THE ROLE OF PROTONATION STEPS IN ELECTRON-TRANSFER REACTIONS IN TRIS-TREATED PHOTOSYSTEM 2 MEMBRANE-FRAGMENTS

The influence of H/D-exchange on the electron transfer from Y-Z to P68 0(+.), and the recombination reaction between P680(+.) and Q(A)(-.) in Tris-treated photosystem 2 (PS2) membrane fragments at pL [L=lyonium ion (H,D)] = 6.5 was investigated by monitoring and numerical analysis of flash-induced absorption changes at 830 and 320 nm, respectively. The H/D-exchange caused retardation by a factor of approximately 3 of the electron transfer from Y-Z to P680(+.). In marked contrast, no sig nificant effect was observed on the kinetics of P680(+.)Q(A)(-.) charg e recombination. In addition, the pH-dependence of P680(+.)Q(A)(-.) re combination kinetics were analysed in samples where Y-Z was functional ly eliminated by exposure of Tris-treated PS2 fragments to strong irra diance. In this case the relaxation kinetics could be fitted by three- exponentials with half lifetimes of 150 mu s (fast), 800 mu s (middle) and 10 ms (slow) at pH = 6.0. The fast and middle kinetics were only slightly dependent on pH in the range from 5.0 to 8.0. On the other ha nd, the normalised amplitudes of these kinetics were markedly pH-depen dent. Furthermore, the normalised extent of the slow kinetics was sign ificantly larger in the absorption changes at 320 nm, reflecting the t urnover of Q(A), than at 830 nm as an indicator of P680(+.) formation and decay. One possible explanation of this feature is provided by an assumption that Tris-washed PS2 membrane fragments exposed to a strong irradiance contain a redox component competing with Q(A)(-.) in the r eduction of P680(+.). Furthermore, the pH-dependent changes of the ove rall kinetics of P680(+.)Q(A)(-.) recombination originated predominant ly from different ratios of the extent of the fast and middle componen ts rather than from marked modifications of the rate constants.