Mechanism of carotenoid singlet excited state energy transfer in modified bacterial reaction centers

Ultrafast transient laser spectroscopy has been used to investigate carotenoid singlet excited state energy transfer in various Rhodobacter (Rb.) sphaeroides reaction centers (RCs) modified either genetically or chemically. The pathway and efficiency of energy transfer were examined as a function of the structures and energies of the donor and acceptor molecules. On the donor side, carotenoids with various extents of π-electron conjugation were examined. RCs studied include those from the anaerobically grown wild-type strain containing the carotenoid spheroidene, which has 10 conjugated carbon-carbon double bonds; the GA strain containing neurosporene, which has nine conjugated double bonds; and aerobically grown wild-type cells, as well as aerobically grown H(M182)L mutant, both containing the carbonyl-containing carotenoid spheroidenone, which has 11 conjugated double bonds. By varying the structure of the carotenoid, we observed the effect of altering the energies of the carotenoid excited states on the rate of energy transfer. Both S₁- and S₂-mediated carotenoid-to-bacteriochlorophyll energy transfer processes were observed. The highest transfer efficiency, from both the S ₁ and S₂ states, was observed using the carotenoid with the shortest chain. The S₁-mediated carotenoid-to- bacteriochlorophyll energy transfer efficiencies were determined to be 96%, 84%, and 73% for neurosporene, spheroidene, and spheroidenone, respectively. The S₂-mediated energy transfer efficiencies follow the same trend but could not be determined quantitatively because of limitations in the time resolution of the instrumentation. The dependence of the energy transfer rate on the energetics of the energy transfer acceptor was verified by performing measurements with RCs from the H(M182)L mutant. In this mutant, the bacteriochlorophyll (denoted B_B) located between the carotenoid and the RC special pair (P) is replaced by a bacteriopheophytin (denoted ø_B), where the Q_x and Q_y bands of ø_B are 1830 and 1290 cm^-1, respectively, higher in energy than those of B^B. These band shifts associated with ø_B in the H(M182)L mutant significantly alter the spectral overlap between the carotenoid and ø_B, resulting in a significant decrease of the transfer efficiency from the carotenoid S ₁ state to ø_B. This leaves energy transfer from the carotenoid S₂ state to ø_B as the dominant channel. Largely because of this change in mechanism, the overall efficiency of energy transfer from the carotenoid to P decreases to less than 50% in this mutant. Because the spectral signature of ø_B is different from that of B_A in this mutant, we were able to demonstrate clearly that the carotenoid-to-P energy transfer is via ø_B- This finding supports the concept that, in wild-type RCs, the carotenoid-to-P energy transfer occurs through the cofactor located at the BB position.