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 BB) located between the carotenoid and the RC special pair (P) is replaced by a bacteriopheophytin (denoted øB), where the Qx and Qy bands of øB are 1830 and 1290 cm-1, respectively, higher in energy than those of BB. These band shifts associated with øB in the H(M182)L mutant significantly alter the spectral overlap between the carotenoid and øB, resulting in a significant decrease of the transfer efficiency from the carotenoid S 1 state to øB. This leaves energy transfer from the carotenoid S2 state to øB 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 øB is different from that of BA in this mutant, we were able to demonstrate clearly that the carotenoid-to-P energy transfer is via øB- 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry