Radiative and non-radiative charge recombination pathways in Photosystem II studied by thermoluminescence and chlorophyll fluorescence in the cyanobacterium Synechocystis 6803

The mechanism of charge recombination was studied in Photosystem II by using flash induced chlorophyll fluorescence and thermoluminescence measurements. The experiments were performed in intact cells of the cyanobacterium Synechocystis 6803 in which the redox properties of the primary pheophytin electron acceptor, Phe, the primary electron donor, P₆₈₀, and the first quinone electron acceptor, Q_A, were modified. In the D1Gln130Glu or D1His198Ala mutants, which shift the free energy of the primary radical pair to more positive values, charge recombination from the S₂Q_A⁻ and S₂Q_B⁻ states was accelerated relative to the wild type as shown by the faster decay of chlorophyll fluorescence yield, and the downshifted peak temperature of the thermoluminescence Q and B bands. The opposite effect, i.e. strong stabilization of charge recombination from both the S₂Q_A⁻ and S₂Q_B⁻ states was observed in the D1Gln130Leu or D1His198Lys mutants, which shift the free energy level of the primary radical pair to more negative values, as shown by the retarded decay of flash induced chlorophyll fluorescence and upshifted thermoluminescence peak temperatures. Importantly, these mutations caused a drastic change in the intensity of thermoluminescence, manifested by 8- and 22-fold increase in the D1Gln130Leu and D1His198Lys mutants, respectively, as well as by a 4- and 2.5-fold decrease in the D1Gln130Glu and D1His198Ala mutants, relative to the wild type, respectively. In the presence of the electron transport inhibitor bromoxynil, which decreases the redox potential of Q_A/Q_A⁻ relative to that observed in the presence of DCMU, charge recombination from the S₂Q_A⁻ state was accelerated in the wild type and all mutant strains. Our data confirm that in PSII the dominant pathway of charge recombination goes through the P₆₈₀⁺Phe⁻ radical pair. This indirect recombination is branched into radiative and non-radiative pathways, which proceed via repopulation of P₆₈₀^* from ¹P₆₈₀⁺Ph⁻] and direct recombination of the ³P₆₈₀⁺Ph⁻] and ¹P₆₈₀⁺Ph⁻] radical states, respectively. An additional non-radiative pathway involves direct recombination of P₆₈₀⁺Q_A⁻. The yield of these charge recombination pathways is affected by the free energy gaps between the Photosystem II electron transfer components in a complex way: Increase of ΔG(P₆₈₀^* ↔ P₆₈₀⁺Phe⁻) decreases the yield of the indirect radiative pathway (in the 22-0.2% range). On the other hand, increase of ΔG(P₆₈₀⁺Phe⁻ ↔ P₆₈₀⁺Q_A⁻) increases the yield of the direct pathway (in the 2-50% range) and decreases the yield of the indirect non-radiative pathway (in the 97-37% range).