We have used flash-detection optical and electron paramagnetic resonance spectroscopy to measure the kinetics and yield per flash of the photooxidation of cytochrome b559 and the yield per flash of the photooxidation of the tyrosine residue YD in Mn-depleted photosystem II (PSII) membranes at room temperature. The initial charge separation forms Yz+ QA−. Following this, cytochrome b559 is oxidized on a time scale of the same order and with the same pH dependence as is observed for the decay of Yz+; under the conditions of our experiments, the decay of Yz+ is determined by the lifetime of Yz+ QA−. In order to explain this observation, we have constructed a model for electron donation in which Yz+ and P680+ are in redox equilibrium and cytochrome b559 and YD are oxidized via P680+. Using our results, together with data from earlier investigations of the kinetics of electron transfer from Yz to P680+ and charge recombination of Yz+ QA−, we have obtained the first global fit for electron donation in Mn-depleted PSII that accounts for the data over the pH range from 5 to 7.5. From these calculations, we have obtained the intrinsic rate constants of all the electron-donation reactions in Mn-depleted PSII. These rate constants allow us to calculate the free energy difference between Yz+ P680 and Yz P680+, which is found to increase by 47 ± 4 mV/pH from pH 5 to 6 and is observed to increase more slowly per pH unit for pH > 6. An important conclusion of our experimental work is that the rates of photooxidation of cytochrome b559 and YD are determined by the lifetime of the oxidizing equivalent on Yz/P680. Extension of our model to oxygen-evolving PSII samples leads to the prediction that the kinetics and yields of electron donation from cytochrome b559 and YD to P680+ will depend on the S2−. or S3-state lifetime.
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