Primary charge separation in closed Photosystem II with a lifetime of 11 ns: Flash-absorption spectroscopy with O 2-evolving Photosystem II complexes from Synechococcus

Abstract

Absorption changes induced by picosecond laser flashes have been studied in purified O 2-evolving Photosystem II complexes from Synechococcus sp. If the first quinone acceptor, Q A, 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 11 ns phase is attributed to the decay of the radical pair, chlorophyll a II + pheophytin a − (Chl a II + Pheo −). It decays by charge recombination to the ground state and a small fraction (approx. 20%) to the triplet state 3Chl a II Pheo via 3(Chl a II + 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 Chl a 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 a II + Q − A 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 Q A, 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.