The structures of S0 spheroidene in the light-harvesting (LH2) complex and S0 and T1 spheroidene in the reaction center of Rhodobacter sphaeroides 2.4.1 as revealed by Raman spectroscopy

The Raman spectrum of S0 spheroidene in the light-harvesting (LH2) complex (LHC) and those of S0 and T1 spheroidene in the reaction center (RC) of Rhodobacter sphaeroides 2.4.1 were recorded. Comparison of the S0 Raman spectrum of the all-trans isomer bound to the LHC of R. sphaeroides 2.4.1 with that free in n-hexane solution suggests that the LHC-bound carotenoid takes a flat, all-trans configuration with possible distortion in the plane of the conjugated chain. On the other hand comparison of the S0 Raman spectrum of the 15-cis isomer bound to the RC with that free in n-hexane solution suggests that the RC-bound carotenoid takes a 15-cis configuration which is twisted around the C15(double bond)C15′ bond and distorted in the (single bond)C15H(double bond)C15′H(single bond) plane. The T1 Raman spectra of the RC-bound spheroidene indicated substantial twisting and in-plane distortion of the conjugated backbone. Based on the above results, a mechanism of triplet-energy dissipation by the RC-bound carotenoid, which involves an internal rotation around the C15(double bond)C15′ bond, is proposed.