Abstract
This chapter discusses the properties of the electron acceptor complex of photosystem II. It discusses electron paramagnetic resonance (EPR) experiments with the photosystem II reaction center that suggest a close analogy with the purple bacterial reaction center. EPR signals from the pheophytin and iron–quinone electron acceptors can be detected, magnetic interaction between them is observed as in purple bacteria, and the redox properties of these components in purified reaction centers also fit the model quite well. It was shown that triplet formation is maximal only below -400 mV indicating the presence of an electron acceptor with Em ≈ 430 mV operating at low temperature. There does not, however, seem to be any effect of reduction of this component on formation of the pheophytin signal following 200K illumination. More experiments are clearly required to understand the different results in algal preparations and in higher plant chloroplasts, and the role of the low potential acceptor affecting reaction center triplet formation.
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