Other electron donors in PS II

The cytochrome is normally in its reduced state in intact dark-adapted material but is very sensitive to changes in its environment, changing from its high-potential form ( n, 380 mV) to its low-potential form ( ,n=80 mV). The redox state of the cytochrome seems to have no direct relationship to the function of the 02-evolving enzyme (e.g. Ref. 190). The oxidized Cyt 6-559 gives rise to characteristic low-spin haem EPR signals [191]. 

Since the cytochrome is in its reduced form in vivo and since it is in close contact with the reaction centre, it might have been expected that it would donate electrons to the highly oxidizing intermediates formed in O2 evolution. However, no role in deactivation of the S states has been demonstrated. 

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