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Primary charge separation in closed photosystem ii with a lifetime of 11 ns flash absorption spectroscopy with oxygen evolving photosystem ii complexes from synechococcus



Primary charge separation in closed photosystem ii with a lifetime of 11 ns flash absorption spectroscopy with oxygen evolving photosystem ii complexes from synechococcus



Biochimica et Biophysica Acta 933(1): 22-34



Absorption changes induced by picosecond laser flashes have been studied in purified O2-evolving Photosystem II complexes from Synechococcus sp. If the first quinone acceptor, QAm is reduced before the flash, we observe a rapid phase with approx. 1 ns lifetime which reflects the decay of excited singlet states of antenna pigments, followed by a clearly separated phase of approx. 11 ns. Based on measurements made between 420 nm and 820 nm the 11ns phase is attributed to the decay of the radical pair, chlorophyll aII+ pheophytin a- (Chl .alpha.II+ Pheo-). It decays by charge recombination to the ground state and a small fraction (approx. 20%) to the triplet state 3Chl .alpha.II Pheo via 3(Chl aII+ Pheo-). From the absence of a significant fluorescence decay component with approx. 11 ns lifetime we conclude that the observed radical-pair recombination does not give rise to regeneration of the excited singlet state of Cl .alpha.II and cannot account for the variable fluorescence. The yield of radical-pair formation is estimated to be approx. 60% .+-. 15% of the yield of Chl .alpha.II+ QA- formation in open reaction centers. The data can be explained by a model which is based on the assumption that the radical pair initially generated in closed reaction centers has a higher free energy relative to that in open centers due to the negatively charged QA, but relaxes rapidly to an energetically more favorable state with 11 ns lifetime. The model predicts that the yield of radical pair formation increases with decreasing antenna size. The triplet state of carotenoids, 3Car, was monitored at 515 nm in closed Photosystem II. 3Car was formed with low yield compared to the yield of radical-pair formation. Its rise time of about 25 ns is attributed to triplet energy transfer from chlorophyll triplets in the antenna.

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Primary charge separation in closed Photosystem II with a lifetime of 11 ns. Flash-absorption spectroscopy with O2-evolving Photosystem II complexes from Synechococcus. Biochimica et biophysica acta: International journal of biochemistry and biophysics 933(1): 22-24, 1988

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