Triplet States in Photosynthetic Reaction Centers of Rb. sphaeroides

The photoexcited triplet state of the primary donor P865 and of the carotenoid spheroidene in the reaction center of Rb. sphaeroides wild type and mutants were investigated by EPR spectroscopy. Different mechanisms of triplet formation were observed: At T = 10 K, Rb. sphaeroides strain R.26−1, 2.4.1 and the double mutant GD(M203)/ W(M260) form a triplet via the radical pair mechanism. Charge separation in these species proceeds exclusively via the A-branch of the reaction centers. In the double mutant LH(M214)/AW(M260) the intersystem crossing mechanism is dominant. At T < 30 K, the radical pair mechanism via the B-branch also contributes to the triplet formation. Moreover, at these temperatures the carotenoid takes over the triplet state from 3P865. Anisotropy of triplet-triplet transfer has been observed that can be traced back to the anisotropy of 3P865 formation from radical pair states. The data confirm that triplet energy transfer from 3P865 to Car proceeds via the accessory BChl, whereby the 3P865→BChla transfer step is rate limiting.