Binding of Carotenoids on Reaction Centers from Rhodopseudomonas Sphaeroides R 26

The carotenoid-less reaction centers isolated from Rhodopseudomonas sphaeroides (strain R 26) bind pure all-trans spheroidene as well as spheroidenone in a nearly 1 : 1 molar ratio with respect to P-870. Neither beta-carotene nor spirilloxanthin, both absent from wild-type Rps. sphaeroides, could be bound in appreciable amounts. Resonance Raman spectra of the carotenoid-reaction center complex indicate that the carotenoid is bound as a cis isomer, its conformation being very close, although probably not identical, to that assumed by the carotenoid in the wild-type reaction centers. The electronic absorption spectra of the carotenoid-reaction center complexes are in good agreement with such a interpretation. When bound to the R 26 reaction centers, spheroidene displays light-induced absorbance changes identical in peak wavelengths and comparable in amplitudes to those observed in the wild-type reaction centers. Thus the binding of the carotenoid to the R 26 reaction centers most likely occurs at the same proteic site as in the wild-type reaction centers. This site shows selectivity towards the nature of carotenoids, and has the same sterical requirement as in the wild type, leading to the observed all-trans to cis isomerisation.