Title: Quantitative EPR studies of the primary reaction of Photosystem I in chloroplasts
Abstract: Quantitative electron paramagnetic resonance studies of the primary event associated with Photosystem I in chloroplasts have been carried out at 25 °K. After illumination of either whole chloroplasts or Photosystem I subchloroplast fragments (D-144) with 715-nm actinic light at 25 °K, equal spin concentrations of oxidized P700 and reduced bound iron-sulfur protein (bound ferredoxin) have been measured. Quantitative determination of the concentration of these two carriers by EPR spectroscopy after illumination at low temperature indicates that Photosystem I fragments are enriched in P700 and the bound iron-sulfur protein as compared with unfractionated chloroplasts. These results indicate that P700 and the bound iron-sulfur protein function as the donor-acceptor complex of chloroplast Photosystem I.