A New EPR Signal Associated with Photosystem 1 of Algal and Plant Photosynthesis1

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Abstract
A new low temperature electron paramagnetic resonance (EPR) signal with a g -value of 1.97 was found in Photosystem-1 particles from a blue-green alga, Anacystis nidulans , illuminated at room temperature. A similar signal was also found in spinach Photosystem-1 particles treated with thiophenol to decrease interference from a signal due to Center A. In the dark, the signal appeared only when the Anacystis particles were at redox potentials lower than −0.5 volts where Centers A and B were also reduced. The signal is most likely due to another iron-sulfur cluster, tentatively designated as Center C. Center C could be photoreduced at low temperatures like Center A when Centers A and B were partially reduced prior to illumination, indicating possible close association of these centers in Photosystem 1 of green plant and algal photosynthesis.