Generation of triplet and cation-radical bacteriochlorophyll a in carotenoidless LH1 and LH2 antenna complexes from Rhodobacter sphaeroides

The LH1 antenna complex and a native form of the LH2 complex were isolated from the carotenoidless R26 and R26.1 mutants of Rhodobacter sphaeroides by the use of a new detergent, sucrose monocholate, One-color, pump-and-probe transient Raman spectroscopy of these complexes using 351 nm, similar to 5 0 ps pulses showed the generation of the triplet state of bacteriochlorophy ll a (BChl a), whereas measurements using 355 nm, similar to 12 ns pulses s howed the generation of BChl a cation radical. Subpicosecond to nanosecond time-resolved absorption spectroscopy using 388 nm, 200 fs pulses for excit ation showed rapid (<1 ps) generation of the triplet state and fast decay ( <10 ps) of the singlet state of BChl a. Microsecond absorption spectroscopy confirmed the generation of BChl a cation radical. EPR spectroscopy using 532 nm, similar to 5 ns pulses for excitation established the generation of BChl a cation radical. The EPR line width suggested that the unpaired elec tron is shared by two BChl a molecules. In LH1, the yield of BChl a cation radical per complex was estimated to be about 80% of that in the reaction c enter, and in LH2 about 50%, Thus, rapid generation of the tripler state, a nd its subsequent transformation into the cation-radical state of BChl a ha ve been shown to be intrinsic properties of B870 and B850 BChl a assembly i n the carotenoidless LH1 and LH2 antenna complexes. In the case of the caro tenoid-containing LH2 complex, the tripler states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet- state BChl a was quenched efficiently by the carotenoid so that no BChl a c ation radical was generated, Thus, the photoprotective function of the caro tenoid in this antenna complex is shown.