Primary Reactions of Photosynthesis: Pho Bound Chloroplast Ferredoxin at Low Tei Detected by EPR Spectroscopy

An electron paramagnetic resonance sig- nal was observed at 25?K in whole spinach chloroplasts after illumination at 77?K. The light-induced epr spec- trum had g-values (g- = 1.86, gy = 1.94, gz = 2.05) and a temperature dependence that were characteristic of the reduced state of a plant-type ferredoxin. The light-in- duced epr spectrum was also observed in broken spinach chloroplasts from which soluble ferredoxin was removed. Chemical analyses showed that both whole and broken spinach chloroplasts contained amounts of nonheme iron and "acid-labile sulfide" consistent with the presence of a bound iron-sulfur protein, at a level of about one molecule per 75 chlorophyll molecules. These results support the conclusion that chloroplasts contain a bound ferredoxin that may serve as a primary low-potential electron acceptor