Flash-induced redox reactions in spinach PS II core particles were investigated with absorbance difference spectroscopy in the UV-region and EPR spectroscopy. In the absence of artificial electron... Show moreFlash-induced redox reactions in spinach PS II core particles were investigated with absorbance difference spectroscopy in the UV-region and EPR spectroscopy. In the absence of artificial electron accepters, electron transport was limited to a single turnover. Addition of the electron accepters DCBQ and ferricyanide restored the characteristic period-four oscillation in the UV absorbance associated with the S-state cycle, but not the period-two oscillation indicative of the alternating appearance and disappearance of a semiquinone at the Q(B)-site. In contrast to PS II membranes, all active centers were in state S-1 after dark adaptation. The absorbance increase associated with the S-state transitions on the first two flashes, attributed to the Z(+)S(1) --> ZS(2) and Z(+)S(2) --> ZS(3) transitions, respectively, had half-times of 95 and 380 mu s, similar to those reported for PS II membrane fragments. The decrease due to the Z(+)S(3) --> ZS(0) transition on the third flash had a half-time of 4.5 ms, as in salt-washed PS II membrane fragments. On the fourth flash a small, unresolved, increase of less than 3 mu s was observed, which might be due to the Z(+)S(0) --> ZS(1) transition. The deactivation of the higher S-states was unusually fast and occurred within a few seconds and so was the oxidation of S-0 to S-1 in the dark, which had a half-time of 2-3 min. The same lifetime was found for tyrosine D+, which appeared to be formed within milliseconds after the first flash in about 10% inactive centers and after the third and later flashes by active centers in Z(+)S(3). Show less
