On the role of exchangeable hydrogen bonds for the kinetics of P680 +. Q A−. formation and P680 +. Pheo −. recombination in photosystem II

Abstract

Possible effects of the hydrogen bond network on the reactions leading to the stabilized ion-radical pair P680 +. Q A −. formation and P680 +. Pheo −. recombination in PS II were analyzed by introducing modifications due to replacement of H 2O by D 2O or the addition of cryoprotectants. The rate constants of primary charge separation and subsequent stabilization were derived from measurements of time resolved fluorescence decay curves in PS II membrane fragments with reaction centers kept in the open state by addition of K 3[Fe(CN) 6]. The numerical analysis of the experimental data was performed on the basis of the exciton radical pair equilibrium model proposed by Holzwarth and coworkers (Schatz, G. et al. (1988) Biophys. J. 54, 397–405). Recombination kinetics of the radical pair P680 +. Pheo −. were measured using D1-D2-cyt b559 preparations resuspensed in either H 2O or D 2O. It was found that: (a) the molecular rate constants k PC of the primary charge separation and that of the subsequent stabilization step, k stab, were about (3 ps) −1 and (300 ps) −1, respectively; (b) these rate constants were only slightly affected by D 2O or cryoprotectants; (c) small H/D isotope exchange effects were also found in PS II core complexes and Tris-washed PS II membrane fragments; (d) the recombination kinetics of the P680 +. Pheo −. radical pair in D1-D2-cyt b559 preparations is almost invariant to replacement of H 2O by D 2O. The results obtained reveal that in PS II the charge separation and recombination processes are only weakly dependent on the network of hydrogen bridges with exchangeable protons, in contrast to pronounced effects observed in anoxygenic purple bacteria (Paschenko et al. (1987) FEBS Lett. 214, 28–34). The measured differences between both types of reaction centers are discussed.