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, approximately 50 ps pulses showed the generation of the triplet state of bacteriochlorophyll a (BChl a), whereas measurements using 355 nm, approximately 12 ns pulses showed the generation of BChl a cation radical. Subpicosecond to nanosecond time-resolved absorption spectroscopy using 388 nm, 200 fs pulses for excitation 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, approximately 5 ns pulses for excitation established the generation of BChl a cation radical. The EPR line width suggested that the unpaired electron 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 center, and in LH2 about 50%. Thus, rapid generation of the triplet state, and its subsequent transformation into the cation-radical state of BChl a have been shown to be intrinsic properties of B870 and B850 BChl a assembly in the carotenoidless LH1 and LH2 antenna complexes. In the case of the carotenoid-containing LH2 complex, the triplet 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 cation radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex is shown.
Limantara
L
L
Faculty of Science, Kwansei Gakuin University, Nishinomiya, Japan.
Fujii
R
R
Zhang
J P
JP
Kakuno
T
T
Hara
H
H
Kawamori
A
A
Yagura
T
T
Cogdell
R J
RJ
Koyama
Y
Y
D001426:Bacterial Proteins; D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism*; D002338:Carotenoids / Q000378:metabolism*; D002412:Cations; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000302:isolation & purification / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000378:metabolism; D013057:Spectrum Analysis / Q000379:methods; D013059:Spectrum Analysis, Raman
C081521:B800-850 light-harvesting complex, bacteria; D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D002412:Cations; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; C094744:light-harvesting complex 1, Rhodospirillum; D002338:Carotenoids