ON THE ROLE OF EXCHANGEABLE HYDROGEN-BONDS FOR THE KINETICS OF P680(-DOT) Q(A)(-CENTER-DOT) FORMATION AND P680(+CENTER-DOT) PHEO(-CENTER-DOT) RECOMBINATION IN PHOTOSYSTEM-II(CENTER)

Possible effects of the hydrogen bond network on the reactions leading to the stabilized ion-radical pair P680(+.) Q(A)(-) formation and P68 0(+.) Pheo(-.) recombination in PS II were analyzed by introducing mod ifications due to replacement of H2O by D2O or the addition of cryopro tectants. The rate constants of primary charge separation and subseque nt stabilization were derived from measurements of time resolved fluor escence 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 a nalysis of the experimental data was performed on the basis of the exc iton radical pair equilibrium model proposed by Holzwarth and coworker s (Schatz, G. et al. (1988) Biophys. J. 54, 397-405). Recombination ki netics of the radical pair P680(+.) Pheo(-.) were measured using D1-D2 -cyt b559 preparations resuspensed in either H2O or D2O. It was found that: (a) the molecular rate constants k(PC) of the primary charge sep aration and that of the subsequent stabilization step, k(stab), were a bout (3 ps)(-1) and (300 ps)(-1), respectively; (b) these rate constan ts were only slightly affected by D2O 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 i s almost invariant to replacement of H2O by D2O, 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 exc hangeable protons, in contrast to pronounced effects observed in anoxy genic purple bacteria (Paschenko et al. (1987) FEES Lett, 214, 28-34), The measured differences between both types of reaction centers are d iscussed.