Synthetic spheroidenes

Farhoosh et al. (1997) have reported on the successful insertion of spheroidene and two synthetic spheroidene analogs with shorter polyene chains,... 

Some of the differences between the electronic spectroscopy of simple polyenes and that of carotenoids are illustrated in Fig. 4 which compares the low-temperature absorption and fluorescence spectra of three heptaenes hexadecaheptaene (7a), a synthetic C30 spheroidene (7b), and an apo-carotene (7c). Spectra of hexadecaheptaene are well resolved with well-defined electronic origins, allowing the... 

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. 

From the above discussion it is clear that it is of vital importance to gain easy access to carotenoids specifically labelled with stable isotopes, especially with C. In the next Section, the tools for synthesizing labelled carotenoids from simple starting materials will be discussed. The synthesis of labelled Cio-synthons suitable for preparing any carotenoid labelled in the central part at predetermined positions will be described. Next, the linear synthesis of the spheroidenes labelled with in the central part will be presented, followed by a new convergent synthetic scheme by which it is possible to label all carotenoids with isotopes in the central part of the molecule. 

Several syntheses of carotenoids isotopically labelled with deuterium have been reported [65-68]. The total synthesis of spheroidenes (97) specifically labelled with deuterium in the central part is based on the synthetic scheme discussed above for the C-labelled spheroidenes [68]. When deuterium-enriched compounds are used, a few modifications are necessary to avoid scrambling and isotope dilution (Scheme 28). 

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