Two Stereoisomers of Spheroidene in the Rhodobacter sphaeroides R26 Reaction Center: A DFT Analysis of Resonance Raman Spectra

From a theoretical analysis of the resonance Raman spectra of 19 isotopomers of spheroidene reconstituted into the reaction center (RC) of Rhodobacter sphaeroides R26, we conclude that the carotenoid in the RC occurs in two configurations. The normal mode underlying the resonance Raman transition at 1239 cm−1, characteristic for spheroidene in the RC, has been identified and found to uniquely refer to the cis nature of the 15,15′ carbon-carbon double bond. Detailed analysis of the isotope-induced shifts of transitions in the 1500–1550 cm−1 region proves that, besides the 15,15′-cis configuration, spheroidene in the RC adopts another cis-configuration, most likely the 13,14-cis configuration.