Photosystem II of green plants: on the possible role of retarded protonic relaxation in water oxidation1

Photosystem II (PSII) of green plants and cyanobacteria uses energy of light to oxidize water and to produce oxygen. The available estimates of the oxidizing potential of P680+, the primary donor of PSII, yield value of about 1.15 V. Two main factors are suggested to add up and engender this high oxidizing potential, namely: (1) the electrostatic influence dominated by Arg-181 of the D2 subunit which elevates the oxidizing potential of P680+ up to 1 V, some 0.1 V above the Em value of a hydrogen-bonded chlorophyll a; and (2) the dynamic component of 0.10-0.15 V due to the experimentally demonstrated retarded protonic relaxation at the P680 site.