Potentiometric titration of Photosystem II fluorescence decay kinetics in spinach chloroplasts

The fluorescence yield of chloroplasts reflects the redox state of the electron acceptor of the Photosystem II reaction center, with increasing yield as the acceptor is reduced. Chemical reductive titrations of fluorescence yield in chloroplasts at room temperature indicate two distinct midpoint potentials, suggesting the possibility of Photosystem II electron acceptor heterogeneity. We have carried out a potentiometric titration of the fluorescence decay kinetics in spinach chloroplasts using a continuous mode-locked dye laser with low-intensity excitation pulses and a picosecond-resolution single-photon timing system. At all potentials the fluorescence decay is best described by three exponential components. As the potential is lowered, the slow phase changes 30-fold in yield with two distinct midpoint potentials, accompanied by a modest (3-fold) increase in the lifetime. The titration curve for the slow component of the fluorescence decay of spinach chloroplasts is best characterized by two single-electron redox reactions with midpoint potentials at pH 8.0 of +119 and ?350 mV, with corresponding relative contributions to the fluorescence yield of 49 and 51%, respectively. There is little change in the fast and middle components of the fluorescence decay. We found that the oxidized form of the redox mediator 2-hydroxy-1,4-naphthoquinone preferentially quenches the fluorescence, causing an anomalous decrease in the apparent midpoint of the high-potential transition. This effect accounts for a significant difference between the midpoint potentials that we observe and some of those previously reported. The selective effect of reduction potentials on particular fluorescence decay components provides useful information about the organization and distribution of the Photosystem II electron acceptor.