FLASH-INDUCED REDOX CHANGES IN OXYGEN-EVOLVING SPINACH PHOTOSYSTEM-IICORE PARTICLES

Flash-induced redox reactions in spinach PS II core particles were inv estigated with absorbance difference spectroscopy in the UV-region and EPR spectroscopy. In the absence of artificial electron accepters, el ectron transport was limited to a single turnover. Addition of the ele ctron accepters DCBQ and ferricyanide restored the characteristic peri od-four oscillation in the UV absorbance associated with the S-state c ycle, but not the period-two oscillation indicative of the alternating appearance and disappearance of a semiquinone at the Q(B)-site. In co ntrast to PS II membranes, all active centers were in state S-1 after dark adaptation. The absorbance increase associated with the S-state t ransitions on the first two flashes, attributed to the Z(+)S(1) --> ZS (2) and Z(+)S(2) --> ZS(3) transitions, respectively, had half-times o f 95 and 380 mu s, similar to those reported for PS II membrane fragme nts. The decrease due to the Z(+)S(3) --> ZS(0) transition on the thir d flash had a half-time of 4.5 ms, as in salt-washed PS II membrane fr agments. On the fourth flash a small, unresolved, increase of less tha n 3 mu s was observed, which might be due to the Z(+)S(0) --> ZS(1) tr ansition. The deactivation of the higher S-states was unusually fast a nd 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 millisecond s after the first flash in about 10% inactive centers and after the th ird and later flashes by active centers in Z(+)S(3).