Electric field effect on the picosecond fluorescence of Photosystem II and its relation to the energetics and kinetics of primary charge separation

Abstract

Across the thylakoid membrane of spinach chloroplasts a diffusion potential of defined magnitude can be created by rapid changes in the KCl concentration. The resulting electric field leads to an increase in the yield of photosystem II (PS II) fluorescence for positive membrane voltages — positive in the inner thylakoid space — and a decrease for negative membrane voltages (Dau and Sauer, 1991, BBA 1098, 49–60). We have how studied this phenomenon by measurement of picosecond fluorescence decay. Based on the kinetic PS II model of Schatz et al. (1988, Biophys. J. 54, 397–405), the fluorescence decays were interpreted in terms of the rate constant of primary charge separation (formation of reaction center cation radical and pheophytin anion radical) primary charge recombination (recombination of primary biradical to chlorophyll singlet state) and secondary charge separation (reduction of primary quinone acceptor). For increasingly positive thylakoid voltages, the results are indicative of a relatively small but significant decrease in the rate constant of primary charge separation (by about 8% per + 100 mV thylakoid voltage) and a much larger increase (by about 50% per + 100 mV) of the rate constant of primary charge recombination. Nevertheless, due to the high sensitivity of the PS II fluorescence yield to changes in the rate constant of primary charge separation, the field-induced increase of fluorescence yield results mainly from the decreased rate constant of primary charge separation, and to a smaller extent (about one third of the increase) from the increased rate constant of charge recombination. The free energy difference of the primary radical pair was found to change by 17 meV per 100 mV thylakoid voltage. The relation between the rate constant of primary charge separation and the free energy difference appears to be linear within the accessible range of free energy changes (−15 meV to +22 meV); the rate constant of primary charge separation increases with increasingly negative free energy differences by 6% per 10 meV. This free energy dependence is compared with model calculations for a sequential two-step and for a super-exchange model of the primary PS II charge separation.