The Carotenoid S1 State in LH2 Complexes from Purple Bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila: S1 Energies, Dynamics, and Carotenoid Radical Formation

Using near-infrared femtosecond absorption spectroscopy, we have determined the S1 energies of the carotenoids spheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S1 energies in the LH2 complexes yield values of 13400 ± 100 cm-1 for spheroidene and 12550 ± 150 cm-1 for rhodopin glucoside, which are very close to the S1 energies obtained for both carotenoids in solution. The 850 cm-1 difference between the S1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2 complexes. The S1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is high enough to allow efficient energy transfer from the S1 state to bacteriochlorophylls, resulting in a substantial shortening of the spheroidene S1 lifetime in the LH2 complex (1.7 ps) compared with the lifetime in solution (8.5 ps). Rhodopin glucoside, which occurs in Rhodopseudomonas (Rps.) acidophila, has an S1 energy in the LH2 complex too low for efficient S1-mediated energy tran...