Effects of pH on reactions on the donor side of Photosystem II

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The effects of pH on the increase of fluorescene yield measured in the microsecond range, and on the microsecond delayed fluorescence have been studied in dark adapted chloroplasts as a function of flash number. 1. (1) At pH 7, the amplitude of the fast-phase of the microsecond fluorescence yield rise oscillated as a function of flash number with period 4 and with maxima on flashes 1 and 5, and minima on flashes 3 and 7. The damped oscillations were apparent over the range between 6 and 8, although the absolute amplitude of the fast phase was diminished at the lower end of the range. At pH 4, there was no fast phase in the rise and, at pH 9, an enhanced fast-phase occurred only for the first flash. 2. (2) The decay of microsecond delayed fluorescence was described by the sum of exponentials with half-times of 10–15 μs and 40–50 μs. Over the pH range 6– <8, the extrapolated initial amplitude and the proportion of the change due to the faster component showed oscillations which were opposite in phase to those observed for the prompt fluorescence yield rise; the slower component showed weaker oscillations of the same phase. At pH 4, there were no oscillations and the slow phase predominated. At pH 9, the delayed fluorescence intensity was diminished on the first flash, and high on subsequent flashes. 3. (3) The results are interpreted in terms of a model in which protons are released during all transitions of the S-states with the exception of S 1 → S 2, and in which there are two sites of inhibition on the donor side of the photosystem at extreme pH values. At pH 4, electron donation to P + occurs with a half-time approx. 135 μs, either by a back reaction from Q −, or from D; electron transport is interrupted between Z 1 and P. At pH 9, electron transport is inhibited between Z 1 and Z 2; rapid re-reduction of P + by Z 1 occurs after 1 flash, and on subsequent flashes electrons from D, an alternative donor reduce P +. The location of the positive charge on states S 2 and S 3 is discussed.