Configuration of spheroidene in the photosynthetic reaction center of Rhodobacter sphaeroides : a comparison of wild-type and reconstituted R26.

We compare the resonance Raman spectra acquired at two excitation wavelengths, 496.5 and 514.5 nm, of spheroidene in the wild-type reaction center of Rhodobacter sphaeroides and reconstituted into the reaction center of the Rhodobacter sphaeroides mutant R26. Our earlier work showed that the reconstituted R26 reaction center holds spheroidene in two configurations: 15,15'-cis and another configuration. Here we show that in the wild-type reaction center only 15,15'-cis spheroidene is present. In the resonance Raman spectra of the reconstituted R26 reaction centers, a transition is identified that arises exclusively from the second configuration. According to density-functional-theory calculations, this transition is specific for the 13,14-cis configuration.