Biophys J, April 2001, p. 1617-1630, Vol. 80, No. 4

Modulation of Primary Radical Pair Kinetics and Energetics in Photosystem II by the Redox State of the Quinone Electron Acceptor Q_A

Krzysztof Gibasiewicz,^* Andrzej Dobek, Jacques Breton,^* and Winfried Leibl^*

 ^*Section de Bioénergétique, DBCM, F-91191 Gif-sur-Yvette Cedex, France; and  Institute of Physics, Adam Mickiewicz University, 61-614 Poznan, Poland

Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q_A either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680⁺Pheo. The maximum transient concentration of the primary radical pair is about five times larger and its decay is about 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 molecular rate constants and energetic parameters for primary reactions in the reaction centers with doubly reduced Q_A as well as with oxidized or singly reduced Q_A. The standard free energy difference G° between the charge-separated state P680⁺Pheo and the equilibrated excited state (Chl_NP680)* was found to be similar when Q_A was oxidized or doubly reduced before the flash (~50 meV). In contrast, single reduction of Q_A led to a large change in G° (~+40 meV), demonstrating the importance of electrostatic interaction between the charge on Q_A and the primary radical pair, and providing direct evidence that the doubly reduced Q_A is an electrically neutral species, i.e., is doubly protonated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in G° than the rate of primary charge separation.

Biophys J, April 2001, p. 1617-1630, Vol. 80, No. 4
© 2001 by the Biophysical Society   0006-3495/01/04/1617/14  $2.00