Magnetic field affects the fluorescence yield in reaction center preparations from Rhodopseudomonas sphaeroides R-26

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Purified photochemical reaction centers from Rhodopseudomonas sphaeroides R-26 were reduced with Na 2S 2O 4 so as to block their photochemical electron-transfer reactions. The magnetic field induced an increase in the emission yield. Our results support the hypothesis that under these conditions, charge recombination in the singlet radical pair composed of the oxidized primary donor and reduced primary acceptor predominantly generates the excited singlet state of the reaction center bacteriochlorophyll. The maximum relative fluorescence change and the value of the magnetic field at which half-saturation of the effect is achieved ( B 1 2 ) at room temperature are 5.5% and 75 G, respectively. For the whole cells of Rps. sphaeroides R-26 these parameters are 1.2% and 120 G. The relative fluorescence change at 600 G, ΔF F(600) , and B 1 2 are studied as functions of temperature. The temperature dependencies of ΔF F(600) for reaction centers and whole cells of Rps. sphaeroides R-26 are qualitatively the same, with the maximum effect (8% for reaction centers) occurring at 230 K. However, the B 1 2 curves for the two preparations are different.