Magnetic-field effects on primary reactions in Photosystem I

Magnetic-field effects on primary reactions of Photosystem I have been studied by measuring changes of the fluorescence yield in preparations isolated from the cyanobacterium Synechococcus elongatus. Photoaccumulation of the reduced phylloquinone A(1) in the presence of dithionite under anaerobic conditions led to an increase of chlorophyll fluorescence yield and appearance of magnetic field effects. A magnetic field effect has been found which is characterized by half-saturation values of the order of similar to 25 G. Addition of salts of monovalent and divalent cations to the suspension significantly modified the profile of the magnetic field dependence. The modification of the magnetic field effect by salts is ascribed to conformational changes in the Photosystem I reaction centre that affect the dipolar and/or spin exchange coupling in the radical pair P(700)(+)A(0)(-). Injection of neutral red into suspensions of samples containing reaction centres in the state P(700)A(0)A(1)(red) gives rise to light induced fluorescence quenching and disappearance of the magnetic field effect. Subsequent incubation in the dark restored the high fluorescence yield and the magnetic field effect. It is inferred that under these conditions the Photosystem I reaction centres are photoaccumulated in the redox state P-700 A(0)(-)A(1)(red) where the anion radical A(0)(-) acts as fluorescence quencher.