Modulation of primary radical pair kinetics and energetics in photosystem II by the redox state of the quinone electron acceptor Q(A)

Time-resolved photovoltage measurements on destacked photosystem II membran es from spinach with the primary quinone electron acceptor Q(A) either sing ly or doubly reduced have been performed to monitor the time evolution of t he primary radical pair P680(+)Pheo(-). The maximum transient concentration of the primary radical pair is about five times larger and its decay is ab out seven times slower with doubly reduced compared with singly reduced Q(A ). The possible biological significance of these differences is discussed. On the basis of a simple reversible reaction scheme, the measured apparent rate constants and relative amplitudes allow determination of sets of molec ular rate constants and energetic parameters for primary reactions in the r eaction centers with doubly reduced Q(A) as well as with oxidized or singly reduced Q(A). The standard free energy difference DeltaG degrees between t he charge-separated slate P680(+)Pheo(-) and the equilibrated excited state (Chl(N)P680)* was found to be similar when Q(A) was oxidized or doubly red uced before the flash (similar to -50 meV). In contrast, single reduction o f Q(A) led to a large change in DeltaG degrees (similar to-+40 meV), demons trating the importance of electrostatic interaction between the charge on Q (A) and the primary radical pair, and providing direct evidence that the do ubly reduced Q(A) is an electrically neutral species, i.e., is doubly proto nated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in DeltaG degrees than the rate of primary charge separation.