How do surrounding environments influence the electronic and vibrational properties of spheroidene?

Absorption and Raman spectra of spheroidene dissolved in various organic solvents and bound to peripheral light-harvesting LH2 complexes from photosynthetic purple bacteria Rhodobacter (Rba.) sphaeroides 2.4.1 were measured. The results showed that the peak energies of absorption and C-C and C=C stretching Raman lines are linearly proportional to the polarizability of solvents, as has already been reported. When comparing these results with those measured on LH2 complexes, it was confirmed that spheroidene is surrounded by a media with high polarizability. However, the change in the spectral width of the Raman lines, which reflect vibrational decay time, cannot be explained simply by a similar dependence of solvent polarizability. The experimental results were analyzed using a potential theoretical model. Consequently, a systematic change in the Raman line widths in the ground state can be satisfactorily explained as a function of the viscosity of the surrounding media. Even when the absorption peaks appear at the same energy, the vibrational decay time of spheroidene in the LH2 complexes is approximately 15-20 % slower than that in organic solvents.