The carotenoid S-1 state in LH2 complexes from purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila: S-1 energies, dynamics, and carotenoid radical formation

Using near-infrared femtosecond absorption spectroscopy, we have determined the S-1 energies of the carotenoids spheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S-1 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 S-1 energies obtained for both carotenoids in solution. The 850 cm(-1) difference between the S-1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2 complexes. The S-1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is high enough to allow efficient energy transfer from the S, state to bacteriochlorophylls, resulting in a substantial shortening of the spheroidene S-1 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 S-1 energy in the LH2 complex too low for efficient S-1-mediated energy transfer and therefore the S-2 state becomes the main energy donor in LH2 complexes containing this carotenoid. In addition, a distinct carotenoid spectral band not observed in solution, was detected at around 960 nm in the LH2 complex of Rb. sphaeroides. This band is assigned to a spheroidene radical cation, which is formed in similar to200 fs and decays within 8 ps. The yield of the spheroidene radical formation is estimated to be in the range of 5-8%.