Modification of the pheophytin midpoint potential in photosystem II: Modulation of the quantum yield of charge separation and of charge recombination pathways

We investigated the dependence of both the quantum yield of charge separation and pathways of charge recombination on the free energy level of the radical pair state P+Ph− in photosystem II. This was done by comparing the basal (F0) fluorescence yield and the recombination rate of the S2QA− state in various strains of Chlamydomonas reindhardtii in which the strength of a H-bond to the pheophytin bound to the D1 subunit has been modified by site-directed mutagenesis. In agreement with previous results with homologous mutants in the cyanobacterium Synechocystis, the quantum yield decreased and the recombination of S2QA− was slowed down when the energy level of the radical pair was increased. The decreased quantum yield is analyzed in terms of a modified equilibrium between exciton and radical pair. The effects on the recombination rate confirm that in the wild type the process involves the P+Ph− state. Analysis of these results shows that, as in the case of Synechocystis, the energy level of the P+Ph− is less negative than currently thought by about 250 meV. An important consequence is a similar upward revision of the potential of the P+/P couple.