POTENTIOMETRIC TITRATION OF FLUORESCENCE YIELD OF SPINACH CHLOROPLASTS

The fluorescence yield of spinach chloroplasts was measured as a function of oxidation-reduction potential under anaerobic conditions at pH 6.0, 7.0, and 8.0. The potentiometric-titration curve showed two fluorescence quenching processes, both quenching in the oxidized state. The complete titration curve could be reasonably fit with a Nernst equation for the sum of two one-electron-transfer reactions with different midpoint potentials. At pH 7.0 the two transitions had midpoint potentials of about -35 mV and -270 mV and both showed a pH dependence of about -60 mV per pH unit. The hypothetical quencher, Q, assumed to be responsible for light-induced fluorescence-yield changes in green photosynthetic systems could be ascribed to the more positive quenching component. © 1969.