Kinetic and Energetic Model for the Primary Processes in Photosystem II

A detailed model for the kinetics and energetics of the exciton trapping, charge separation, charge recombination, and charge stabilization processes in photosystem (PS) II is presented. The rate constants describing these processes in open and closed reaction centers (RC) are calculated on the basis of picosecond data (Schatz, G. H., H. Brock, and A. R. Holzwarth. 1987. Proc. Natl. Acad. Sci. USA. 84:8414–8418) obtained for oxygen-evolving PS II particles from Synechococcus sp. with ∼80 chlorophylls/P680. The analysis gives the following results. (a) The PS II reaction center donor chlorophyll P680 constitutes a shallow trap, and charge separation is overall trap limited. (b) The rate constant of charge separation drops by a factor of ∼6 when going from open (Q-oxidized) to closed (Q-reduced) reaction centers. Thus the redox state of Q controls the yield of radical pair formation and the exciton lifetime in the Chl antenna. (c) The intrinsic rate constant of charge separation in open PS II reaction centers is calculated to be ∼2.7 ps-1. (d) In particles with open RC the charge separation step is exergonic with a decrease in standard free energy of ∼38 meV. (e) In particles with closed RC the radical pair formation is endergonic by ∼12 meV. We conclude on the basis of these results that the long-lived (nanoseconds) fluorescence generally observed with closed PS II reaction centers is prompt fluorescence and that the amount of primary radical pair formation is decreased significantly upon closing of the RC