-390... 

The spectroscopic and photochemical properties of the synthetic carotenoid, locked-15,15 -cA-spheroidene, were studied by absorption, fluorescence, CD, fast transient absorption and EPR spectroscopies in solution and after incorporation into the RC of Rb. sphaeroides R-26.1. High performance liquid chromatography (HPLC) purification of the synthetic molecule reveal the presence of several Ai-cis geometric isomers in addition to the mono-c/x isomer of locked-15,15 -c/x-spheroidene. In solution, the absorption spectrum of the purified mono-cA sample was red-shifted and showed a large c/x-peak at 351 nm compared to unlocked all-spheroidene. Spectroscopic studies of the purified locked-15,15 -mono-c/x molecule in solution revealed a more stable manifold of excited states compared to the unlocked spheroidene. Molecular modeling and semi-empirical calculations revealed that geometric isomerization and structural factors affect the room temperature spectra. RCs of Rb. sphaeroides R-26.1 in which the locked-15,15 -c/x-spheroidene was incorporated showed no difference in either the spectroscopic properties or photochemistry compared to RCs in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 RCs which naturally contain spheroidene. The data indicate that the natural selection of a c/x-isomer of spheroidene for incorporation into native RCs of Rb. sphaeroides wild type strain 2.4.1 was probably more determined by the structure or assembly of the RC protein than by any special quality of the c/x-isomer of the carotenoid that would affect its ability to accept triplet energy from the primary donor or to carry out photoprotection. 

In this study we have investigated the mechanism of the solvent induced shift in the absorption spectrum of two carotenoids, i.e. spheroidene and trans- 3-carotene. Carotenoids belong to the class of polyenes and they are essentially hydrophobic. In vivo the carotenoids are non-covalently bound to specific pigment-protein complexes. Our working model is the red-shifted spectra of carotenoids in vivo is mainly due to dispersive interactions of the carotenoid and the surrounding, and consequently the shift is due to the polarizability of the surrounding medium. The measurements reported here support this model. 

Fig. 3. Absorption spectrum of B800-850 antenna complex (Rb. sphaeroides) and spheroidene in quinoline.

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.,... 

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. 

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