Spheroidene, structure

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),... 

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). 

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]. 

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... 

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. 

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]... 

Fig. 3.21 The chemical structure and the principal axes of spheroidene (top), and the conformation of 3Car(l) assumed and those of3Car(R) and 3Car(l I) determined by comparison between the observed and the...

Proteobacteria (Imhoff, 1995). The functions of carotenoids in photosynthetic bacteria have been investigated in most detail in the Rhodospirillaceae (other chapters in this book). Their RC resembles that of PS 11 of green plants. Their major BChl is BChl a or b. The RC was firstly crystallized from Bla. (previously, Rhodopseudomonas) viridis, and the localization of one carotenoid, 1,2-dihydroneuro-sporene, four BChl b and two bacteriopheophytin b molecules was determined (Deisenhofer et al., 1995). A similar localization of spheroidene in the RC of Rba. sphaeroides has also been described (Yeates et al., 1988 Ermler et al., 1994). The fine crystal structure of the LH II antenna complex from Rps. acidophila strain 10050 has shown the localization of one rhodopin glucoside and three BChl a molecules per ap monomer (McDermott et al., 1995). A similar localization of lycopene in the LH II complex from Rsp. molischiamm has also described (Koepke et al,... 

The bacterial RCs contain a non-covalently bound carotenoid molecule that is located at the B branch. The lacking of a symmetrically related cofactor in the A branch introduces an element of asymmetry into the RC s architecture. The structural effect of this asymmetry on the Co chains of the L- and M-subunits is not drastic as can be seen in Fig. 5. The carotenoid is spheroidene in the RC of Rb. sphaeroides and 1,2-dihy oneurosporene in the RC of Rp. viridis. In both RCs, the carotenoids interact with helices A, B, C (see below) and cd of the M-subunit and the plane spanned by the carotenoid molecule is oriented parallel to the plane of the membrane. The carotenoid shows a c 5-geometric isomeric form and its binding pocket is mainly formed by hydrophobic residues, especially in the Rb. sphaeroides RC where six phenylalanines and five tryptophans are found within a radius of 5 A around the spheroidene molecule. In... 

Arnoux B, Ducruix A, Reiss-Husson F, LutzM, Morris J, Schiffer M and Chang CH (1989) Structure of spheroidene in the photosyntheticreactioncenterfromR/jorfoi aciersp/jaeroirfes Y. FEBS Lett 258 47-50... 

Heterologous reconstitution experiments show some structural restraints Spheroidene from Rb. sphaeroides can be reconstituted into a complex with LHl polypeptides from Rhodospirillum rubrum, restoring spectral properties of native Rs. rubrum LHl and carotenoid-BChl energy transfer whereas spirilloxanthin is unable to incorporate into a reconstituted complex with LHl apoproteins from Rb. sphaeroides (Davis et al., 1995). 

Figure 1 shows the chemical structures ofcarotenoids which will be dealt with in this Section. The length and position of the conjugated system are different from one carotenoid to another. All the carotenoids, except for /3-apo-8 -carotenal, neurosporene, and spheroidene, contain a complete central structural motif which is terminated by the 6 and 6 carbon atoms. The dependence of the H-NMR, electronic absorption and resonance-Raman spectroscopic properties on the cis-trans configurations of the symmetric )3-carotene molecule can change when there is a lack of a complete central structural motif in a carotenoid or when the carotenoid is asymmetric. In the following three subsections, the spectroscopic properties of the carotenoid, )3-carotene, will be described first, and then those of other carotenoids will be mentioned. 

Fig. I. Chemical structures of carotenoids, the dependence ofthe ground-state properties on their m-tra/ij configurations being described in Sec. II, (a) p-Carotene, (b) canthaxanthin, (c) zeaxanthin, (d) lutein, (e) /3-apo-8 -carotenal, (f) lycopene, (g) neurosporene, (h) spheroidene, (i) spirilloxanthin and (j) okenone. For each carotenoid, the number of conjugated C=C plus C=0 bonds is shown.

Tables 1 and 2 also list the H chemical shifts of the dil-trans isomers of other carotenoids. The chemical-shift values in the region from lOH to lO H are conserved for carotenoids having a complete central structural motif in either benzene or chloroform solution. Even in )3-apo-8 -carotenal, neurosporene and spheroidene that lack this structural motif, the chemical-shift values are conserved in the complete...

However, the finding of a new electronic level (the B" state) in-between the B and the 2A states in crystalline all-fran spheroidene (Sashima et al, 1998b) has revealed that the energy-transfer mechanisms can be much more complicated than anticipated. A discussion based on the detailed X-ray structures of the LH2 complexes is given by R. Cogdell in Chapter 4. 

The Structure of the RC-Bound Spheroidene in the T, State and a Possible Mechanism of Triplet-Energy Dissipation... 

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