| Abstract
| - Using near-infrared femtosecond absorption spectroscopy, we have determined the S1 energies of the carotenoidsspheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S1 energies in the LH2 complexesyield values of 13400 ± 100 cm-1 for spheroidene and 12550 ± 150 cm-1 for rhodopin glucoside, which arevery close to the S1 energies obtained for both carotenoids in solution. The 850 cm-1 difference between theS1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2complexes. The S1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is highenough to allow efficient energy transfer from the S1 state to bacteriochlorophylls, resulting in a substantialshortening 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 theLH2 complex too low for efficient S1-mediated energy transfer and therefore the S2 state becomes the mainenergy donor in LH2 complexes containing this carotenoid. In addition, a distinct carotenoid spectral bandnot observed in solution, was detected at around 960 nm in the LH2 complex of Rb. sphaeroides. This bandis assigned to a spheroidene radical cation, which is formed in ∼200 fs and decays within 8 ps. The yield ofthe spheroidene radical formation is estimated to be in the range of 5−8%.
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